Biomedical Engineering Reference
In-Depth Information
d
n
3
r
4
n
g
|
7
Figure 10.15
Schematic for the preparation of brush-co-polymer gradients:
(A) homopolymer molecular weight gradient using a syringe pump
and microchannel; (B) microfluidic static mixer with two different
monomers to create a compositional gradient from monomer A to
monomer B.
Figure modified from Fasolka et al.
148
poly(
L
-lysine)-g-poly(ethylene glycol) solution.
205
By controlling the ad-
sorption time across the substrate, a gradient in polymer density could be
created. Subsequent backfilling with a second polymer or different chem-
istry, or additional functionality, provided a saturated polymer gradient.
Larsson and Liedberg
206
have produced PEG thickness gradients ranging
from 0 to 500 Å through graft polymerisation. Firstly, a cyclo-olefin polymer
is exposed to an air plasma to create peroxides and other reactive species on
the surface.
207
Subsequent UV irradiation leads to the generation of surface
radicals, which interact with the PEG monomers in the solution. During UV
irradiation a shutter is gradually slid across the surface. This results in the
formation of a polymer brush gradient, with the thickness depending upon
the length of UV exposure. Human Fn was found to penetrate into the PEG
layer for
.
200 Å whereas human serum albumin was unable to penetrate
into the PEG layer at all.
B
10.2.2.5 Gradients of Surface Wettability
As discussed earlier, it is too simplistic to view the contact angle as a major
parameter that determines cellular response as there are often multiple
factors (functional group chemistry, surface topography, etc.) influencing the
contact angle observed. Nonetheless, many researchers have investigated
wettability gradients using a wide variety of functional groups and polymer
compositions. Hydrophobic groups are most commonly comprised of car-
bon chains, ranging from a single carbon to long chains. Hydrophilic groups
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