Biomedical Engineering Reference
In-Depth Information
limited thermal stability, primarily polymers and
biomaterials, was not considered to be possible
until the processing temperatures of some ALD
processes were pushed down to ranges that can
be tolerated by such sensitive substrates.
The initiating work for this novel application
field of ALD was the publication from the group
of George
[13]
, describing the coating of a poly-
mer bottle with Al
2
O
3
at temperatures as low as
33 °C. From the thermal point of view, the pro-
cess showed great promise for application to
biological substrates and biomaterials. Of course,
several other factors play a role, such as the
required vacuum, which could easily lead to
destruction of a biological substrate by dehydra-
tion, but also the unknown reactivity of the pre-
cursor with the biomaterial, which in the worst
case would be destructive to proteins.
The first experiments applying ALD to bioma-
terials were based on trial-and-error approaches
but performed surprisingly well. The remainder
of this section summarizes the work of the past
years, in which ALD was applied to structural
and/or functional biomimicry, biocompatibility,
and biomineralization.
its intrinsic physical properties but merely to the
chemical functionality the molecule offers. The
CNT itself cannot be easily coated by ALD, since
it lacks functional anchor groups for chemisorp-
tion of the precursor. If deposited, the films usu-
ally do not adhere very well to the CNTs and
show enhanced surface roughness due to the
initial island growth on defect sites
[37]
. To ena-
ble a coating of the CNT, Lu
et al
. wrapped the
CNT with DNA and the adhesion was provided
via stacking of
π
-electrons
[18]
. The chemical
functionalities of the DNA subsequently acted
as anchor groups for the ALD process. Those
groups enabled a uniform coating of the CNT
with the high dielectric-constant material HfO
2
,
which, without the wrapped DNA molecules, is
not easily possible (
Figure 16.4
). However,
approaches using DNA as a template for ALD
coating do not truly relate to structural mimicry,
because the resulting material does not reflect
the original DNA structure.
Some peptides in certain experimental condi-
tions form nanostructure networks through
self-assembly. With diphenylalanine, even
millimeter-long fibers can be obtained by elec-
trospinning
[38]
. Organogel formation will
result in ribbons with lengths of hundreds of
micrometers, widths of some hundreds of
nanometers, and thicknesses of some tens of
nanometers. Removal of the solvent (e.g., chlo-
roform) will yield xerogels.
Kim and coworkers made use of such ribbons,
which they obtained after gelation of diphenyla-
lanine, as templates for a coating with TiO
2
[39-
41]
. The TiO
2
coating was deposited at 140 °C
and the ribbons were subsequently calcinated at
temperatures exceeding 300 °C. Thermal degra-
dation of the peptides and recrystallization of the
resulting hollow TiO
2
coat as anatase occurred in
a way similar to that shown earlier with electro-
spun polymer fibers as templates
[42]
. The choice
of TiO
2
for this approach was based on two
important aspects: (1) TiO
2
is one of those
materials that can be easily handled by ALD and
also deposited at low temperatures, and (2)
16.2.1 Structural Mimicry
The use of ALD for mimicking the structural prop-
erties of biological substrates is the most common
route during the past several years. A very com-
mon case of a biological nanostructure is DNA,
which resembles a one-dimensional fiber. DNA is
considered a promising template for the synthe-
sis of metallic nanowires
[35]
, but technological
applications with DNA-based nanowires are still
lacking. Due to the great stability of DNA, a coat-
ing of the molecule by ALD appeared feasible.
DNA has been used as a template for metal-oxide
deposition, either for curiosity
[36]
or as means of
functionalization of a substrate
[18]
.
The latter approach may be used to produce
a carbon nanotube (CNT)-based transistor. The
function of the DNA in this case is not related to
