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4 FABRICATION OF HIERARCHICAL SYSTEMS The benefits, in both function and performance, from the development of synthetic systems with hierarchical architecture have been illustrated in previous sections of this report. However, realization of this potential has been limited by available processing technology. 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. In order to be economical, synthetic processes need to be able to be accomplished at a much greater rate and scale. Although the use of synthetic hierarchical concepts is at an early stage, many structural variables can be altered more readily in synthetic materials than in natural materials. The following variables can be altered (though, for the most part, not independently) through control of fabrication processes: . . elemental composition and structure (including tailored lattices); molecular structure; nanostructures and boundaries; dislocation and other defect structures; cells and other substructures (size, morphology, structure, orientation); 73

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74 . . Hierarchical Structures in Biology as a Guide for New Afatenals Technology sizes, distributions, and morphologies of constituents and phases; grain sizes and morphologies; crystallographic orientation; orientation distributions; phase relations (including transformations); interfaces at all levels; and microstructure. In this chapter, the present state of the art of fabrication technologies for synthetic hierarchical systems is discussed. Processes to produce many of the systems described in detail in Chapter 3 are outlined, with emphasis on the structural variables that can be affected through process controls. Emerging and innovative processes or techniques to provide control of structural variables at multiple size scales or to enhance the ability to produce synthetic hierarchical systems are also discussed. For example, methods exist for production of multilayer ceramics (Otsuka, 1993) and for computer-aided modeling of parts from photo- polymerizable resin (Jacobs and Reid, 1992~. These kinds of methods could provide organization down to the milli- and micro-scale. For organization on the nanoscale, self-assembly of the constituents would be necessary. 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. Direction of self-assembly might depend on additives that modify phase separation to favor particular sizes and shapes. Some of the processing methods can be borrowed from the semiconductor industry, but to form metals, polymers, and ceramics of many types, there is a need to extend the range of materials that can be deposited, especially where materials of different melting points, etc., are to be co-deposited. Many methods are available or can be envisaged for the control of structures in films. Sequential application of these techniques, or the simple stacking and fusing of films, can be used to make three

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Fabrication of Hierarchical Systems 7s dimensional parts with a high degree of structural control. Many complex biological growth processes can also be viewed as a combination of a close control over a surface, plus progressive extension on the third dimension, to build up a solid body. 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. Subsequent orientation of this spherulitic structure in the solid state (drawing) leads to a microfibrillar microstructure that is characterized by highly oriented noncrystalline chains, a crystalline lamellar thickness that reflects the stress-temperature history imparted by the draw process, and a structural retention of both the original entanglement network present in the polymer prior to crystallization and the interaction present in the spherulitic structure. The transformation of spherulitic structure during drawing has been treated in detail by Peterlin (1971, 197S, 1979, 1983), who suggested that the final structure is a microfibrillar hierarchy, with the size and connectivity of the hierarchical elements a function of the starting structure (see Figure 4-1~. Fiber morphology induced in this fashion tends to show high orientation in both crystalline and noncrystalline regions. In most cases, the observation of drawn spherulitic structure shows only the 100 A microfibrils. If fibers are produced under spinning conditions that impart a net strain to the molecular chains prior to or cluring crystallization (i.e., conditions that lower the entropy of the ground state of the melt or solution), the morphology of the initial crystals produced is fibrillar rather than spherulitic. Depending on the nature of the starting

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76 Nierarc~u'cal Stares ir' Biology as a Guide for New Materials Technology ~Af ~ l ~Amf ~ I] _e All'-' A,' / / / CR~ BLOCKS ABEL (BUNDLE Of Ml~lFll~ILSJ ZONE OF ~ M~EC~ ///// STACK ~ PARALLEL RAE FIGURE 4-1 Schematic of cold drawing process with transformation of the lamellar texture into a microfibrillar structure. Source: Peterlin, 1972. polymer and the time-temperature-stress profile of the spin line, the concentration of these fibrillar crystals (often referred to as line nuclei) varies from making up essentially all of the fiber microstructure to relatively few fibrils being formed. The concentration of fibrils is controlled by the stress imposed during spinning. In general, line-nucleated structures tend to be characterized by a high degree of preferred molecular orientation in crystalline regions and a lower degree of orientation in noncrystalline regions. In summary, it may be stated that low entropy starting states (high orientation) give rise to fibrillar crystals (line nuclei), while high entropy starting states (random coil) give rise to spherulitic crystals (point nuclei). Fiber processes that favor line nucleation offer the opportunity by separating the levels of orientation in the various structural elements to separate mechanical and thermal fiber performance.

