Biomedical Engineering Reference
In-Depth Information
different material. In
solution-based
(bio)templat-
ing (the focus of this chapter),
liquid
precursors
are used to coat the surface of a template or infil-
trate the void space surrounding a template struc-
ture (
Figure 14.4
). After solidification of the
precursor to form a robust material, the surround-
ing template is selectively removed, leaving
behind a stable, self-supporting structure contain-
ing all architectural features of the original tem-
plate. The final structure is called the
(bio)replica.
Hollow vs. solid replicas
. A hollow replica is a
shell-like copy of the original structure and is
fabricated by depositing a thin coating around
the biotemplate and subsequently removing this
biotemplate (see
Figure 14.4
b). A solid replica is
a positive or negative copy of the original tem-
plate, composed of a completely filled frame-
work. Solid replicas are fabricated by completely
backfilling the free space around a template with
a new compound and removing the original
structure (see
Figures 14.4
c and 14.4d).
Negative vs. positive replicas
. A negative rep-
lica is an inverse copy of the original biotem-
plate; i.e., the original structure is inscribed into
the new compound as a framework made of air
(see
Figure 14.4
c). A positive replica is an exact
(or true) copy of the original biotemplate (see
Figure 14.4
d). Apart from being made from dif-
ferent materials, the replica and original frame-
works are identical. A positive replica can be
fabricated simply by repeatedly forming a neg-
ative replica. A negative of the negative rep-
lica—the positive replica—is created by making
a negative replica of the original template and
using this structure as the new template.
FIGURE 14.4
A given template structure (a) can yield
three different types of replicas (b-d) through various infil-
tration and template-removal routes.
architectural features into a desirable material.
Depending on its structural properties, this can
be accomplished by imprinting, casting, molding,
infiltration, coating, and several other techniques.
Molding and imprinting are fast and power-
ful ways to transfer two-dimensional surface
features into polymeric and ceramic replicas
[15-18]
. For example, using this approach, it is
possible to replicate the intricate surface struc-
tures of lotus leaves and other plants
[15, 16]
,
insect eyes
[17, 18]
, and cicada wings
[19]
.
Importantly, the replicated samples not only
retain the surface structures of these templates,
but they display desirable properties such as
superhydrophobicity and antireflectivity.
Transferring more complex three-dimensional
frameworks—such as those found in butterfly
wings, bird feathers, and wood samples—
requires infiltration and coating techniques
[10-
12]
. For the latter, the empty space inside a given
biological structure is infiltrated with a precur-
sor species, and chemical and/or physical pro-
cesses are used to convert this precursor into a
14.2.2 General Infiltration and
Templating Methods
An important route to new materials with unprec-
edented properties involves converting unique
biological structures into positive and negative
copies, or replicas
[10-14]
. The general strategy
for achieving such new materials is to use the
biological structure as a template and transfer its
