Solution-Based Techniques for Biomimetics and Bioreplication - Engineered Biomimicry - page 359

solid compound surrounding the biological tem-
plate. In the final step, the biological template is
removed by etching or pyrolysis, leaving behind
a material with the same structural features as
the biological template but composed of an
entirely different material.
In general, infiltration-and-coating-based
biotemplating processes can be divided into
low-temperature deposition/evaporation meth-
ods and solution-based methods [11] . Among
the former, atomic layer deposition [20-22] and
the physical vapor deposition method called con-
formal evaporated film by rotation [23-25] have
proven particularly suitable for bioreplication.
The common feature of these methods is the use
of vapors or gaseous precursors in the infiltra-
tion step. These precursors then transform into a
solid coating on the biotemplate surface by a
step-wise atom-by-atom growth mechanism. Dep-
osition/evaporation-based infiltration methods
are thus mainly used to produce shell-like rep-
lica structures (after removal of the biotemplate).
Atomic layer deposition is discussed in detail in
Chapter 16 by Zhang and Knez; evaporation
methods are presented in Chapter 15 by Martín-
Palma and Lakhtakia.
In contrast, solution-based infiltration meth-
ods use liquid precursors and generally form
solid negative copies of the original biological
template. In these methods, liquid precursor
solutions are infiltrated into the template void
space by capillary forces. After solvent evapora-
tion, the precursor species transform into a solid
network through various chemical pathways,
depending on the type of precursor and reaction
conditions. The most widely applied solution-
based bioreplication method is sol-gel chemis-
try using molecular [26-30] and colloidal
nanoparticle [10, 31] precursors.
Apart from producing bioreplicas with
different structural features (shell-like vs. solid
frameworks), solution and deposition/
evaporation-based infiltration methods also differ
greatly in terms of instrumentation, processing
conditions, and accessibility. For example, sol-gel

chemistry methods require only a few inexpensive
precursors and solvents, a chemical lab bench,
and a furnace. Atomic layer deposition or
conformal evaporated film-by-rotation methods,
in contrast, require expensive instrumentation,
gas, and vacuum lines and highly trained users.
On the other hand, the precision and reproducibility
of bioreplica samples obtained by evaporation/
deposition-based methods are exceptional and
represent a big advantage over most solution-
based techniques.
For example, using atomic layer deposition,
the degree of infiltration and thus the thickness
of the inorganic replica shell can be controlled
on an atomic-layer scale. This predictable
control of shell thickness can be used to create
replicas with precisely tuned optical properties.
This outcome is depicted in Figure 14.5 for the
photonic structure of butterfly-wing scales
replicated into aluminum oxide. By carefully
controlling the aluminum oxide shell thickness,
for example, Wang and co-workers tuned the
reflection color of the replica samples from
green to yellow, pink, and purple (note that
the original color of the butterfly-wing was
blue) [20] .
Regardless of the method used, there are a
few key requirements that must be fulfilled for
a system to be suitable for bioreplication: (1)
simultaneous replication of large and small fea-
ture sizes; (2) preservation of framework geom-
etry and lattice parameters; and (3) avoidance of
crack formation, structural damage, and loss of
long-range features.
The first requirement—simultaneous replica-
tion of features both large and small—is particu-
larly important in bioreplication since an
attractive aspect of many biological structures is
their hierarchical structure, with feature sizes
often spanning several orders of magnitude. In
particular, in solution-based methods, the pres-
ervation of fine features is often challenging,
since surface tension during solvent evapora-
tion can easily smooth out nanoscale structures.
Additionally, fine features can also be lost during