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OCR for page 93
5
CONCLUSIONS AND RECOMM~NI)ATIONS:
SCIENTIlilC AND TECHNOLOGICAL OPPORTUNITIES
Biological structures are characterized by hierarchical
architectural designs in which organization is controlled on length
scales that range from the molecular to the macroscopic. These
materials are multifunctional and are produced in situ at room
temperature and atmospheric pressure. Many such structures are self-
healing and remarkably durable, and many display properties that
change in response to a changing environment; features of biological
materials 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 these
same materials again and again to realize an astonishing range of
structure and function.
The utility of many synthetic hierarchical materials is limited at
the present time by shortcomings in fabrication technology and
resultant finished-part costs that are high. This is especially true for
very high performance materials, that is, continuous fiber-reinforced
composites (polymer, ceramic, metal matrix, etc.), materials designated
for use under environmental extremes, and parts that need to function
reliably for extended time periods. Similarly, there is always a need
for more-efficient and more-sophisticated system designs, for
applications ranging from improved performance aircraft and
spacecraft to faster switching communication devices.
93
OCR for page 94
94
Hierarchical Structures in Biology as a Guide for New Aiatenals Technology
The Committee on Synthetic Hierarchical Structures concludes
that the study of biological materials will provide many lessons for use
in the development of new materials technologies. In some instances,
this may take the form of direct utilization of biological materials or
biosynthetic pathways; in other circumstances, biology will provide
inspiration for the creation of new designs and new methods of
fabrication. Some preliminary successes of this kind are described in
chapters 3 and 4 of this report. The analysis of lessons learned from
natural material systems could lead to the development of new classes
of synthetic materials, improved processing technology, and
innovative design and analysis approaches.
MATERIALS
The hierarchical architectures of biological materials
systems rely on critical interfaces that link structural
elements of disparate scale. The study of such systems
reveals extraordinary combinations of performance
properties, as well as limitations due to the modest thermal
and chemical stabilities of biological molecules.
A pplication 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.
Specific opportunities and needs for materials development that
were identified in previous chapters include:
.
.
synthetic constituents to produce hierarchical materials
with useful performance over a broad range of
environmental conditions;
low-friction and wear-resistant materials for joints and
bearings;
OCR for page 95
Conclusfons and Recommendations
.
95
· adhesives that mimic the tenacity and toughness of
biological adhesives; and
composites with high volume fractions of reinforcements.
PROCESSING
Biological structures are fabricated via highly
coupled, often concurrent, synthesis and assembly In the
conception and, evaluation of synthetic and processing
schemes for new materials systems, the prospects for
integrated system fabrication should be carefully
considered.
Specific needs to realize the full promise of integrated
fabrication methods include:
concurrent materials synthesis and structural assembly;
processes to fabricate highly specific synthetic membranes
and filters;
use of cells to synthesize and deposit materials;
biosynthetic pathways to the cost-effective manufacturing
of new classes of shaped hybrid composites; and
biosynthetic concepts and materials for self-repair of
critical components and devices.
DESIGN AND ANALYSIS
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
OCR for page 96
96
Hierarchical Structures in Biology as a Guide for New Materials Technology
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.
Several universities have recognized the promise of this
approach and have developed programs to address these needs.
Although, due to the diversity of the field, bioengineering curricula
may vary in detail, they all strive to bring together the biologists'
knowledge of physiology, anatomy, biochemistry, and molecular
biology with the engineers' knowledge of design and structure
(Watanabe, 1993~.
Lessons to be learned from the design of natural systems
include:
strong, durable interfaces between hard and soft structural
components;
· tribological joints with low friction coefficients and
remarkable durability;
mechanistic understanding and analysis methods for
deformation and failure of complex systems;
energy-absorbing mechanisms of rigid biological
composites;
platelet and surrounded plate analytical concepts; and
moisture-friendly synthetic systems.
SCIENTIFIC OPPORTUNITIES
The hierarchical structures observed in biological systems
represent potential solutions to the problems of materials choice,
OCR for page 97
Conclusions and Recommcnda~'ons
97
materials fabrication, and component or system design that are
currently limiting the utility and implementation of many modern
materials and design concepts. Although the prospects for new
biologically inspired materials technologies are real, full exploitation
of this approach will 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 methodology, instrumentation, modeling,
and theory to enhance the development and applications of hierarchical
systems that are based on natural analogies. Some instances in which
the translation of hierarchical structures found naturally might
significantly impact materials science and technology are described
below.
