Biomedical Engineering Reference
In-Depth Information
9.4.1
Light Illumination as External Stimulus
Light is one of the most important external stimuli [
140
]. A variety of photo-
responsive inorganic oxides and organic polymers undergo the transition from larger
CAs to relatively lower CAs upon ultraviolet (UV) illumination, reverting to the
original states with larger CAs after the UV-irradiated films are placed in the dark
or exposed to visible (Vis) illumination. This alternating transition can repeat for
several cycles under this environment.
TiO
2
is one of the most studied and widely applied photo-sensitive semi-
conductors, because of its impressive self-cleaning property under sunlight and
natural rainfall. The mechanism of self-clean can be ascribed to the intrinsic
photocatalytic and photo-stimulated properties. In 1997, Fujishima et al. [
141
]
have found reversible wetting behavior on anatase TiO
2
polycrystalline films upon
UV illumination. In his report, the CAs on TiO
2
polycrystalline films change
between 72
1
ı
. Therefore, based on the principle of roughness-induced
superhydrophobicity, there is increasing attention on the introduction of fine surface
textures to TiO
2
films to prepare superhydrophobic TiO
2
films. Tadanaga et al.
[
142
] have prepared a rough TiO
2
gel layer on flowerlike Al
2
O
3
film modified
with hydrolyzed fluoroalkylsilane. By combining microstructure and chemical
modification, CA larger than 150
ı
was achieved on such surfaces. In addition,
wetting transition from superhydrophobicity to superhydrophilic was realized after
exposure to UV irradiation. Similar result has been reported on superhydrophobic
TiO
2
nanorod films with dual-scale hierarchical structures. The CA modulation
range is from 154
1
ı
and 0
˙
˙
1.3
ı
to about 0
ı
and this process can be repeated several
cycles before and after UV irradiation (Fig.
9.7
)[
143
]. Fujishima et al. have
reported another simple and reproducible method to roughen TiO
2
surfaces by
adopting a CF
4
plasma etching technique [
144
,
145
]. The discontinuous TiO
2
wedge-like microtextural films were formed via plasma etching for 30 s, after that
the surface was coated with an octadodecyl phosphonic acid monolayer to exhibit
superhydrophobicity. The sample originally showed water CAs larger than 165
ı
and were converted to superhydrophilic ones with CA about 0
ı
upon UV irradiation
for 1 h.
ZnO is another prominent photo-responsive inorganic oxide that possesses
several favorable properties on optical, electronic, and acoustic. It has been found a
similar wetting behavior and converting mechanism as the TiO
2
surface [
146
,
147
].
On a flat ZnO substrate, the maximum water CA is only 109
ı
. Considering the
roughness enhanced on superhydrophilicity/superhydrophobicity, a large amount of
efforts have been devoted to fabricate ZnO film with varied micro/nanostructures,
including nanoporous ZnO films [
108
], aligned ZnO nanorod array [
148
]to
nanocolumnar ZnO surfaces [
149
], and to hierarchically structured ZnO film [
80
]
and nanoprotrusions [
150
], and so on. Combining these structured surfaces with
low-surface-energy materials, switchable superhydrophobic and superhydrophilic
wetting states can be achieved on ZnO-based film upon alternating UV irradiation
and storage in the dark. Besides, much more photo-responsive inorganic oxide
˙
