Biomedical Engineering Reference
In-Depth Information
Again, the ribbon structures performed better
than TiO
2
powders. The best power conversion
efficiency was obtained with the mixed phase
and amounted to 3.8%. In comparison, the pow-
der-based approach showed an efficiency of
3.6%. The large overall surface of the ribbons,
together with a large crystallite size, the reduced
grain boundaries, and densely packed crystal-
lites, are assumed to be the sources of the
improvement.
ALD was also implemented on collagen-fiber
networks. Although it is not very pure, such a
collagen-fiber network can easily be obtained
after peeling an avian egg. The soft tissue
located between the exterior calcite shell and
the egg white protects the embryo from bacte-
rial invasion while allowing for effective gas
exchange. The main constituent of this mem-
brane is collagen. Such a membrane was coated
by ALD with TiO
2
or ZnO at temperatures rang-
ing from 70 °C to 300 °C. The main objective was
to figure out whether or not the ALD process
will affect the structural morphology
[45]
. The
higher deposition temperatures lead to
denaturation, but it was found that the initial
deposition of a protective layer at lower
temperatures results in stabilization of the
structure for subsequent deposition at higher
temperatures.
Investigations of the crystallinity of the ZnO
coating confirmed that crystalline features of a
wurtzite type occur at a processing temperature
as low as 70 °C. This is an important observa-
tion because, although polycrystalline or even
nanocrystalline, the ZnO coating may readily
exhibit photocatalytic effects. In contrast, TiO
2
coatings only show reasonable indications of
anatase at processing temperatures above
160 °C.
The photocatalytic efficiency of the coating
was deduced from the bactericidal effect the
membrane exhibits upon illumination with ultra-
violet light. The membranes were built into cells
containing
E. coli
bacteria. Samples of the bacte-
ria-containing solution were taken at diverse
instants of time and cultured to count the popula-
tions. As expected from the crystallinity data, the
membranes coated with ZnO at 100 °C already
have a good photocatalytic efficiency, whereas
TiO
2
-coated membranes required processing
temperatures exceeding 160 °C for a similar effect.
Thus, ZnO is apparently a reasonable alternative
for TiO
2
, particularly if the substrate is thermally
sensitive.
The mechanical properties of the ALD-treated
collagen-based membranes changed. Tensile tests
showed a simultaneous increase in strength and
ductility of the membranes, thus increasing the
toughness threefold
[46]
. Such a behavior is
remarkable and unusual in physics, as discussed
in Section
16.3.2
.
A further class of fibers used as substrates
for ALD processing consists of cellulose fibers.
Initial work on cellulose fibers derived from
paper was performed by coating with TiO
2
or
bilayers of Ir/Al
2
O
3
or Ir/TiO
2
, respectively
[47, 48]
. Cellulose shows much higher resist-
ance to thermal treatment than most protein-
based materials, thereby enabling higher
processing temperatures. The metal oxides
were deposited at 150 °C or 250 °C and iridium
at 250 °C, below the decomposition tempera-
ture of the cellulose. The TiO
2
coatings con-
sisted of crystalline anatase and were
photocatalytically active. Additional Ir coating
even improved the photocatalytic activity.
A protective effect, induced by the metal-
oxide coating, was observed. The byproduct
during the Ir deposition process is atomic
oxygen, which is expected to decompose the cel-
lulose. But the metal oxide prevents the decom-
position, possibly due to the chemical reactivity
of the metal oxide with the atomic oxygen.
The group of Persons performed a more
directed approach toward functionalization of
cellulose-based fibers. Their target was cotton
fibers, which, after coating with thin inorganic
films by ALD should result in fabrics with
enhanced wear resistance. They initially depos-
ited a 50-nm-thick coating of Al
2
O
3
at 100 °C
[49]
.
