Click for next page ( 2


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
ExEcuTnE SUMMARY Great fleas have little fleas, upon their backs to bite 'em, and little fleas have lesser fleas, and so on infinitum, and the great f leas themselves, in turn, have greater fleas to go on, While these again have greater still, and greater still, and so on. William DeMorgan (1839-1917) Hierarchical structures are assemblages of molecular units or their aggregates that are embedded or intertwined with other phases, which in turn are similarly organized at increasing size levels. Such multilevel architectures are capable of conferring unique properties to the structure. Hierarchical structures can be prepared from metals, ceramics, or polymers, or from hybrids of various classes of these materials. The unifying theme for all types of materials is the pervasiveness of hierarchical structures in practically all complex systems, particularly naturally occurring ones. Many materials systems found in nature exhibit combinations of properties not currently found in synthetic systems. The unique performance of natural materials arises from precise hierarchical organization over a large range of length scales. The hierarchical architectures of cellulose aggregates in wood or collagen aggregates in cartilage or tendon provide excellent examples of natural composite 1

OCR for page 1
2 Hierarchical Structures in Biology as a Guide for New Materials Technology materials designee! for multifunctional applications. Even the ultrasoft membranes surrounding cells exhibit exceptional properties that emanate from structure on many length scales. Studies of materials of biological origin invariably yield surprises that demonstrate clearly that these properties have been refined by slow evolutionary engineering. These hierarchically structured materials display unique properties that are affected by structure and generative processes at all levels of the biological structural hierarchy. Hierarchical materials systems in biology are characterized by: . . . recurrent use of molecular constituents (e.g. collagen), such that widely variable properties are attained from apparently similar elementary units; controlled orientation of structural elements; durable interfaces between hard and soft materials; sensitivity to and critical dependence on-the presence of water; properties that vary in response to performance requirements; fatigue resistance and resiliency; controlled and often complex shapes; and capacity for self-repair. Nature has a very limited range of materials with which it works. In most biological tissues, the constituents are proteinaceous. In rigid composites, they tend to be calcium carbonates, calcium phosphates, and silica. While natural composites exhibit outstanding combinations- of properties, these materials systems and their constituent components exhibit the properties over a temperature range too narrow for most engineering designs. Thus, although rules about adhesion, architecture, and composite elements in mechanical collaboration are useful to learn from nature and to apply to other material components in order to produce analogous synthetic structures, the natural constituents themselves have performance deficiencies. It is the unique interfaces in natural composites that provide unusual mechanical performance in such materials.

OCR for page 1
Execanve Summary 3 Although the use of synthetic hierarchical concepts is at an early TV she range of materials is potentially great. In addition, many ~ - ~O _, - . _ ~ . . ~ ~ 1 _ _1~_ ~_1 ~_A ALGA 1~? ~ Our-th~;^ ma; structural variables can ne altered more rebury ~ byItilibLl~ Ilia`~o than in natural materials. Through control of fabrication processes, materials variables such as atomic structure; molecular structure; nanostructures and boundaries; dislocation and other defect structures; cells and other substructures; size, distribution, and morphology of constituents and phases; grain size and morphology; orientation distributions; phase relations (including transformations); interfaces at all levels; and microstructure can be altered (although, for the most part, not independently). However, the realization of the potential of synthetic hierarchical structures has been limited, because available processing technology does not provide methods for precise control of materials variables over all levels of structural arrangement. When synthetic materials are manufactured with an emphasis on tailoring their properties through microstructural control, the extent of this control is generally at a specific length scale. For instance, the mechanical properties of most metallic materials are controlled through the manipulation of dislocation dynamics at the nanometer length scale, whereas the mechanical properties of ceramic materials are controlled through the propagation of cracks that are initiated from defects of micrometer length scales. For composites that are composed of two constituents, which are generally of quite different character, the controls are much more complex. At the request of the Department of Defense and the National Aeronautics and Space Administration, the National Materials Advisory Board convened the Committee on Synthetic Hierarchical Structures to conduct case-studies by selecting natural material systems to be used as models for synthetic efforts; review state-of- the-art synthetic techniques and processes for assembling synthetic hierarchical structures; characterize properties, unusual characteristics, and potential end-use applications for these synthetic systems; and recommend research that will expedite the understanding of the complex phenomena involved, lead to increased coordination among disciplines, and provide direction for future activities in the field. Although a broad range of functions are represented in biological

