Solution-Based Techniques for Biomimetics and Bioreplication - Engineered Biomimicry - page 371

FIGURE 14.11 Schematic of the double-templating bioreplication route. The diamond-based photonic crystal structure of
the weevil L. augustus (top) is converted into a titania positive replica via an intermediary silica negative replica. Gray: air;
green: dielectric (silica or titania). (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this topic.)

intermediary, negative replica out of silica,
which then serves as a sacrificial template in a
second replication step ( Figure 14.11 ). Galusha
et al. showed the highest-fidelity negative replica
structures are obtained by using a hybrid
organic-inorganic silica sol for the infiltration
process [26, 27] . The added polymer component
lends higher flexibility to the framework, and
thus reduces cracking during the drying stage.
Following solidification of the silica-based
framework, this biological template was removed
by etching with a mixture of concentrated per-
chloric and nitric acids. This method was chosen
over pyrolytic removal of the template to keep
shrinkage of the framework to a minimum, with
around 5% (acid treatment) compared to 30%
(heat treatment).
The silica-based inverse replica was then
used as the new (intermediary) template and

was infiltrated with a liquid titania sol-gel pre-
cursor. Titania is the compound of choice for
photonic crystals operating in the visible spec-
trum since it combines a high refractive index of
2.1-2.8, depending on the type and degree of
crystallinity, with excellent optical transparency
(thereby satisfying the second requirement for a
refractive index greater than 2.3 for the high-
dielectric component).
Infiltration of the precursor solution was
followed by heating the silica/titania composite
to 500 °C to induce nanocrystallization of titania.
This infiltration/heating cycle can be repeated to
adjust the final filling fraction of titania (achieving
the third requirement of a filling fraction between
30% and 40%) and creating a dense framework.
In the final processing step, the intermediary
silica-based template was selectively removed
by hydrofluoric-acid etching, leaving behind a