Biomedical Engineering Reference
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lateral diffusion coefficient and mobile fraction of the lipids were
high in this system, which, however, was very unstable in the ab-
sence of cholesterol. All above polymer-supported lipid bilayers
were deposited on non-conducting supports and, consequently,
they are not amenable to electrochemical measurements. These
bilayers were often patchy and with several defects.
Polymer-cushioned lipid bilayers on conducting supports have
been investigated only rarely. Spinke et al. 227,228 described a poly-
mer-supported lipid bilayer anchored to gold. These authors used a
methacrylic terpolymer consisting of a hydrophilic main chain that
acts as a spacer, a disulfide unit that anchors the polymer to the
gold surface, and a hydrophobic lipid-like part that forms a first
lipid monolayer upon self-assembly. A second lipid monolayer
was formed on top of the first by fusion with DMPC vesicles. The
fusion was followed by SPR, which revealed a maximum increase
in film thickness by about 2 nm after 4 h.
Polymer-cushioned lipid bilayers have frequently been inves-
tigated on the indium-tin-oxide (ITO) semiconductor. In case of a
lipid bilayer deposited on a regenerated (and thus hydrophilic)
cellulose cushion, the swollen polymer film behaves like an aque-
ous electrolyte. Selective ion transport via ion channels and carrier
proteins incorporated in the membrane was, therefore, quantita-
tively evaluated by determining the electric resistance of the mem-
brane. 229 On ITO electrodes, the presence of polymer cushions
significantly reduces local defect densities and results in an electric
resistance from 5 to 50 times higher than that of a lipid bilayer
directly deposited on ITO. 8,138,139
A hydrophobic cellulose derivative provides not only an envi-
ronment to a lipid monolayer, but also acts as an electric insulator
with an electric resistance as high as 20 M: cm 2 . When a lipid
monolayer is deposited on a semiconductor surface pre-coated
with such a polymer insulator, the whole system consists of a con-
ductor-insulator-semiconductor set-up. As the film thickness can
be controlled with nm accuracy, the potential drop across the pol-
ymer insulator can be controlled precisely. The system can be
tuned so as to detect the charging and discharging of the polar
heads of the lipid monolayer by monitoring changes in the semi-
conductor space charge capacitance. When optimized, this simple
device should reach a sensitivity of about 1 e/30 nm 2 . 230
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