Page 380 - Biomimetics : Biologically Inspired Technologies

Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c014 Final Proof page 366 6.9.2005 12:41pm

366 Biomimetics: Biologically Inspired Technologies

functional webs to entrap prey; (b) organic–inorganic composite structures found in sea shells to
form highly engineered, hard, tough materials; (c) surfaces such as shark skin to reduce hydro-
dynamic friction; (d) modes of ‘‘sticking’’ to surfaces used by the gecko to produce strong
adhesives; and (e) muscles in the human body to create highly engineered actuators. These
examples, optimized through evolution, provide a range of topics for inspiration in materials
designs and functions. They also provide generic insight into the underlying principles employed
by biological systems to achieve remarkable plasticity in materials structure and function.
Each of the topics listed above is reviewed with a focus on what is currently understood in
terms of structure and function, a mechanistic view of the system, and the current state of the art
in mimicking these systems. It will be obvious at the end of this chapter that the learning curve is
barely past the lag phase (microbial growth curve perspective). It is also worth considering that
we are inherently limited in gaining additional insight into these systems due to the complexity of
the biological systems of interest, our current limited understanding of their structure and
function, and our preconceived bias of how to understand these systems due to training or
perspective from more traditional materials science and engineering approaches. The excitement
with Nature as a guide to materials science and engineering is that this is only the beginning and
there is a lot to be learned in order to elucidate the ‘‘rules’’ that govern the processes involved.

14.2 COMPARISONS: BIOLOGICAL MATERIALS
AND SYNTHETIC MATERIALS: SYNTHESIS AND ASSEMBLY

At the core of this chapter are the novel ‘‘rules’’ that govern materials formation in Nature. These
rules originate from the template-based synthesis driven by genetic blueprints. Furthermore, the
building blocks (e.g., amino acids, sugars, nucleic acids) are linked (via enzymatic coupling
reactions) into polymers with control of stereochemistry to affect regularity in chemistry and thus
higher order interactions (intra and interchain). These polymeric building blocks (proteins, poly-
saccharides, nucleic acids, and other biological macromolecules) are therefore ‘‘programmed’’
(chemically and physically) to self-organize into more complex materials through hierarchical
structural complexity that gives rise to novel materials performance. The control of this structural
hierarchy initiates with the regularity in structure at the individual monomer and chain levels, and is
propagated up length scales from the molecular (chains), through the mesoscopic (mesophases), and
finally to the macroscopic (material ultrastructure) level. Remarkably, these processes occur within a
complex mixture of small and large molecules inside and outside of the sites of synthesis (cells).
Compartmentalization helps in these processes, along with membrane interfaces. Most of the
details involved in these processes are largely unexplored territory scientifically. The entire materials
assembly process is governed by the interplay between genetic programs, environmental conditions
inside and outside of the cells, and the remarkable specificity and control achieved through enzym-
atic processes. Historically, these hierarchical interactions have been studied from the ‘‘top-down’’
or at the macro-scale, using electron microscopy to interrogate ultrastructure, or by testing mechan-
ical properties of the materials and using this to interpret structural organization. In recent years, the
focus of inquiry has shifted to the ‘‘bottom-up’’ paradigm, molecular-level interactions.
Polymer assembly as the basis for structural hierarchy and function in biology is most often
governed by many weak bonds (hydrogen bonding, van der Waal). It is the high frequency and
location of these types of bonds that allow assembly or disassembly of these material systems
within reasonable energy demands to permit functions (e.g., such as denaturation and renaturation
(replication fork) of DNA during semiconservative replication). These processes are mediated by
water, structure, and location, with respect to the organic components and features such as
hydrophobic hydration play a major role in the processes. General themes to consider that contrast
the process of materials formation and assembly in Nature vs. in the laboratory via synthetic
approaches are listed in Table 14.1.