Synthetic Methodology
As discussed in Chapter 3, the design and preparation of
hierarchical materials will place a new premium on the synthesis of
macromolecules of precisely defined primary structure and complex
chemical composition. At present, the only methodology available for
the preparation of such polymers involves the use of gene synthesis
and recombinant-DNA technology to create artificial structural
proteins. 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 improve materials
performance. However, it is clear that the thermal and hydrolytic
sensitivities of proteinaceous materials will limit their applicability in
many important synthetic materials applications. Generalization of the
methods of controlled synthesis to new classes of monomers thus
becomes an important objective.
Some initial developments along these lines may be foreseen. It
has been known for many years that certain analogues of the natural
amino acids can be incorporated into bacterial proteins with high
OCR for page 98
98
Hierarchical Structures in Biology as a Guide for New Materials Technology
fidelity (Cowie and Cohen, 1957), and it is likely that additional
examples will continue to be identified. As further information
becomes available regarding the mechanisms of transfer RNA
charging and proofreading, the rational design of amino acid
analogues useful in protein biosynthesis can be anticipated.
Some first steps toward templated polymerization of other
monomers have also been reported. Schultz and coworkers (Noren et
al., 1989) have recently described a method whereby suppressor
transfer RNAs that are chemically acylated serve to deliver unnatural
amino acids to messenger RNA templates in an in vitro translation
system. In this initial report, the method succeeded for X-amino acid
analogues but failed when applied to ,B-amino acids. Despite this
limited success, such methods offer a basis for systematic studies of
templated polymerization processes. Progress in this area must be
accompanied by advances in cell-free translation methodology if any
impact on materials synthesis is to be made, since current cell-free
methods are limited to the preparation of submilligram quantities of
material.
Looking beyond templated polymerizations, one sees little
current evidence of real progress toward efficient synthesis of
genuinely uniform chain populations. Nevertheless, recent advances
in living ionic and metathesis polymerizations have been substantial
and may in time lead to higher-order control of chain length,
sequence, and stereochemistry. Issues such as environmental impact
of the manufacture and disposal of polymers, along with 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 of the chemistries presently
in the waste stream. To achieve this, while preserving or expanding
the current product diversity available with commercial polymers,
increased interest in the effects of synthetic polymer primary
specificity of structure of synthetic polymers on cost and performance
will be manifest.
OCR for page 99
Conclusions and Recon~ncr~ations
99
Cellular Synthesis of Materials
It should be possible to develop cell "epitaxy" methods whereby
biological cells are employed to fabricate thin layers (of organics or
minerals) on synthetic material substrates, perhaps providing persistent
maintenance and regulation as "epidermis." The objective is to use
organisms as "microengineers" for structuring 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. Not only
could these advances create 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 that make up 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
properties, by combining outstanding strength and stiffness with
improved fracture toughness compared with that of monolithic
materials. In addition to practical and cost-effective fabrication
techniques, an understanding of deformation mechanisms and the
OCR for page 100
loo
Hierarchical Structures in Biology as a Guide for New Alatenals Technology
ability to optimize composite structures through mechanical modeling
are critical to the successful development of such materials.
Adhesives and Interfaces
Adhesives and interfaces play important roles in both synthetic
and natural composites. Although much has been done in adhesion
science and technology, there are opportunities to tailor new synthetic
adhesives and unique structural architectures through mimicry of
natural systems. Adhesives play a critical role 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.
Naturally occurring marine adhesives are analogous to composite
thermosets in that they are made up of fiber, filler, and catalyst
molecules dispersed in a cross-linked resin. Resin proteins display a
regular, repetitive structure that is thought to be related to their
function (Waite, 1990~. For example, the East Coast blue mussel
(Mytilus edulis) uses a resin polymer made up of approximately 80
repeats of a decamer. The West Coast mussel (Mytilus californianus)
has evolved in an environment with greater wave action than the East
Coast mussel and has resin virtually identical in structure except that
two residues have been interchanged and a serine has been replaced by
a threonine, a very conservative exchange. This small change in
structure, however, produces an adhesive that is two to three times
stronger than the East Coast adhesive (Waite, 1986~. This observation
lends support to the view that mussel adhesive might be used as a
model to systematically investigate the relationship between molecular
structure and adhesive function, which could lead ultimately to a
generic glue that can be modified at the molecular workbench for any
number of different moist environments. If broad application is to be
OCR for page 101
Conclusions and Recommendatwns
101
realized for such adhesives, however, functional durability over a
wider range of temperatures must be achieved.