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Fabacanon of Hierarchical Systems 77 An alternative to straightening flexible chains through the application of stress to the spin line is to start with a stiff, "rod-like" molecule. Such molecules tend to be nematic liquid crystals, examples of which are the lyotropic poly~p-phenylene-terephthalamide; Kevlar_) and the thermotropic copolyesters (Vectra - I. Such molecules have little tendency to chain fold and show highly fibrillar, hierarchical microstructure in the solid state (Sawyer and Jaffe, 1986), 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. Possibilities include "crystallization," reflection of previously generated entanglement network, or the fracture of larger-diameter species During processing. All oriented polymers and all synthetic fibers are characterized by a microfibrillar morphology with a diameter of about 100 A. While the origin of this structure is qualitatively understood, quantitative understanding of its formation is lacking. The relationship of the elements of the hierarchy to fiber properties is reasonably in hand, with properties that are predicted from mechanical models correlating well with measured data (mostly axial mechanical performance). What is missing are the quantitative theories and models necessary to relate fiber formation conditions to microfibrillar (hierarchical) detail. In conclusion, a fundamental difference in the driving forces that control the formation of fibrillar hierarchies in natural and synthetic fibers should be noted. In nature, the origin of level and size of hierarchical structure is primarily driven by chemistry (specific interchain and intrachain interactions). In contrast, structure formation in synthetic systems is driven by physics, which leads to less defined structures, which are often better described by size distributions than single size parameters. Hence the appearance of hierarchy in nature is "by design" to satisfy a given performance need, while synthetic polymer hierarchies are not present through the designing of structure for performance but rather because of underlying process physics criteria, the impacts of which are often not

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78 Hierarchical Structures In Biology as a Guide for New Afatenals Technology 5 ~EXTP'U0P`TES OUTER "TRUE" SKIN 1 * Macro T ~-~- INNER SKIN Fibrils F briis Micro LAYERS \ 5 ,um 0 5 ,um Fibrils UNORIENTEC COOP * FjbCIO Fills FMbG''O MOLDINGS 5,um 0.05,um Orientation depends upon: - polymer - draw ratio - diameter - process I:' ~- .` " . ,. l.Omm (A FIGURE 4-2 LOP polymer structure model of extrudate and molding. Source: Sawyer and Jaffe, 1986.

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Fabacatfon of Hierarchical Systems 79 fully appreciated. A significant opportunity therefore exists to learn from natural systems to produce the next level of sophistication in synthetic fiber products. Multilayer Processing An obvious way to build a complex hierarchical structure is to construct it as a series of layers. Multilayer processing includes 1 lamination of structural composites, polymer coextrusion, and step- wise deposition processes. Based on design criteria derived from nacre, as discussed in Chapter 3, the processing of ceramic/metal and ceramic/polymer laminated composites through tape casting and liquid infiltration techniques, specifically with boron carbide/aluminum and boron carbide/polymer composites, respectively, has been accomplished. These laminated composites can be formed by one of three basic methods: (1) 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) nonsintered ceramic tapes of different porosity are laminated (stacked and pressed), partially sintered, and then infiltrated. In these cases, the resulting structure is a ceramic/metal or ceramic/polymer laminated composite with metal or polymer at intra- and interlayers. The reinforcement content of laminated samples is altered by changing the ratio between the matrix-rich and boron carbide-rich layers in the microstructure. The effect of changing the thickness of the laminae on both fracture strength and fracture toughness is in agreement with the Hall-Petch relation (Eq. 3-1~. The coarsening of the microstructure by increasing the tape thicknesses degrades the mechanical properties to values approaching those for isotropic samples. Structures with more-finely graded laminates have not been processed at this time because of the difficulty in casting and handling tapes thinner than 15 nm. Attempts to form ultrafine laminated layers

