Biomedical Engineering Reference
In-Depth Information
nature
[6-8]
. These colors and effects arise from
a delicate interplay of light and periodically
organized architectures with feature sizes of a
few hundreds of nanometers and are in part the
result of
structural
colors
[9]
, discussed in detail
in Chaper 11 by Dushkina and Lakhtakia. For
example, it is the periodic variation of biopoly-
meric compounds embedded into wings and
exoskeletons that lends many butterflies, birds,
beetles, and marine animals their iridescent
appearance (
Figure 14.3
). Mimicking or replicat-
ing the structure in these compounds can result
in entirely new materials with fascinating prop-
erties, as we discuss in detail in Section
14.3
.
Nature's ability to generate structurally com-
plex architectures with feature sizes covering
several length scales under rather simple envi-
ronmental conditions and with limited resources
is still largely unmatched by our synthetic abili-
ties. However, by unraveling the wonders of
nature's design, scientists have developed bio-
mimetic and biotemplated materials with
entirely new capabilities.
Biomimetic materials research draws inspira-
tion from nature to address technological issues
or, more fundamentally, to reveal knowledge
about a biological structure of interest for a spe-
cific application
[10]
. As such, biomimicry relies
on expertise from chemists, biologists, physi-
cists, materials scientists, and engineers to unlock
mechanisms and design principles in nature.
Nature creates fine structures based on the
self-assembly of component materials. For bio-
mimetic materials, self-assembly processes
seen in biological systems are leveraged for the
fabrication of advanced materials. These bot-
tom-up self-assembly routes use soft chemis-
try-based techniques to generate hybrid
materials. Among these methods, the sol-gel
process is a versatile technique to express
organic or even biological species in new mate-
rials derived from precursors.
Today researchers understand how many of
these structures look and behave but, in many
cases, still lack nature's synthetic capabilities in
marrying elegant structures with complex func-
tionality. Unlike biomimetic structures, biotem-
plated materials borrow from nature's blueprints
for our own technological needs, quite often
resulting in new materials with altogether dif-
ferent chemical composition and function
[11-
14]
. In biotemplating, natural systems are used
as scaffolds to combine complex structural char-
acteristics with specific functions. Biotemplating
is achieved by duplicating a specific structure or
by extracting design principles encoded in natu-
ral structures.
Using templates such as organic molecules,
supramolecular aggregates, colloids, nanoparti-
cles, and their assemblies, biotemplated materi-
als can be made with technologically significant
structures at nanometer length scales. Depend-
ing on the template's structural properties and
desired dimensionality, this can be accomplished
by imprinting, casting, molding, infiltration,
coating, and several other techniques. These
materials have ordered pores, reactive sites, and
other attractive features advantageous for appli-
cations in catalysis, drug delivery, photonics,
and molecular electronics.
Regardless of the synthetic technique
employed for bioreplication, there are a few key
requirements: simultaneous replication of large
and small feature sizes; preservation of frame-
work geometry and lattice parameters; and
avoidance of crack formation, structural dam-
age, and loss of long-range features. In general,
inorganic materials with hierarchical structures
based on the biotemplating of plants or animals
are promising for lightweight structural materi-
als, filters, and catalysts, or photonic devices.
14.2 BIOREPLICATION
TE
CHNIQUES AND PROCESS
ES
14.2.1 Some Definitions
In bioreplication, terms such as
hollow and solid,
inverse, true, negative
and
positive replicas
are
typically chosen to describe the structural
