Biomedical Engineering Reference
In-Depth Information
substrates can be achieved, even in a patterned
way
[66, 67]
. Gerasopoulos
et al
. combined those
strategies to produce TMV patterns, which were
metallized and subsequently coated by ALD
[68]
. The metallization enhanced the stability of
the viruses for the subsequent deposition of TiO
2
or Al
2
O
3
so that the processing temperature
could be increased to 220 °C for Al
2
O
3
and 150 °C
for TiO
2
. The resulting patterns are attractive for
applications in catalysis, dye-sensitized solar
cells (DSSC), etc.
We stated earlier that the ALD deposition is
not directed (non-line-of-sight) but that all avail-
able surfaces are coated because of the chemical
saturation mechanism. Nevertheless, there are
limitations for the uniformity of coatings, which
are mainly related to the diffusivity of the pre-
cursor molecules. Long, narrow channels require
a seriously longer diffusion time for the precur-
sor to traverse. The diffusion time increases as
the diameter of the channels decreases with each
ALD cycle. As excess precursor is applied to the
substrate and the diffusion of the excess precur-
sor out of the channels needs to be considered,
the purging time is affected as well; it can easily
increase the whole processing time by several
orders of magnitude.
A very recent approach to apply ALD to bio-
materials with extreme aspect ratios made use
of bristles of a sea mouse, a species in the genus
Polychaeta
[69]
. The bristles exhibit even, centim-
eter-long, parallel, hollow channels with diam-
eters of around 200 nm. The structures served as
templates for Al
2
O
3
deposition with the goal to
synthesize high-aspect-ratio nanotubes. The
wall thickness of the resulting nanotubes was
around 20 nm, but the total length of the nano-
tubes could not be determined, since it was dif-
ficult to completely release the nanotubes from
the template. It remains for future work to
enhance the efficiency of ALD for coating such
extremely high-aspect-ratio structures.
Compared to all the aforementioned biomate-
rials, biominerals are much easier to process by
ALD because they consist of inorganic materials.
Among the most prominent and beautiful
structures are the exoskeletons of diatoms
[70]
.
Many exoskeletons show perfect arrangements
of pores and protrusions. A coating by ALD is
easily possible for such templates, even at higher
temperatures and with aggressive precursors
[71]
. After ALD processing, the pore size of the
frustule valves shrinks from 40 nm to about 5 nm,
with applications of such pore arrangements in
molecular separation. A more demanding
approach involved an initial modification of the
biosilica with germanium and the formation of
a photonic crystal slab
[72]
. The results appear
promising for future investigation and optimiza-
tion, particularly with respect to emission color
and brightness.
16.2.2 Functional Mimicry
16.2.2.1 Optical Properties
Many natural materials show fascinating opti-
cal effects, whether for attracting insects (e.g.,
to flowers) or warning off enemies. In the case
of flowers, one can observe a plethora of colors
during the spring blooming period. The phys-
ics behind the coloration is more complex than
usually considered by the observer. The human
eye can recognize coloration in the visible spec-
tral regime but is not aware of optical absorption
or reflection in other spectral regimes. Some of
those attract certain insects.
The optical properties of a red rose petal come
from two coloration effects: the chemicals or
pigments and the structural colors. Whereas the
chemical coloration influences the optical appear-
ance of the petal in the visible spectral regime,
the structural colors are primarily based on the
nano- and microstructure of the petal surface and
are seen in the ultraviolet regime. Structural
colors are discussed in detail in Chapter 11 by
Dushkina and Lakhtakia. By twofold inversion
of the rose petal with a Ni mold and a polymer,
the structure of the rose petal was copied from a
polymer, but without the pigmentation
[73]
.
