Biomedical Engineering Reference
In-Depth Information
bilayer and the fusing vesicles. The repulsion among the charged
lipid molecules of the SiO 2 -supported bilayer may also favor the
entrapment of inorganic ions in the polar head region and their
subsequent movement along small fluctuating membrane-spanning
pores, with a resulting decrease in R m .
A sBLM formed by fusing DMPC-cholesterol vesicles on the
optically transparent semiconductor indium tin oxide (ITO) was
investigated by EIS. 139 The vesicles also contained 10 mol% of a
positively charged lipid to favor their interaction with the negative-
ly charged ITO surface. The impedance spectrum was analyzed
with the same equivalent circuit used for the Si/SiO 2 substrate. In
this case, gramicidin was incorporated in the ITO-supported lipid
bilayer by incubating it with gramicidin-containing vesicles. Upon
incorporating gramicidin, the resistance R m of the lipid bilayer was
strongly reduced in the presence of a Na + salt with an organic ani-
on not permeating membranes, whereas its capacitance did not
change remarkably. Conversely, R m was practically unaffected in
the presence of a Cl - salt with an organic cation not permeating
membranes. This experiment confirms the functional activity and
ion selectivity of gramicidin, which is known to be highly selective
toward monovalent inorganic cations. The outer membrane pro-
teins OmpF and OmpA from Escherichia coli were reconstituted
in vesicles that were fused on ITO. 139 In an aqueous solution of
NaCl, the resistance of the ITO-supported lipid bilayer was strong-
ly reduced by incorporating OmpF, whereas it was only slightly
decreased by incorporating OmpA. Such a behavior was explained
by the fact that only OmpF is a pore-forming protein, whereas
OmpA does not form pores. Note that the functional activity of
OmpF was preserved because, on the cytosolic side of the bacterial
membrane, it has no extramembrane domain that might be endan-
gered by direct contact with the ITO surface.
Freshly formed surfaces of Pt, Au, Ag, Cu, Ni or stainless
steel are hydrophilic. Self-assembly of a lipid bilayer on these sur-
faces is realized by cutting the end of a Teflon-coated hydrophilic
metal wire while keeping it dipped in a decane or squalene solu-
tion of the lipid, and by then immersing the freshly cut metal sur-
face in an aqueous solution for 5-10 min. 140 During this period, the
lipid solution in excess creeps between the metal wire and its Tef-
lon coating, leaving a self-assembled lipid bilayer on the metal
surface. The differential capacitance of this film, 0.3-0.5 PF cm 2 ,
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