Hierarchical Structures in Biology as a Guide for New Materials Technology (1994) / Chapter Skim
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5 CONCLUSIONS AND RECOMMENDATIONS: SCIENTIFIC AND TECHNOLOGICAL OPPORTUNITIES
5 CONCLUSIONS AND RECOMMENDATIONS: SCIENTIFIC AND TECHNOLOGICAL OPPORTUNITIES
Pages 93-110
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From page 93... ...
Nature is parsimonious in its use of constituent materials, it returns to these same materials again and again to realize an astonishing range of structure and function. The utility of many synthetic hierarchical materials is limited at the present time by shortcomings in fabrication technology and resultant finished-part costs that are high.
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From page 94... ...
MATERIALS The hierarchical architectures of biological materials systems rely on critical interfaces that link structural elements of disparate scale. The study of such systems reveals extraordinary combinations of performance properties, as well as limitations due to the modest thermal and chemical stabilities of biological molecules.
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From page 95... ...
PROCESSING Biological structures are fabricated via highly coupled, often concurrent, synthesis and assembly In the conception and, evaluation of synthetic and processing schemes for new materials systems, the prospects for integrated system fabrication should be carefully considered. Specific needs to realize the full promise of integrated fabrication methods include: concurrent materials synthesis and structural assembly; processes to fabricate highly specific synthetic membranes and filters; use of cells to synthesize and deposit materials; biosynthetic pathways to the cost-effective manufacturing of new classes of shaped hybrid composites; and biosynthetic concepts and materials for self-repair of critical components and devices.
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From page 96... ...
Although, due to the diversity of the field, bioengineering curricula may vary in detail, they all strive to bring together the biologists' knowledge of physiology, anatomy, biochemistry, and molecular biology with the engineers' knowledge of design and structure (Watanabe, 1993~. Lessons to be learned from the design of natural systems include: strong, durable interfaces between hard and soft structural components; · tribological joints with low friction coefficients and remarkable durability; mechanistic understanding and analysis methods for deformation and failure of complex systems; energy-absorbing mechanisms of rigid biological composites; platelet and surrounded plate analytical concepts; and moisture-friendly synthetic systems.
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From page 97... ...
Synthetic Methodology As discussed in Chapter 3, the design and preparation of hierarchical materials will place a new premium on the synthesis of macromolecules of precisely defined primary structure and complex chemical composition. At present, the only methodology available for the preparation of such polymers involves the use of gene synthesis and recombinant-DNA technology to create artificial structural proteins.
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From page 98... ...
Progress in this area must be accompanied by advances in cell-free translation methodology if any impact on materials synthesis is to be made, since current cell-free methods are limited to the preparation of submilligram quantities of material. Looking beyond templated polymerizations, one sees little current evidence of real progress toward efficient synthesis of genuinely uniform chain populations.
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From page 99... ...
For example, the nacreous material in mollusk shell is a segmented composite with a very low volume fraction of matrix phase in very thin layers. The ability to design and fabricate synthetic structures with similar characteristics, as well as the ability to mimic adhesion between the phases, could lead to composites with remarkable properties, by combining outstanding strength and stiffness with improved fracture toughness compared with that of monolithic materials.
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From page 100... ...
This small change in structure, however, produces an adhesive that is two to three times stronger than the East Coast adhesive (Waite, 1986~. This observation lends support to the view that mussel adhesive might be used as a model to systematically investigate the relationship between molecular structure and adhesive function, which could lead ultimately to a generic glue that can be modified at the molecular workbench for any number of different moist environments.
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From page 101... ...
These challenges include preparation of "self-healing" capsular materials that possess tunable and "motile" properties; methods for assembly of soft organic and hard material interfaces that are mechanically, chemically, and electrically compatible; and development of membrane composites that are based on fluid-surfactant interfaces that are supported by tethered polymer networks that possess permeability restriction and mechanical strength. An example of the potential impact of soft-tissue understanding is the reduction in energy needled to move a body through water when its drag is reduced.
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From page 102... ...
The ability to design synthetic systems capable of assembling in an analogous fashion would have obvious practical impact. For the purpose of this report, the determination of shape in biological systems needs to be considered at the level of hierarchical matrix formation.
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From page 103... ...
Up to now such cells, usually bacteria, yeasts, or insect cells, have been grown in suspension culture, without any specific orientation. For example, the previously mentioned bacteria that secrete cellulose are grown in suspension culture, which produces a random tangle of cellulose fibrils.
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From page 104... ...
The successful translation of these principles to synthetic materials could lead to the integration of the materials synthesis and processing steps of part fabrication. TECHNOLOGICAL OPPORTUNITIES Biomedical Materials There is a recognized societal and economic need for synthetic hierarchical materials with appropriate mechanical and functional performance characteristic properties for use in biomedical applications.
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From page 105... ...
These cells manufacture and organize the molecular building blocks and maintain the collagen-proteglycan extracellular solid matrix around themselves by a slow but balanced metabolic process. At the tissue-scale, articular cartilage possesses a set of unique nonlinear, anisotropic and nonhomogeneous material properties that seem to have been specifically designed to provide excellent long-term tribiological (friction, lubrication, and wear)
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From page 106... ...
, the material properties are probably insufficient for use in diarthrodial joints where the applied stresses are very high. Development of strong, cohesive, porous, permeable, resorbable gels that are capable of sustaining high stresses and strains and of providing a supporting and protecting environment for the seeded cells is a major challenge for future biomedical researchers interested in developing synthetic hierarchical materials for clinical use.
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From page 107... ...
Examples of smart materials applications include load and vibration alleviation systems, failure sensing and repair, and shape memory. Challenges in sensor development and integration of sensing and response functions with practical structures need to be addressed to realize the potential of smart materials (NRC, 1 994)
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From page 108... ...
Surface gradient techniques may find applications in processing, which will allow selective deposition or coating processes, or in tailored membrane or sensor applications. The development of functionally gradient materials is still in its early stages.
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From page 109... ...
It is instructive to compare the "steps" of synthetic and biological fabrication technologies. SYNTHETIC BIOLOGICAL Produce Reinforcing Fiber Treat Fiber Surface Impregnate Fiber with Matrix Line Up Prepeg Plies in Mold Cure/Shane Part Produce Matrix "Scaffold" for Part Form Crystal Directing Surface Fill with "Gel" Replace Gel with Oriented "Fiber"
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From page 110... ...
Understanding and, where appropriate, mimicking the structure and manufacturing logic of natural hierarchies offer an opportunity to leapfrog current composite technologies and realize the promise of synthetic composites, which has proven elusive to the materials community for more than two decades. The toughest materials are known to raise the energy required for tearing by diverting cracks away from their preferred directions of propagation.
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