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A Path to the Next Generation of U.S. Banknotes: Keeping Them Real (2007)

Chapter:Appendix D Long-Term Feature and Feature Platform Descriptions

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Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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D
Long-Term Feature and Feature Platform Descriptions

This appendix has in-depth feature descriptions of the longer-term features and feature platforms—that is, features that can be implemented in a time frame of more than 7 years. These features are also discussed in Chapter 5. Each feature description includes subheadings dealing with various aspects of the feature:

  • Description—An explanation of the physical principle(s) on which the feature is based. Also, the feature application as visible, machine-readable, applicable to the visually impaired, forensic applicability, and so on is described. Furthermore, the benefits and limitations of the feature are presented; graphics may be included to depict the feature and its operation.

  • Feature Motivation—A summary of the reasons why the feature is highly rated by the committee and reference to its uniqueness.

  • Potential Implementations—A description of scenarios that provide examples of how the feature could be employed to deter counterfeiting.

  • Materials and Manufacturing Technology Options—A summary of the materials and manufacturing process that could be used to produce the feature as well as initial thoughts on how the feature could be integrated into a Federal Reserve note.

  • Simulation Strategies—A discussion of potential ways in which a counterfeiter could simulate or duplicate the feature, and the expected degree of difficulty in attempting to do so.

  • Key Development Risks and Issues: Phase I—A discussion of the durability challenges, feature aesthetics, anticipated social acceptability, and descrip-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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tion of the key technical challenges that must be addressed during the first phase of the development process to demonstrate the feasibility of the feature idea: that is, demonstrate feature capabilities and determine the usefulness in counterfeit deterrence. (The development phases are defined in Chapter 6.)

  • Development Plan: Phase I—A characterization of the current maturation level of the feature technology, key milestones to be achieved during Phase I, known current and planned related developments external to the Bureau of Engraving and Printing (BEP), and a high-level schedule for Phase I.

  • Estimate of Implemented Production Costs—An initial assessment of additional BEP operational steps that would be required at the BEP to produce a banknote with the feature, incremental cost (higher, lower, the same) relative to the cost of the current security thread, and an indication of whether additional BEP capital equipment would be required for production.

  • References and Further Reading—Selected references relating to the feature and its associated components. Such references could include, for example, papers and conference proceedings for background on any work done relating to this feature. These lists are not exhaustive but are intended to provide a snapshot of current work related to the feature concept.

The features described in this appendix are as follows:

  • Anomalous Currency Space

  • Chemical Sensors

  • Digitally Encrypted Substrate

  • Engineered Cotton Fibers

  • e-Substrate

  • NiTi Shape Memory and Superelastic Responsive Materials

  • Smart Nanomaterials

  • Tactilely Active Electronic Features

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ANOMALOUS CURRENCY SPACE

Description

The “anomalous currency space,” or ACS (pronounced “ace”), is a materials-based approach that can serve as a platform for a wide range of anticounterfeiting strategies by providing a region or regions that differ entirely in materials composition from the banknote substrate. The primary objective is to provide an eye-catching visual feature that would also possess tactile properties. It is expected that the unique structure and composition of advanced materials incorporated into an ACS would assist forensic investigation as well.

This empty space can notionally be thought of as a clear plastic window. However, it does not have to be shaped like a typical window. For example, the shape could be a strip that runs the full length or width of the banknote, or a strip that runs along any or all of the edges of the note, or a series of regions dispersed throughout the currency note. Also, the materials composition of this region does not necessarily need to be a clear plastic or other polymer.

In this context, “anomalous” is used to emphasize that there is a physical space within the banknote that differs dramatically in terms of materials composition and behavior from the rest of the bill. Because Federal Reserve notes (FRNs) and their analogues already use a multiplicity of features that differ dramatically in materials structure and properties, the term “anomalous” refers to a macroscopic region of radical discontinuity relative to the bulk composition of the bill or note.

Feature Motivation

This feature platform offers numerous ways to create an eye-catching visual feature that could also possess other properties, such as a distinctive feel. The ACS provides for the incorporation of heterogeneous materials into the FRN in a manner that would not allow the ACS region to be inconspicuously removed or tampered with; also, these materials would have durability for the lifetime of the banknote.

Potential Implementations

The clear plastic window or variations on this theme are already in use in some foreign currencies, so it appears that this feature platform concept has been successfully introduced. The concept here is to extend the scope of this feature significantly. The polymeric material itself may be further modified in any number of ways, including the following:

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
  • Direct integration of electro-optical or other types of materials within the polymer. The material could be anything up to and including complete integrated circuitry.

  • Surface etching or other physicochemical modifications to one or both surfaces (back and front) of the ACS feature to create novel electro-optical or other effects.

  • Complete perforation of the polymer to enable a number of effects, from patterned microholes for physical identification to a type of diffraction grating.

  • Incorporation of “smart” materials (including nanomaterials) whose unusual properties are based on new compositions and/or structures and that are capable of dynamic interaction with the environment: for example, memory polymers that shape-shift on the basis of a change in a physicochemical parameter (for example, temperature).

  • Use of dumb materials (including nanomaterials) whose unusual properties are based on new types of composition and/or structure but do not respond to environmental stimuli: for example, an ultratough polymeric strip that traverses the entire border of the FRN and is impossible to tear.

  • Inclusion of other materials in or on the polymer to create composites with various properties (for example, holographic metal strips).

  • Employment of other materials with unique active and/or passive properties, structures, or behaviors.

The window could be composed of two (or more) layers. Polymeric layers, for example, could have any or all of the properties described above. Further, the space(s) between the polymeric layers could contain additional materials. In the simplest case, two polymeric layers would be embedded flush with the two surfaces of the FRN, with each layer being less than 50 percent of the total thickness of the note itself. For example, assume a thickness of ~100 micrometers for U.S. currency and a thickness of 40 micrometers for a single polymeric layer. Assuming the polymer layers do not collapse and adhere to each other, the linear space between them is 20 micrometers in the center region (the ends would be embedded into the substrate). If the “window” is 1 centimeter square, the three-dimensional space between the two polymeric layers creates a volume of 2 microliters. This volume could be (fully or partially) filled with a novel material or composite, including current microscale and nanoscale materials and those in development as part of the National Nanotechnology Initiative (NNI). These materials and composites could be smart or dumb, active or passive, and so on. A larger window or thinner polymer layer would increase the available interlayer volume.

Given that U.S. currency exhibits desirable characteristics of materials strength, toughness, and so on with a thickness of ~100 micrometers, it is not unreasonable

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

to assume the existence of materials that provide the same level of performance at less than half that thickness which, in turn, creates the interlayer space discussed about. Since future research will be conducted on supertough materials, it is reasonable to assume that layer thicknesses down to 10 micrometers or even thinner might be achievable. Such thinness would create the opportunity for larger interlayer volumes and/or multiple layers within the window. In the latter case, holographic-like effects and/or color-changing effects should be possible.

There are many ways in which a banknote designer could apply the ACS feature platform concept. A few ideas include the following:

  • Strip along outer edge. A strip of ultratough materials around the outer edge of the bill could be easily detected by its distinct look and feel—this region could not be torn or perforated.

  • Distribution of small ACS features throughout the banknote. Distribution of smaller ACSs throughout the bill could produce easily recognizable patterns that could be used for visible, tactile, and possibly instrument-based detection. Such patterns could exhibit dynamic as well as passive behavior if, for example, memory metals or polymers were used.

  • Memory polymers. Memory materials fall into a larger category of smart materials that exhibit unique behavior. Memory polymers (often constructed of bulk copolymers) are capable of dynamic movement and associated shape-shifting when the variable of state is applied (often a change in temperature). For example, spatial distribution of a memory polymer within a certain space could allow the surface to change when it fell above or below body temperature. A simple case would involve reversible surface stippling that would manifest as a change in roughness, which could be detected qualitatively by touch and quantified by instrumentation. Embedded circuitry is specifically excluded from consideration here, so the behavior of these types of smart materials would be strictly dependent on the composition of the material itself and would require no dedicated power source.

As a further example, memory polymers have been produced for several biomedical applications, including self-tying surgical sutures. A memory polymer can exist in either of two states: elongated (two-dimensional fiber) or contracted (three-dimensional cylindrical). This material, produced by block copolymerization, could be incorporated into a clear plastic window in such a manner that the cylindrical watermark-like image could be created by the variation in light transmission produced by the three-dimensional state. Suppose the transition temperature was adjusted to ~35°C. The simulated watermark would appear in the window at temperatures below 35°C and disappear above 35°C. It is reasonable to suppose that U.S. currency spends more than 90 percent of its time at temperatures

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

below 35°C. Therefore, FRN verification could occur by vigorously rubbing the window between the thumb and forefinger. Because the window would be ≥100 micrometers thick, frictional heating would rapidly raise the temperature, causing the image to disappear. As soon as the window was allowed to cool, the image would reappear.

A simpler alternative might be to construct the window so that the image disappeared when the window was physically stretched in one (or any) direction relative to the xyz coordinates of the FRN itself. This could be accomplished by controlling the orientation of the coiled memory polymers during fabrication so that the image was formed much like crosshatched pen and ink work or the engraving process itself. Since the image would disappear rather than becoming distorted, this trait could not be simulated by simply using a window made of an elastic material.

Materials and Manufacturing Technology Options

The ACS, by definition, has no specific material requirement other than being a completely different material from the bulk of the FRN. The requisite manufacturing technologies would depend entirely on the feature design and the materials selected for the feature application. The ACS provides a flexible mechanism whereby these advanced materials can be incorporated into currency with minimum disruption to the production process. Like the silicon wafer facilities required for integrated-circuit fabrication, extremely-high-technology equipment would be required for the production of some of these new materials, but once the manufacturing process was online the cost per unit would drop to the level of a bulk commodity. In other cases, the properties will depend on exact nanofabrication that will not be possible to counterfeit or simulate without complete knowledge of the molecular structures of the components, once again putting counterfeiting out of the reach of all but the most sophisticated criminals. The spectrum of physicochemical properties that could be incorporated into the ACS is as large as the spectrum of 21st-century materials, which means that the unique property could be physical, chemical, optical, electromagnetic, and so on.

Simulation Strategies

The ability to simulate an ACS feature will depend on the behavior of the material(s) selected. It is assumed that the wide range of materials that are under development will make it possible to select material performance traits that will make simulation extremely difficult, requiring resources above the level of the opportunist counterfeiter.

Using the example of the shape-shifting memory polymer: temperature-sensitive, reversible changes in surface roughness would be extremely difficult to

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

simulate. The reason is that the properties of the feature are based on both the unique composition of the material and the method in which it is integrated into the ACS. Likewise, the ultratough, flexible, lightweight materials currently under development in places such as the Massachusetts Institute of Technology’s Institute for Soldier Nanotechnologies cannot be simulated, since they are new materials with novel properties that are dependent on yet-to-be-created processing methods. It is highly probable that many of these methods will require a very high initial investment in sophisticated instrumentation. Therefore, initially there will be no analogues available for simulation and (for properties such as ultrahigh tensile strength) no way to simulate them.

Key Development Risks and Issues: Phase I

Since a wide range of highly durable materials will be available for the ACS, as well as rigorous testing methods by which this durability may be characterized, durability will need to be evaluated but probably will not be a major constraint. The ability to integrate a window or other ACS into the FRN is really a question of whether sufficiently strong bonding can be formed between the cotton-linen paper and the material(s) used to create the ACS. Given current and future methods for creating materials composites, it is highly probable that the ACS can be integrated into the FRN in a secure and durable manner, but the details would be part of the development program.

In terms of aesthetics, many advanced materials could provide a high-technology gloss to the FRN that would make it appealing to many users. Paper currency that possesses a region that changes shape or contains a perimeter that cannot be ripped, cut with a knife, or even perforated with a bullet would probably be received favorably by most of the public. By specifically limiting any new property to an ACS, the Bureau of Engraving and Printing (BEP) can retain most of the traditional look of the “American greenback.” Retention of the traditional craft involved in creating U.S. currency and the reliability that has accrued to the “brand identification” are, in and of themselves, highly desirable aesthetically.

Another key implementation consideration involves the selection of the materials to ensure that they would not pose an environmental or health hazard. Great care will need to be exercised in the selection of any advanced material for the ACS. One simple yet profound example is an ACS capable of degrading into its component nanoparticles. Little is known about the toxicology of nanoparticles. Further, any specific nanofabricated material will have its own set of physicochemical properties. An ACS might meet with all reasonable durability standards for use yet, when burned, release nanoparticulates that could be inhaled (say by a child). Until there is a great deal more experience with these types of materials, the potential to set

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

off an irrational panic response based on the incorporation of any new, advanced material (nanotechnology-based or otherwise) must be considered.

The specific key technical challenges with respect to the ACS will depend entirely on the materials selected for it. However, one of the advantages of choosing an ACS component for currency is that many key technical challenges for the materials themselves will be driven by other extremely-high-value applications such as medical implants or biodefense. As a result, most of the research and development (R&D) on the materials themselves can be leveraged by the BEP. Certain technical challenges will be intrinsic to the specific use of these advanced materials in FRNs. The most obvious is the physical incorporation of the ACS into the FRN.

Can any ACS feature be physically incorporated into the FRN? Using the clear plastic window as a simple example, it is reasonable to assume that if the material used in the ACS can be fabricated to conform with the physical dimensions of the FRN, then it may be incorporated into the FRN through some form of compositing. It is more likely that the key decision will be the cost of integrating any such compositing step into the current FRN production process. The BEP already has significant experience in incorporating anomalous materials into specific regions of the FRN (for example, specific regions of color-shifting ink and the security thread). In many ways, the ACS may be viewed as a logical extension of such features.

Development Plan: Phase I

Activity during Phase I must address the key technical challenges with respect to the use of the ACS in U.S. banknotes. There are many opportunities for features within this technology area, so Phase I activities can determine which potential features are of greatest interest for counterfeit deterrence. Feasibility experiments can be conducted, through a combination of laboratory-based work and modeling and simulation analysis, to select the most promising future directions for currency applications.

A thorough review of current related work would be the first priority. Several important development programs can be leveraged to create ACS-based features rapidly. This materials revolution is expected to produce new materials with properties that could be of tremendous usefulness in anticounterfeiting efforts. The NNI has already been mentioned. Specifically, the Department of the Treasury is already a member of the Nano-scale Science, Engineering and Technology (NSET) Subcommittee of the National Science and Technology Council that provides a mechanism by which the BEP can obtain information about candidate materials for an ACS. In addition to or in coordination with the NNI, advanced materials are under development at all major government agencies funded by an equally wide range of missions. Obvious candidate agencies would be the Department of

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

Defense (for example, the Institute for Soldier Nanotechnologies), NASA, and the basic sciences division of the Department of Energy.

Since there are so many possibilities of new features, multiple feature concepts can be pursued. Therefore, there could be a base level of activity that is continually at work in Phase I to further develop new ACS ideas. Then, specific ideas that have been determined to have attractive counterfeit-deterrence benefits could be spun out of the base program into distinct, defined projects that would proceed through all the requisite development steps.

Estimate of Implemented Production Cost

Based on the examples of other currency notes shown to the Committee on Technologies to Deter Currency Counterfeiting, there appear to be no major technical hurdles to the introduction of clear plastic windows and other ACS-based features. However, the introduction of a window or other ACS will likely affect the manufacturing operations at the BEP.

Since the ACS is a concept of a feature platform rather than any particular technology, it is not possible to provide estimates of cost. However, many of the technologies that could be leveraged for an ACS-type feature are under development for large-scale industrial, medical, and military applications. So it is reasonable to assume that cost-effective manufacturing (including cost minimization) is a major goal of many of these advanced materials-based projects. Therefore, it is possible that at least some ACS-based features will have the same cost as that for the current security thread.

