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OCR for page 1
ExEcuTnE SUMMARY
Great fleas have little fleas,
upon their backs to bite 'em,
and little fleas have lesser fleas,
and so on infinitum,
and the great f leas themselves,
in turn, have greater fleas to go on,
While these again have greater still,
and greater still, and so on.
William DeMorgan (1839-1917)
Hierarchical structures are assemblages of molecular units or
their aggregates that are embedded or intertwined with other phases,
which in turn are similarly organized at increasing size levels. Such
multilevel architectures are capable of conferring unique properties to
the structure. Hierarchical structures can be prepared from metals,
ceramics, or polymers, or from hybrids of various classes of these
materials. The unifying theme for all types of materials is the
pervasiveness of hierarchical structures in practically all complex
systems, particularly naturally occurring ones.
Many materials systems found in nature exhibit combinations of
properties not currently found in synthetic systems. The unique
performance of natural materials arises from precise hierarchical
organization over a large range of length scales. The hierarchical
architectures of cellulose aggregates in wood or collagen aggregates in
cartilage or tendon provide excellent examples of natural composite
1
OCR for page 2
2
Hierarchical Structures in Biology as a Guide for New Materials Technology
materials designee! for multifunctional applications. Even the ultrasoft
membranes surrounding cells exhibit exceptional properties that
emanate from structure on many length scales. Studies of materials of
biological origin invariably yield surprises that demonstrate clearly
that these properties have been refined by slow evolutionary
engineering. These hierarchically structured materials display unique
properties that are affected by structure and generative processes at all
levels of the biological structural hierarchy.
Hierarchical materials systems in biology are characterized by:
.
.
.
recurrent use of molecular constituents (e.g. collagen),
such that widely variable properties are attained from
apparently similar elementary units;
controlled orientation of structural elements;
durable interfaces between hard and soft materials;
sensitivity to and critical dependence on-the presence
of water;
properties that vary in response to performance
requirements;
fatigue resistance and resiliency;
controlled and often complex shapes; and
capacity for self-repair.
Nature has a very limited range of materials with which it
works. In most biological tissues, the constituents are proteinaceous.
In rigid composites, they tend to be calcium carbonates, calcium
phosphates, and silica. While natural composites exhibit outstanding
combinations- of properties, these materials systems and their
constituent components exhibit the properties over a temperature
range too narrow for most engineering designs. Thus, although rules
about adhesion, architecture, and composite elements in mechanical
collaboration are useful to learn from nature and to apply to other
material components in order to produce analogous synthetic
structures, the natural constituents themselves have performance
deficiencies. It is the unique interfaces in natural composites that
provide unusual mechanical performance in such materials.
OCR for page 3
Execanve Summary
3
Although the use of synthetic hierarchical concepts is at an early
TV she range of materials is potentially great. In addition, many
~ - ~O _, - . _ ~
. . ~ ~ 1 _ _1~_ ~_1 ~_A ALGA 1~? ~ Our-th~;^ ma;
structural variables can ne altered more rebury ~ byItilibLl~ Ilia`~o
than in natural materials. Through control of fabrication processes,
materials variables such as atomic structure; molecular structure;
nanostructures and boundaries; dislocation and other defect structures;
cells and other substructures; size, distribution, and morphology of
constituents and phases; grain size and morphology; orientation
distributions; phase relations (including transformations); interfaces at
all levels; and microstructure can be altered (although, for the most
part, not independently). However, the realization of the potential of
synthetic hierarchical structures has been limited, because available
processing technology does not provide methods for precise control of
materials variables over all levels of structural arrangement.
When synthetic materials are manufactured with an emphasis on
tailoring their properties through microstructural control, the extent
of this control is generally at a specific length scale. For instance, the
mechanical properties of most metallic materials are controlled
through the manipulation of dislocation dynamics at the nanometer
length scale, whereas the mechanical properties of ceramic materials
are controlled through the propagation of cracks that are initiated
from defects of micrometer length scales. For composites that are
composed of two constituents, which are generally of quite different
character, the controls are much more complex.
At the request of the Department of Defense and the National
Aeronautics and Space Administration, the National Materials
Advisory Board convened the Committee on Synthetic Hierarchical
Structures to conduct case-studies by selecting natural material
systems to be used as models for synthetic efforts; review state-of-
the-art synthetic techniques and processes for assembling synthetic
hierarchical structures; characterize properties, unusual characteristics,
and potential end-use applications for these synthetic systems; and
recommend research that will expedite the understanding of the
complex phenomena involved, lead to increased coordination among
disciplines, and provide direction for future activities in the field.
