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OCR for page 13
1
INTRODUCTION
Biological materials, such as tendon, bone, wood, and many
others, are characterized by hierarchical architectural designs in which
organization is controlled with striking precision on many discrete
length scales, which range from the molecular to the macroscopic.
These hierarchically structured materials display properties that are
affected by processes operating at all levels of the length-scale
spectrum. Most such materials are multifunctional and are produced
in situ at room temperature and atmospheric pressure through
ecologically balanced processes. Many are self-healing and thus
remarkably durable even under high cyclic loading (e.g., a human
knee joint), and many display properties that change, either abruptly
or gradually, in response to a changing environment. Equipped to
prepare only limited sets of constituent materials, organisms have
evolved an astonishing array of architectural strategies to realize a
broad range of structure and function.
Virtually all biological materials are bounded systems that are
synthesized and processed by cells at the nanoscale. Nature makes
very different systems out of similar macromolecular and inorganic
constituents through the process of differentiation during
development. All of these materials systems have specific hierarchical
composite structures. Starting with very similar macromolecular
designs, each system (e.g., tendon, intestine, cornea, bone, etc.) is
assembled to serve distinct, highly specific functions.
These natural systems follow three rules for complex assemblies
(Beer et al., 1992~. First, the structure is organized in discrete levels
13
OCR for page 14
14
Hierarchical Sauctures in Biology as a Guide for New Aiatenals Technology
or scales. Virtually all biocomposite systems are found to have at least
one distinct structural level at each of the molecular, nanoscopic,
microscopic, and macroscopic scales.
Second, the levels of structural organization are held together by
specific interactions between components. Considerable evidence
indicates that strong surface-to-surface interactions occur, which are
caused by intermolecular covalent bonds at specific active sites or by
strong van der Waals forces. Whatever the nature of the bonding
between levels, adequate adhesion is required for system structural
integrity.
Finally, these highly interacting levels are organized into an
oriented hierarchical composite system that is designed to meet a
complex spectrum of functional requirements. Furthermore, as
composite systems increase in complexity, they can function at higher
levels of performance. The so-called intelligent materials and
adaptive composite systems result from this type of complex
architectural arrangement. A hierarchical biocomposite is more than
just a material out of which larger objects can be built; it is a complete
structural system in itself.
Synthetic materials must be described in different terms. Free
of the constraints imposed by biosynthetic pathways, materials
scientists and engineers have created entirely new classes of metals,
ceramics, polymers, and electronic materials with extraordinary
properties. Nevertheless, many desirable features of biological
materials have not yet been attained in synthetic systems. For
example, synthetic materials may perform well with respect to a single
figure of merit (e.g., strength) but fail to meet a more complex set of
performance requirements that may include permeability, optical
clarity, or frictional properties. Many synthetic materials must be
processed at elevated temperatures and pressures or with the aid of
environmentally burdensome organic solvents. In addition, self-
healing materials and materials capable of controlled environmental
response remain elusive.
This situation has prompted growing interest within the
materials community in the lessons that might be gleaned from a
careful study of biological structures and of the processes by which
OCR for page 15
Introduction'
15
they are made. At the same time, biologists have begun to bridge the
materials-biology gap through the application of increasingly powerful
engineering analyses of natural structures and through the direct use
of organisms to make interesting new materials or their building
blocks.
This report focuses on a ubiquitous feature of biological
materials systems their hierarchical architectural design. What are
the advantages of biological materials that are organized on many
different length scales? What mechanical properties emerge from such
designs? How does the architecture relate to the fabrication of the
structure by the organism? Can synthetic materials systems be made
this way? If so, what advantages might be realized?
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 examine these issues and to review techniques related to
preparing hierarchical structures that possess useful and unusual
physical properties and to assess the opportunities for these structures
in civilian and military applications. Although a broad range of
functions are represented in biological systems, the committee
concentrated on structural material systems and their properties.
The purpose of this study was to conduct case-studies by
selecting natural material systems to be used as models for synthetic
efforts; characterize properties, unusual characteristics, and potential
end-use applications for these synthetic systems; review state-of-the-
art synthetic techniques and processes for assembling synthetic
hierarchical structures; 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.
OCR for page 16
Representative terms from entire chapter:
hierarchical structures
