Biomedical Engineering Reference
In-Depth Information
Subsequent work focused on the wetting behav-
ior of the fabrics as a function of the thickness
of the ALD coating
[50]
. An interesting observa-
tion was that a single ALD cycle of TMA/water
abruptly switched the wetting behavior of cot-
ton from wetting to nonwetting. Processing with
further cycles caused a reverse switch.
The effect can be explained as follows: Native
cotton fibers contain nanoscale fibrils on the sur-
face. Because of the initial nucleation of the inor-
ganic film upon processing, the first ALD cycles
might increase the surface roughening. Further
cycling will merge nucleation clusters and even-
tually result in a continuous film, which will
become smoother with an increasing cycle num-
ber. The initial surface roughening might be the
reason for the rather hydrophobic surface, and
the smoothing during the continuation of the
process might be the reason for the surface
becoming hydrophilic again.
The
S layer
is a two-dimensional biological
template frequently used in nanoscience or
materials science
[51]
. It is a molecular sheet
with regularly arranged pores that self-
assembles from proteins of cell envelopes of
certain bacteria, if assembly conditions are
properly chosen. As in many other approaches
besides ALD, the goal here was also to use the
S layer as a mask for the synthesis of nanodots.
The difficulty, however, is that ALD is a non-
line-of-sight deposition method but relies on
the surface functionality of the substrate. More-
over, the S layer consists of proteins with a
plethora of functional groups, and thus it was
expected that the coating would not be selec-
tive to the pores. To avoid deposition on the
protein sheet, the functional sites were passi-
vated with octadecyltrichlorosilane (ODTS)
and the subsequent deposition of HfO
2
occurred
exclusively on the underlying Si wafer through
the tiny pores
[52]
. After the process, the S layer
was removed by thermal treatment at 600 °C in
air. The resulting nanodots on the Si wafer sur-
face had diameters of about 9 nm and a very
regular distribution.
Many biological materials are nanostructures
with precise sizes, structures, and content.
Fer-
ritin
is among biological nanoparticles of great
interest to materials scientists. Ferritin is a glob-
ular protein assembled of 24 subunits, with a
diameter of 12 nm, and it contains a cavity with
a diameter of approximately 7 nm. In nature,
ferritin acts as a storage container and transport
vehicle for iron, which is stored within the fer-
ritin cavity as ferrihydrite. The iron-containing
core can be removed from the protein capsule
through small, 3-4 Å channels, which are located
at the boundaries of the protein subunits. The
hollow ferritin remaining is named
apoferritin
.
ALD deposition of Al
2
O
3
or TiO
2
on spread
layers of ferritin resulted in freestanding metal-
oxide films with embedded ferritin
[29, 36]
. Given
the fact that numerous materials were wet-
chemically synthesized within the apoferritin
cavity, ALD promises a good approach to thin
films with embedded luminescent, magnetic, or
plasmonic nanoparticles. The importance of this
work lies in the fact that the ferritin proteins
appear to be robust enough to withstand the
process conditions, which are considered rather
harsh for biomaterials. A follow-up work showed
that indeed apoferritin is not destroyed during
the process
[53]
.
The channels perforating the protein shell
deposit TiO
2
within the hollow cavity. Control of
the deposition toward either the outer surface of
the apoferritin or the inside of the cavity was
made possible by varying the pretreatment con-
ditions of the apoferritin (
Figure 16.5
). The
amount of water bound to the surface of the
above-mentioned 3-4 Å channels was of crucial
importance for the precursors to enter the cavity.
If present, the water hydrolyzes the precursor
already at the entrance of the channel, and fur-
ther diffusion of precursor molecules is hin-
dered due to clogging. Dehydrating the channels
leads to an increased lifetime of the precursors
and enhanced possibility to enter the cavity.
The described clogging can easily happen
if the channel diameters are sufficiently tiny.