Although a broad range of functions are represented in biological
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4
Hierarc)ucal Structures in Biology as a Guide for New Afatenals Technology
systems, the committee concentrated on structural materials systems
and their properties.
CONCLUSIONS AND RECOMMENDATIONS
Biological structures are characterized by hierarchical
architectural designs in which organization is controlled on length
scales ranging from the molecular to the macroscopic. These materials
are multifunctional and are produced in situ at room temperature and
atmospheric pressure, although at slow rates. Many such structures are
self-healing and remarkably durable, and many display properties that
change in response to a changing environment; features that represent
desirable, and as yet unattainable, objectives in the design and
manufacture of synthetic materials systems. Nature is parsimonious
in its use of constituent materials; it returns to the same materials
again and again to realize an astonishing range of structure and
function.
The hierarchical architectures of biological materials systems
rely on critical interfaces that link structural elements of disparate
scale. The study of such systems reveals extraordinary combinations
of performance properties, as well as limitations due to the modest
thermal and chemical stabilities of biological molecules. Application
of hierarchical design concepts to more-robust synthetic building
blocks provides promising routes to high performance adhesives and
composites, biomedical materials, highly specific membrane and
filtration systems, low friction bearings, and wear-resistant joints.
Biological structures are fabricates! via highly coupled, often
concurrent, synthesis and assembly. Although these assembly
processes provide valuable lessons for synthetic processing, they
generally occur at rates that are too slow to be economically viable.
However, in the conception and evaluation of synthetic and processing
schemes for new materials systems, the prospects for integrated system
fabrication should be carefully considered. lo
The prospects for new biologically inspired materials
technologies are real, however, full exploitation of this approach will
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EiCecanve Summary
s
require advances in engineering, education, and enabling science.
Although there is a broad range of technologies that may contribute
to the understanding of biomaterials, the committee recommends
concentration on developments in structural biology, interface science,
synthetic methods for polymers with controlled sequences and methods
for producing patterned structures by localized chemical synthesis,
instrumentation, modeling, and theory (in materials, chemistry,
biology, mechanics, etc.) to enhance the development and applications
of hierarchical systems that are based on natural analogies.
Biological structures perform as parts of integrated systems and
undergo continuous evaluation and refinement based on system
performance. In analogous fashion, considerations of integrated
systems design and performance will take on increasing importance in
the high-technology materials-related industries of the future.
Interdisciplinary teams of scientists and engineers will be required to
effectively design and develop structural systems with such complex
architectures. The committee recommends that the academic and
industrial sectors of the materials community prepare for this
development through implementation of appropriate educational and
engineering programs that are based on systems concepts.
SCIENTIFIC OPPORTUNITIES
Examples of hierarchical structures in synthetic materials range
from those that are deliberately biomimetic (e.g., ultrathin layered
composites) to those whose relation to natural systems is coincidental
(e.g., highly orientec! polymers). The utility of many synthetic
hierarchical materials is currently limited by the availability of
fabrication technology, excessive fabrication times, and high cost for
finished parts. This is especially true for very high performance
materials, that is, continuous fiber reinforced composites materials
(polymer, ceramic, metal-matrix, etc.) designated for use under
environmental extremes, or parts which must function reliably for
extended time periods. Similarly, there is always a need for more-
efficient and more-sophisticated system designs, from improved
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6
Hierarchical Structures in Biology as a Guide for New Alatenals Technology
performance aircraft and spacecraft to faster-switching
communication devices. The scientific and technological opportunities
identified below represent examples of areas in which biological
hierarchical paradigms may be effectively utilized to satisfy societal
needs and solve existing problems.
Synthetic Methodology
The design and preparation of hierarchical materials will place
a new premium on the synthesis of macromolecules of precisely
defined primary structure and complex chemical composition. At
present, the only methodology available for the preparation of such
polymers involves the use of gene synthesis and recombinant DNA-
technology to create artificial structural proteins. This methodology
is powerful and may lead not only to the creation of polymeric
materials with functions not obtainable through conventional synthetic
methods but also to an understanding of how control of molecular
structure and function can impact improved materials performance.
It is clear, however' that the thermal and hydrolytic sensitivities of
proteinaceous materials will limit their usefulness in many important
synthetic materials applications. Generalization of the methods of
controlled synthesis to new classes of monomers thus becomes an
important objective.
