Biomedical Engineering Reference
In-Depth Information
eyes limits the practical applicability, since
competing strategies are currently more cost
effective
[79]
.
For those experiments, ALD plays an impor-
tant role because the distortion of the structural
properties of the substrate should be reduced to
a minimum to discriminate the impact of the
surface chemistry and the structure on the
water-repelling effect. In the cases of both
the butterfly wings and the water-strider legs,
the thin Al
2
O
3
film seriously modified the sur-
face chemistry, whereas the structure was only
barely changed. The outcome shows that, with
such experiments, one can quite easily deter-
mine the contributions of chemistry and struc-
ture to wetting phenomena of a surface.
16.2.2.2 Wetting Behavior
One of the most important properties of
structured surfaces in nature is the wetting
behavior. The very delicate structure of butterfly
wings shows water-repelling properties, which
is important for the survival of the insect.
A remarkable experiment showed the use
of amorphous Al
2
O
3
, which is intrinsically
hydrophilic, for replication of hydrophobic
structures—namely, butterfly wings and water-
strider legs. The wetting properties of the
coated wings were subsequently compared to
coated water-strider legs
[80]
. In spite of bulk
Al
2
O
3
being hydrophilic, the Al
2
O
3
replicas
were hydrophobic.
The hydrophobic nature of structured sur-
faces is characterized by two model states, the
Cassie state
[81]
and the Wenzel state
[82]
,
which differ in their contact-angle hysteresis
[83]
. For antiwetting behavior, the Cassie state
is favored. Both the butterfly wing and the
water-strider leg show a Cassie-like behavior
prior to ALD coating. The thin film of alumina
on top of the structures induces some changes.
Whereas the coated water-strider leg persists
in the Cassie state, the coated butterfly wing
changes to the Wenzel state. The reason for
this differential behavior can be found in the
chemical and structural composition of those
two differing materials. In the case of the but-
terfly wing, the surface is natively coated with
wax, which strongly contributes to the hydro-
phobic behavior. In contrast, the wetting
behavior of the water-strider leg appears to be
dominated by the structural properties.
Namely, the aspect ratio of the structures in the
water-strider legs is much higher than in the
case of the butterfly wings, enabling the trap-
ping of air, which strongly favors the super-
hydrophobic behavior.
16.2.3 Biocompatibility
Biocompatible coatings play an increasingly
important role for biology and medicine, as dis-
cussed in detail in Chapters 7 and 8. Artificial
substrates are commonly used for growth of
cells and tissue or for implants. In many cases,
those artificial materials can satisfy the required
preconditions in terms of mechanical stability,
weight, etc., but are not biocompatible. On the
other hand, biocompatible materials are often
too expensive, too heavy, or too brittle. A good
example is furnished by titanium implants,
which show very good biocompatibility and
stability but are very expensive.
Thin-film coatings provide one possible solu-
tion for this problem. An implant or a substrate
may be designed from a material that is not
biocompatible but satisfies all other require-
ments. A thin film could be applied on top for
biocompatibility, which could seriously decrease
the cost of the production, provided that the thin
film is biocompatible, stable, free of pinholes,
and firmly attached to the substrate.
The latter two requirements are intrinsically
provided by ALD. The thin-film coating will be
conformal and pinhole free because of the self-
limiting growth mechanism described in Section
16.1
. The firm attachment of the coating to the
surface of the substrate is provided by the chemi-
cal anchoring of the precursors to the functional
