Biomedical Engineering Reference
In-Depth Information
the step height using these methods the surface can be masked with a
flexible polymer prior to polymer deposition. Following deposition the mask
can be peeled off and the step height measured. Alternatively a scratch can
be made on the deposited polymer surface. Although AFM is potentially
slightly more accurate, the experimental effort is higher than using profi-
lometry. Techniques such as ellipsometry and quartz crystal micro-
gravimetry could also be used to determine the thickness of the polymer;
however, prior knowledge such as the refractive index or density of the
polymer is required to use these techniques.
7,99,100
Interferometric reflect-
ance spectroscopy, UV-visible interferometry and XPS can also be used to
determine film thickness within limitations.
d
n
3
r
4
n
g
|
7
10.2.2.3 Electrografting Gradients
Electrografting is very useful for generating surfaces of a wide variety of
functional groups through electrochemical attachment onto pSi. Electro-
chemical reactions of pSi to form Si-C bonds on pSi can be performed using
organohalides, alkynes or Grignard reagents.
191
Organohalides, most commonly bromides or iodides, react with pSi upon
electrochemical reduction to form Si-C bonds as summarised in
Figure 10.13. The lithium iodide (LiI) present in the reaction reduces the
organo halide to form a radical in the first step of the reaction.
192
This alkyl
radical then removes a H
radical from the Si-H surface, thereby creating
Si radicals.
193
Subsequently, the Si radicals react with the alkyl radicals to
form a Si-C bond. This cathodic electrografting reaction is prone to surface
oxidation arising from moisture or oxygen in the environment and hence,
the reaction must take place under an inert atmosphere.
A wide variety of functional groups can be attached to pSi using
this method (summarised by Gurtner et al.
192
) including: trifluoroacetyl-
amino,
126
ethyl ester,
126
pentyl acetate,
107
bipyridines
194
and hydrocarbon-
based molecules.
107,195
Some functional group terminated alkyl halides,
however, can react with the Si-H surface. These groups include phenol
groups, alcohols or terminal acids, and they compete with the Si-C bond
formation.
192
These unwanted side reactions could be overcome by using
an ester functional group, which can be later reduced to a terminal acid or
alcohol.
192
The stability of a pSi surface can be further improved by
methylating or 'end-capping' the unreacted Si-H functional groups with
.
Figure 10.13 Mechanism for the attachment of organic functional groups onto pSi
by the electrochemical reduction of organohalides.
Adapted from Gurtner et al.
192
Search WWH ::
Custom Search