Biomedical Engineering Reference
In-Depth Information
been successfully applied to opal templates
[37,
38]
. An advantage of this method is that metal
deposition starts at the conducting surface and,
as deposition time continues, gradually and uni-
laterally fills the template void space. By con-
trolling deposition time and the current applied
during the deposition process, it is thus possible
to precisely control the degree of metal infiltra-
tion into the template.
An alternative to creating inorganic replicas is
template infiltration with organic monomers or
prepolymer solutions. In the presence of polym-
erization initiator compounds, the precursor spe-
cies is then cross-linked by exposure to ultraviolet
(UV) light or mild heating, yielding a negative
polymeric replica of the original structure. Typical
polymers used with this technique are elasto-
meric perfluoropolyether (PFPE), polydimethyl-
siloxane (PDMS), polymethyl methacrylate
(PMMA), and epoxy resins. This technique has
been extensively applied in replica molding, a
soft-imprint lithographic method used to repli-
cate two-dimensional structures such as, the sur-
face relief of leaves, wings, and eye
[15-17]
.
An extension of polymer templating to three-
dimensional structures has been successfully
demonstrated by replicating opaline structures.
For this method, opal templates were infiltrated
with UV-curable polyurethane or poly(acrylate—
methacrylate) copolymer precursors
[39]
. After
polymerization, the template was selectively
removed by either etching (hydrofluoric acid for
silica templates) or solvent extraction (toluene for
polystyrene templates). Since mild processing
temperatures and no or minimal amounts of sol-
vent were required, replicas of high quality (with
little framework shrinkage and damage) could be
obtained. Furthermore, this method opens the
door to fabricating bioreplica samples out of
functional yet inexpensive organic compounds.
precursor species into a solid material via a “sol”
(colloidal particle suspension) to “gel” (cross-
linked particles) pathway
[40, 41]
. Most sol-gel
reactions start with molecular precursors, which
first undergo a hydrolysis step followed by
condensation of these molecular species into
polymeric networks.
The most commonly used molecular precur-
sors are metal alkoxides (M-OR;
M
is a metal atom
and
R
is an organic group such as methyl, ethyl,
isopropyl, etc.). These compounds readily react
with water in a nucleophilic substitution reaction
to form metal hydroxides as intermediary species
in the first step of the sol-gel process; thus:
M−
(
OR
)
X
+
X
· H
2
O → M−
(
OH
)
X
+
X
· ROH.
(14.1)
The index
x
denotes the available alkoxide
moieties for a given metal atom; for typical sol-
gel compounds such as silica, alumina, titania,
and zirconia,
x
has the following values:
x
=
3
(aluminum),
x
=
4 (silicon, titanium, zirconium).
For example, for the most commonly used sol-
gel compound, silica (or silicon dioxide), and for
a typical starting compound such as silicon
tetraethoxide (also referred to as tetraethyl-
orthosilicate, TEOS), reaction
(14.1)
becomes
SI
(
OCH
2
CH
3
)
4
+ 4H
2
O → SI
(
OH
)
4
+ 4CH
2
CH
3
OH,
(14.2)
with orthosilicic acid and ethanol formed as
intermediary products.
These formed metal hydroxides then undergo
a condensation reaction and combine either
with other hydroxides moieties (oxolation, reac-
tion
14.3
) or with unhydrolized alkoxides moie-
ties (alcoxolation, reaction
14.4
). In both cases,
dimeric species are formed with two silicon
atoms connected by an oxo-bridge.
M−
(
OH
)
X
+M−
(
OH
)
X
→
(
HO
)
X−1
−M− O −M−
(
OH
)
X−1
+ H
2
O,
(14.3)
14.2.4 Sol-Gel Chemistry
The term
sol-gel chemistry
encompasses all
processes involved in the transformation of
M−
(
OH
)
X
+M−
(
OR
)
X
→
(
HO
)
X−1
−M− O −M−
(
OR
)
X−1
+ ROH.
(14.4)