OCR for page 1
4 Hierarc)ucal Structures in Biology as a Guide for New Afatenals Technology systems, the committee concentrated on structural materials systems and their properties. CONCLUSIONS AND RECOMMENDATIONS Biological structures are characterized by hierarchical architectural designs in which organization is controlled on length scales ranging from the molecular to the macroscopic. These materials are multifunctional and are produced in situ at room temperature and atmospheric pressure, although at slow rates. Many such structures are self-healing and remarkably durable, and many display properties that change in response to a changing environment; features that represent desirable, and as yet unattainable, objectives in the design and manufacture of synthetic materials systems. Nature is parsimonious in its use of constituent materials; it returns to the same materials again and again to realize an astonishing range of structure and function. 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. Application of hierarchical design concepts to more-robust synthetic building blocks provides promising routes to high performance adhesives and composites, biomedical materials, highly specific membrane and filtration systems, low friction bearings, and wear-resistant joints. Biological structures are fabricates! via highly coupled, often concurrent, synthesis and assembly. Although these assembly processes provide valuable lessons for synthetic processing, they generally occur at rates that are too slow to be economically viable. However, in the conception and evaluation of synthetic and processing schemes for new materials systems, the prospects for integrated system fabrication should be carefully considered. lo The prospects for new biologically inspired materials technologies are real, however, full exploitation of this approach will

OCR for page 1
EiCecanve Summary s require advances in engineering, education, and enabling science. Although there is a broad range of technologies that may contribute to the understanding of biomaterials, the committee recommends concentration on developments in structural biology, interface science, synthetic methods for polymers with controlled sequences and methods for producing patterned structures by localized chemical synthesis, instrumentation, modeling, and theory (in materials, chemistry, biology, mechanics, etc.) to enhance the development and applications of hierarchical systems that are based on natural analogies. Biological structures perform as parts of integrated systems and undergo continuous evaluation and refinement based on system performance. In analogous fashion, considerations of integrated systems design and performance will take on increasing importance in the high-technology materials-related industries of the future. Interdisciplinary teams of scientists and engineers will be required to effectively design and develop structural systems with such complex architectures. The committee recommends that the academic and industrial sectors of the materials community prepare for this development through implementation of appropriate educational and engineering programs that are based on systems concepts. SCIENTIFIC OPPORTUNITIES Examples of hierarchical structures in synthetic materials range from those that are deliberately biomimetic (e.g., ultrathin layered composites) to those whose relation to natural systems is coincidental (e.g., highly orientec! polymers). The utility of many synthetic hierarchical materials is currently limited by the availability of fabrication technology, excessive fabrication times, and high cost for finished parts. This is especially true for very high performance materials, that is, continuous fiber reinforced composites materials (polymer, ceramic, metal-matrix, etc.) designated for use under environmental extremes, or parts which must function reliably for extended time periods. Similarly, there is always a need for more- efficient and more-sophisticated system designs, from improved

OCR for page 1
6 Hierarchical Structures in Biology as a Guide for New Alatenals Technology performance aircraft and spacecraft to faster-switching communication devices. The scientific and technological opportunities identified below represent examples of areas in which biological hierarchical paradigms may be effectively utilized to satisfy societal needs and solve existing problems. Synthetic Methodology 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. This methodology is powerful and may lead not only to the creation of polymeric materials with functions not obtainable through conventional synthetic methods but also to an understanding of how control of molecular structure and function can impact improved materials performance. It is clear, however' that the thermal and hydrolytic sensitivities of proteinaceous materials will limit their usefulness in many important synthetic materials applications. Generalization of the methods of controlled synthesis to new classes of monomers thus becomes an important objective. Looking beyond templated polymerization, there is little current evidence of real progress toward! efficient synthesis of genuinely uniform polymer chain populations. Nevertheless, recent advances in living ionic and metathesis polymerization have been substantial and may in time lead to higher-order control of chain length, sequence, and stereochemistry. The committee believes that issues such as environmental impact of the manufacture and disposal of polymers and the need for continuing improvement of cost/performance within the polymer industry will cause polymer science to move in directions that will tend to minimize the numbers of monomers (raw materials) utilized by the industry and hence reduce the number and volume of the offending chemicals presently in the waste stream. To achieve