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80 Hierarchical Strictures in Biology as a Guide for New Materials Technology in hard and soft steel sandwich composites by deformation processing resulted in break-up of the layers. An inexpensive method for producing laminated plastic films directly through coextrusion of two or more polymers has shown great flexibility (Alfrey and Schrenk, 1980~. The coextrusion process, shown schematically in Figure 4-3, consists of introducing molten parallel streams of polymer through feed ports and passing them through a die to produce a thin, wide sheet. To maintain the parallel orientation of polymer layers, the transition region from the feed stream through the die is particularly important. Complex polymer films are commonly extruded with five or more layers in order to provide barriers to permeation of different gases in food packaging. Baer and co-workers have shown that novel properties can be induced in polymer sheets made by coextruding two polymers as hundreds of very thin alternating layers of, for instance, soft and hard polymers (Ma et al., 1990a, by. 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. The last twenty years have seen the development of a battery of techniques for depositing materials in thin layers from the vapor phase. These include sputtering, chemical-vapor deposition, and molecular-beam epitaxy (Dresselhaus, 1987; Sinjo and Takada, 1987~. The resulting films can be a few atomic layers thick and can be patterned down to a scale of a few microns. Vapor phase methods will probably always be preferred for thin films, but for layers of 100 nm or more, it is also sensible to utilize methods of deposition from liquid solution or suspension. Methods that could be applied to the buildup of patterned-layer structures include photopolymerization, electropolymerization, epitaxial crystallization on modified surfaces, metalorganic deposition, localized particle attachment, Langmuir film deposition, and standard casting or coating methods. In combination, these methods could be used to process complex composites that contain a wide range of organic and inorganic materials.

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Fabacanon of Hierarchical Systems Transition channels Lay: (number of layers is equal to number of feed ports) 81 /y '1 , , _ ~ , ~ ~ 11 , , , , ~ , ~ J Next/> . 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. Source: Alfrey and Schrenk, 1980. The formation of polymeric and composite materials in biology is an excellent example of control of structure. However, the constituents of biological materials have been limited to organic polymers plus phosphates, silica, iron oxides, and carbonates. Current work is extending the principle of in situ precipitation to other oxide ceramics, metals, and sulfides (Calvert and Mann, 1988; Calvert, 1994~. Related efforts are developing the ability to locally deposit minerals from solution on patterned surfaces (Rieke et al., 1993~. This promises to permit complex structures to be built up by a series of precipitations, in the same way that integrated circuit technology allows complex structures to be built on a silicon wafer. However, deposition from aqueous solutions at room temperature will permit a much wider range of materials to be incorporated into the structures. Injection Molding Sequential growth of complex structures requires revolutionary approaches to manufacturing and cannot be expected to make a major

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82 . Hierarchical Structures in Biology as a Guide for New Afatenals Tec)umlogy impact in the short term. An important direction for immediate application to production of hierarchical structures is by more- sophisticated molding processes. Progressive refinements in plastic molding methods, including injection molding and extrusion, have allowed complex blends of materials to be formed directly. Also, injection molding of reinforced polymers and reinforced reaction injection molding have allowed higher fiber contents and longer fibers to be introduced into larger parts while still retaining the advantages of mass production. 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 one- dimensional hierarchy is introduced in liquid crystal polymer fibers through flow orientation. Similarly, injection molding of a liquid crystal polymer or a reinforced composite results in a graded structure, with high preferred orientation in the direction of flow (mold filling direction) near the mold walls and decreasing orientation toward the part interior. A schematic of this graded structure is shown in Figure 4-4. The properties of the molded parts can be influenced when the part and mold are designed by controlling the flow parameters within the mold through placement of gates and risers. One-dimensional hierarchies can be introduced by complex film dies, by building up stacks of polymers during extrusion, or by fusing a series of films during a rolling step. It is intrinsically quite feasible to construct a similar two-dimensional arrangement into an extruded rod or pipe. For instance, reinforcing threads could be coextruded with a tough matrix, and could be spirally arranged, to form pipe. Such coextrusion of polymers is standard procedure. Short reinforcing fibers can be blended into the polymer, but there is little control over their orientation or local concentration. Finer reinforcements with better axial ratios are available, but it is not yet possible to reinforce on the same scale as is seen for the mineral in bone. Ceramic, glass, and metal reinforcements could be added by in situ reaction (with orientation control by applied electric field or currents, etc.), which might offer more control than simple additions of fibers or flakes.