Soft-Tissue-Based Materials
Nature has developed exceptional designs for "ultrasoft"
materials and for interfacing soft and hard materials with capabilities
well beyond present day technology. 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. Therefore, the challenges are to extract design
lessons from nature especially for development of material
technologies that are inaccessible at present. These challenges include
preparation of "self-healing" capsular materials that possess tunable
and "motile" properties; methods for assembly of soft organic and hard
material interfaces that are mechanically, chemically, and electrically
compatible; and development of membrane composites that are based
on fluid-surfactant interfaces that are supported by tethered polymer
networks that possess permeability restriction and mechanical strength.
An example of the potential impact of soft-tissue understanding
is the reduction in energy needled to move a body through water when
its drag is reduced. Also, in order to prevent detection, there is
interest in reducing the hydrodynamic noise a body makes moving
through water. Turbulence caused by a moving hull raises drag and
hydrodynamic noise. There is no theory at present that links the
viscoelastic properties of the surface of a body to its drag or its ability
to reduce the amount of hull surface that causes turbulence. Dolphins
have a peculiar skin overlying blubber and a collagenous subdermal
sheath, and each of these tissue layers has different elastic and
damping properties. More importantly, dolphins swim faster and
farther using less metabolic energy than calculations would lead one
to expect, and it is not clear how they do it. Further studies of the
control of damping properties by the micro-, ultra-, and molecular
structure of skin, blubber, and peripheral connective tissues of
dolphins and of the boundary-layer conditions over their swimming
OCR for page 102
102
Hierarchical strictures in Biology as a Guide for New lldaterials Technology
bodies could lead to new methods for reducing turbulence, drag, and
noise.
A second soft tissue is muscle, a soft tissue that has no clear
synthetic analogue. It is a transducer, converting electrical signals and
chemical energy into mechanical motion. The characteristics of
interest are response time, power density in terms of force developed
per gram of muscle, and efficiency of power conversion. Piezoelectric
transducers can do a similar job but give quite small motions for large
applied voltages. Generally, engineers and designers must resort to
small electric motors when building, for example, the arms for robots.
Water-swollen cross-linked polymer gels can respond to electric
fields by contracting in a way that is superficially similar to muscle.
The field drives out a mobile counter-ion. The resulting neutralization
of acid groups in the gel causes contraction. Devices have been made
with these gels, but the response is slow (1-10 seconds compared with
10-100 milliseconds for muscle), and the power density is low. A soft,
light, powerful actuator would have many applications in mechanical
engineering ranging from actuators to engines. Intriguing progress
toward these objectives has been reported (Urry et al., 1992~.
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 anti 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.
For the purpose of this report, the determination of shape in
biological systems needs to be considered at the level of hierarchical
matrix formation. This is between the level of the component
macromolecules, whose shapes determine their possible hierarchical
interactions, and the level of whole cells, which appear almost
OCR for page 103
Conclusions arid Recommendations
103
shapeless after isolation from their attachments to other cells and to
adjacent extracellular matrix.
Although the shape of an industrially produced article is defined
by a set of dimensions that is measured with an externally applied
scale, biological shape is determined by the history of internal
manufacture of the object, as described in Chapter 2. A hallmark of
biological matrix formation is the interaction beween matrix and the
cell that adheres to it while manufacturing more of the same material.
Previously it was mentioned that cells can be bioengineered to produce
and secrete specific macromolecules. Up to now such cells, usually
bacteria, yeasts, or insect cells, have been grown in suspension culture,
without any specific orientation. For example, the previously
mentioned bacteria that secrete cellulose are grown in suspension
culture, which produces a random tangle of cellulose fibrils.
However, if such cells were provided with cell surface receptors that
attach to cellulose, then they could initially orient by attaching to
previously made cellulose fibers. Their matrix production would
thereby become vectorial with respect to the substrate to which they
attach. The initial substrate that is provided to the cells could be
structured by weaving, knitting, etc., and the subsequent matrix would
be built on to that. The activities of the attached cells could be
modulated in various ways, for example, to suppress or enhance cell
replication locally and thereby produce patterns of increased
concentrations of manufacturing cells. Cells have been engineered to
express new receptors at their surfaces by inserting suitable genetic
information into them. Furthermore, composite materials could be
produced by replacing the initial manufacturing cells later by another
set that have been engineered to produce a different matrix
macromolecule.
It was previously noted that biological shape emerges partly due
to local sculpting. Highly controlled enzymes, such as some
collagenases, and their inhibitors, are secreted by cells for limited,
local and short-lived action. These enzymes may only modify their
substrate, rather than totally destroy it. Limited etching, removal of
material, is well established in the manufacture of microelectronic
OCR for page 104
104
Hierarchical Stn~ca~res ir' Biology as a Guide for New Afatenals Technology
devices and might have its counterpart in the above scheme of
vectorial matrix production.