Further Reading

While no specific references are given here, numerous examples of smart materials under development may be found at the National Nanotechnology Institute’s Web site at <www.nano.gov>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×

CHEMICAL SENSORS

Description

Sensors embedded in banknotes could detect a human-produced or gadget-produced chemical and generate a human-detectable signal. Passive sensors would change their appearance directly, while active sensors would require a power source that could be either self-generated on the banknote or obtained from a battery, for instance at the point-of-sale. Active sensors can trigger visual, audible, or tactile responses (see the section below entitled “e-Substrate”). This feature class can enable features for unassisted use or assisted use with simple devices, as well features for the blind.

The sensed chemical could be an exhalation gas, activated by breathing on the sensor. Expired air has typically 3.6 percent carbon dioxide as compared with 0.03 percent in ambient air, making it a good target. Expired air also has 6.2 percent water given an atmospheric air water content of 0.5 percent.1

The sensed chemical could also be one of the constituents of perspiration, activated by touching the sensor. Perspiration is 98 to 99 percent water, but also contains (per 100 ml perspiration): lactic acid (45 mg to 452 mg), chloride (30 mg to 300 mg), sodium (29 mg to 294 mg), and potassium (21 mg to 126 mg) as well as numerous other organic and inorganic compounds in smaller quantities.2 The sensed quantity could also be acidity or alkalinity. As an example, a lactic acid sensor could be triggered when someone touched the note, then either the sensor’s appearance would change or the sensor would supply an electric current to activate a light or sound. Care would be required when designing the sensor to ensure that it is reversible, that is, that the sensor returns to its original state after the trigger chemical is removed.

Alternatively, a gadget similar to the commonly used “starch pen” could be designed to contain a chemical that produces a temporary change in the banknote’s appearance or to produce an audible or tactile signal. An example could be a pen filled with vinegar or some other inexpensive, nonhazardous substance that could trigger a reversible response when drawn across the banknote.

1

For additional information on the human respiratory system, see <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pulmonary.html>. Accessed February 2007.

2

For additional information on latent fingerprint composition, see the information from the Victoria Forensic Science Centre, Victoria Police, Australia, available at <http://www.nifs.com.au/F_S_A/Latent%20fingerprint%20composition.pdf>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Feature Motivation

This feature platform received a high rating from the committee because of its potential to deter opportunist, petty criminal, and professional criminal counterfeiters, and because of its potential usefulness for the unassisted general public, cashiers, tellers, and the blind, either unassisted or with the use of an inexpensive device. The primary benefit of this feature platform is the difficulty of reproducing or simulating it. Chemical sensors are difficult to reproduce by opportunist counterfeiters and petty criminals because neither the sensors nor the materials required to make them are readily available in the marketplace. Professional criminal counterfeiters would also be deterred by the difficulty of reproducing these sensors well.

The primary limitation of this feature platform is its potential complexity, which might make it expensive to manufacture and limit its robustness over the expected life of the banknote. The sensors must be activated by all people (for example, young, old, healthy, sick) in the full variety of habitable environmental conditions (for example, wide ranges of humidity and temperature).

The effectiveness of chemical sensors for banknote authentication depends entirely on how sensitive and robust the sensors are and on the specific implementation of the human-detectable response. In general, active features (those that change in response to a stimulus) should be easily noticed by the general public and should even generate interest in observing the note.

Potential Implementations

Passive sensors could be formed from chemically activated optical materials sandwiched between porous plastic films; for instance, the material’s refractive index could normally render it transparent, but the material would change to opaque upon exposure to lactic acid or carbon dioxide. Or, the material could change color, polarization, or thickness.

Electrical sensors could be formed from electronic elements whose electrical properties would change reversibly when the concentration of a specific chemical was changed. These elements could be transistors, diodes, or passive components. This class of sensor does not produce a detectable signal itself, but rather it provides a trigger to activate a separate, human-sensible device such as a buzzer, light, or raised bump.

Specific examples of these sensors are given below.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Scenario 1a. Passive Sensors: Liquid Crystals

Liquid crystals sandwiched between porous plastic films could be fabricated so that one end of each crystal was attracted to a specific chemical and the other end was repelled. The plastic films could be finely grooved to ensure that the liquid crystals were initially aligned with their long axes oriented parallel to the plane of the banknote. The presence of the target chemical penetrating through the film would rotate the crystals by 90 degrees, rendering them perpendicular to the film. This rotation could be used to produce a number of features: (1) If the plastic films were linearly polarized and formed a transparent window through the banknote, the liquid crystals could be aligned so that they had crossed polarization in their initial state, forming a dark window, and then would lose their polarization in their perpendicular orientation, forming a bright window. (2) The liquid crystals could be chiral and oriented so that they were transparent in one orientation and would create a spectrally pure interference color in the other (switching from transparent to brightly colored). This feature could be used either in reflection or transmission, allowing its use as either a windowed feature or a patch.

Scenario 1b. Passive Sensors: Optical Interference

A material can be sandwiched between porous plastic films in such a way that the films form an interferometer known as a Fabry-Pérot cavity. One film must be highly reflective, the other partially so. Light reflected back through the partially reflective surface will have a characteristic, spectrally pure interference color. Upon exposure to a trigger chemical, either the refractive index of the sandwiched material will be altered or the material will swell slightly. Either mechanism will cause a change in the color of the reflected light.

For another implementation, the sandwiched material chosen can have a refractive index initially the same as that of the films but that changes upon exposure to a trigger chemical. An internal reflection that can be viewed obliquely would exist in the triggered state. The reflection would disappear upon removal of the chemical. This effect might also be used to enable and thwart thin-film interference or to render the material opaque in its high-index state.

Scenario 1c. Passive Sensors: pH-Sensitive Dyes

Material whose color changes with changing pH can be selected for sandwiching between porous plastic films. A person’s touch would change the color of a patch of this material.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Scenario 1d. Passive Sensors: Fluorescent Molecules

Material phosphorescence is highly sensitive to local oxygen concentration. In general, the less oxygen, the brighter the phosphorescence. A sensor can be made that has bright phosphorescence under ultraviolet (UV) illumination, but much less bright phosphorescence under the same illumination immediately after exposure to an exhaled breath. This feature requires a gadget, namely, a UV light source.

Many organic compounds are highly fluorescent, and they can be designed so that their fluorescent spectra and/or brightness can change upon exposure to a specific chemical. These changes can be observed under UV illumination in the same way as that described above for oxygen quenching.

Scenario 2a. Electrical Sensors: Transistors

Transistors are commonly used for chemical sensors because of their sensitivity and selectivity. Very briefly, a transistor operates by allowing electrons across a barrier in a controlled fashion. When the transistor is designed to enable specific chemicals to enter the device, the chemicals can alter its resistivity and hence affect the electron flow—that is, the transistor becomes a chemical sensor. These devices are now being made on the microscale, and research is ongoing on nanoscale devices (for example, see Wang et al., 2006). Nanofabricated transistors offer the potential for very low power consumption and highly robust operation on flexible substrates.

Scenario 2b. Electrical Sensors: Diodes

Thin-film Schottky diodes have recently begun to be used as chemical sensors (Gergen et al., 2001). Gas molecules impinge on a thin metal layer and generate electric current. The diodes are built so that they can be chemically selective. Currently, Schottky diodes require electrical heating to function and thus are likely not practical for banknote use. Research continues to reduce the power draw, so they may become viable. Since these diodes are best used as gas sensors, they could be good for detecting exhaled breath.

Scenario 2c. Electrical Sensors: Passive Components

Passive components such as resistors and capacitors can also be used as chemical sensors, but their sensitivity is generally very low.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Materials and Manufacturing Technology Options

Passive sensors are integrated into a banknote by the embedding of an optical material between two porous plastic films. These films can form a window or they can be adhered to one surface of the banknote. (See the preceding section, “Anomalous Currency Space.”)

Electrical sensors can be formed by printing different kinds of organic semiconductors in arrays using the multiple nozzles of an ink-jet printer. Or they can be embedded into a banknote by depositing flexible, coated wires.3 (See the section “e-Substrate,” below.)

Simulation Strategies

Claiming that the sensor is broken is probably the best way to “simulate” this feature, for all classes of counterfeiters. Even if the selected sensor has near-perfect reliability, the public may not believe this and may be very willing to accept that all things electronic break.

Otherwise, as chemical sensors become ultrareliable and inexpensive, they may become available commercially and simply be removed from other products for use on banknotes. A target market for the widespread use of chemical sensors is that of food packaging, for the detection of spoilage. Another growing market is biological sensors for homeland security. Care must be taken in selecting an activation chemical for banknote use that is unlikely to be used by these industries.

Key Development Risks and Issues: Phase I

Significant development issues must be addressed for this feature to be viable as a counterfeit deterrent. The most critical issues are the following:

  • The selection of the human-produced chemical to detect that covers the range of human variability.

  • The long-term durability of the sensor and its power source.

  • The cost of the sensing system.

  • The accuracy of the sensor over the range of operating conditions and multiple sensing attempts.

  • The establishment of the fact, with extremely high confidence, that the sensor will not collect and transmit human pathogens.

3

For additional information on woven transistors, see <http://www.coe.berkeley.edu/labnotes/0204/lee.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Development Plan: Phase I

The committee determined that the development challenges for chemical sensors were sufficiently high, compared with those for the other feature concepts, that pursuing a development program at this stage would be premature.

Estimate of Production Costs

The production costs cannot reasonably be estimated at this point. The concept of chemical sensors as counterfeit-deterrent features is quite immature at the present time.

Further Reading

Assisi, F.C. 2005. UC Berkeley’s Vivek Subramanian Invents Electronic Nose. Available at <http://www.indolink.com/printArticleS.php?id=120105035208>. Accessed February 2007.

Gergen, B., H. Nienhaus, W.H. Weinberg, and E.W. McFarland. 2001. Chemically induced electronic excitations at metal surfaces. Science 294: 2521-2523.

Wang, L., D. Fine, D. Sharma, et al. 2006. Nanoscale organic and polymeric field-effect transistors as chemical sensors. Analytical and Bioanalytical Chemistry 384(2): 310-321.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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DIGITALLY ENCRYPTED SUBSTRATE

Description

The concept of the digitally encrypted substrate involves adding small-diameter optical-fiber segments to the substrate. These fiber segments, when illuminated by laser light or narrow spectrum illumination, create a unique signature that can be tagged to the specific note or material. This feature can be used by a robust authenticating machine reader. To employ this feature, optical fibers, or more preferably fiber segments, are placed in the substrate. As the substrate is manufactured, these fiber segments become mixed in the paper batch before the paper is dried. Since the mixing is a random process, the fiber pattern in the substrate will be very unique from note to note.

When the finished substrate is illuminated with light, especially laser light, the fibers will light up as the incident light emanates from the ends of the fibers. The first deterrent example would be for a user to notice the speckles of light from the substrate when it is illuminated. Figure D-1 illustrates the fibers embedded in the substrate. See the section “Further Reading,” below, for further illustration of the concept and use of this technique.

When the manufacturing cycle of the banknote was nearly completed, a selected region of the note would be scanned or digitally photographed. This image would then be converted to a secure, two-dimensional bar code that would be printed on the banknote. In order to authenticate the note, a machine reader would compare the image of the selected region to that stored in the encrypted bar code. This method would be extremely difficult to copy, since each note is unique, and re-creating the exact fiber pattern in the substrate would be virtually impossible. The limitation of this approach is the need to scan, photograph, or otherwise capture a picture of the substrate and process the picture.

FIGURE D-1 Fiber-infused substrate.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Feature Motivation

This feature has a high rating from the committee owing to the difficulty of exactly duplicating the feature—namely, the fibers in the substrate—and the utility of the highly robust image analysis of the fiber placement. Furthermore, this feature would not be reproducible using electronic printing and scanning techniques and hence would frustrate nearly all counterfeiters. It is expected that only a few very persistent counterfeiters would attempt to generate their own substrate with fibers in it. If they did, they still could not duplicate the exact fiber structure in authentic currency substrate materials.

Potential Implementations

The discussion under “Description,” above, provides the scenario of interest. A less secure version of this feature is summarized in Appendix C, in the section “Fiber-Infused Substrate.”

Materials and Manufacturing Technology Options

The manufacturing requirements for this feature would involve paper manufacturing and integrating the appropriate-sized fiber fragments into the paper or other substrate material. The fibers could be glass, plastic, or micro- or nanomaterials, with custom design of the properties as required.

It is not expected that adding the fibers during the papermaking process would be a difficult operation, although some tooling and process changes would no doubt be required at the substrate manufacturing plant. Once the substrate was produced, note production at the BEP could proceed as usual until the end, when two additional steps would be needed: pattern scanning of the selected region and printing of the encoded unique image on the banknote.

Simulation Strategies

Simulation of this feature by would-be counterfeiters would be nearly impossible. Furthermore, only the professional criminal or state-sponsored counterfeiter would have any hope of even embedding fibers in the substrate. Since the BEP could also control the fiber materials in the substrate, the counterfeiter would be faced with the difficult task of creating the fibers as well as making the substrate, which would rule out the vast majority of criminals; only state-sponsored counterfeiters would have any plausible chance of attacking this feature. While would-be counterfeiters could attempt to put something on or in the currency to simulate the effects of the fibers, in most cases the counterfeiter may not know what is being

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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evaluated when the banknotes are properly authenticated. Therefore, while simulating the look and feel of real banknotes may be attempted, authentication by an approved device or instrument being used would be an exercise in frustration for most counterfeiters, even those who are state-sponsored. Ultimately, the security of this feature would depend on the strength of the encryption algorithm and the allowable tolerance in the degree of pattern match.

Key Development Risks and Issues: Phase I

The durability of this digitally encrypted substrate feature is unknown, but it would depend on the lengths of fiber embedded in the currency as well as on the continued sharpness of the unique image as the banknote became worn. For instance, if the fibers broke or became disbonded from the substrate, the image could change significantly. No degradation of the fibers themselves is anticipated. Should some of the fibers be conductive—that is, metallic—breakage may not be an issue but the fibers should not be able to stick out of the note by being too long. Combinations of optical- and electronic-fiber characteristics could compound the difficulty faced by the would-be counterfeiter. It is also possible that semiconducting fibers could be used in conjunction with the e-substrate methods (see the section entitled “e-Substrate,” below).

There should be no aesthetic issues with this feature. Until illuminated, the note would look and feel identical to one without the feature. Even when illuminated, the feature should not detract from the note’s appearance. The two-dimensional bar code would not be very large—perhaps 1 mm × 1 mm—and should be printed in an inconspicuous location such as the margin of the note. Thus, this feature would be aesthetically neutral, conforming to the look and feel of current notes.

Similarly, there should be no social acceptability issues surrounding this feature, with the caveat that the public must be convinced that scanning the note and authenticating it would not reveal any privacy information.

The key technical challenges include the following:

  • The selection of the optical fiber that possesses the necessary characteristics for incorporation into the substrate.

  • The selection of the appropriate three-dimensional random pattern and encryption of the pattern in such a way that an authenticator can be printed directly on the banknote.

  • The ability of the fibers to survive the high-pressure intaglio process without unacceptable breakage.

  • The durability of the pattern itself, and of the printed encryption of the pattern.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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  • The implementation of a method to read the pattern quickly and to compare the results with the printed authenticator.

  • The development of a hierarchy of effects that provide increasing security with increasing authentication capabilities.

  • The cost of implementing the feature.

Development Plan: Phase I

The maturity level of this technology is relatively low. The science behind its use and verification is known, but this technique has not been implemented in high-volume, low-cost applications such as that envisioned here.

Key milestones would include the following:

  • Place fiber fragments in paper substrates to see the applicability of the technique and any operational or manufacturing difficulties that might arise.