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Fabacanon of Ilierarcincal Systems ~ 20 micron ~ 1 ~ subloger /// ~ f ibrous Conner tlons be Preen ~Icroioger s dater ~ 0.2 micron lock nrr.PrP~ mtcrotogers _ boundary levier at) a. LCP matrix 83 0.4-0.6 micron ~ Aileron 7 T t0-50~1cron 500-700 micron 10 to ~ Fee hundred ~1 L micron .- Mo(d- f itting direction -- ~ . . . . b. LOP composite . i,., skin 1 FIGURE 4-4 Proposed hierarchical model of injection-molded (a) unreinforced liquid- crystal-polymer resin material and (b) its short-fiber reinforced composite. Source: Reprinted from Weng et al., 1989, p. 278 by courtesy of Marcel Dekker, Inc. Thus, many already available molded materials are hierarchies. 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. For example, block copolymers can promote fine-scale mixing in polymer blends, since the copolymer may be required, as a consequence of its structure, to remain at an interface. At larger scales, the required hierarchy must be processed directly. A number of research efforts involving layer-wise syntheses of complex structures are underway. These efforts resemble mineralization in large biological structures in which minerals are

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84 Hierarchical Structures in Biology as a Guide for New Afatenals Technology deposited by chemical precipitation at a "moving front" that passes through the bone or shell matrix. The uniquely high volume fraction of mineral phase that is attainable in biological composites and the local variations in properties are due to this strategy. Such methods are known as rapid prototyping or free-form manufacturing and have recently received much attention. Polymer parts are prepared by laser photopolymerization in a layer of monomer at the surface of a part. The laser is computer driven to produce the required cross-section in each successive layer. As a result, a prototype can be rapidly produced from a computed design. Variations of this process are being developed for ceramic powder deposition by local fusion of polymer-bound particles (Marcus and Bourell, 1993) and by patterned deposition of slurries that uses an ink-jet printer (Sachs et al., 1992~. It is already a common practice to make ceramic packages for microelectronics by sintering a stack of shaped green sheets. It is clearly a small step to depositing each layer from a slurry rather than from a pre-cast soft sheet. 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. In polymers, it appears likely that cellular synthesis of artificial proteins or polysaccharide will be possible. These macromolecules should be capable of self-assembly into higher-order structures by reason of the detailed sequence of units on the chain. However, at present, biological synthesis and processing are far from completely understood. The following paragraphs describe several aspects of biological synthesis and processing that appear to offer prospects for new developments in materials technology.

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Fabrication of Hierarchical Systems 85 Macromolecular Synthesis Production of hierarchical materials requires synthesis of the constituent molecular and more often macromolecular species. Organic composites, for example, consist of polymeric matrices and either particulate or fibrous reinforcing materials, and laminates are generally formulated from preformed polymers. 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. For example, existing synthetic methodologies allow the preparation of polymeric materials characterized by well-defined and predictable d istributions of molecular weights but cannot afford chain populations of uniform molecular weight. Similar statements hold for the other important molecular parameters. Molecular heterogeneity is important in optimizing polymer processing characteristics (e.g., melt viscosity) while maintaining acceptable mechanical and thermal characteristics. However, it is not yet clear whether aspects of biological structures, such as self-assembly, can be reproduced without going to synthetic methods that require full control of the sequence of units on a polymer chain. Biosynthetic Routes to New Polymeric Materials As outlined above, conventional approaches to the synthesis of polymers lead to populations of chains characterized by relatively broad distributions of length, composition, stereochemistry, etc. In contrast, the structural proteins of higher animals (e.g., silk, collagen, and elastin) are synthesized under direct genetic control and are, as a result, essentially uniform in chemical structure. Because natural

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86 Hierarchical Structures fit Biology as a Guide for New Laterals Technology hierarchical architectures emerge from complex and as yet poorly understood processes of molecular assembly, a high premium is placed in nature on precise control of macromolecular structure. Full exploitation of such assembly processes in the creation of new synthetic materials will require similar control and should stimulate exploration of new routes to polymers of well-defined structure. Perhaps the most straightforward approach to this problem is to adapt directly the chemistry of protein biosynthesis to the creation of new artificial proteins with useful structural properties. Several successful reports on this approach have appeared (Capello et al., 1990a; Creel et al., 1991), and it seems likely that general strategies for genetic engineering of new structural materials will emerge rapidly. Indeed, attention is already shifting from biological problems (e.g., the stabilities of artificial genes and proteins in microorganisms) associated with synthesis to the engineering of the physical (Tirrell et al., 1991) or functional (Cappello and McGrath, 1994) properties of the product polymers. Biological syntheses of other classes of polymers are also growing in importance. Poly(,6-hydroxyalkanoate~s (Dot, 1990), cellulose (Johnson et al., 1989; Ben-Bassat et al., 1986) and a wide variety of enzymes and chemical intermediates are now being made in substantial quantity by microbial fermentation. In vitro enzymatic catalysis of polymerization is also being pursued (Wallace and Morrow, 1989). Processing of Biological Polymers Most synthetic polymers are processed by melting followed by extrusion or molding. 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. This could become an important component of processing for complex hierarchical materials where fine-scale control is the essence. The synthesis and assembly of

