Biomedical Engineering Reference
In-Depth Information
light source, either radiation or electrons. An additional surface treatment procedure
is demanded to render the surface's superhydrophobicity. Structures prepared by
photolithography can be easily characterized and they are frequently applied in the
surface modeling.
For instance, Furstner et al. [ 90 ] fabricated silicon wafers with ordered patterns of
spikes by X-ray lithography. Subsequent sputtering of a layer of gold and immersion
into a hexadecanethiol solution make the wafer hydrophobic. The spikes have a
width ( d )of1or2 m, the distance between the spikes ( a ) range from 1 to 5 m,
and the height of the spikes ( h ) was also different, from 1 to 4 m. CA of the surfaces
ranged from 113 to 161 ı determined by the pillar size, the clearance as well as the
height of the spikes. And the best self-cleaning surface was found with d / a
D
2and
h / d
4, while the self-cleaning property of the specimen was measured by applying
an artificial fog.
Martines et al. [ 92 ] have produced nanopillars and nanopits on silicon wafers
by E-beam lithography. The center-to-center distance among pitches is kept in a
constant value at 300 nm. The diameter ( d ) and height ( h ) of the pillars and pits
are varied for different samples. The hydrophobic nanopatterns were obtained by
silanization, covering of a surface through self-assembly with silane-like molecules,
the octadecyltrichlorosilane. All treated surfaces exhibit improved hydrophobicity
in contrast to the flat control surface. The contact angle behavior of samples with
nanopits falls in the Wenzel regime. Samples with pillars are found in Wenzel and
Cassie-Baxter states, being ascribed to the differences in aspect ratio ( h / d ). They
displayed that a given spacing with increasing aspect ratio can remarkably stabilize
the Cassie-Baxter regimes.
D
9.3.1.3
Plasma Treatment of Surfaces
Plasma treatment on surfaces involves plasma etching, a dry etching technique
where reactive atoms or ions (such as oxygen, chlorine, and fluorine) are engendered
in a gas discharge. The ions are expedited in the border layer between plasma and
substrate with high directionality. They are thus capable of creating deep grooves
with steep walls to yield reactive ion etching. Due to this anisotropic etching of the
surface layers, plasma treatment can induce a considerable change in the surface
structure.
Fresnais et al. [ 68 ] treated low-density polyethylene (LDPE) with plasma
etching. The surfaces were treated by LDPE with oxygen and subsequent CF 4
plasma, then surfaces with CA of 170 ı and low hysteresis less than 5 ı and with
the roughness within the range of 20-400 nm were fabricated [ 68 ].
Minko et al. [ 97 ] fabricated self-adaptive surfaces using oxygen plasma
treatment of PTFE. The resulting plasma-etched PTFE exhibited a water CA
about 160 ı and was observed with CA hysteresis. The superhydrophobic PTFE
surfaces then undergo sequent treatment of ammonia plasma and a mixed
polymer brush consisting of two carboxyl terminated incompatible polymers:
carboxyl-terminated
poly(styrene- co -2,3,4,5,6-pentafluorostyrene)
(PSF-COOH)
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