Biomedical Engineering Reference
In-Depth Information
ȍ cm
2
. Hence, the electrical properties of the partially remaining
barrier oxide need to be considered in data analysis.
On these particular alumina substrates that were again func-
tionalized with CPEO3, the fusion process of LUVs leading to in-
sulating lipid bilayers was finished after 3 hours, which is signifi-
cantly faster. Also the success rate of the preparation increased to
80 %. We suggest that the process is driven by the bilayer that is
formed on the solid part of the alumina substrate, which then acts
as a nucleation site for membrane formation in the adjacent region.
It was shown by others that the border of a lipid bilayer catalyzes
the growth of bilayers in adjacent regions when liposomes are
added.
39,40
To analyse the impedance data, an equivalent circuit was de-
veloped that takes the two different areas (
A
1
and
A
2
) of the porous
substrate into account (
Fig. 7
).
The barrier oxide layer is represented by a resistance
R
ox
and a
capacitance
C
ox
, on which a lipid bilayer characterized by a mem-
brane resistance
R
m,c
and capacitance
C
m,c
is deposited. To get ac-
cess to
R
ox
and
C
ox
, the electrical response of the substrate prior to
the selective removal of the barrier oxide layer was measured by
EIS for each individual substrate. To account for the electrical
properties of the lipid bilayer covering the open-pore array, a par-
allel
RC
-element with the membrane resistance
R
m,o
and capaci-
tance
C
m,o
is added in series, whereas
R
el
represents the Ohmic be-
haviour of the electrolyte in the bulk and the open-pore array. This
equivalent circuit, however, contains too may parameters to be
able to fit all of them independently. Two approaches were fol-
lowed to simplify the equivalent circuit and reduce the number of
parameters. For both, prior to the fitting routine,
R
ox
and
C
ox
were
individually determined and kept constant during the following
fitting routine. In approach (1), we assumed that the area related
values
R
m,sp
and
C
m,sp
obtained from lipid bilayers on gold elec-
trodes are similar to those of membranes attached to the barrier
oxide. This approach implies that the barrier oxide is fully covered
by a lipid bilayer, whereas the coverage of the open pores is not
defined, thus the parameters
R
m,o
and
C
m,o
as well as
R
el
are free in
the fit routine. In approach (2), the specific membrane capacitance
and membrane resistance were assumed to be equal on the open-
pore array and on the barrier oxide. This was rationalized by the
electrical properties of CPEO3-DPhPC/DOPC bilayers on planar
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