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Fabr~canon of Hierarchical Systems 87 silk fibers provide excellent examples of the kinds of biological materials processing that merit increased attention from the materials community. Silks are produced by a variety of organisms, including the domesticated silkworm (Bombyx mori) and orb-weaving spiders. Silks from these organisms are characterized by an antiparallel beta sheet secondary structure stabilized by hydrophobic and hydrogen bonding. 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. In the silkworm, and presumably in spiders, the polypeptide is synthesized and exported from epithelial cells that line the lumen of the posterior region of the major ampullate gland (Fossey et al., 1991; Fraser and MacRae, 1973~. After synthesis and export into the lumen of the posterior region of the gland, the polypeptide moves to the middle portion of the gland for storage. Here, the polypeptide is at a concentration of about 20 percent, the viscosity is high, the shear rate is low and the pH is in the 5.6 to 5.0 range. After passing into the anterior region of the silk gland, the protein concentration rises to around 30 percent, viscosity is again low despite the higher protein content, and the pH drops to below 5.0. During the latter process, and as the polypeptide is spun into air through the spinneret into the final silk fiber, the beta sheet conformation is realized, and an insoluble fiber is formed (Kerkam et al., 1991~. The mechanisms involved in this natural system for processing polypeptide in an aqueous environment at ambient temperatures are of interest. The result of this process is insoluble fibers with unusually high tensile strength and global alignment. The properties of birefringence and relatively low viscosity at high concentrations of polypeptide are characteristic of these materials during this processing.

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88 Hierarchical Structures in Biology as a Guide for New Afatenals Technology Structural Polysaccharide: Chitin, Chitosan, and Celluose The polysaccharide, chitin, is associated with naturally occurring composites such as cell walls of filamentary fungi and insect and crustacean exoskeletons. Electron micrographs taken of cuticle-secreting cells of the locust indicate that chitin synthesis occurs at the cell surface. Coupled polymerization and assembly (crystallization) processes occur, as with cellulose, although the details of this process are not understood. Studies with fungal preparations have demonstrated that chitosan fibers can be formed in vitro (Ruiz- Herrera and Bartnicki-Garcia, 1974~. It is generally accepted that in vivo assembly of chitin and chitosan fibers involves the tandem action of a series of enzymes. Based on the insolubility of chitin, it is also presumed that assembly occurs in association with the cell wall. Two enzymes, chitin synthase (membrane bound) and chitin deacetylase (soluble), are key to the polymerization of the N-acety1glucosamine monomers into chitin followed by deacetylation to form chitosan. The deacetylase is inactive against crystalline chitin, and tightly coupled polymerization and assembly processes are also seen in the fungal system. Cellulose, the most abundant polysaccharide in nature, is produced by plants and bacteria. Cellulose-producing bacteria include the genera Acetobacter, Rhizobium, Agrobacterium, and Sarcina. Most of the clues to cellulose biosynthesis and assembly are derived from studies on bacterial systems, since cellulose is synthesized and assembled independently from other polysaccharide and matrix components. Cellulose biosynthesis in bacteria couples synthesis with assembly; otherwise a random disorganized fibrous matrix would be expected. Instead, a highly crystalline fibri} structure is formed at the cell surface. 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. The cellulose synthase complexes are localized in aggregates at the cell surface to assure rapid and ordered assembly into larger microfibril at the sites of synthesis.