OCR for page 1
ExecaDve Summary 7 this, while preserving or expanding the current product diversity available with commercial polymers, increased interest in the effects of specificity of the primary structure of synthetic polymers on cost and performance will be manifest. Cellular Synthesis of Materials Biological cells can be employed to fabricate thin layers (of organic materials or minerals) on synthetic material substrates. The objective is to employ organisms to structure materials on difficult-to-manage length scales and with difficult-to-synthesize chemistries. The cellular mechanism is capable of organizing fibrous networks, for instance, with functional hydrogel components to produce low-friction, durable, fatigue-resistant joint bearings. Cellular responses to environmental effecters, such as mechanical stress or hormones, can beneficially change the composition and assembly of these materials. This is enabled through the coupling of specific protein synthesis and degradation with the constant monitoring of mechanical function and the state of need of the organism. Long-term cellular activity within the material can enable the repair of the material upon damage by reactivation of matrix formation. These advances could create not only new membrane and biomaterial technologies but also new insights for structuring hard materials. Rigid Structural Composites Many of the rigid structural materials found in nature are composites comprising unusual compositions and configurations. 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

OCR for page 1
8 Hierarchical Structures in Biology as a Guide for New Afatenals Technology properties, by combining outstanding strength and stiffness with improved fracture toughness compared with that of monolithic materials or current composites. In addition to practical and cost- effective fabrication techniques, an understanding of deformation mechanisms and the ability to enhance composite structural response through mechanical modeling are critical to the success of these materials. Adhesives and Interfaces Adhesives and interfaces play important roles in both natural and synthetic composites. Although much has been done in adhesion science and technology, there are opportunities to tailor new synthetic adhesives and unique structural architectures by way of mimicry of natural systems. Adhesives are critical in the formation, strength, and durability of composite materials as agents responsible for bonding between matrix and reinforcing phases. Advances in composites have emphasized the need for durable adhesives that would work in wet environments. Adhesives produced by organisms, especially marine organisms, suggest themselves as candidates for study, because they cure in the presence of water and resist its subversive effects. Soft-Tissue Based Materials Exceptional designs for "ultrasoft" materials and for interfacing soft and hard materials are found in nature with capabilities well beyond present day technology. The committee feels that exposing the physical and chemical principles that underlie the special features of these materials is certain to stimulate new approaches to design of synthetic materials, parts, and systems. Such approaches may include preparation of "self-healing" capsular materials that possess tunable and motile properties; general methods for assembly of soft organic and hard material interfaces that are mechanically, chemically, and electrically compatible; and development of membrane composites that

OCR for page 1
E'cecutz've Summary 9 are based on fluid~urfactant interfaces supported by tethered polymer networks that possess permeability restriction and mechanical strength. Control of Size and Shape (Assembly, Self-Assembly) Inherent in the behavior of natural proteins is their assembly into structures of a given size and shape to allow the performance of a specific end-use function. This formation of parts and systems is driven by local geometry and molecular forces and does not require additional "shaping and machining" steps. The ability to design synthetic systems capable of assembling in an analogous fashion would have obvious practical impact. Some natural self-assembling systems have a defined size, such as some vesicles, while others are indefinite in extent, such as unstrained crystals. Proper function requires that system size be controlled as well as system shape. A key factor in control of system size and shape is the identification of switching mechanisms that govern, for example, the size and shape of nacreous platelets in abalone shells, as well as the thickness of the protein layers that separate the layers. TECHNOLOGICAL OPPORTUNITIES Biomedical Materials There is a recognized societal and economic need for synthetic materials with appropriate mechanical and functional performance characteristic properties for use in biomedical applications. The challenges in developing a manufacturing process to produce synthetic hierarchical materials with these required mechanical properties and functional characteristics are great. First, biologic tissues have very complex compositions and ultrastructural organizations. Second, the tissue is manufactured by tissue-specific cells in situ by as yet