Some natural self-assembling systems have a defined size, such
as some vesicles, while other self-assembling systems are indefinite in
extent, such as unstrained crystals. Proper function requires that
system size be controlled as well as system shape.
Some examples of methods to control and limit growth are
.
controlling the amount of material available for the
transformation (the control mechanism is a time-
dependent chemical potential);
controlling the molecular geometry of vesicles;
termination, due to build up of strain, of synthesis
reactions within the molecule being created; and
diffusion limited aggregation.
The successful translation of these principles to synthetic
materials could lead to the integration of the materials synthesis and
processing steps of part fabrication.
TECHNOLOGICAL OPPORTUNITIES
Biomedical Materials
There is a recognized societal and economic need for synthetic
hierarchical materials with appropriate mechanical and functional
performance characteristic properties for use in biomedical
applications. This need represents a motivating opportunity for the
scientific community to develop these materials.
A brief summary of articular cartilage and diarthrodial joints
has been presented (see Chapter 2) as a paradigm for hierarchical
materials and structures with nanoscale, ultrascale, microscale,
tissue-scale, and macroscale features that enable the performance
critical to function. Nanoscale structures such as the charged groups
OCR for page 105
Conclusions arid Recomm~ndanons
105
on the proteoglycan molecules provide the electrochemical bases for
Donnan osmotic pressure and charge-to-charge repulsion. Ultrascale
structures of proteoglycan aggregates and the collagen network
provide the organization for the microporous solid matrix and its
essential material properties. Microscale structures include specific
cell types (chondrocytes), pericellular, territorial, and interterritorial
extracellular matrix organization. These cells manufacture and
organize the molecular building blocks and maintain the
collagen-proteglycan extracellular solid matrix around themselves by
a slow but balanced metabolic process. At the tissue-scale, articular
cartilage possesses a set of unique nonlinear, anisotropic and
nonhomogeneous material properties that seem to have been
specifically designed to provide excellent long-term tribiological
(friction, lubrication, and wear) functions at extremely high loads.
Finally, at the macroscale, articular cartilage is the bearing material
that provides the smooth, near frictionless function required of
diarthrodial joints.
The challenges in developing a manufacturing process to
produce synthetic hierarchical
materials with these required
mechanical properties and functional characteristics are great. First,
articular cartilage and other biologic tissues have very complex
compositional make-ups and ultrastructural organizations. Second, the
tissue is manufactured by tissue-specific cells in-situ. These cells are
regulated by as yet unknown control processes, which control the
production and assembly of the biomacromolecules and organize these
macromolecules into an exquisite fabric that is the tissue. It is
unlikely that any synthetic process can be developed in the near future
that will duplicate the ability of the specific cells to manufacture and
organize a hierarchical material with such fine ultrastructural features.
However, a hybrid approach has been taken by some researchers,
where synthetic grafts have been produced that are made of
biocompatible resorbable matrices such as polylactic acid or
copolymers of lactic and glycolic acids. These grafts serve as scaffolds
for the specifically seeded cells (Cima et al., 1991~. These synthetic
matrices are not subject to immune reactions. Other gels made of
collagen and glycosaminoglycan seeded with cells also show promise
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106
Hierarchical Stn~ca~res in Biology as a Guide for New Materials Tecl~logy
as graft materials for skin and blood vessels. However, for most gels
currently available (e.g., fibrin clot that is used for joint surface
repair), the material properties are probably insufficient for use in
diarthrodial joints where the applied stresses are very high.
Development of strong, cohesive, porous, permeable, resorbable gels
that are capable of sustaining high stresses and strains and of
providing a supporting and protecting environment for the seeded
cells is a major challenge for future biomedical researchers interested
in developing synthetic hierarchical materials for clinical use.
Improved Membranes and Membrane-based Devices
Improved membrane selectivity is desirable in the areas of water
purification, protective clothing for 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 and mechanisms,
in order 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. For a better understanding of membrane
structures in terms of processing and assembly, the incorporation of
responsive channels is key. Additional questions lie in the realm of
suitable substitutes for water as plasticizers in these materials and in
approaches to biomimetic membrane design.
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 self-preservation or 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 (Rogers, 1992~. Passively smart materials
OCR for page 107
Conclusions and Recommendations
107
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 (Newnham and Ruschau, 1990; Newnham,
1993).
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. Challenges in sensor development and
integration of sensing and response functions with practical structures
need to be addressed to realize the potential of smart materials (NRC,
1 994).