Looking beyond templated polymerization, there is little current
evidence of real progress toward! efficient synthesis of genuinely
uniform polymer chain populations. Nevertheless, recent advances in
living ionic and metathesis polymerization have been substantial and
may in time lead to higher-order control of chain length, sequence,
and stereochemistry. The committee believes that issues such as
environmental impact of the manufacture and disposal of polymers
and the need for continuing improvement of cost/performance within
the polymer industry will cause polymer science to move in directions
that will tend to minimize the numbers of monomers (raw materials)
utilized by the industry and hence reduce the number and volume of
the offending chemicals presently in the waste stream. To achieve
OCR for page 7
ExecaDve Summary
7
this, while preserving or expanding the current product diversity
available with commercial polymers, increased interest in the effects
of specificity of the primary structure of synthetic polymers on cost
and performance will be manifest.
Cellular Synthesis of Materials
Biological cells can be employed to fabricate thin layers (of
organic materials or minerals) on synthetic material substrates. The
objective is to employ organisms to structure materials on
difficult-to-manage length scales and with difficult-to-synthesize
chemistries. The cellular mechanism is capable of organizing fibrous
networks, for instance, with functional hydrogel components to
produce low-friction, durable, fatigue-resistant joint bearings.
Cellular responses to environmental effecters, such as mechanical
stress or hormones, can beneficially change the composition and
assembly of these materials. This is enabled through the coupling of
specific protein synthesis and degradation with the constant
monitoring of mechanical function and the state of need of the
organism. Long-term cellular activity within the material can enable
the repair of the material upon damage by reactivation of matrix
formation. These advances could create not only new membrane and
biomaterial technologies but also new insights for structuring hard
materials.
Rigid Structural Composites
Many of the rigid structural materials found in nature are
composites comprising unusual compositions and configurations. For
example, the nacreous material in mollusk shell is a segmented
composite with a very low volume fraction of matrix phase in very
thin layers. The ability to design and fabricate synthetic structures
with similar characteristics, as well as the ability to mimic adhesion
between the phases, could lead to composites with remarkable
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8
Hierarchical Structures in Biology as a Guide for New Afatenals Technology
properties, by combining outstanding strength and stiffness with
improved fracture toughness compared with that of monolithic
materials or current composites. In addition to practical and cost-
effective fabrication techniques, an understanding of deformation
mechanisms and the ability to enhance composite structural response
through mechanical modeling are critical to the success of these
materials.
Adhesives and Interfaces
Adhesives and interfaces play important roles in both natural
and synthetic composites. Although much has been done in adhesion
science and technology, there are opportunities to tailor new synthetic
adhesives and unique structural architectures by way of mimicry of
natural systems. Adhesives are critical in the formation, strength, and
durability of composite materials as agents responsible for bonding
between matrix and reinforcing phases. Advances in composites have
emphasized the need for durable adhesives that would work in wet
environments. Adhesives produced by organisms, especially marine
organisms, suggest themselves as candidates for study, because they
cure in the presence of water and resist its subversive effects.
Soft-Tissue Based Materials
Exceptional designs for "ultrasoft" materials and for interfacing
soft and hard materials are found in nature with capabilities well
beyond present day technology. The committee feels that exposing the
physical and chemical principles that underlie the special features of
these materials is certain to stimulate new approaches to design of
synthetic materials, parts, and systems. Such approaches may include
preparation of "self-healing" capsular materials that possess tunable
and motile properties; general methods for assembly of soft organic
and hard material interfaces that are mechanically, chemically, and
electrically compatible; and development of membrane composites that
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E'cecutz've Summary
9
are based on fluid~urfactant interfaces supported by tethered
polymer networks that possess permeability restriction and mechanical
strength.
Control of Size and Shape (Assembly, Self-Assembly)
Inherent in the behavior of natural proteins is their assembly
into structures of a given size and shape to allow the performance of
a specific end-use function. This formation of parts and systems is
driven by local geometry and molecular forces and does not require
additional "shaping and machining" steps. The ability to design
synthetic systems capable of assembling in an analogous fashion would
have obvious practical impact. Some natural self-assembling systems
have a defined size, such as some vesicles, while others are indefinite
in extent, such as unstrained crystals. Proper function requires that
system size be controlled as well as system shape. A key factor in
control of system size and shape is the identification of switching
mechanisms that govern, for example, the size and shape of nacreous
platelets in abalone shells, as well as the thickness of the protein layers
that separate the layers.
TECHNOLOGICAL OPPORTUNITIES
Biomedical Materials
There is a recognized societal and economic need for synthetic
materials with appropriate mechanical and functional performance
characteristic properties for use in biomedical applications. The
challenges in developing a manufacturing process to produce synthetic
hierarchical materials with these required mechanical properties and
functional characteristics are great. First, biologic tissues have very
complex compositions and ultrastructural organizations. Second, the
tissue is manufactured by tissue-specific cells in situ by as yet
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10
Hierarchical Structures ir' Biology as a Guide for New Materials Technology
unknown processes, which control the production and assembly of the
constituent biological macromolecules.
