Biomedical Engineering Reference
In-Depth Information
6. Polymer-Cushioned Bilayer Lipid Membranes (pBLMs)
The spaciousness of the ionic reservoir of tBLMs may not be suf-
ficient to accommodate bulky extramembrane domains of mem-
brane proteins. The problem is particularly serious with cell-
adhesion receptors, whose functional extracellular domains can
extend to several tens of nanometers. Thus, the spreading of prote-
oliposomes doped with human platelet integrin D IIb E 3 and ATP
synthase on quartz or glass substrates results in inhomogeneous
patches of pinned proteins. 208,209 This problem can be circumvent-
ed by separating the lipid bilayer from the solid substrate using
soft polymeric materials of typically less than 100 nm thickness,
which rest on the substrate and support the bilayer. These stratified
films are often referred to as polymer-cushioned or polymer-
supported bilayer lipid membranes (for a review, see Ref. 210).
This approach reduces the non-specific binding of proteins to the
solid support and the frictional coupling between proteins and the
support, preventing the risk of protein denaturation due to direct
contact between protein subunits and the bare support surface. In
some cases, the cushion may assist self-healing of local defects in
lipid bilayers deposited on macroscopically large supports.
To form a thermodynamically stable polymer-lipid composite
film on a solid support in aqueous solution and to avoid the for-
mation of polymer blisters, several requirements should be ful-
filled. Specifically, the spreading pressures should be positive for
the lipid bilayer on the polymer and for the polymer on the solid
support. 211 From a thermodynamic viewpoint, a stratified structure
is only stable if each additional layer results in a gain in the total
surface free energy. Moreover, the interaction between the lipid
bilayer and the substrate surface must be repulsive. In fact, if the
net force acting per unit area (disjoining pressure) is negative, con-
tinuous thinning of the interlayer results in the film collapse, i.e.,
dewetting, giving rise to regions of tight local contact between the
lipid bilayer and the substrate surface (the so-called pinning cen-
ters). Polymer-cushioned membranes are fairly unstable if the at-
tractive interfacial forces between the polymer and the lipid bilayer
are relatively weak. In this case, the bilayer can easily detach from
the polymer cushion. On the other hand, too strong attractive forc-
es may decrease the lateral mobility of the bilayer. A compromise
should, therefore, be found between a sufficient stability of the
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