Biomedical Engineering Reference
In-Depth Information
means of the Langmuir-Blodgett and/or Langmuir-Schäfer tech-
nique, or, more easily, self-assembly processes are exploited.
While Langmuir-Blodgett films offer the control over the compo-
sition and lateral pressure of both leaflets, self-assembly tech-
niques are more versatile and the bilayers are easier to prepare. In
the latter case, lipids are attached via covalent or quasi-covalent
linkage, which appears to be an attractive solution to the problem
of membrane immobilization. However, at the same time, the lat-
eral mobility and a sufficient distance between solid support and
the surface-facing membrane leaflet needs to be maintained, which
is a prerequisite for a functional reconstitution of membrane pro-
teins. Only few examples are given that demonstrate the feasibility
of incorporating transmembrane proteins in solid supported mem-
branes owing to the close proximity of the membrane to the solid
substrate.
7-9
To overcome these difficulties, other strategies com-
prising the separation of a solid supported membrane by a thin,
water-swollen polymer cushion or anchoring of lipids to the sup-
port by spacer units (tethered bilayers) to increase the distance be-
tween the membrane and the substrate have been developed,
10,11
which allowed monitoring of ion flows through reconstituted
channels.
7,12-15
However, the ion currents are hampered by the ca-
pacitive coupling of the membrane to the substrate and, thus, sin-
gle channel ion currents cannot be detected.
To combine the merits of robustness of membranes attached
to a support with the advantage of freestanding bilayers, which al-
low an easy insertion of transmembrane proteins without a capaci-
tive coupling, substrates with pore arrays or even porous materials
have been introduced as membrane supports.
16-19
Membranes
spanning these holes with diameters between 50 nanometres and
several tens of micrometres take advantage of the natural proper-
ties of a cell membrane, where integrated ion channels are mobile,
and transport of ions from one side of the membrane to the other is
not hindered by the vicinity of a substrate.
Here, we give an overview of the potential of pore-suspending
membranes for electrical monitoring of ion channel and transporter
activities. We will focus on our established pore-suspending mem-
brane systems based on functionalized porous alumina substrates
with pore diameters of only 60 nm. The presented systems exhibit
membrane resistances which allow for integral electrical readouts
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