For example, important lessons can be learned by studying how
sea urchins control the material properties of their bodies as a function
of the local environment. Sea stars, sea urchins, and sea cucumbers (in
fact, all echinoderms) can control the viscosity of their body wall and
other connective tissues. Such tissues can be stiff enough to act as
ligaments at one moment and undergo 30 percent extension the next.
The change in stiffness and extension is reversible but not elastic, and
an animal can cycle through these different states of stiffness-and-
compliance, stretching-and-recovery dozens of times a day throughout
its multiyear life without the materials showing wear or fatigue. The
viscosity of the tissue is modulated by divalent cations, chiefly
calcium, which form labile links in a mixture of collagen and
glycoprotein molecules. A ganglion of nerve cell bodies whose long
fingerlike processes (axons) carry chemicals to all parts of the ligament
sits on each ligament. Thus, an important aspect of "smart materials"
is sensing. In the case of echinoderms, these biological sensors
(receptors) detect neurotransmitters from the nerve cells and in
response allow a local increase in the concentration of divalent cations.
An interesting class of nonbiological sensors is that of
membranes or thin films adsorbed onto a surface. The chemical sensor
consists of arrays of these films or membranes, each sensitive to
different materials. There is every expectation that these sensors can
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Hierarchical Stn~c~rcs in Biology as a Guide for New Afatenals Technology
be made with architectures similar to those of presently available
microchips, with competitive cost/performance characteristics.
Functionally Gradient Materials
Functionally gradient materials (FGM) are defined as 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 (Ramesh
and Markworth, 1993~. 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 or orientation of constituents. For example, articular
cartilage exhibits gradients in collagen/proteoglycan concentrations
and in collagen fiber orientation. Often, as with cartilage, FGMs are
used to provide an interracial transition between dissimilar materials
or to provide multiple functions.
Synthetic FGMs can be produced from mixtures of metals,
polymers or ceramics in virtually any combination. A large part of the
research in FGMs has focused on coatings and transitions for high-
temperature aerospace applications like hypersonic aircraft and
advanced turbine engines (Perepezko, 1991; Ramesh and Markworth,
1993~. Generally these systems transition from high-temperature
resistant ceramics to metallic structural alloys. FGMs have been
produced using vapor-phase synthesis, powder techniques, thermal
spray processes, and self-propagating high-temperature synthesis.
Another interesting area of research is surface gradients, where
the nature of the surface is varied continuously with position (Elwing
and Golander, 1991~. Gradients in surface areas have been produced
that cause water to move uphill (Chaubhury and Whitesides, 1992~.
Surface gradient techniques may find applications in processing,
which will allow selective deposition or coating processes, or in
tailored membrane or sensor applications.
The development of functionally gradient materials is still in its
early stages. The biggest challenge is in scaling the processes to
OCR for page 109
Conclusions and Recomm~ndanons
109
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 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 methodologies 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 finished part
shape and surface characteristics required for the application. 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. It is instructive to compare the
"steps" of synthetic and biological fabrication technologies.
SYNTHETIC
BIOLOGICAL
Produce Reinforcing Fiber
Treat Fiber Surface
Impregnate Fiber with Matrix
Line Up Prepeg Plies in Mold
Cure/Shane Part
Produce Matrix "Scaffold" for Part
Form Crystal Directing Surface
Fill with "Gel"
Replace Gel with Oriented "Fiber"
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Hierarchical Structures in Biology as a Guide for New Materials Technology
In both cases, the materials employed and the final composite
structure will contain key hierarchical structural elements. In the
natural system, however, the hierarchy is a key to performance.
Understanding and, where appropriate, mimicking the structure and
manufacturing logic of natural hierarchies offer an opportunity to
leapfrog current composite technologies and realize the promise of
synthetic composites, which has proven elusive to the materials
community for more than two decades.
The toughest materials are known to raise the energy required
for tearing by diverting cracks away from their preferred directions
of propagation. A horse's hoof is difficult to split vertically (in the
direction up the horse's leg). Hoof material contains keratin (the
major protein in hair, finger nails, feathers, and rhino horn) in an
ordered three-dimensional array such that a crack initiated by a
vertical cut will turn and split the material at right angles to the
vertical direction (circumferentially in the hoof). As a result, if a split
does occur, it will cause the loss of a thin strip of the hoofs three-
dimensional fiber array. The relationships between the array and the
resistance to impact and control of crack propagation could illustrate
new mechanisms of fracture toughness to be used in designing new
synthetic materials. In addition, study of the mechanisms of synthesis
of hoof material in the horse can be expected to provide hints for the
industrial fabrication of such complex three-dimensional fibrous
materials. However, improvements over current analysis methods are
needed to take full advantage of these toughening mechanisms.
Representative terms from entire chapter:
smart materials
