Biomedical Engineering Reference
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oriented parallel to the surface; 137 the resulting ordered monolayer
resembles that formed by alkanes. With time, the molecules reori-
ent and the monolayer is transformed into a hemimicellar film. In
the presence of a high vesicle concentration in solution, the hem-
imicellar state is transformed further into a bilayer.
A sBLM formed by fusing vesicles consisting of a mixture of
50 mol% cholesterol and 50 mol% dihexadecyl-dimethyl-
ammonium bromide on boron doped (p-type) silicon covered with
native oxide was investigated by EIS. 138 The positively charged
lipid was chosen to favor vesicle spreading on the negatively
charged Si/SiO 2 surface. The impedance spectrum was simulated
by an equivalent circuit consisting of two RC meshes in series,
with the resistance R : of the aqueous solution in series with them.
One RC mesh simulates the electrode, regarded as a combination
of the Si space-charge region, the SiO 2 oxide and the thin water
layer interposed between the semiconductor surface and the lipid
bilayer; the other RC mesh simulates the lipid bilayer. The re-
sistance and capacitance of the electrode amount to about 19 M:
cm 2 and 2.2 PF cm -2 , respectively, those of the lipid bilayer to 0.98
M: cm 2 and 0.75 PF cm -2 . Values of the two capacitances are
close, but the resistance of the electrode is much greater than that
of the lipid bilayer, due to the 1.3-1.4 nm thick silicon oxide. The
time constants of the two RC meshes are, therefore, appreciably
different and the corresponding frequency ranges are sufficiently
apart to allow an accurate estimate of the corresponding parame-
ters. Incorporation of gramicidin from an aqueous solution de-
creases the resistance of the lipid bilayer by more than one order of
magnitude in the presence of Na + ions, and even more in the pres-
ence of K + ions. This agrees with the cation selectivity scale of
gramicidin. When the Si/SiO 2 substrate was coated with a lipid
bilayer consisting of a mixture of 1,2-dioleoyl-3-trimethyl-
ammonium propane (DOTAP) and DOPC, the resistance R m of the
bilayer was found to attain a maximum value for 10 mol% of the
positively charged lipid. 8 Electrostatic forces are those of longest
range between a lipid bilayer and a substrate, but other forces are
also operative, such as attractive van der Waals forces and repul-
sive hydration and steric forces. In view of the major role played
by electrostatic forces, the rapid decrease of R m for DOTAP mole
fractions greater than 10% can be tentatively explained by electro-
static repulsion between the partially formed SiO 2 -supported lipid
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