It is unlikely that any synthetic process can be developed in the
near future that can duplicate the ability of the specific cells to
manufacture and organize a hierarchical material with very fine
ultrastructural features. However, a hybrid approach has been taken
by some researchers, where synthetic grafts which serve as scaffolds
for the specifically seeded cells, have been produced from
biocompatible resorbable matrices, such as polylactic acid or
copolymers of lactic and glycolic acids. Gels made of collagen and
glycosaminoglycan seeded with cells also show promise as graft
materials for skin and blood vessels. Development of gels that are
strong, cohesive, porous, permeable, and resorbable and that are
capable of sustaining high stresses and strains and providing a
supporting and protecting environment for the seeded cells is a major
challenge for future biomedical researchers interested in developing
synthetic hierarchical materials for clinical use.
Improved Membranes and Membrane-based Devices
Improved membrane selectivity is desirable in the areas of water
purification, clothing to protect those handling hazardous materials,
outdoor clothing and shelters, gas separations, industrial purification
processes, etc. Coupled with this is a need for improved stability and
increased lifetime for these membranes, as well as for mechanisms to
reduce fouling. One approach to solving these problems is to
incorporate responsive channels and self-repair ("living membranes")
or self-cleaning attributes that are patterned after natural membrane
systems. This requires a better understanding of membrane structures
in terms of their processing and assembly. Additional areas for
development include suitable substitutes for water as plasticizers in
these materials, and in approaches to biomimetic membrane design.
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Erecunve Summary
11
Smart Materials
Natural systems have the ability to sense their surroundings and
to respond to impulses or changes in conditions by changing properties
or initiating repair responses. The development of smart materials,
which integrate the functions of sensing, actuation, and control, can
benefit greatly from lessons gleaned from the studies of these
biological systems. Passively smart materials respond to external
change without assistance often through phase changes or transitions
in fundamental properties. Actively smart materials utilize feedback
loops to recognize changes and initiate appropriate responses.
Opportunities for application of smart materials systems in
structural applications generally focus on reduced component mass and
adaptive functionality aimed at improving structural efficiency,
durability, and safety. Examples of smart materials applications
include load and vibration alleviation systems, failure sensing and
repair, and shape memory. Developments in sensor development and
integration of sensing and response functions with practical structures
are necessary to realize the potential of smart materials.
Functionally Gradient Materials
Functionally gradient materials (FGM) are materials in which a
continuous spatial change in composition or microstructure gives rise
to position-dependent physical and mechanical properties that can
extend over microscopic or macroscopic distances. Natural materials
with functional gradients abound. Examples of materials with
functional gradients that are discussed in this report include articular
cartilage and bone. FGM can result in changes in composition or
orientation of constituents.
Synthetic FGMs can be produced from mixtures of metals,
polymers or ceramics in virtually any combination. FGMs whose
properties vary in the dimension of their thickness (through thickness
variations) can provide a transitional interface between dissimilar
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Hierarchical Structures in Biology as a Guide for New Afatenals Technology
materials or serve as coatings with optimized environmental resistance
and adhesion to selected substrates. Another interesting area of
research is surface gradients, where the nature of the surface is varied
continuously with position. Surface gradient techniques, allowing
selective deposition or coating processes, may find applications in
processing or in tailored membrane or sensor applications.
The development of FGMs is still in its early stages. The
biggest challenge is to scale the processes to practical size components
while maintaining the precise control and consistency needed. The
study of gradients in natural materials may provide direction for
architectural design, fabrication processes, and potential applications
for synthetic FGMs.
Design and Assembly of Complex Composite Parts
Competitive composite parts require three structural elements to
be controlled in a manner that leads to a finished part that possesses
the desired mechanical, thermal, and environmental properties in three
dimensions. These elements are matrix uniformity, fiber orientation,
and fiber-matrix surface interaction. Current fabrication methods are
highly labor intensive, are not amenable to complex shape formation,
and present significant problems in performance assessment. Often,
machining, polishing, etc., is necessary to achieve the required
finished part shape and surface characteristics. In contrast, biological
systems often contain complex and sophisticated fiber-reinforced
composite "parts" (examples range from trees to bones), which exhibit
superb performance over extended lifetimes, are capable of healing
and are produced directly as finished parts from cell-based
manufacturing plants. Examination of these natural fabrication
methods may provide guidance in development of "smart" composites,
net shape processing, multifunctional composites, controlled
orientation, and FGMs.
Representative terms from entire chapter:
smart materials
