Biomedical Engineering Reference
In-Depth Information
started spreading and forming lamellipodia,
which are actin projections on some motile cells
involved in the process of cell migration. In spite
of being a complicated process, the deposition
of HA by ALD is a promising first step toward
truly biocompatible thin films. Further optimi-
zation and simplification of the process will be
required to make this approach economical.
Further investigations on the biocompatibility
of ALD-deposited coatings involve structured
substrates in addition to the chemical properties
of the film. The wetting properties of surfaces
have a strong influence on the biocompatibility.
The wetting properties can be modified through
the chemical functionalities on the surface but
also strongly depend on the structural features
of the surface [81, 82] .
Porous anodic alumina (AAO) is a material
with nanoscale pores on the surface and is used
commonly by biomedical research groups
worldwide. Besides the wet-chemical modifica-
tion of AAO, porous alumina also has been
coated with Pt, TiO 2 , or ZnO by ALD. The coated
porous alumina was tested for the proliferation
of neonatal human epidermal keratinocytes
[86-88] . Improvement in cell proliferation was
observed when AAO was coated with TiO 2 and
ZnO but not with platinum.
Hyde et al . investigated the functionalization
of cotton fabrics by ALD for the sake of improved
biocompatibility [89] . The cotton fabrics were
coated with thin films of TiN at process tempera-
tures around 150 °C. Because cotton provides
hydroxyl groups, the TiN film gets covalently
bound to the cotton fibers. The TiN coating
showed some oxidation, presumably on the sur-
face, after storage in air for 10-15 days. Therefore,
the films are better described as titanium oxy-
nitride. The biocompatibility of the coated fabrics
was investigated by the adherence of human
adipose-derived adult stem cells (hADSC). The
coating thickness and the wetting properties of
the surface were taken into consideration. The
biocompatibility was good for all coating
thicknesses and the cell adhesion was maximized
on the most hydrophobic surfaces with only
about 2 nm thickness of TiN. It is not clear, how-
ever, to which extent the surface of the coating
consisted of the nitride or the oxy-nitride.
TiO 2 is known to be biocompatible; thus, the
designation of the nitride for biocompatibility
has to be considered with care in this particular
case. Nevertheless, with respect to the unlimited
natural resources for cotton and the easy han-
dling of the material, this approach seems to be
very promising and, upon further optimization,
could indeed lead to the inexpensive production
of biocompatible substrates.
16.3 FUTURE PERSPECTIVES OF
ALD IN BIOMIMETICS
16.3.1 Enzyme Mimetics
Thus far, our discussion has been confined to
various approaches toward structural or func-
tional mimetics involving thin films produced
by ALD. One important aspect of the ALD tech-
nology is the processing of catalytically active
materials. The example of the photocatalytically
active ZnO and TiO 2 coatings applied to colla-
gen was already mentioned. The resulting bio-
inorganic composite was able to generate H 2 O 2
under ultraviolet irradiation and exhibited an
antibactericidal effect [45] .
Similarly, photocatalytically active structures
were produced from pods (legumes) as templates
[90] . The used legume contained microscaled
tubular trichomes, which were coated with Al 2 O 3
and ZnO. After calcination, a ZnAl-mixed metal-
oxide framework (MMO) formed, showing crys-
talline phases of ZnO and ZnAl 2 O 4 . The latter
ternary compound falls under the family of spi-
nels, a group of minerals with cubic crystal struc-
ture and a chemical formulation of AB 2 O 4 with
A and B being metal ions and O being oxygen.
The formation of ZnAl 2 O 4 during calcination is
 
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