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OCR for page 73
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
OCR for page 74
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
OCR for page 75
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
OCR for page 76
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.
OCR for page 77
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
OCR for page 78
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.
OCR for page 79
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
OCR for page 80
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.
OCR for page 81
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
OCR for page 82
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.
OCR for page 83
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
OCR for page 84
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.
OCR for page 85
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
OCR for page 86
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
OCR for page 87
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.
OCR for page 88
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.
OCR for page 89
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.
OCR for page 90
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.
Representative terms from entire chapter:
injection molding