  • Scan and visually observe the fragments according to the referenced techniques to see how difficult the practical implementation and verification of this feature might be.

  • Evaluate different encryption algorithms using criteria defined by the development team, and select the best.

  • Determine the best bar code and location for printing the encrypted image.

  • Initiate a discussion with vendors regarding the prototype scanners and associated processing software.

Similar methods were used in placing fiber-embedded placards on missiles for treaty verification. If a placard was removed, the fibers would be broken in places and the imaging pattern from the placard would be altered, making it obvious that the placard was no longer authentic.

Estimate of Production Costs

The optical fibers would be embedded by the substrate manufacturer. This should be a low-cost operation. At least two additional steps would be required at the BEP for the “signing” of the note based on the digital data acquired from the fiber substrate. This might alter the printing procedures or serial number generation, but the operation could be highly automated. It would require new capital equipment at the BEP.

The cost of adding the fibers and signing the note should be approximately the same as that of the current security thread.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Further Reading

Chen, Y., M.K. Mihcak, and D. Kirovski. 2005. Certifying authenticity via fiber-infused paper. ACM SIGecom Exchanges 5(3): 29-37.

DeJean, G., and D. Kirovski. 2006. Certifying authenticity using RF waves. Paper presented at IST Mobile Summit, Myconos.

National Research Council. 1993. Counterfeit Deterrent Features for the Next-Generation Currency Design. Washington, D.C.: National Academy Press, pp. 74-75, 117-120.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ENGINEERED COTTON FIBERS

Description

The cotton fiber is a complex biological structure engineered by both natural selection and intensive plant breeding. Cotton is the premier natural fiber for textile applications. It is a biological composite of cellulose, small quantities of hemicellulose, pectins, and proteins that provides excellent wearability and aesthetics. Cotton fiber is normally hollow, although many fibers collapse after drying or later during processing.

Engineered cotton fibers are not a feature but rather a set of tools that may be employed to generate a wide array of potential features. Throughout this description, reference to cotton fibers is understood to mean fibers treated in a manner similar to that for the materials currently used in FRNs. These cotton fibers have already undergone extensive processing, first to form clothing and then to form the composite paper substrate used by the BEP. The durability and other materials characteristics of the cotton fiber used in FRN production and the test modes for determining them are well known. The current substrate material is effectively a cotton-linen composite. Using recombinant deoxyribonucleic acid (rDNA)-based or conventionally bred cotton fibers would not change that.

The various ways of engineering a cotton fiber include adding new materials to the fiber lumen, modifying the cellulosic material that forms 90 percent of the fiber itself, modifying the proteins associated with the fiber, or a combination of these methods.

For example, a second biopolymer can be synthesized within the fiber lumen without affecting fiber wall integrity. Researchers have been able to use genetic engineering to fill the hollow center (lumen) with a natural thermoplastic polyester compound, poly-D-(−)-3-hydroxybutyrate (PHB) for synthesis in fiber (Maliyakal and Keller, 1996). The new cotton fibers exhibited measurable changes in thermal properties that suggested enhanced insulation characteristics. The engineered fibers conducted less heat, cooled down slower, and took up more heat than conventional cotton fibers.

Feature Motivation

Engineered cotton fibers offer the opportunity for the FRN to retain that unique, highly distinctive feel that is frequently cited as a major attribute of the U.S. dollar, while also promising application as counterfeit-deterrent features. Custom-engineered cotton varieties could be developed for any number of applications, from physical identification by the end user to embedded, cryptic forensic features. Among the advantages of engineered cotton fibers are the following:

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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  • The impossibility of counterfeiting or simulating resulting features by all but state-sponsored counterfeiters.

  • The unique spectroscopic signatures that can be achieved by modifying protein R-groups.

  • The possibility of “filling” fiber with other compounds, which opens up significant possibilities to specialize the fiber further.

  • The significant amount of current research regarding enhancing properties of the fiber.

A wide range of naturally occurring materials show physicochemical properties that could be useful as features. The feature could involve an addition of new materials to the fiber lumen, modification of the cellulosic material that forms 90 percent of the fiber itself, modification of the proteins associated with the fiber, or some combination. These include the following:

  • Proteins that contain iron and other electromagnetic and/or paramagnetic metals (for example, hemoglobin).

  • Proteins that contain optical and/or electro-optical properties (for example, bacteriorhodopsin).

  • Proteins that form fibers to add strength, toughness, or stiffness to the fiber (for example, actin).

  • The large list of natural compounds with useful properties that could be “loaded” into the lumen. When one adds in the possibility of creating synthetic genes that encode synthetic proteins or other materials for lumen loading, the possibilities become extremely large.

Potential Implementations

This subsection describes some examples of potential applications for custom-engineered cotton fibers. One possible implementation is the creation of a series of cotton varieties whose fibers have surface-associated proteins genetically engineered to display novel epitopes (molecular structures) sufficiently different from other cotton varieties so that rapid, powerful immunoassays could be used for identification purposes. Immunodetection tools, in turn, vary both in sensitivity and ease of use. Simple tools could be developed for the end user—for example, an “immunoassay pen” similar to the starch-detecting pen. This type of instrument would give a quick (seconds) colorimetric reaction that would be highly accurate. However, a positive reaction might be simulated by, for example, grinding up a single FRN and dusting multiple counterfeit bills. In such a case, the immunoreaction would obviously be weakened, so successful detection would depend on the rigor with which the “immunoassay pen” was used. Engineering of multiple novel

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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epitopes into a single cotton variety or the blending of multiple varieties would create an extremely complex immunoreactive profile for which a complementary mixture of monoclonal antibodies could be generated for law enforcement and other rigorous detection applications. In this latter case, an immunological profile would be generated with sufficient redundancy to make counterfeiting or simulation totally impossible. The sensitivity of immunoreactions is such that nondestructive microsampling could be employed even in crucial forensic applications.

Another implementation would take advantage of the structure of a normal cotton fiber that is similar to a hollow tube. rDNA technology is currently being used to create fibers whose hollow center (lumen) is filled with various types of chemical compounds. The properties of any specific material used to fill the lumen will, in turn, affect the properties of the fiber and, ultimately, the FRN. Once again, a range of features is ultimately possible. One example would be that loading the lumen with, for example, a small iron-containing protein such as hemoglobin would ab initio introduce certain measurable electromagnetic and paramagnetic properties. This property might be manipulated further, for example, by passing the FRN through an electromagnet powerful enough to align the spins of all the molecules in a region of the bill or even of the entire FRN. This type of feature would be highly amenable to machine reading. Incorporation of virtually any unique molecule into the lumen will create an equally unique molecular fingerprint for forensic applications. With current instrumentation, only microsampling would be required to see this molecular profile via, for example, gas chromatograph-mass spectrometry.

Proteins associated with the surface of the cotton fiber could also be modified via rDNA technology to provide electromagnetic, electro-optical, or other unique signatures. A simple example would involve changing the numbers and types of R-groups on these proteins to specifically change certain fluorescence and/or absorbance properties. These modified proteins would, in turn, change the “signature” of the FRN with respect to its interaction with certain forms of electromagnetic radiation. Some of these signature changes could be relatively simple and amenable to hand detection technology: for example, tryptophan fluorescence. Other changes in signature could be more complex: for example, changes in circular dichroism spectra based on changes in amount of helicity in the proteins present on the fiber surface.

Regarding nonspecific physicochemical characteristics of the cotton fiber, any large plant-breeding program will produce and retain a large number of individuals (genotypes) whose target phenotype (in this case the cotton fiber) is undesirable because this individual expresses some other desirable phenotype—for example, insect resistance or drought tolerance. Therefore, it is highly probable that a substantial number of genotypes already exist that produce unusual cotton fibers. As a result, it should be possible to rapidly create new, stable varieties of commercial

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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cotton that produce unusual or abnormal fiber phenotypes (measurable physical traits such as fiber length, strength, roughness, lumen size, and so on). Because the cotton fibers produced by certain engineered genotypes were undesirable for commercial applications, the resulting cotton varieties were either put aside by breeders and molecular geneticists or only used in early crosses during the development of commercial varieties. Some of these traits may be of interest for FRN production—for example, fibers with increased surface roughness. In almost all such cases these traits will be characterized by the breeding programs only in terms of classical genetics, meaning that the genes involved have not been identified or even mapped at the molecular level. As a result, a situation emerges where key enabling technology—the gene or genes that encode the trait—remains unknown. The BEP could control information about, or even patent, the genes for a fiber trait useful for currency production as they are cloned and characterized.

As an example, suppose that the cotton plant could be engineered to produce fibers with bacteriorhodopsin. Bacteriorhodopsin is found in the intensely purple cell membrane of a bacterium called Halobacterium salinarium, which grows in salt marshes. Illuminating the protein triggers a photochemical reaction cycle, which transports protons along a channel spanning the cell membrane. The membrane’s purple color comes from a bacteriorhodopsin component called retinal, which is strongly bound to an amino acid inside the membrane channel. Unbound retinal in solution is pale yellow. Alternating laser light of two different wavelengths on the protein molecule can switch it back and forth between its purple and yellow forms. That behavior has prompted research on the use of bacteriorhodopsin as the light-sensitive element in artificial retinas and as memory or processing units.4

Fibers containing genetically engineered bacteriorhodopsin could be throughout the paper or in specific areas, thereby creating a color-changing technology that would be virtually impossible to counterfeit or simulate, especially if multiple forms of bacteriorhodopsin were used so that two or more colors changed simultaneously. Work on the bacteriorhodopsin system was the subject of a chapter in a recent report from the National Research Council (NRC, 2001).

The rating of any particular genetically engineered trait will depend entirely on the materials engineered into the fiber and the properties these materials display. Inherent in the genetic engineering strategy is the degree of difficulty required to isolate, synthesize, and clone the genes to produce cotton in such a manner that the desired materials are expressed and properly targeted to the fiber.

The ability of any particular genetically engineered (or even naturally occurring) trait to deter counterfeiting will depend entirely on (1) the properties resulting from the incorporation of the materials into the cotton and (2) any properties resulting from the processing of the engineered cotton into the paper for

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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the FRN. As an example, consider a bacteriorhodopsin-based trait that will deter counterfeiting by exhibiting complex dynamic behavior that is only displayed by these classes of materials (see the following subsection). The bacteriorhodopsin would be engineered so that in natural or artificial light, one color (or spectrum of colors) is displayed, whereas covering the FRN (or placing it in darkness) will terminate the color-generating process and cause the bacteriorhodopsin to revert to its baseline color. It is assumed that the ability to synthesize and fabricate such complex, dynamic materials will not be possible for any but the most sophisticated counterfeiter—that is, state-sponsored. Obviously, if there are other synthetic dyes that are similar in behavior to bacteriorhodopsin, then the feature could be simulated. It is assumed that such dyes would be equally complex to synthesize and/or obtain, so that only the most well-funded sophisticated counterfeiter could simulate this trait.

One scenario involves genes encoding engineered bacteriorhodopsin (rBR) being incorporated into a specific cotton line and maintained under appropriate security. The plant line has been engineered so that the rBR is expressed and incorporated to high levels in the lumen of the cotton fiber. While the light-blocking properties of such placement would have to be determined, it is probable that if the FRN contained a large proportion of this type of fiber, the FRN would display (at the least) a plainly visible background color that changed when the bill went from light to dark. Use of rBR genes engineered for different color-shift properties might allow two or even more color-shifting background tones to be obtained.

A second scenario involves genes encoding engineered rBR being incorporated into a specific cotton line and maintained under appropriate security. The plant line has been engineered so that the rBR is expressed and incorporated on the surface of the cotton fiber at levels sufficient to produce a highly visible color shift but low enough to allow the fiber to maintain its general physicochemical properties (as manifested by length, strength, toughness, and so on). It is probable that if the FRN contained a large proportion of this type of fiber, the FRN would display a plainly visible surface color that changed when the bill went from light to dark. Use of rBR genes engineered for different color-shift properties might allow two or even more color-shifting colors to be obtained.

There are a number of possibilities that range from simple to highly complex. On the simple side, modification of cotton fiber proteins to contain additional fluorescent biological R groups (for example, tryptophan) creates an opportunity to incorporate complex fluorescence patterns directly into the paper itself. This includes taking advantage of fluorescence shifts of up to 20 nm by modifying the environment of the engineered fibers during paper formation. On the complex end, the creation of cotton plants that produce fibers containing nonbiological components—metal nanoparticles and so on—offers the opportunity to create patterns with unique or uniquely complex electromagnetic or optical properties.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Materials and Manufacturing Technology Options

The features discussed above are in the long-range category, so it is not possible at this time to describe specific manufacturing strategies for currency applications. However, some strategies are already in development for other applications. For example, protein-linked quantum dots are currently in use as fluorescent and/or color-generating tags for a number of applications, with more in development.5

Simulation Strategies

The rBR-based feature described above should be impossible to simulate by all but state-sponsored counterfeiters. To simulate this feature, the counterfeiter would have to (1) know the specific gene construct and what it encoded or be able to deduce it from the behavior of the FRN, and (2) reproduce the effect created in a bona fide FRN via the use of rDNA-based cotton fiber.

Key Development Risks and Issues: Phase I

It is reasonable to assume that at least some of the useful phenotypes that may be created in a cotton fiber by rDNA technology will be as durable as those displayed by the natural fibers. For example, materials incorporated into the lumen of the cotton fiber should, in general, remain intact as long as the fiber itself remains intact. Likewise, novel epitopes created by engineering fiber-associated proteins should, in general, display the same wear properties as those of the fiber-associated proteins in natural cotton. The final molecular structure of these epitopes will be formed as fully hydrated biomolecules are extracted, processed, and ultimately dried down to form paper. Just as normal FRN paper wears and smoothes with age, FRNs containing fibers produced by engineered cotton will be expected to show normal wear characteristics, except where modification involves exotic or extreme changes in the biochemistry and/or supramolecular structure of the fiber. The stability of these more extreme products would need to be closely examined, and appropriate cost-benefit analyses would need to be conducted with respect to any trade-off between anticounterfeit value and decreased durability.

In terms of reasonable analogies, there are now numerous examples of the use of rDNA to modify or enhance desirable plant phenotypes. Because cotton is a major source of both food and fiber in the United States, genetic engineering of this plant is now routine. Most of the commercial work has focused on relatively simple single-gene traits such as herbicide or pest resistance (for example, Monsanto’s

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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incorporation of a gene that allows the plant to survive the application of the herbicide glyphosate). Given the existence of an efficient transformation system (which does exist in cotton), the speed with which new traits may be incorporated into this plant will be a direct function of the complexity of the molecular genetics underlying that trait and whether any or all of the genes involved have been cloned and characterized. However, there is certainly a great deal of technology both in the public sector (U.S. Department of Agriculture [USDA]) and private sector that could be used to leverage and accelerate progress.

In terms of aesthetics, the use of genetically engineered plants has been generally accepted in the United States, especially for nonfood applications, so there is little probability that any type of panic would be created if GM cotton fibers were incorporated into U.S. currency. However, this possibility cannot be entirely discounted. The use of rDNA technology might even impart a high-technology gloss to the FRN that would be appealing to some sectors of the public. In addition, the use of genetically engineered cotton has the potential to enhance the security of the FRN while allowing the BEP to retain most of the traditional look and feel of the “American greenback.” Retention of the traditional craft involved in creating U.S. currency and the reliability that has accrued to the dollar’s “brand identification” are, in and of themselves, highly aesthetically desirable. Obviously, any change in fiber characteristics will need to be evaluated for its impact on the look and feel of the FRN. Traits specifically designed to affect the look and feel (for example, increased surface roughness) will need to be evaluated in even more detail for aesthetic impact.