OCR for page 1
10 Hierarchical Structures ir' Biology as a Guide for New Materials Technology unknown processes, which control the production and assembly of the constituent biological macromolecules. It is unlikely that any synthetic process can be developed in the near future that can duplicate the ability of the specific cells to manufacture and organize a hierarchical material with very fine ultrastructural features. However, a hybrid approach has been taken by some researchers, where synthetic grafts which serve as scaffolds for the specifically seeded cells, have been produced from biocompatible resorbable matrices, such as polylactic acid or copolymers of lactic and glycolic acids. Gels made of collagen and glycosaminoglycan seeded with cells also show promise as graft materials for skin and blood vessels. Development of gels that are strong, cohesive, porous, permeable, and resorbable and that are capable of sustaining high stresses and strains and 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. Improved Membranes and Membrane-based Devices Improved membrane selectivity is desirable in the areas of water purification, clothing to protect those handling hazardous materials, outdoor clothing and shelters, gas separations, industrial purification processes, etc. Coupled with this is a need for improved stability and increased lifetime for these membranes, as well as for mechanisms to reduce fouling. One approach to solving these problems is to incorporate responsive channels and self-repair ("living membranes") or self-cleaning attributes that are patterned after natural membrane systems. This requires a better understanding of membrane structures in terms of their processing and assembly. Additional areas for development include suitable substitutes for water as plasticizers in these materials, and in approaches to biomimetic membrane design.

OCR for page 1
Erecunve Summary 11 Smart Materials Natural systems have the ability to sense their surroundings and to respond to impulses or changes in conditions by changing properties or initiating repair responses. The development of smart materials, which integrate the functions of sensing, actuation, and control, can benefit greatly from lessons gleaned from the studies of these biological systems. Passively smart materials respond to external change without assistance often through phase changes or transitions in fundamental properties. Actively smart materials utilize feedback loops to recognize changes and initiate appropriate responses. Opportunities for application of smart materials systems in structural applications generally focus on reduced component mass and adaptive functionality aimed at improving structural efficiency, durability, and safety. Examples of smart materials applications include load and vibration alleviation systems, failure sensing and repair, and shape memory. Developments in sensor development and integration of sensing and response functions with practical structures are necessary to realize the potential of smart materials. Functionally Gradient Materials Functionally gradient materials (FGM) are materials in which a continuous spatial change in composition or microstructure gives rise to position-dependent physical and mechanical properties that can extend over microscopic or macroscopic distances. Natural materials with functional gradients abound. Examples of materials with functional gradients that are discussed in this report include articular cartilage and bone. FGM can result in changes in composition or orientation of constituents. Synthetic FGMs can be produced from mixtures of metals, polymers or ceramics in virtually any combination. FGMs whose properties vary in the dimension of their thickness (through thickness variations) can provide a transitional interface between dissimilar

OCR for page 1
2 Hierarchical Structures in Biology as a Guide for New Afatenals Technology materials or serve as coatings with optimized environmental resistance and adhesion to selected substrates. Another interesting area of research is surface gradients, where the nature of the surface is varied continuously with position. Surface gradient techniques, allowing selective deposition or coating processes, may find applications in processing or in tailored membrane or sensor applications. The development of FGMs is still in its early stages. The biggest challenge is to scale the processes to practical size components while maintaining the precise control and consistency needed. The study of gradients in natural materials may provide direction for architectural design, fabrication processes, and potential applications for synthetic FGMs. Design and Assembly of Complex Composite Parts Competitive composite parts require three structural elements to be controlled in a manner that leads to a finished part that possesses the desired mechanical, thermal, and environmental properties in three dimensions. These elements are matrix uniformity, fiber orientation, and fiber-matrix surface interaction. Current fabrication methods are highly labor intensive, are not amenable to complex shape formation, and present significant problems in performance assessment. Often, machining, polishing, etc., is necessary to achieve the required finished part shape and surface characteristics. In contrast, biological systems often contain complex and sophisticated fiber-reinforced composite "parts" (examples range from trees to bones), which exhibit superb performance over extended lifetimes, are capable of healing and are produced directly as finished parts from cell-based manufacturing plants. Examination of these natural fabrication methods may provide guidance in development of "smart" composites, net shape processing, multifunctional composites, controlled orientation, and FGMs.