Biomedical Engineering Reference
In-Depth Information
a vast selection of compositions and geometries. The diculty with some
gradients is that several parameters are altered in the course of the investi-
gation.
87
For example, the chemical composition and/or phase separation can
alter with a variation in film thickness.
99
In such cases it can be dicult to
assign a single surface property to the observed cell-surface interactions, with
a combination of factors potentially leading to the observed result. Therefore,
adequate characterisation of the deposited surface is essential to determine
the main parameters influencing the cellular response. The vast array of
chemical gradient preparation techniques existing today have been expertly
reviewed by Genzer and Bhat,
83
Morgenthaler et al.
84
and others.
11,79
d
n
3
r
4
n
g
|
7
10.2.2.1 Self-assembled Monolayer Gradients
There are two distinct categories of self-assembled monolayer (SAM) gradi-
ent preparation techniques; those that use silane chemistry on glass or Si
substrates, and those that use alkanethiols on gold or silver substrates. The
first reported preparation of a chemical gradient by Elwing et al.
86
in 1987
utilised silane chemistry to create gradients of methylsilanes on Si. The
methyl silane was bedded under a xylene solution containing an immersed
silicon wafer and the methyl silane slowly diffused through a xylol region
and was simultaneously bound to the surface. A modification of this pro-
cedure was described by Chaudhry and Whitesides,
89
whereby decyltri-
chlorosilane in paran oil was positioned next to a Si wafer with the vapours
being allowed to diffuse along the surface, adsorbing and therefore gener-
ating a gradual change in silane coverage.
This technique has been readily employed to create silane gradients
with semi-fluorinated,
166
terminal alkene,
167
chlorine,
167
amine,
89
hydro-
carbon,
86
and specially modified protecting groups for patterned UV
irradiation.
168-170
Many of these functional groups can be further modified
or irradiated to generate COOH terminated surfaces, peptide gradients
and even carbon nanotube density gradients.
167,171,172
In most of these in-
stances, backfilling was employed, either as the second step to fill vacant
spaces or as the primary step to create a fill template for subsequent sila-
nisation of the functional groups of interest.
Another method for generating silane chemistry gradients is through UV
irradiating a uniform silane coating for varying periods of time as described
by Gallant et al.
173
(Figure 10.10). In the first step, uniform n-octadi-
methylchlorosilane coatings were deposited on a Si wafer. The sample was
then placed under a motorised UV lamp. Varying the UV exposure time
across the sample led to variations in the extent of oxidation of the hydro-
carbon chains. Such method produced gradients with contact angles ran-
ging from 251 to 951. Subsequent functionalisation using 'click chemistry'
174
led to the generation of an RGD density gradient. As an extension to this
study, Kennedy et al.
175
adsorbed Fn on the UV irradiated surfaces. It was
found that Fn-mediated cell attachment was faster on the more hydrophobic
surface than on the hydrophilic surfaces. This finding was also supported by
.
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