The environmental consequences of growing genetically engineered crop plants has been under constant evaluation for more than 25 years. Appropriate federal and state regulations have been developed that, in all probability, will cover the type of limited acreage required by the BEP for FRN paper substrate. Security considerations—that is, the need to protect the cloned genes, the cotton germplasm (seeds), the plants in the field, and so on—would provide additional de facto insurance against any unintended release of novel plant genes into the environment.

The key technical challenges will depend entirely on the trait(s) selected by the BEP for genetic engineering. As discussed above, the manufacturing issues directly associated with genetic engineering of a new phenotype into cotton have been largely addressed. Likewise, the parameters involved in determining the stability of a plant phenotype during the creation of a “true breeding” commercial plant variety are extremely well known and applied routinely. In other words, as with all major crop plants in the developed world, the infrastructure for cotton production and processing already exists as a mature manufacturing industry with well-established Technology Qualification Application procedures, and so on.

The outcome of this study will allow a rational decision-making process with respect to what (if any) type of program should be initiated in this area.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Development Plan: Phase I

Activity during Phase I must address the key technical challenges mentioned above. There are many opportunities for features within this technology area, so Phase I activities can determine which potential features are of greatest interest for counterfeit deterrence. Feasibility experiments can be conducted, through a combination of laboratory-based work and modeling and simulation analysis, to select the most promising future directions for currency applications.

A thorough review of current related work would be the first priority. For instance, it is likely that multiple candidate features exist as phenotypic traits that have been produced as by-products of long-term, ongoing cotton breeding programs throughout the nation and the world. This is equivalent to data mining of existing inventories (libraries) of chemical compounds by pharmaceutical companies when a new potential application is identified or new screening technology developed.

Because cotton has been the subject of intensive, long-term conventional breeding programs, it is possible, even probable, that conventional genetic methods have already produced fibers with phenotypes that could translate into useful features for currency. These traits could be characterized and the fibers used without ever engineering the genes involved, so long as the germplasm and plants remained secure. Engineering genes offers the further possibility of introducing completely novel biomolecular structures that fall outside of the metabolic capabilities of the cotton genome (for example, genes that encode a system that forms fibers where the lumen is filled with paramagnetic proteins).

There are development programs underway in many countries in both the public and private sectors with the goal of using biotechnology to modify various aspects of cotton fiber structure and development. The United States is currently the leader. The cotton fiber itself is the target of much of this work. In addition, there is an effort to create synthetic genes that encode proteins with unusual traits and/or cellular production of, for example, protein-nanoparticle composite materials (biomolecular-materials composites), some of which would undoubtedly be useful as FRN features.

The BEP could work directly with the appropriate departments within the USDA (for example, the Agricultural Research Service [ARS]) and/or with appropriate private-sector consultants to survey the current state of knowledge and capability with respect to the modification of cotton fibers (including lumen loading). Also, the nanomedicine initiative at the National Institutes of Health (NIH) involves a significant effort in the creation of biomolecular-materials composites.

The technical requirements for producing engineered cotton are well known. A realistic scenario is that the novel GM cotton varieties would be produced in an existing facility using existing equipment. By analogy, the phenotype encoded by the new genes would most likely be verified and parameterized by existing

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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research facilities. One possible path would be a research partnership with the USDA or its ARS. Alternatively, the work could be done by private industry or even in a university research facility, so long as appropriate security precautions were maintained.

Use of engineered or genetically unique cotton is a long-term project even if varieties currently exist that produce fibers with novel phenotypes that could provide the basis for novel features. Since there are so many possibilities of new features, multiple feature concepts can be pursued. Therefore, there could be a base level of activity that is continually operating in Phase I to further develop new engineered cotton fiber options. Then, specific ideas that have been determined to have attractive counterfeit-deterrence benefits could be spun out of the base program into a distinct, defined Phase I, II, and III project.

The time and effort to develop the new line depends on a number of factors, which include the following: the complexity of the phenotype selected, whether the genes encoding that phenotype have already been engineered, the extent to which the products encoded by these genes have already been characterized, and so on. For instance, if cotton genotypes useful for counterfeit deterrence already exist, the feasibility of incorporating those new fibers into the currency substrate could be demonstrated within 3 years. If they do not exist, the time line could easily be doubled to 6 years.

Estimate of Production Cost

The major technical challenges to the incorporation of engineered or genetically novel cotton fibers into FRNs involve the substrate producer. Also, modification of the physicochemical properties of the fiber could change the substrate enough to impact BEP operations. This compatibility issue should be addressed during Phase II of the development process as the BEP updates the substrate material specification. Once the new cotton line is developed, the incremental production cost should be extremely low, less than the cost of the current security thread. It is highly unlikely that the BEP would need to invest in any new capital equipment in order to generate new features based on engineered cotton fibers.

References and Further Reading

Maliyakal, E.J., and G. Keller. 1996. Applied biological sciences metabolic pathway engineering in cotton: Biosynthesis of polyhydroxybutyrate in fiber cells. Proceedings of the National Academy of Sciences 93(November): 12768-12773.

National Research Council. 2001. Opportunities in Biotechnology for Future Army Applications. Washington, D.C.: National Academy Press. Available at <http://www.nap.edu/catalog/10142.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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e-SUBSTRATE

This feature can be subdivided into passive and active electronic substrates, here called “e-substrates.”

Passive e-Substrate
Description

Passive electronic substrates—that is passive e-substrates—refer to classes of features that are enabled by techniques and materials currently used in large-area electronics (for example, in liquid-crystal televisions) or low-cost electronics (for example, in radio-frequency identification tags [RFIDs]), as well as those that are being explored for newer devices such as flexible displays. This type of feature consists of passive structures (i.e., those without electronic functionality) fabricated with techniques and materials similar to those used for these electronic systems, in the form of security threads or patches integrated with the paper substrate. The collections of materials—dielectrics, conductors, semiconductors—that can be patterned using these approaches and the resolution and registration accuracy that can be achieved in single and multilayer configurations provide visible as well as invisible attributes that have value for currency applications owing to the high levels of difficulty in replication or simulation. In this version of the e-substrate feature, the patterns do not offer any form of electronic or optoelectronic operation. The subsection below on “Active e-Substrate” examines the functional possibility.

Feature Motivation

Currency is now produced, almost exclusively, by conceptually old printing techniques (for example, offset, intaglio, and so on) that have historically dominated the conventional printed paper industry. The nature of emerging threats and the powerful technologies that are becoming available to the counterfeiter suggest that it might be useful to examine whether this basic approach to currency production will likely continue to provide a path to secure currency indefinitely into the future. In particular, one can argue that intrinsic limits in resolution, registration, pattern layouts, and inks associated with printing methods may make it necessary to consider radically different manufacturing concepts. Through the passive e-substrate feature platform, a fundamentally new approach is examined here that adapts, for the production of currency or currency features that offer no electronic functionality, tools and fabrication facilities principally designed for electronics used in large-area applications, such as liquid-crystal televisions and computer monitors, or low-cost devices, such as RFID tags. The latter devices use circuits that

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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consist of patterns of dielectrics (for example, silicon dioxide and silicon nitride), metals (for example, aluminum), and semiconductors (for example, amorphous silicon) formed on large-area glass substrates with layouts designed to switch individual pixels in these displays. The basic approaches are, however, sufficiently adaptable that they can be implemented with a wide range of other substrates (for example, plastics), materials (for example, polymers), and designs that could be useful for passive features in currency in the form of patterns or images with overt or covert security purposes.

The technologies for producing this kind of electronics, sometimes referred to as macroelectronics (Reuss et al., 2005, 2006), are advancing rapidly, as measured in production capacities, costs per unit area, yields, and electrical performance. Although the main application drivers are flat-panel displays, emerging applications in structural health-monitoring equipment, x-ray imagers, sensors, and other systems also exist and are increasing in significance. As a result, in addition to commercially established fabrication approaches, newer techniques and materials are under development at many small and large companies worldwide, with goals of further increasing and lowering the areas and costs, respectively. Taken together, these recent developments suggest that it may be possible to consider existing or emerging macroelectronic-like processing techniques as next-generation methods for producing passive images or patterns for currency features or, ultimately, the entire currency note itself.

This approach has many attractive features. First, the fabrication techniques offer levels of resolution (~500 nm) and pattern alignment (~500 nm) that are better, by at least 10 times, than anything that is likely achievable with established printing techniques. Second, the wide range of materials that can be processed into patterns, in single-layer formats or complex multilayer stacks, lie outside of the possibilities associated with printed inks. Third, the approach provides a scalable manufacturing platform with user-definable levels of complexity in the materials and pattern layouts, as well as new active forms of currency features, such as those that require active electronics, light emitters, solar cells, sensors, and other systems with diverse functionality, as described in the subsection below, “Active e-Substrate.” Fourth, the high capital costs and technical sophistication of the fabrication facilities would prevent any counterfeiter, other than those associated with well-financed, state-sponsored organizations, from reproducing (or simulating, depending on the design) currency or currency features produced in this fashion. Fifth, the current technology status and future trends in costs, capabilities, and production capacities indicate that this approach to currency production is, for certain implementations such as security strip features, feasible now, with costs that are expected to continue to decrease in the future.

In macroelectronics, the area coverage defines the metric by which progress is measured, rather than critical feature sizes or integration densities as in conven-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE D-2 Large-area circuits in the Gen 7 format (see discussion in text). SOURCE: Reuss et al. (2005). © 2005 IEEE.

tional wafer-based integrated circuits. Some of the first commercial macroelectronic systems, which appeared in 1987, used substrates (glass) with sizes of 270 mm × 200 mm, known as Gen 0 glass, and were implemented in 8.4 inch liquid-crystal displays. Currently, Gen 7 glass is in production (1,870 mm × 2,200 mm), and some Gen 8 (2,200 mm × 2,500 mm) fabrication facilities became operational in 2006.6 Figure D-2 provides an image of a completed circuit on Gen 7 glass. These large-area systems are patterned using photolithographic processes with step-and-repeat stages capable of stitching together multiple images. Whereas the integrated-circuit industry uses mainly single-crystal silicon in wafer form, the semiconductor of choice for existing macroelectronic systems is sputtered thin-film amorphous silicon (a-Si). Deposition, etching, and patterning of this material, and other vacuum-deposited materials needed for the circuits (for example, gate insulators,

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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metal interconnects and electronics, encapulants, and passivation coatings), can be accomplished over very large size scales. Figure D-3 shows an optical micrograph of one of the many millions of thin-film transistors that exist on substrates like the one shown in Figure D-2. A device of this type typically requires the sequential deposition and patterning of between four and six layers of material.

The proposed concept is that these, or other classes of material structures, could be used as passive, visible or invisible, patterns or multilayer-structured elements on a currency note. Alternatively, they could be used in conjunction with conventional pick-and-place methods to integrate separately processed substrate pieces using strategies similar to those used for RFID tags. This approach provides another pathway to the passive e-substrate feature. Very recently, the costs of both routes to such systems have reached levels that can be contemplated for currency or currency features. The most realistic possibility, as argued subsequently, involves the use of such approaches for the fabrication of new, passive features for conventional printed paper notes, where these features, in the form of narrow strips, are cut from macroelectronic substrates and integrated with the paper using approaches currently employed for digital optically variable devices and security strips.

Potential Implementations

In the simplest implementation, the patterns formed using the materials and patterning techniques of macroelectronics provide passive features that have a distinct appearance (for overt operation) or that incorporate hidden information that is invisible to the unaided eye (for covert operation). The design flexibility, in

FIGURE D-3 Optical micrograph of one of the many millions of transistors in a liquid-crystal display.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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terms of pattern layouts and materials choices, are large. The methods are already well developed for existing applications in large-area and low-cost electronics, as outlined above. Implementation in currency would involve, then, the specification of designs that provide the desired levels of overt and covert functionality, with attributes that maximize the difficulty of simulation. Creating these designs represents the first phase of work in implementing this class of feature.

Materials and Manufacturing Technology Options

The cost structure of the passive e-substrate features is critically important to their implementation in currency. The following discussion is limited to existing, commercial-scale fabrication approaches based on thin-film processing for the passive features. (Some of the same arguments apply to technology options for the electronics and/or interconnects, the active features described in the subsection below, “Active e-Substrate.”) The level of detail on new methods for fabricating electronics that use printable materials and techniques similar to printing is relatively low, thereby preventing a quantitative analysis of their possible use for these applications. These methods will, however, become increasingly important as future technology options as their state of development improves.

The photolithographic process for patterning is central to the fabrication of all forms of electronics that have achieved widespread use. This process involves passing ultraviolet light through a mask to expose, in a patterned geometry, a thin layer of photosensitive polymer known as a photoresist. Washing away the exposed (or unexposed, depending on the chemistry of the photoresist) regions creates a pattern of photoresist that can then serve as a sacrificial mask for spatially directing the deposition or etching of other materials to produce the final structures. As implemented in large-area electronics, this process offers extremely high resolution (~500 nm) and layer-to-layer registration capabilities (~500 nm), when compared with conventional printing techniques. The methods used to dice the large-area substrates into smaller pieces for device integration are also well developed. Although most available data apply to patterns formed on the sorts of glass substrates that are used in displays, the basic processes are applicable to a range of other substrate types, including flexible plastics or paper substrates with suitable planarizing and protective coatings. These types of materials should, if sufficiently planar and smooth, allow resolution that is comparable with that achievable on glass. Mounting such substrates onto glass carriers for ease of handling would allow them to be processed with minimal modifications to the tooling. The registration capabilities, however, would likely be degraded somewhat owing to dimensional instabilities (for example, from thermal expansion and contraction and other mechanisms) in the substrates. Also, current systems are most fully developed for patterning only on one side of the substrate. New handling equipment would have

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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to be developed for processing both sides. Given the intrinsically high resolution and low levels of distortion associated with the patterning techniques, it is reasonable to expect that suitable mechanical designs and optical registration schemes (that is, alignment marks in transparent regions of the substrate) could yield good front-to-back registration.

The materials used in addressing circuits for liquid-crystal displays—amorphous silicon, silicon dioxide, silicon nitride, metals (for example, aluminum, copper, and so on), and others—are typically deposited by physical or chemical vapor deposition or electroplating (electrolytic or electroless). Deposition through openings in photoresist masks followed by removal of the resist leaves patterns of these materials. Patterning can also be accomplished by using these masks, to prevent etching with wet chemical baths or dry plasmas in reactive ion etching tools. Versions of these and other deposition and etching methods can be used—for example, those used to coat plastics with metals to improve hermeticity in food packaging or to provide optical substrates for data storage (for example, CDs and DVDs) or reflective diffractive optical elements that are at present used in currency. The photoresist masking procedures are applicable to broad classes of other materials that might be considered for currency applications. In addition, many materials themselves can be designed to be photodefinable, for direct patterning by photolithography, without the etching or deposition steps. For example, conducting polymers, semiconducting small molecules, electroluminescent polymers, and dye doped polymers have all been successfully patterned in this manner. Multilayer stacks of patterned materials can be produced by repeating any of these processes. Detailed technical requirements (for example, for resolution, registration, defect density, yield, and so on) for passive e-substrate features would likely be much different, and more relaxed, than those for functional circuits, depending on the designs. As a result, the range of materials and substrates that could be used is likely to be broad.

Simulation Strategies

The difficulty of simulating a passive e-substrate would be a strong function of the design of the feature. The manufacturing approaches for these features provide, as described above, a level of flexibility in materials choices and of patterning capabilities that would be difficult or impossible to reproduce by all classes of counterfeiter (with the possible exception of the state-sponsored class), owing to the high capital cost and technical sophistication of the manufacturing systems. Effective implementation in covert features would be straightforward. Use in overt or visible features would require optimizing designs to frustrate various simulation strategies.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Development Risks and Issues: Phase I

For the production of currency or currency features, the adaptation of processing systems similar to those used for electronics represents a radical departure from conventional printing-based manufacturing. Such a radical change has associated risks. The approaches described here are, however, inherently technically feasible for passive features. The durability of a passive e-substrate strip would be comparable to, or better than, that of a diffractive optically variable device (DOVD) or conventional security strip.