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Fabacanon of Hierarchical Systems 89 Self-assembly processes are involved that are indirectly controlled by the polymerization steps and the morphology of the cell (ordered-granule hypothesis; Ross et al., 1991~. In plants and algae, analogous steps related to synthesis and assembly are indicated. A process to produce bacterial (A.xylinum) cellulose as a fine, continuous, cross-linked network instead of separate fibers has recently been developed (Ben-Bessat et al., 1986~. The small diameter (0.1 to 0.2 Am versus fibers with diameter of 30 Am that are produced from wood pulp), coupled with the network structure, provide improved surface area for a number of potential applications where high water-holding capacity is critical. Self Assembly An impressive feature of biological materials is that the structure is well engineered on length scales from microscopic (a 1 nm) up to macroscopic dimensions (>1,000 nary). Nature appears to be adept at "processing" in the difficult intermediate (mesoscopic) length-scale regime of 10-100 nm. A method employed to great advantage in nature is based on the split-chemical tendencies of surfactant molecules, that is, amphiphilic preference for interfaces between hydrophilic and hydrophobic regions. Recognized as the "self-driven" mechanism for assembly of lamellar-bilayer membranes, amphiphilic characteristics lead to other mesoscopic architectures (e.g., cubic, hexagonal, biocontinuous, subtypes of lamellar structures, etc.~. In association with other cosurfactants, these systems create three- dimensional lattices, lamelIar arrays, and hexagonal stacks of "tubes," all with spatial periods of tens of nanometers. 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. From the viewpoint of industrial processing, the immediate concern will be cost: biologically derived surfactants are expensive.

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go fl~erarchical Structures in Biology as a Guide for New Materials Technology However, the physical-chemical mechanisms that drive the phase behavior are beginning to be understood and can be reproduced with synthetic diblock copolymers to achieve similar mesophases. These synthetic amphiphiles will be especially useful in processes where the chemical and physical conditions are harsh. Vesicle Mediated Multicomponent Processing Intravesicular precipitation of inorganic, crystalline particles is common in nature. Nanometer-sized magnetite particles, for example, are fabricated in intracellular vesicles by certain types of bacteria that have precise control over particle morphology and orientation (Franker and Blakemore, 1984~. In addition, single component particles can be precipitated within synthetic vesicles as a model system for the study of biomineralization (Mann and Williams, 1983; Mann and Hannington, 1988~. Particle precipitation within vesicles has several fundamental differences from bulk precipitation methods due to the unique properties of the lipid bilayer. 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~. Diffusion rates of anions, on the other hand, are significantly higher than for cations (Bangham et al., 1965), but are still low (10-' cm/s for CITE. This characteristic produces a system in which cations are essentially "trapped" within the phospholipid cage until precipitation can occur (Mann et al., 1986~. This could enhance chemical homogeneity within the system and facilitate the aqueous precipitation of water-soluble phases (such as Ba(OH)2~. Figure 4-5 shows a typical transmission electron micrograph of the vesicle-formed particles using the yttrium, barium, copper, and silver nitrate precursors. The particle size is smaller than the corresponding vesicle size.

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Fabricator' of Hierarchical Systems TIME" 202LSEC Rid 64 en = to 32 4B 16 O i _ Nl 91 lOe':!ch Ibl ~.~,A, ~ ~ . ~, , nut' 5.00 10.00 15.00 E N E R G Y [KeV] E O FIGURE 4-5 (a) Transmission electron micrograph of multicomponent particle formed within vesicle and (b) energy dispersive spectra of single particle. Source: Liu et al., 1991. In summary, this biomimetic system is truly multifunctional in that it simultaneously acts as: (1) a reaction cell for particle precipitation, (2) an ion selective membrane that affects precipitation kinetics, (3) a barrier to prevent spontaneous agglomeration of the ultrafine particles, and (4) 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. The general protocol that is envisaged is the isolation of cells with the potential for a specific phenotype, which may then be incorporated into a support matrix and finally be transplanted as an in vitro-generated replacement tissue in a patient. The motivation for this approach in the medical community is the

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92 Hierarchical Structures in Biology as a Guide for New Afatenals Technology application of these engineered tissues for use as biological grafts to replace diseased, damaged, or aged parts that cannot be replaced easily or satisfactorily with man-made materials. This can cover a range of applications, from the improvement of current treatment modalities to the opportunity of offering life-saving therapy. 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. The availability of in vitro-generated skin could offer life-saving treatment to many patients (for example, burn victims with little remaining skin). Skin is a highly organized and differentiated tissue, but careful basic science studies have developed this technology into an example of success for in vitro-generated tissues. A successful system has been developed for the production of a type of skin in vitro that appears to result in a well-differentiated tissue and that has the potential to be clinically usable. There appears to be enormous potential for the application of in vitro-generated tissues seeded with cells. Recent scientific and technological advances make this a topic with achievable goals rather than an idealized hypothesis.