Biomedical Engineering Reference
In-Depth Information
Such biotemplate-specific features require
additional pre-infiltration steps, such as remov-
ing the component of interest from the organism
by cracking, polishing, or cutting to provide
access to enclosed structural frameworks. In
addition, many biological structures have thin
protective layers on their surface. Very often,
these layers are composed of waxy, hydrophobic
molecules to prevent water from wetting the
surface and/or entering the inner void space of
these structures. Since solution-based biotem-
plating often uses hydrophilic solvents (e.g.,
water or alcohols) and infiltration of the void
space is based on capillary forces, these protec-
tive layers need to be removed through treat-
ment with organic solvents or acids.
Once these preprocessing steps have been
completed and the desired biological structure
can be accessed, a given biotemplate is ready for
infiltration with a precursor-containing solution.
In principle, any of the techniques used to repli-
cate synthetic templates can also be used for bio-
logical structures. These methods include colloidal
nanoparticle sols, molten or supersaturated salt
casting, electrochemistry, polymerization tech-
niques of organic monomer species, and molecu-
lar sol-gel chemistry. The following section
provides brief descriptions of these methods with
a more detailed discussion of molecular sol-gel
chemistry—the most widely used solution-based
bioreplication technique.
Typical colloidal nanoparticle sols are aque-
ous or alcoholic solutions/suspensions of oxide
nanoparticles with sizes of a few nanometers to
tens of nanometers. An important parameter
for this templating technique is concentration
of the nanoparticle sol. On one hand, this con-
centration should be as high as possible to
ensure a dense, solid framework after solvent
evaporation: The lower the nanoparticle con-
centration, the higher the nanoporosity. On the
other hand, high nanoparticle concentration
results in enhanced particle-particle interac-
tions, leading to an increase in the solution vis-
cosity. Since capillary forces drive template
optoelectronics, catalysis, separation, and sorp-
tion, or as scaffolds in battery electrodes, energy
absorption, and tissue engineering. In addition,
solution-based methods extend the variety of
accessible structures from hollow to solid repli-
cas of the original template with a negative or
positive (true replica) framework.
The process of solution-based templating has
been known for thousands of years and has been
applied to replicate a multitude of synthetic and
natural structures with features ranging from
nanometers to several meters. For example, solu-
tion-based templating can be used to replicate
the nanometer-sized three-dimensional pore net-
work of colloidal crystals into polymers and
semiconductors. At much longer length scales,
these same templating principles have been used
for millennia to cast meter-sized statues, figures,
and bells from metals and precious alloys.
These very same techniques are used in solu-
tion-based bioreplication but instead rely on
biological structures such as wood, bones, insect
scales, feathers, and marine animals as tem-
plates. The motivation for using biological tem-
plates lies in the enormous multitude of complex
structures found in nature
[6, 32-34]
. Some of
these structures, such as the intricate hierarchi-
cal architectures of diatoms and marine sponges
[32]
or the three-dimensional photonic crystal
lattices of certain butterfly wings and weevil
scales
[6, 26]
, are still beyond our synthetic capa-
bilities. Therefore, these complex materials are
intriguing additions to our existing synthetic
structure portfolio.
A drawback of using biotemplates is that they
are not (in most cases) freestanding, unlike syn-
thetically generated frameworks that are easily
accessible. Biological structures designed to ful-
fill given functions within an organism are inte-
grated into larger systems (feathers, wings, hair,
skin, bones, exoskeleton, etc.). In these cases, the
structural features of interest for replication are
often buried and hidden, or embedded into a
structure-less and sometimes impermeable
cover matrix.