Development Plan: Phase I

The implementation of the passive e-substrate feature would require adapting for this application the existing processing approaches outlined above. A key part of the development is the definition of suitable feature designs and materials that frustrate simulation strategies. Also, the modes of manufacturing would need to be considered. The preferred approach would be to create a dedicated manufacturing facility based on a design similar to a flat-panel-display fabrication line but different in key ways. Other issues, such as durability, should be examined. The durability of the passive e-substrate feature is estimated by the committee to be comparable with, if not better than, current holographic features.

Estimate of Implemented Production Cost

The costs and capacities for manufacturing of large-area electronics have recently reached levels at which they can begin to be considered for currency or currency-feature fabrication. The latest facilities, some of which first became operational in 2006, use glass substrates that measure 2,200 mm × 2,500 mm (Gen 8). Initial configurations of these manufacturing plants have capacities of ~15,000 Gen 8 substrates per month, with projections of about 30,000 to 50,000 substrates per month at full capacity.7 The most promising integration pathway uses passive patterns formed on thin plastic substrates (mounted on the glass or a rigid substrate for processing) that are then integrated into conventional printed paper currency through the use of strips woven into or bonded to the paper. If these strips are 5 mm wide and 5 cm long, then ~20,000 of them can be cut from a single Gen 8 substrate. This number corresponds to a monthly production capacity of ~1 billion strips per month, from a single fabrication facility in its initial configuration, and about 2 billion to 4 billion strips per month at full capacity. These numbers are

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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conservative estimates, since many currency implementations (for example, passive images or structures) would require only one or two patterned layers instead of the four to six that are used for circuits. Some of this advantage disappears, however, if both sides of the substrate must be patterned.

In addition to the production capacities, the costs are critically important. Figure D-4 presents the cost per square meter of fully formed pixel switching circuits for color liquid-crystal display (LCD) modules, as a function of year, based on estimates associated with a Gen 7 fabrication line. For 2006, these estimates correspond to costs of $0.08 per strip if it is assumed that the additional costs of processing on plastic sheets and of cutting and packaging the elements for currency are no more (or less) than those for making the displays. These costs represent upper bounds for the passive features, because they require the following; (1) only one or two patterned layers, in the simplest embodiments, which is three or four times fewer than those needed for the circuits; (2) only modest yields and levels of registration, compared with those needed for active circuits; and (3) classes of plastic substrates that have less demanding requirements than the ultraflat glass plates used for circuits. These differences might, the committee speculates, reduce the costs by a factor of about 5 times—for example, 2 times for (1) and another of 2.5 times for (2) and (3)—bringing the estimates to ~$0.015 per strip. Finally, adopting such a manufacturing approach for currency or currency-feature fabrication would happen, at the earliest, about 5 years from now. Projections as shown in Figure D-4 suggest that costs in 2010 could be about 30 percent lower than they are today, leading to costs of $0.01 per strip and about $0.45 per note. The cost of any

FIGURE D-4 Estimated cost per square inch of circuits in the Gen 7 format as a function of production year, including projections, based on model-generated data. SOURCE: iSuppli, Inc., Flat Panel Display Manufacturing Cost Model: TFT-LCD (February 2005).

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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specific implementation would scale accordingly, by area. Successful efforts in the development of newer and potentially lower cost approaches to forming large-area electronics could lead to further reductions in cost. These numbers indicate cost feasibility for the strip embodiment of the passive e-substrate feature.

References and Further Reading

Reuss, R., B.R. Chalamala, A. Moussessian, M.G. Kane, A. Kumar, D.C. Zhang, J.A. Rogers, M. Hatalis, D. Temple, G. Moddel, B.J. Eliasson, M.J. Estes, J. Kunze, E.S. Handy, E.S. Harmon, D.B. Salzman, J.M. Woodall, A.A. Alam, J.Y. Murthy, S.C. Jacobsen, M. Olivier, D. Markus, P.M. Campbell, and E. Snow. 2005. Macroelectronics: Perspectives on technology and applications. Proc. IEEE 93(7): 1239-1256.

Ruess, R.H., D.G. Hopper, and J.-G. Park (eds). 2006. Macroelectronics. MRS Bulletin 31(6).

Active e-Substrate
Description

Active electronic substrates—that is active e-substrates—refer to classes of features that, like the passive e-substrate features, are enabled by techniques and materials emerging from developments in large-area and/or low-cost electronics.

Unlike the passive features, however, the active e-substrate provides functional electronic or optoelectronic devices (for example, light emitters, chemical sensors, or actively programmable surface topography) that interact actively with the cash handler (for example, by emitting light or sound or by sensing chemicals and emitting a signal of some sort) or with a machine reader (for example, by responding to a radio frequency or to microwave radiation). This active functionality provides, together with the structures themselves, extremely high levels of security. This subsection provides some conceptual designs for active structures, including ideas on the power generation required and the production of human perceptible responses. Some information on manufacturing options is also included. The committee stresses that the technical challenges in meeting the durability and cost requirements of such features for currency applications are extreme. The purpose of this subsection is only to introduce some notional concepts.

Feature Motivation

The active e-substrate feature platform represents a fundamentally new approach to currency feature design in which parts of the note are able to interact

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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actively with the general public or with specialized machine readers. The specialized manufacturing approaches, which are similar to those described for the passive e-substrate feature, together with the active functionality lead to features that offer extremely high levels of security. Duplication or simulation would be possible only by large, state-sponsored counterfeiters. This class of feature, while extremely challenging technically, can begin to be considered owing to the many recent advances in large-area and low-cost electronics, as described in the subsection above, “Passive e-Substrate.”

Potential Implementations

Potential implementations of the active e-substrate feature have considerations that overlap and extend beyond those associated with the passive embodiments. The active functionality could provide significant benefits, if successfully implemented. Achieving acceptable cost structures and device durability and providing adequate power supply are formidable technical obstacles. Potential scenarios for the three primary e-substrate components for active functionality (power generation, circuitry, and human-perceptible response) are described below. Specific features would use one method from each component type—for example, a piezoelectric film embedded within the banknote could generate a current that is delivered to an organic light-emitting diode (LED) by means of inorganic flexible electronics that produces a “twinkling eye” in the note’s portrait when the banknote is squeezed between a cash handler’s thumb and finger.

Scenario 1a. Power-Generating Devices: Piezoelectric
  • Source of energy: Pressing (squeezing) or shaking the banknote.

  • Principle of operation: Certain materials generate electrical power when compressed. The amount of power is proportional to the piezoelectric coefficient and thickness of the material, and to the applied force. Piezoelectric films, disks, and rods are activated by compression (pressing or squeezing the banknote), and piezoelectric cantilevered beams are activated by shaking. Common piezoelectric materials include quartz and lead zirconate titanate, and polymers are now being developed as piezoelectric elements for microelectromechanical systems (MEMS) devices.8

  • Power-generation capability: It has been reported that 200 microwatts of power have been generated from a microfabricated piezoelectric cantilevered device vibrated by resting on the outside case of an operational

8

For more information on the integration of piezoelectric materials into MEMS devices, see <http://www.npl.co.uk/materials/functional/pdf/mems_metrology.pdf>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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microwave oven (Roundy, 2003). Brief power spikes of 120 milliwatts for 0.1 millisecond have been generated from 15-micron-diameter fibers embedded in a 5.85-millimeter slab by dropping a 34-gram ball bearing from a height of 10 centimeters (Mohammadi et al., 2003).

  • Durability/manufacturing considerations: Monolithic piezoelectric devices are commercially available as rods or cylinders and can be embedded into the banknote in the same way as the current security threads, except now lead wires are required. The lead wires and piezo rods would be embedded during the same manufacturing step. Alignment tolerances are a function of the cross-sectional area of the piezo rod. Good electrical contact is required, but the orientation of the wire and piezo rod is not critical. Alternatively, the piezoelectric rod and wires could be surface printed on the banknote using standard ink-jet technology. A microfabricated piezoelectric cantilever could be similarly embedded into the banknote, but in this case the wire must be precisely aligned with the piezo device. This device could also be surface printed but requires three-dimensional control using, perhaps, rapid prototyping technology.

  • Potential research topics: Research will be needed to investigate the production of materials with higher piezoelectric coefficients.

Scenario 1b. Power-Generating Devices: Reverse Peltier (Thermoelectric) Device
  • Source of energy: One possible energy source is the temperature gradient between the user (either directly or through an input device) and the ambient environment—possibly by means of holding a banknote between the fingers, breathing on the banknote, or placing it in a warm or cold location.

  • Principle of operation: The reverse Peltier effect, known as the Seeback effect, discovered in 1822, produces a voltage when the device is exposed to a temperature gradient, with the voltage produced proportional to the temperature difference. A traditional Peltier device converts electrical current into temperature gradient. As an electrical current is passed through a junction made up of two materials with different conductivities, heat is either produced or absorbed at the junction. A second junction, which completes the electrical circuit, displays the opposite heating or cooling. This thermoelectric effect, which is much more dramatic in semiconductors than in metals, is currently seeing use in a range of heating and cooling applications—for example, in small coolers that can be run off an automobile’s 12 volt power outlet.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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  • Power-generation capability: The power-generating capability of a thermoelectric device is not entirely clear. A number of issues affect the potential for small-size applications such as in currency applications. The power density of a thermoelectric device is inversely proportional to the thickness (distance between the hot and cold surfaces). A number of companies are currently using nanotechnology, including quantum dots, to produce micron-scale devices.9 Such devices are currently being used for small-scale cooling, but would conversely be capable of scavenging energy. One research effort currently has the following goal for a high-density nanoengineered thermoelectric material: generate 100 microwatts of power at 3 V with 1°C temperature change on a 6 cm2 surface.10

  • Durability/manufacturing considerations: Manufacturing and durability considerations similar to those for piezoelectric devices would apply for thermoelectric devices integrated into currency, with the exception that it is unlikely that ink-jet technology would be capable of directly printing thermoelectric devices onto currency.

  • Potential research topics: Extensive research is ongoing to improve thermoelectric figures of merit and efficiencies. Research should focus on the ability of current technologies to produce devices capable of powering low-power outputs such as LED for currency applications.

Scenario 1c. Power-Generating Devices: Solar
  • Source of energy: External light source (sunlight, flame, or electric light).

  • Principle of operation: Solar cells convert energy from light into electrical current either directly through the photovoltaic effect or indirectly through the generation of heat. Most solar cells use the photovoltaic effect and generate a voltage across a p-n junction by using the energy from incident photons to release trapped electrons, allowing them to flow and generate a current through a circuit. The photons can come from any radiant source, provided they have sufficient energy (brightness).

  • Power-generation capability: Typical conversion efficiency of commercial single-crystal solar cells is about 16 percent, with a theoretical maximum of about 28 percent and laboratory-demonstrated efficiencies of about 24 percent (Moslehi, 2006). Silicon crystalline solar cell wafers about 0.3 mm

9

See <http://www.nanocoolers.com> and <http://www.evidenttech.com>. Accessed February 2007.

10

For additional information, see Biophan Technologies at <http://www.biophan.com/2004meeting/riedlinger_TEBio_presentation.ppt>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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thick, with a diameter of 10 cm to 15 cm, generate approximately 35 mA of current per cm2 area at a voltage of 550 mV at full illumination (Lenardic, 2001).

  • Durability/manufacturing considerations: Emerging thin-film technologies based on the deposition of very thin films of photovoltaic materials on plastics could offer much-lower-cost alternatives to silicon-based cells. A large group of materials is being considered for thin-film solar cells, including cadmium telluride, cadmium sulphide, and copper indium diselenide. A compound semiconductor-based approach uses single-crystal gallium arsenide and its alloys, such as gallium-indium phosphide. Other technologies being pursued include low-cost plastic solar cells (Moslehi, 2006). Recently, titanium dioxide thin films have been developed for potential photovoltaic cell construction. These transparent films are particularly interesting because they can also serve double duty as windows (Parry-Hill et al., 2006).

  • Potential research topics: Extensive research is ongoing to improve photovoltaic conversion efficiency and durability.

Scenario 1d. Power-Generating Devices: Capacitive
  • Source of energy: Pressing (squeezing) or shaking the banknote.

  • Principle of operation: An electrical capacitor stores electronic charge for release when required. As envisioned for banknote power generation, the capacitor can amplify a small initial voltage by harvesting mechanical energy obtained by squeezing or shaking the banknote. A capacitor consists of two charged plates separated and insulated from each other by a dielectric material. The amount of capacitance, C, is inversely proportional to the thickness of the gap, and can thus be dynamically changed by changing the gap via squeezing or shaking. Note that the gap can be either in the plane of the banknote or along its thickness. The voltage across the capacitor is inversely proportional to the capacitance, so the voltage is proportional to the gap thickness.

  • Power-generation capability: An optimized, in-plane gap, microfabricated variable capacitor has been demonstrated to generate 100 microwatts per cubic centimeter (Roundy, 2003).

  • Durability/manufacturing considerations: Variable capacitors can be manufactured using traditional MEMS fabrication methods. The durability issues are similar to those for thermoelectric devices.

  • Potential research topics: Research is needed to improve the durability of a microfabricated device for banknote use.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Scenario 1e. Power-Generating Devices: Inductive
  • Source of energy: Rubbing two ends of the banknote together, or passing the banknote across an external magnet, or shaking the banknote.

  • Principle of operation: Current is generated by moving a coiled wire past a magnet. Both the magnet and wire can be embedded in or on the surface of the banknote, and a current can be generated by moving one across the other. The wire can be on one side of the note and the magnet on the other, and current would be generated rubbing the ends across each other. Alternatively, the magnet could be an external, point-of-sale device and current would be generated by swiping the note across the magnet. Finally, the coil and magnet can be integrated into a microfabricated device embedded within the note, and current would be generated by shaking the note. The amount of current is proportional to the number of winds in the coil, the coil diameter, and the strength of the magnetic field.

  • Power-generation capability: For a fixed magnetic strength, the voltage generated (V) is proportional to the number of coil winds (N), the strength of the magnet (B), and the rate of change with time of the area common to the coil and the magnet (A): V = −N BAt). This voltage is likely to be very small for a banknote-embedded device, because the limited size of the magnet will limit both the magnetic strength and available area. The number of coils can be made large using microfabrication techniques.

  • Durability/manufacturing considerations: The “wire” can be created using microfabrication techniques to provide a large number of “coils” in a very small area. Containing the wire in a very small area improves durability by reducing the local radius of curvature for even severe crumples. Alternatively, very flexible wire can be used and distributed over a relatively large portion of the banknote. “Rubber metal,”11 gold-wound threads, and other technologies being developed for electronic textiles12 are applicable. The Department of Defense sponsors research in this area that might be relevant.

  • Potential research topics: Research is needed to improve the durability of a microfabricated device for banknote use.

11

For more information on rubber metal, see <http://www.sciencentral.com/articles/view.php3?type=article&article_id=218392354>. Accessed February 2007.

12

For more information on electronic textiles, see <http://www.research.ibm.com/journal/sj/393/part3/post.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
×
Scenario 1f. Power-Generating Devices: High Power Density Batteries
  • Source of energy: Chemical or nuclear.

  • Principle of operation: Nuclear microgenerators spontaneously emit high-energy electronics that can be used to generate paired electrons and holes to produce current across a diode. Nickel-63 is a safe choice because its relatively low energy beta particles are easily absorbed by a 25-micrometer layer of plastic; they are also absorbed by the thin dead-skin layer covering our bodies (Lal and Blanchard, 2004).

