Hierarchical Structures in Biology as a Guide for New Materials Technology (1994) / Chapter Skim
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4 FABRICATION OF HIERARCHICAL SYSTEMS
4 FABRICATION OF HIERARCHICAL SYSTEMS
Pages 73-92
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From page 73... ...
The methods for precise control over all levels of structural arrangement simply are not available. While there are important lessons to be derived from studying how nature produces systems with precise control at all levels of hierarchy, the time scales involved in these processes would generally be prohibitive in synthetic fabrication.
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From page 74... ...
For self-assembly processes, the component materials would need to be delivered to the appropriate sites via the gas phase or the liquid phase and could be in a molecular form, as a precursor or as a submicron particle. Positioning could be determined by masking or by photo-induced reactions.
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From page 75... ...
SYNTHETIC PROCESSING Fiber Processing Many of the advances in synthetic fiber property control of the past several decades originate from the separation of those processing steps involving orientation, crystallization, and "structural" perfection. For example, spinning polymer under conditions of low net chain orientation gives rise to point nucleated lamellar crystals, which emanate from the point with radial symmetry.
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From page 76... ...
give rise to fibrillar crystals (line nuclei) , while high entropy starting states (random coil)
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From page 77... ...
, as shown in Figure 4-2. While the existence of a fibrillar hierarchy in liquid crystalline polymer fibers has been established, it remains unclear what the origin of the hierarchical elements are.
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From page 78... ...
,. l.Omm (A FIGURE 4-2 LOP polymer structure model of extrudate and molding.
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From page 79... ...
partially sintered ceramic tapes are sandwiched with metal or polymer sheets and then heated to induce infiltration of the metal or the polymer; (2) nonsintered ceramic tapes are stacked, partially sintered, and then infiltrated with metal or polymer; and (3)
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From page 80... ...
This polymeric "millefeuille" structure could be extended to more-complex arrangements and more than two polymers to make a hierarchical structure. The final film properties depend on the constituent polymer properties, the layer thicknesses, and the nature of the interfaces between layers.
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From page 81... ...
Aim,. ~ ~ fir _ ~ ._ ~ `~ ~ 1 ~ /' ~ ~ _~ Di rection of flow Feed ports meter layers of two or more polymers FIGURE 4-3 Schematic diagram of the feedblock method of coextruding multilayer polymer streets end firma.
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From page 82... ...
An example of structural hierarchy that results from molding processes is the injection molding of a liquid crystal polymer (LCP; Weng et al., 1989~. The previous section discussed how onedimensional hierarchy is introduced in liquid crystal polymer fibers through flow orientation.
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From page 83... ...
An analysis of the advantages to be gained from flexible structural control, coupled with a broad approach to the manufacture of multimaterial composites, is required. Three-Dimensional Manufacturing Self-assembly directed by highly structured copolymers can be expected to yield controlled fine structures on the scale of 10-100 nm.
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From page 84... ...
By carrying out the chemistry necessary for ceramic particle formation or polymer deposition on each layer in turn, and by using lithographic methods to pattern each layer, complex hierarchical structures can be built up. BIOLOGICALLY INSPIRED PROCESSING The growth of biotechnology is opening the way to the use of biological processing for the manufacture of materials.
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From page 85... ...
In each case, production of these constituent materials requires relatively advanced manufacturing technology in order to ensure control and repeatability of critical molecular parameters such as length, composition, stereochemistry, branching, and cross-linking of the polymer chain. The present state of the art of macromolecular synthesis is such that these critical molecular parameters are indeed subject to control, but only in a statistical sense.
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From page 86... ...
One intriguing aspect of biological polymers is the fact that insoluble materials can be formed in an organized fashion at room temperature. This implies some way of manipulating the material in a temporarily soluble form.
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From page 87... ...
Some of the fibers made up of silk polypeptide are characterized by a combination of high strength and high extensibility. Some silk polypeptide are high molecular weight, over 300 kilodaltons in the case of major ampullate gland silk, which forms dragline silk, or the strongest of the silks produced by most orb-weaving spiders.
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From page 88... ...
A single bacterial cell can incorporate up to 200,000 glucose monomers per second into a growing cellulose polymer chain. The final assembly step involves the formation of a ribbon of fibrils with about 1,000 glucan chains.
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From page 89... ...
These special features of lyotropic mesophases expose a potential for use as structural "templates" in preprocessing of hard materials, thereby extending control of the microstructure to much larger dimensions. Possible applications range from presintering processes for ceramics and ceramic catalysts to fabrication of intercalated composites.
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From page 90... ...
In addition to forming a reaction cell that limits particle size, the bilayer serves as a semipermeable membrane to ion diffusion. Generally, phospholipid vesicles are nearly impermeable to cations, with typical permeability coefficients between 1 on to 10 i4 cm/s (Johnson and Bangham, 1969; Hauser et al., 1972; Papahadjopoulos, 1971~.
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From page 91... ...
a lubricant/dispersant that facilitates particle rearrangement during particle consolidation. Cell Seeding Cell seeding, or cell transplantation, for the development of specific tissues in vitro or in viva, has become a highly attractive and exciting prospect.
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From page 92... ...
Cell-matrix transplantation appears to be particularly attractive for the replacement of tissues such as skin and cartilage. Skin is the largest organ of the body, and although the body has developed highly efficient mechanisms to repair skin, major events of trauma and surgery can often require skin grafting.
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