  • Power-generation capability: One nuclear battery has been shown to generate about 3 nanowatts of power from 0.1 millicuries of nickel-63 (Lal and Blanchard, 2004). Additional power has been generated from devices combining radioactive-decay with piezoelectric amplification (Lal and Blanchard, 2004).

  • Durability/manufacturing considerations: Nuclear batteries are currently expensive: 1 millicurie of nickel-63 costs about $25, but the cost of tritium for a single microbattery might be as low as a few cents because it is a by-product from some nuclear reactors (Lal and Blanchard, 2004).

  • Potential research topics: Research is needed to reduce the cost and increase the power density of nuclear microbatteries.

Scenario 2a. Electronics: Printed Electronics

One strategy for forming the electronics component of an active e-substrate feature involves the deposition and patterning of the circuit layers directly on the substrate (for example, plastic for a security strip). The development of semiconductors that can be deposited over large areas and the patterning techniques for building circuits out of them are central to this approach. The most-established technology is based on amorphous silicon (a-Si), deposited by physical vapor deposition, for the semiconductor and photolithographic tools for the patterning. This process, which involves vacuum deposition steps and high temperatures, is extremely well developed for glass substrates (for example, for active matrix liquid-crystal displays), and it has also been demonstrated, in slightly modified forms, on flexible plastic substrates. The main disadvantage is that the performance is relatively low, limited by the poor mobilities in amorphous silicon (~1 cm2/Vs). The performance can be enhanced considerably (mobilities >100 cm2/Vs) by the use of pulsed laser annealing techniques to convert the amorphous silicon into large, oriented-grain polycrystalline silicon. Although this method must be applied with care to avoid thermal damage (for example, thermal buffer layers are often used), polycrystalline silicon-based circuits have been achieved on plastic substrates (that is, polyimide) and flexible steel foils in this manner. Other semiconductor

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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materials include organic polymers and oligomers, as well as organic-inorganic hybrid materials, which exhibit mobilities of 0.1 to 1 cm2/Vs. Transistors formed with solution-deposited inorganics (that is, nanoparticles or nanowires) have similar performance: effective device mobilities (as determined empirically using parallel plate capacitances defined by the physical dimensions of the channel) of ~1 cm2/Vs for the nanoparticles and ~2 cm2/Vs for the nanowires. The intrinsic mobilities of the nanowires can have values approaching those of wafer scale sources of material. Films of solution-grown inorganics, such as CdSe, represent another possibility; the mobilities in this case are ~1 cm2/Vs and higher, depending on the materials and deposition techniques. Finally, carbon nanotubes, in the form of thin films of aligned arrays of tubes or random networks of them, can also be used for these systems. Devices built with such semiconductors can exhibit, in some cases, close to the extremely high intrinsic mobilities of the tubes (several thousand cm2/Vs). Table D-1 summarizes a few of the options. Figure D-5 shows some flexible circuits formed using inorganic and organic semiconductors. The judgment of the committee is that the technology that is already well developed for active matrix liquid-crystal televisions represents the most promising potential path to circuits for the e-substrate feature. The newer approaches that use printable semiconductors and other materials are currently not sufficiently well developed for serious consideration for currency applications.

Progress in these areas will, of course, potentially change this situation. Figures D-6 and D-7 show circuits formed by printing.

TABLE D-1 Summary of Selected Semiconductor Materials That Have Been Implemented in Flexible Electronic Systems

Semiconductor Material

Mobility (cm2/Vs)

Processing Technique

Technology Demonstrations

Technology Status

Small molecule organics

<5

Evaporation

Large areas, circuits

Small-scale fabrication

Polymers

<1

Solution printing

Large areas, circuits

Small-scale fabrication

Hybrid organic-inorganic

~1

Solution printing

Small areas, devices

Inactive

Amorphous silicon (a-Si)

~1

Evaporation

Large areas, circuits

Commercial

Laser annealed a-Si

~100

High temperature

Small areas, circuits

Research

Solution CdSe, SnS2

~10

Solution printing

Small areas, devices

Research

Grown nanopart

~1

Solution printing

Small areas, devices

Research

Grown nanowires

~1

Solution assembly

Small areas, devices

Research

Nanotube networks

~50

Solution printing or transfer printing

Large areas, circuits

Research

Nanotube arrays

~200

Transfer printing

Small areas, devices

Research

Microstructured silicon

~500

Transfer printing

Large areas, circuits

Research

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE D-5 Flexible electronics based on (left) inorganic (Service, 2006) and (right) organic semiconductor materials (Drury et al., 1998). SOURCES: (Left) Courtesy of I. Chun Cheng, Princeton University. (Right) Reused with permission from Drury et al. (1998), ©1998, American Institute of Physics.

FIGURE D-6 Images and performance characteristics of a large-area printed circuit, formed with a laser thermal printing technique, and an organic small-molecule semiconductor. SOURCE: Reused with permission from Blanchet et al. (2003), ©2003, American Institute of Physics.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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FIGURE D-7 Flexible, paperlike displays that integrate electrophoretic inks with printed flexible electronic backplane circuits. The images show (top left) an initial demonstration and (bottom left) a printed backplane circuit (Rogers et al., 2001); and (right) an example of the current state of the art (<http://www.plasticlogic.com>).

Scenario 2b. Electronics: Silicon Blocks

There are several methods for dicing wafers into very small pieces (for example, 100 × 100 μm, or in some cases much smaller—2 μm × 100 nm) and then integrating them on plastic substrates to form flexible, low-cost devices. The most well-established technique uses robotic “pick-and-place” tools to move these elements from the source wafer to the target substrate. Many existing forms of RFID tags, smart cards, and related applications are formed in this manner. Newer approaches rely on solution suspensions of these elements and guided self-assembly techniques13 to achieve this pick-and-place outcome without the need for precision robotics. At least one company is manufacturing RFID tags in this manner, although the cost per device is relatively high. Recent research publications describe yet another approach, in which soft elastomeric stamps pick up many ultrathin

13

For more information, see <http://www.alientechnology.com>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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elements (sometimes referred to microstructured silicon, μs-Si) chemically etched from a source wafer in a parallel fashion and transfer them, in a single-step printing process, to a target substrate. High-performance devices and circuits have been demonstrated using this approach, on a range of substrates including rubber and thin plastic sheets. For all three of these approaches, the semiconductor is usually single-crystal silicon, although many other materials are possible. Electrical interconnects are typically patterned on the device substrate to interface the elements with one another or with other subsystems such as the power sources described in the previous subsection.

Scenario 3a. Human-Perceptible Response: Light-Emitting Elements

With the “twinkling eyes” feature—a light shines from the banknote (the eyes “twinkle”)—the light could be a single-colored point, or an array with a variety of colors. One approach would be to use an organic light-emitting-diode (OLED) powered by a MEMS piezoelectric device activated by shaking. OLEDs consist of electroluminescent organic layers sandwiched between a transparent anode and a substrate-mounted cathode. Excluding the substrate, which may be <1 mm thick, the thickness of the rest of the layers is typically 20 nm to 50 nm total. OLEDs are being developed for video displays, and some demonstration devices have been achieved on flexible surfaces. Power requirements are given assuming large pixelated arrays and are a function of brightness. State of the art seems to be 250 mW for movie-theater-equivalent brightness for 852 pixels × 3 pixels × 600 pixels. Others quote “low current at 2-10 volts.” A single pixel might require only ~250/(852 × 3 × 600) = ~0.2 microwatts. OLEDs can emit anywhere in the visible spectrum, but reds and greens are more efficient than blues. A major challenge with OLEDs in existing devices is their limited lifetime and reliability and the temporal drift in their properties. A promising alternative path uses ultrasmall inorganic LEDs. Such devices can be formed with sufficiently small dimensions for integration onto flexible substrates. They have the advantage of avoiding many of the challenges associated with OLEDs. This inorganic approach might represent the most promising embodiment of light-emitting elements.

Scenario 3b. Human-Perceptible Response: Visible, Nonemissive Elements

Liquid-crystal devices, electrochromic materials, and electrophoretic cells represent some of the non-emissive visible elements that could be considered. These systems have the advantage of requiring much less power than that needed by the types of emissive elements described above. Several demonstrators of these types of systems, on flexible substrates, exist and appear to be moving toward commer-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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cialization for handheld electronic devices such as electronic books, personal digital assistants, and cellular telephones.

Materials and Manufacturing Technology Options

The cost structure of the active e-substrate features is critically important to their implementation in currency. The analysis provided in the “Passive e-Substrate” subsection (above) provides one possible framework for estimating the costs associated with the circuit component of an active e-substrate feature. Costs of the active features could be expected to be 10 times higher than the simple passive structures, when fabricated with thin-film processing techniques. An attractive alternative manufacturing approach for the active features could exploit the integration of small-scale chips of processed single-crystal silicon, similar to those elements used in RFID tags, as described in the previous subsection. For the foreseeable future, such systems likely offer the most promising means to meet the demanding requirements on reliability, performance, and cost. In this approach, most or all of the device processing and materials sets are borrowed directly from the well-established silicon integrated-circuit industry. The transfer printing or pick-and-place methods then integrate these fully formed devices or circuit blocks onto a thin strip on which other printing techniques form the necessary electrical interconnects. These printing methods can range from well-developed screen printing techniques that are used for printed circuit boards, to newer techniques that use laser thermal transfer (see Figure D-6).

These chips can not only provide electrical functionality, but they can also be integrated with sensors, LEDs, or other elements. As with the circuits, there is a range of possibilities. For example, newer classes of small-molecule or polymer organic electroluminescent materials or conventional inorganic micro-LEDs could be used for the “twinkling eyes” feature. The most robust and lowest-cost systems, especially for applications in currency, where the number of active elements might be much smaller than those in a commercial display device, might be achieved with the most-established technology, that is, inorganic micro-LEDs in this case. These devices, like the silicon circuit elements, can be integrated using assembly or transfer printing techniques.

Simulation Strategies

The active e-substrate feature has the potential to provide overt features that would be very difficult to simulate. Indeed, this technology would be such a radical departure from the current production methods that it would decouple advances in digital printing technology from enhanced counterfeiting tools.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Key Development Risks and Issues: Phase I

The development risks associated with the active e-substrate feature platform are significant. A long-term development program is required.

Development Plan: Phase I

There are many opportunities for features within this technology area. Phase I activities should collect sufficient information and data to determine which potential features are of greatest interest for counterfeit deterrence.

Key milestones for Phase I would include the following:

  • Define power requirements for a class of human-perceptible responses—for example, design an OLED having sufficiently small size and durability properties; then determine the current and voltage requirements to drive it.

  • Define packaging specifications for the power-generation device and electric conductors to withstand expected durability tests.

  • Design the prototype system, including power generation, conductors, and packaging, specifying candidate materials and processes.

  • Analytically demonstrate the required power generation.

Estimate of Implemented Production Cost

The costs for an active e-substrate feature are difficult to estimate. Based on the detailed costing analysis for the passive e-substrate feature, the committee estimates a minimum of $0.10 per strip.

References and Further Reading

Alien Technology Corporate Web site. 2005. Available at <http://www.alientechnology.com>. Accessed February 2007.

Blanchet, G., Y.-L. Loo, J.A. Rogers, F. Gao, and C. Fincher. 2003. Large area dry printing of organic transistors and circuits. Applied Physics Letters 82(3): 463-465.

Drury, C.J., C.M. Mutsaers, C.M. Hart, M. Matters, and D.M. de Leeuw. 1998. Low-cost all-polymer integrated circuits. Applied Physics Letters 73(1): 108-110.

Lal, S.A., and J. Blanchard. 2004. The daintiest dynamos. IEEE Spectrum, September: 36-41.

Lenardic, D. 2001. Solar cells. Available at <http://www.pvresources.com/en/solarcells.php>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Loricchio, D.F. 1992. Key force and typing performance. Pp. 281-282 in Proceedings of the Human Factors Society 36th Annual Meeting. Santa Monica, Calif.: Human Factors Society.

Mohammadi, F., A. Khan, and R.B. Cass. 2003. Power generation from piezoelectric lead zirconate titanate fiber composites. Pittsburgh, Pa.: Materials Research Society. Materials Research Society Symposium Proceedings, Vol. 736: D5.5.1-D5.5.6.

Moslehi, B. 2006. Harvesting the Sun: Renewable power generation from photovoltaic solar cells. Micro Magazine, June. Available at <http://www.micromagazine.com/archive/06/04/reality.html>. Accessed March 2007.

Parry-Hill, M.J., R.T. Sutter, and M.W. Davidson. 2006. Solar Cell Operation. National High Magnetic Field Laboratory, Florida State University. Available at <http://micro.magnet.fsu.edu/primer/java/solarcell/>. Accessed February 2007.

Plastic Logic Corporate Web site. 2000. Available at <http://www.plasticlogic.com>. Accessed March 2007.

Rogers, J.A., Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V.R. Raju, V. Kuck, H. Katz, K. Amundson, J. Ewing, and P. Drzaic. 2001. Paper-like electronic displays: Large area, rubber stamped plastic sheets of electronics and electrophoretic inks. Proceedings of the National Academy of Sciences USA 98(9): 4835-4840.

Roundy, S. 2003. Energy Scavenging for Wireless Sensor Nodes with a Focus on Vibration to Electricity Conversion. Ph.D. dissertation, Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, May. Available at <http://engnet.anu.edu.au/DEpeople/Shad.Roundy/EnergyScavenging.pdf>. Accessed February 2007.

Service, R. 2006. Inorganic electronics begin to flex their muscle. Science 312: 1593-1594.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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NiTi SHAPE MEMORY AND SUPERELASTIC RESPONSIVE MATERIALS

Description

Shape memory materials, most commonly based on the intermetallic alloy NiTi (often referred to as nitinol), offer a number of phenomena that could be used to produce active responses useful for developing human-detectable features for currency security. These phenomena are based on the reversible austenite-to-martensite phase transformation in these alloy systems. Three phenomena based on the thermoelasic martensite have potential for currency security features:

  • Shape memory effect (SME)—in which deformed structures are recovered with heating.

  • Transformation superelasticity (TSE)—in which structures can be deformed to large strains but recover their shape when the stress is released.

  • Two-way shape memory effect (TWSME)—in which a reversible shape change is induced by changes in temperature.

A number of currency features based on this set of technology can be envisioned:

  • Superelastic features: These could be wires or thin-foil-based patterns (dots, eagles, numbers, buildings, and so on) that could be easily deformed but would spring back to shape. This technology is the same as that used in some eyeglass frames.

  • Shape memory features: Similar patterned features can be deformed at relatively low temperature, then recovered at higher temperature, such as under the heating from a human finger, incandescent light, or low-temperature heat source. Applied to currency, this approach may be more problematic, owing to the relatively broad ambient temperature range in which currency must be able to work (for example, −20°C to +40°C).

  • Two-way shape memory effect: Patterned features will change shape when touched, moving in one direction when the human touch increases the temperature and the other direction when the human touch decreases the temperature. However, this effect may be difficult to achieve effectively within the temperature range available using purely human inputs; it may require external heating and cooling.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Feature Motivation

Such features would be quite effective at allowing the general public to identify genuine currency. The features would require the user to input a stress and identify a response, so some education would be required. However, the response should be robust and easy to identify. Such features would also be very useful for the visually impaired.

The shape memory/superelastic nature of NiTi thermoelastic alloys is well documented and understood. Such alloys are used in a range of industrial, medical, and consumer applications. However, their cost is typically rather high. Considerable industrial and academic research is involved with developing these materials for cost-effective, higher-volume applications. While the base metals in these alloys are relatively inexpensive in the base forms in the amounts needed for currency applications, processing can be quite expensive. Thus, the key to these alloys finding application in currency is to be able to produce relatively thin foils and/or films in high volumes at a reasonable cost.

The active nature of features based on thermoelastic martensite features would be a strong deterrent to currency counterfeiting for all but the most sophisticated counterfeiters sponsored by national governments. Primitive, hobbyist, and petty criminal counterfeiters would be hard-pressed to simulate the active response combined with the metallic nature of the features. The professional criminal counterfeiter would have difficulty duplicating the chemistry (stoichiometry) and processing to achieve a suitable response. State-sponsored counterfeiters would need considerable time to duplicate the processing and stoichiometry control.

Potential Implementations

As noted above, thermal elastic martensite alloys would allow verification of a note by means of user input and/or user-detected output. For superelastic alloys, the user would fold or bend or deform a foil or wire and observe, either by feel or vision, the feature recovering its shape upon unloading. Shape memory features would require the observation of a shape recovery (either tactile or visual detection). Two-way shape memory would require either heating or cooling to induce a user-detected shape change.

Scenario 1

A specific example of a superelastic feature would be a wire or security strip integrated into the substrate. The user could fold the feature over. With release of the load, the strip would snap back into its original shape. Similarly, a foil em-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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bossed with a recognizable symbol could be deformed but would retain its shape with unloading.

Scenario 2

A shape memory alloy feature could entail a metallic security strip or foil that would show permanent deformation with use. This deformation might come from either the specific user or through general circulation. To verify the authenticity of the note, the user would heat the note (possibly by means of user body heat or with an inexpensive external source). The feature would recover its original, undeformed shape.

Scenario 3

A two-way shape memory feature could entail a foil with an embossed feature or pattern of bumps. This pattern would appear and disappear with changes in temperature. These temperature changes could be user-induced (for example, heating through touch) or via an external device (such as a lamp for heating, a ice cube for cooling).

Materials and Manufacturing Technology Options

Thermal elastic marteniste alloys based on NiTi are readily available from commercial sources. However, implementation of these alloys into currency requires overcoming significant processing challenges. Once produced, it is anticipated that the features would be relatively easy to integrate into currency. Currently, security threads are integrated into U.S. currency, and a number of international currencies have displayed metallic strips in the past (for example, higher-denomination British notes have included both silver and gold strips). Thus, it is anticipated that NiTi metallic strips could be readily integrated using current technology. Likewise, because many currencies currently include holographic foils, one would expect to be able to integrate NiTi foils readily into notes.

Simulation Strategies

Primitive and opportunist counterfeiters would not be able to duplicate and would have a very difficult time simulating the effects of thermoelastic alloy features. Most simulations would most likely be based on using elastic polymers to simulate the superelastic effect. However, the general response of such features would be significantly different in terms of the mechanical response. Furthermore, the metallic nature of NiTi foils and strips would be lost when using polymers.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Shape memory features could be simulated using shape memory polymers (for example, those used in shrink-wrap and shrink fittings). Again, these would not be metallic in nature. In both cases, these counterfeiters would be challenged to produce the undeformed/recovered features accurately if these were embossed.

Petty criminals would be challenged in attempting to counterfeit thermoelastic alloy features. While a range of shape memory and superelastic materials are available commercially, integrating these into notes in a realistic manner with accurate responses would be difficult. More likely, such criminals would attempt to raise the denomination of notes. However, with the specific shapes and topologies of features on each note (for example, embossing of the denomination), along with strategic placement among the notes, raising notes could be made less effective.

Professional criminals would be very challenged by the processing and alloy control necessary to obtain good duplication of the features and responses. State-sponsored counterfeiters could, with time, reverse-engineer the chemistry necessary to obtain the proper temperature, and response behavior. More time would be required to develop the specific processing that would be required to employ a thermoelastic feature in currency.

Key Development Risks and Issues: Phase I

Manufacturing NiTi to the desired shape (strip, embossed foil) requires careful processing with relatively precise temperature control. Furthermore, the transformation temperatures of these alloys are a strong function of Ni-Ti stoichiometry, tertiary alloys content, and alloy purity. Processing bulk alloys to the thin foils required for currency applications requires precise thermal mechanical processing and high-strength rolling mills. Direct production of thin foils using sputtering and other approaches is currently being carried out in the laboratory but will require technological advances for production scale-up to provide the proper chemistry control in conjunction with the large volume necessary for full-rate currency production.

NiTi thermoelastic alloys can recover approximately 7 to 8 percent elastic deformation in the martensitic phase. Within this strain range these alloys can survive millions of deformation cycles. The key to durability in currency applications will be in avoiding deformation beyond this critical strain in the folding and crunching of the note. However, it should be noted that the elastic strain (εE) at the top or bottom of a beam (foil) is proportional to the thickness (t) of the member and inversely proportional to the bending radius (ρ)—that is, εE = t/2ρ. Thus, for the thin foils anticipated in currency applications, fairly small bending radii should be sustainable; for example, a foil 20 microns thick will not reach a permanent deformation strain of 8 percent until it is bent to a bending radius of 0.125 mm. Furthermore, it is anticipated that some of the deformation from folding and bend-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ing will be accommodated in the substrate. Thus, disbanding of the NiTi from the substrate also needs to be evaluated.

NiTi features would have to be attached to and integrated with a note in a manner similar to holographic foils or woven security strips. From an aesthetic point of view, NiTi features would impact the note in the same manner as these features.

NiTi features should not raise significant social acceptability concerns, as they are not electronic and contain components found in coins (Ni) and consumer products (both Ni and Ti). NiTi alloys are currently accepted by the public in eyewear.

Development Plan: Phase I

The basic technology of superelastic and shape memory alloys based on thermoelastic martensite is well developed and understood. A range of industrial and consumer products are currently manufactured with these alloys. The critical activities to be addressed in Phase I include the following:

  • Identification of the specific operation temperatures for the features and development and/or identification of the alloy compositions that meet these needs. This is a straightforward task for superelastic alloys, as a number of alloy compositions display superelasticity around ambient temperature. However, developing alloy compositions that display shape memory and recovery or two-way shape memory at ambient temperature, and in particular that will activate with simple human input such as touch, will be more difficult.

  • Development of robust, repeatable manufacturing to produce thin-foil TiAl alloys with the necessary stoichiometry control. As noted, this may entail ingot processing or direct sputtering, both of which currently have some limitations. In particular, ingot processing is subject to variations in stoichiometry across an ingot and difficulty in reducing the ingot size down to the thin-foil dimensions needed for currency applications. Direct sputtering of thin foils is complicated by the difficulty in controlling and reproducing exact alloy stoichiometries, owing either to uneven sputtering rates of Ni and Ti from alloy targets or to uneven spatial distribution from single-component targets, if co-sputtering is used.

A number of university and industrial programs continue to develop NiTi thin-film technology. For example, both General Motors and Johnson Wax currently have active programs.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Estimate of Implemented Production Cost

It is anticipated NiTi security features will be produced by outside vendors and incorporated into currency by the BEP or the substrate manufacturer Crane. As noted previously, wire/foil features might be integrated into notes in the same manner used for the current security threads. Likewise, foil patches could be integrated with notes in the same manner currently used to attach holographic patches on many non-U.S. currencies. This latter approach would require some modification of the BEP production process and would require additional capital investment.

Further Reading

Duerig, T.W., K.N. Melton, D. Stöckel, and C.M. Wayman (eds.). 1990. Engineering Aspects of Shape Memory Alloys. Burlington, Mass.: Butterworth-Heinemann.

Otsuk, K., and X. Ren. 2005. Physical metallurgy of Ti-Ni based shape memory alloys. Progress in Materials Science 50: 511-678.

Zhang, Y., Y.-T. Cheng, and D.S. Grummon. 2005. Indentation stress dependence of the temperature range of microscopic superelastic behavior of nickel-titanium thin films. Applied Physics Letters 98: 033505.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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SMART NANOMATERIALS

Description

The field of “smart” nanomaterials may provide the foundation for security features that are simultaneously extremely complex to fabricate and easy to use. Smart nanomaterials are being developed for a wide range of applications. Many of the smart nanomaterials projected to come out of the nanotechnology revolution will be created by some variation of molecular manufacturing (MM). These materials are expected to exhibit a wide range of dynamic behaviors that may be useful for anticounterfeit features.

Many research programs in smart nanomaterials have targeted the use of extremely high technology manufacturing systems to create materials capable of independent dynamic responses. The final output of these responses may be both human-perceptible (detected by simple bioassay) and Boolean—that is, their state would be represented by a simple yes/no answer to the bioassay. If such a smart nanomaterial can be designed in a manner that fits within the physical and fiscal constraints required for currency, the result would be an anticounterfeiting feature almost impossible to counterfeit or simulate, yet as simple to use as color-shifting ink or a watermark.

In the broadest sense, self-assembly describes the natural tendency of physical systems to lose energy to their surroundings and assume patterns or structures of lower energy. Random thermal motions bring constituent particles together in various configurations, so that stable configurations (those with the most binding energy) form, tend to persist, and eventually become predominant. Self-assembly is generally driven by a reduction in the Gibbs free energy for the total system of interest. When properly harnessed, self-assembly offers one of the most reliable, reproducible operating mechanisms known to science and engineering. Many self-assembly events may be viewed as analogous to spontaneous chemical reactions and, as such, the information necessary to produce self-assembly is contained within the fundamental physicochemical structure of the components themselves. Through this simple operation of physical law, a pattern or structure arises in a bounded system with the input of relatively little information from outside. A system slowly approaching equilibrium will assume a simple repetitive structure, while a dynamic system may generate structures of great complexity. For example, molecules in a cooling bucket of water will self-assemble as simple ice crystals, while the same molecules in a turbulent cloud with ever-changing temperature and humidity will self-assemble as complex snowflakes in enormous variety. Many fascinating structures in the natural world around us are self-assembled.

Chemists and biologists often use the term self-assembly in a more restricted sense to describe structure formation in a fluid containing various types of mol-

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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ecules, particularly organic molecules that form weak chemical bonds with a strength that is exquisitely sensitive to molecular shape and orientation. The strongest bond between such molecules often occurs when the molecules fit together in a “lock-and-key” fashion. Biological molecules such as proteins have evolved complex three-dimensional topographies that can create complex higher-order structures based on a “lock-and-key” fit utilizing hundreds or thousands of specific atomic contacts. For example, the bacterial ribosome—a complex molecular machine consisting of about 55 different protein molecules and several ribosomal RNA molecules—will, under appropriate conditions, self-assemble in a test tube. It is important to note that these reactions may occur under conditions that are relatively far from those found in the cytoplasm of living cells. A great deal of nanobiotechnology research is focused on developing ways to stabilize and utilize complex biomolecular self-assembly for processes outside the living cell.

Smart materials whose macroscopic structure depends on the molecular self-assembly of engineered molecular structures may offer a unique combination of “ultrahigh technology” fabrication that produces simple yet uncounterfeitable Boolean behavior. Molecular self-assembly (MSA) is being explored as a manufacturing base for the large-scale industrial processing of consumer items such as computer chips and scaffolds for bioengineering applications like in vitro tissue and organ growth. Large-scale industrial demand for MSA/MM-based facilities may bring the cost security features using the same technology into line with the economic constraints of FRN production the near future. In a sense, these security features could be considered as a spin-off of the National Nanotechnology Initiative.

Feature Motivation

There is a general assumption in the security-device industry that increased technical sophistication results in a decrease in either the simplicity of operation for the end user or the durability of the device itself. For example, the incorporation of electromagnetic security devices requires the use of an instrument capable of reading the signal generated by that device. In the latter case, the incorporation of holograms or other passive optical features increases security via direct visual bioassay, but the materials used to create the holographic image are both subject to physical degradation with repeated handling and may be counterfeited or simulated, since holographic technology is widely available. Smart nanomaterials created by MSA and/or MM may have the ability to combine high-technology, high-durability fabrication methods beyond the capability of most counterfeiters with simple bioassay-based operation.

Nanomaterial-based features are fabricated and/or operate via molecular self-assembly or other dynamic properties that are intrinsic to the composition of the

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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materials themselves. These materials exhibit dynamic behavior, but only in a limited range, triggered by changing a simple physicochemical variable such as thermal energy or the presence (or absence) of a specific chemical or biological compound. The result is posited to be a human-perceptible phenomenon easily and rapidly recognized by the targeted class of users. The usefulness of such a feature is that it recaptures the ease of counterfeit detection inherent in the macroscopic optical security features of FRNs prior to the commercialization of cheap yet sophisticated reprographic products. In addition, because it is the materials themselves that create the feature, the potential exists for bioassays other than those involving vision. This possibility could be useful to the vision-impaired.

Potential Implementations

The Institute for Soldier Nanotechnologies (ISN) at the Massachusetts Institute of Technology (MIT) provides a convenient example of how nanofabrication methods may be used to harness MSA and MM to create technology platforms. There is significant similarity between the performance specifications required for many advanced materials under development for soldier technologies and those required for use as anticounterfeiting features in banknotes. Under field conditions, many of these smart materials will need to be highly durable. In addition, because of the immediacy of the battlefield, many applications (for example: Is there a toxin present? Has a ballistic impact occurred?) will require an almost-instantaneous, unequivocal yes or no, and human-perceptible manifestation. These same traits would make these materials of potential use for anticounterfeit applications.

Example 1: Mechanical Actuators Capable of Switching Between States

The ISN at MIT is “developing nanomaterials that are capable of mechanical actuation and dynamic stiffness.”14 Either or both of these properties could be incorporated into a windowed currency feature that is both visible and tactile. As part of the soldier’s battlesuit, these adaptive multifunctional materials will improve soldier survivability. According to the ISN:

Mechanical actuators embedded as part of a soldier’s uniform will allow a transformation from a flexible and compliant material to a non-compliant material that becomes armor, thus protecting the soldier by distributing impact. Soft switchable clothing can also be transformed into a reconfigurable cast that stabilizes an injury such as a broken leg. Contracting materials can be made to apply direct pressure to a wound, function as a tourniquet, or even perform CPR when needed. Mechanical

14

For further information on this research, see <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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actuators can also be used as exo-muscles for augmentation of a soldier’s physical strength or agility and as wound compresses.15

Smart, self-assembling materials capable of switching between states may be adaptable as a security feature in future generations of FRNs.

Example 2: Active, Reactive Fiber Coatings

ISN Team 3 is developing smart nanomaterials that will provide protective measures to enable the future soldier to detect and respond to chemical and biological threats.16 The strategy is to develop

protective fiber and fabric coatings for integration in the battlesuit. These surfaces will neutralize or significantly decrease bacterial contaminants as well as chemical attack agents such as nerve gas and chemical toxins. For example, some investigations include responsive nanopores that “close” upon detection of a biological agent. In addition, novel organic-inorganic hybrid nanocomposites, consisting of nanoparticles and formed using simple dip processing methods that combine sensing and reactive components.17

While the goal of this work is to develop smart nanomaterials that can act as reactive or responsive protective coatings for fibers and fabrics for soldier technologies, any material that can exhibit “dynamic, reversible behavior” that is human-perceptible (that is, may be bioassayed) is obviously a candidate for a security feature in currency.

Materials and Manufacturing Technology Options

These features would use materials and manufacturing methodology that is expected to emerge from recent, large-scale investments in nanoscience and nanotechnology. Importantly, these investments are being driven by the need to nanoscale the manufacturing of components for major industries such as the manufacture of defense-related products, of semiconductor devices for computers and communications, more generally, as well as of bioengineered devices for health care applications.

Also, the ISN is developing the technology to

enable the synthesis of nanotechnologies developed by ISN to provide soldier protection in the field … [using a] broad-based approach to developing processing and fabrication technologies for novel nanomaterials. These technologies must be capable of effectively processing a wide range of components: nanoscale fibers and

15

See <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007.

16

For further information on this research, see <http://web.mit.edu/ISN/research/team03/index.html>. Accessed February 2007.

17

See <http://web.mit.edu/ISN/research/team02/index.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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films; multilayered materials; membranes and microdevices; microfluidic devices; functional hollow fibers; and field-responsive materials and devices. Team 5 has as its goal the fabrication and integration of hierarchically structured materials to achieve multiple and synergistic property combinations.18

Simulation Strategies

Smart materials, such as those under development at the ISN, will be virtually impossible to counterfeit or simulate without the ability to nanofabricate—that is, to build at the molecular level with atomic precision. The power of a molecular self-assembly-based material or device lies in its simplicity of operation. This simplicity is based on the fact that certain forms of MSA occur spontaneously if and only if one can nanofabricate the materials components of the device ab initio. It is unlikely that this type of technology will become available to any but the most sophisticated government-sponsored counterfeiters for at least a generation.

Nanofabrication facilities necessary to create microprocessor chips or molecular scaffolds for tissue engineering are unlikely to evolve into a format that will make them common household items anytime in the near future. Rather, it is probable that such instrumentation will be controlled by large corporations and major government and/or university research facilities.

Key Development Risks and Issues: Phase I

Smart nanomaterials comprise an emerging area, and a number of possible features can be imagined. The adaptation of the nanomaterials work to counterfeitdeterrent features leverages ongoing programs. Thus, a key risk that would hinder the future development of these novel features would be the termination of the NNI. A second key risk is related to the need to understand any risks associated with environmental, health, and safety effects of nanotechnology. This is an ongoing program at the national level, and it would be beyond the scope of a currency security feature R&D program. The third risk would be an unfocused program that expended resources without achieving concrete results.

Development Plan: Phase I

This feature platform would require a long-term commitment to R&D. The most important activity during Phase I would need to be a comprehensive survey of ongoing work and an analysis of the most-promising targets of opportunity for future counterfeit-deterrent features. Once these targets of opportunity were

18

See <http://web.mit.edu/ISN/research/team05/index.html>. Accessed February 2007.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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selected, a specific plan could be developed for each one. The most desirable situation would be for the BEP to be able to highly leverage ongoing work so that BEP funding would be needed only to adopt the technology, not to invent it.

Estimate of Implemented Production Cost

This technology is too immature to make an estimate regarding production cost. A number of different directions could be pursued. It is possible that this technology could be implemented at some future time for less cost than that of the current security thread. However, usual experience is that new technology costs somewhat more, and a decision must be made about whether the benefit would be worth the added cost.

Further Reading

See the Web site for the Institute for Soldier Nanotechnologies (ISN) at <http://web.mit.edu/ISN/research/> and for the National Nanotechnology Initiative at <www.nano.gov>. Accessed February 2007.

National Research Council. 2006. A Matter of Size—Triennial Review of the National Nanotechnology Initiative. Washington, D.C.: The National Academies Press.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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TACTILELY ACTIVE ELECTRONIC FEATURES

Description

A number of material classes offer the potential to develop currency features with tactilely active responses—that is, they will produce local changes in shape or tactile nature as a result of a user input. Two promising classes of material for such features are piezoelectric crystals and electroactive polymers.

A piezoelectric crystal develops a voltage when strained. Conversely, if a voltage is applied to a piezoelectric crystal, it can respond with a strain resulting in a shape change or deflection. This effect can be used in currency applications to produce a user-generated and detectable change in the tactile feel at a specific location on a note.

In the simplest sense, one can envision piezoelectric bumps that would raise or lower on the note when a voltage is applied. The voltage might be supplied by an external source, such as a small battery, but it might also be supplied by an internal source (see the discussion above in the section “e-Substrate”).

In practice, while it may be difficult to produce large enough piezoelectric deflections within the constraints of a note, patterns of fine bumps could be produced to generate changes in the tactile nature of the patterned area. The patterning sequence could be varied on notes of different denominations so that each denomination had a unique, readily identifiable pattern.

Since most piezoelectric materials require relatively high voltages for activation, the materials would have to be carefully selected for application in currency. At the present time, it is not clear if tactilely active piezoelectric features can be practically integrated into notes. Such features will require significant development to integrate them effectively into notes with electronic substrates.

Electroactive polymers (EAPs) are currently being developed for application as artificial muscles and in other actuator applications. The phenomena that make these polymers attractive as active materials, that is, they deform under the influence of an applied external voltage, may also have application for active-response features in currency. Upon the application of the external voltage, an EAP will deform over the period of a few seconds.

As an example of the potential of these materials,19 strips 5 cm × 0.6 cm × 0.02 cm (~0.05 inches thick) can display deflections in the range of 60 to 90 degrees and can displace loads in the range of 50 times the weight of the strips at the ends of the strips. The EAP will regain its original shape upon the reversing of the polarity of the voltage. This process can be repeated over a large number of cycles with

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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no apparent degradation in response. The voltages necessary to drive the response are quite low, on the range of a few volts, in the range offered by dry cell batteries (D, AA, AAA, C).

In a currency application, one might envision a number of types of active-response features developed from EAPs. These could be in the form of an active bending security strip or strips or, with more development, features that changed shape or topography (including changes in tactile nature). The note response would be designed to be unique to the denomination of the note. Clearly, the response would require a voltage input that could either be self-contained on the note or provided by the user in the form of a readily available battery. Both of these power options would require integration with some level of an electronic substrate (see the section above entitled “e-Substrates”).

Feature Motivation

The active nature of features based on piezoelectric crystals and electroactive polymers would be a strong deterrent to currency counterfeiting at all levels of counterfeiting. Primitive, opportunist, and petty criminal counterfeiters would not be able to simulate the active response of the features accurately. Furthermore, feature configurations and designs specific to each denomination would render of dubious value attempts to “raise” notes through bleaching and reprinting higher denominations. Professional criminal and state-sponsored counterfeiters would be challenged to duplicate the proper materials and processing to obtain the required response. Furthermore, integration with the necessary electronic substrate would be extremely difficult to duplicate.

Active-response features would have a number of advantages for use by the general public. The novel nature of the features would attract attention from the public, leading to people’s making use of the features. Currency users could readily detect shape changes and changes in the tactile nature of the features. Denomination-specific placement and patterning of the features would allow simple identification of the note values, making such features very useful for visually impaired users.

Potential Implementations

As described above, tactilely active features would allow point-of-sale user verification of a note’s authenticity and denomination by facilitating a user-input/ feature-output response. The user would primarily use the change in shape and tactile nature to evaluate the note. However, other human-perceptible indicators such as change in visual reflectivity might also be detectable.

In the simplest form, EAP strips could be integrated into a banknote. Such

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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strips would change shape when the note was folded across the contacts of an AA or AAA battery (both provide 1.5 volts and are pervasively available). Different denominations would have both the thread and contacts in different locations on the notes, leading to very specific and publicly identifiable denominating.

More complex integration into a note would include patterns of either EAPs or piezoelectric crystals that would change local topography on the note with the voltage applied. This patterning could be designed to produce recognizable effects such as buildings, eagles, or numbers specific to the denomination, as illustrated in Figure D-8. The changes in topography would be recognizable either visually or

FIGURE D-8 Illustration of the change that would occur in a banknote as the surface relief rose with the application of voltage. In practice the response could be in any direction and still lead to changes in relief.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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by changes in their tactile nature. Although the schematic in Figure D-8 illustrates the surface relief rising with the application of voltage, in practice the response could be in any direction and still lead to changes in relief. Thus, the electroactive material would not have to be orientation-controlled during processing. Ideally, the user input could be generated by an on-note power supply.

The changes in tactile nature on the note would be a function of a number of BEP- or supplier-controllable variables, including electroactive material, dot size, and coating. Primitive, opportunist, and petty criminal counterfeiters would not be able to simulate this active change in tactile response accurately. State-sponsored and some professional criminal counterfeiters would be able to reverse-engineer the feature and in time simulate the response.

This feature is primarily targeted to assist currency verification by the general public and point-of-sale merchants. The outstanding benefit of this feature is that it is user-induced (preferably by an integrated power source) and detectable by humans. If the power source can be integrated into the note, the authenticity of the note can be subtly checked without visual inspection, allowing a wide range of users to verify the notes, including the visually impaired.

Materials and Manufacturing Technology Options

Piezoelectric materials are used in a wide range of industrial, consumer, and laboratory applications, and extensive research is currently producing and studying nanoscale piezoelectric materials. For example, piezoelectric print heads are often used as print heads in ink-jet printers; these heads are in fact manufactured through an ink-jet process. Consequently, processing the piezoelectric materials is well established. The key in the current application would be to make the best selection of piezoelectric material to obtain the intended response and to properly integrate the patterned feature with the electronic substrate.

A wide range of electroactive polymers is being developed, including ferroelectric polymers, dielectric EAPs, electrostrictive graft elastomers, ionic polymer gels, and ionometic polymer-metal composites, all with advantages and disadvantages. Many of these require the polymer to be saturated with water or immersed in water. Clearly, the need for water to achieve the mechanical response has implications for currency applications. Further advances in EAP technology or flexible encapsulation will be necessary for some EAPs to be integrated into currency. Thus, this is a relatively immature technology that will require considerable development.

Simulation Strategies

While piezoelectric disks are available for less than $1 at hobby stores, engineered patterns of piezoelectric features would be difficult for many counterfeiters

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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to simulate. The proper patterning and integration with the appropriate leads and power supplies would require processing beyond the realm of all but professional criminal or state-sponsored counterfeiters.

Primitive and opportunist counterfeiters would not be able to duplicate and would have a very difficult time simulating the active response of EAPs. Simulations would most likely be based on using elastic polymers to simulate some aspects of the response. However, it is unlikely that such simulations would be reversible, and they would not hold up under even moderate scrutiny. These efforts would be further complicated by more complex patterns that would result in topography changes in the notes. Petty and professional criminals would be challenged in attempting to counterfeit EAPs. The polymer chemistry and processing would require significant technology and equipment to duplicate. It is difficult to envision other approaches to achieving the active response of EAPs that would be any less challenging to the criminal. State-sponsored counterfeiters could, with time, reverse-engineer the technology necessary to obtain the EAPs. However, this would not be a trivial matter. Thus, if proper technology control were in place, state-sponsored counterfeiters would be extremely challenged by active-response features such as EAPs.

Key Development Risks and Issues: Phase I

The integration of electroactive material features would have some impact on the aesthetics of U.S. currency. It is envisioned that such features would be contained in a moderate-sized strip or patch, approximately the size of the current security strip or of a fingertip. The feature would be noticeably different from the rest of the note, in the same manner that holographic films, clear windows, or metallic ink are noticeably different from the surrounding areas on current currencies. Furthermore, it is envisioned that both internal or external power sources would require recognizable features that would impact the aesthetics of the note. Although aesthetics is a subjective issue, it is likely that such features would be judged to degrade the aesthetics of the currency.

Electroactive features would be socially neutral, although any incorrect suggestion that the feature might actually record or track the fingerprints or DNA, or add what you would like, of anyone who touched the note could affect its social acceptability. Many products have been doomed by false rumors about them.

Most electroactive polymers are in the early stages of development and are a considerable way from being used as artificial muscles. However, other EAPs are at or near implementation in other actuator technologies. In order for EAPs to be integrated into currency, a number of issues would need to be addressed. Viable EAPs that would be useable in a range of environments, from dry to damp, would need to be developed—possibly requiring new types of EAPs or the encapsulation

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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of the EAPs. The durability of EAPS should be quite high, given the thin sections and large deflections currently reported. However, this might be compromised to a degree by encapsulation.

The durability of tactilely active piezoelectric features is unknown and would have to be assessed as part of the development of such features. Because such features would be electronic in nature, durable electronic platforms would be necessary, in addition to durable active features. It is envisioned that considerable redundancy of leads and fine-scale piezoelectric bumps would be necessary.

Many of the key technological challenges to developing electroactive currency features go hand in hand with those of developing electronic substrates. For example, currently piezoelectric patterns can be produced in manufacturing settings. The difficulty would be to produce patterns integrated with the electronic substrate. Furthermore, the need to power the devices effectively, either through external sources, or preferably, through integrated power sources, would take significant development in order to integrate either EAPs or piezoelectric crystals into currency.

Development Plan: Phase I

All of the functions critical to implementing electroactive features into currency—piezoelectric devices, EAPs, the electronic substrate, and power supplies—have been demonstrated individually. However, integration of the technologies has not been developed in a currency platform.

There are significant industrial and university programs developing e-substrate technologies that would be well suited to developing electroactive features for currency. Also, at the present time some work is going on to develop a number of different variable-friction surface technologies. However, these do not appear to be targeted to currency applications.

A key milestone in the development of this technology would be the patterning and encapsulation or coating of the piezoelectric or EAP features in a durable form suitable for integration in currency. At that point the effectiveness of the active changes in shape and tactile nature could be assessed.

Estimate of Implemented Production Cost

This technology is very immature for currency applications, and thus an analysis of the implementation cost should be included as part of the development program during Phase I. However, it would be expected that features based on this technology will be significantly more costly compared with the current security thread. Since the devices would be embedded in the substrate, the substrate manufacturer would have to make significant investments in process technology.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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The impact on manufacturing operations within the BEP might be minimal, but it is premature to state this unequivocally.

Further Reading

Bar-Cohen, Y. 2001. ElectroActive Polymers—EAPs. Available at <http://www.azom.com/details.asp?ArticleID=885#_Ferroelectric_Polymers>. Accessed February 2007.

Bar-Cohen, Y. 2006. WorldWide Electroactive Polymer Actuators Webhub. Available at <http://eap.jpl.nasa.gov>. Accessed February 2007.

Cohen, J.Y. Electroactive Polymers as Artificial Muscles—A Primer. Available at <http://www.polysep.ucla.edu/Research%20Advances/EAP/electroactive_polymers_as_artifi.htm>. Accessed February 2007.

Free electricity from nanogenerators. MIT Technology Insider. Available at <http://www.techreview.com/read_article.aspx?id=16746&ch=nanotech>. Accessed February 2007.

Galassi, C., M. Dinescu, K. Uchino, and M. Sayer (eds.). 2000. Piezoelectric Materials: Advances in Science, Technology and Applications. NATO Science Partnership Sub-Series: 3: High Technology, Vol. 79. Norwell, Mass.: Kluwer Academic Publishers.

Price, B., and C. Blankenship. 2003. NASA Langley Research Center is offering to license its intellectual property in electroactive polymers to prospective industrial clients for commercialization opportunities. Available at <http://www.teccenter.org/electroactive_polymers/index.html>. Accessed February 2007.

Wang, Z.L., and J. Song. 2006. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 312(April 14): 242-246.

Suggested Citation:"Appendix D Long-Term Feature and Feature Platform Descriptions." National Research Council. 2007. A Path to the Next Generation of U.S. Banknotes: Keeping Them Real. Washington, DC: The National Academies Press. doi: 10.17226/11874.
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Next: Appendix E An Example Using a Flow Model for Counterfeiting »
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The rapid pace at which digital printing is advancing is posing a very serious challenge to the U.S. Department of the Treasury’s Bureau of Printing (BEP). The BEP needs to stay ahead of the evolving counterfeiting threats to U.S. currency. To help meet that challenge, A Path to the Next Generation of U.S. Banknotes provides an assessment of technologies and methods to produce designs that enhance the security of U.S. Federal Reserve notes (FRNs). This book presents the results of a systematic investigation of the trends in digital imaging and printing and how they enable emerging counterfeiting threats. It also provides the identification and analysis of new features of FRNs that could provide effective countermeasures to these threats and an overview of a requirements-driven development process that could be adapted to develop an advanced-generation currency.

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