Biomedical Engineering Reference
In-Depth Information
min, the orientation of the erythrocyte membrane was identified
with two immune-fluorescence labels. This labeling proved that
polymer-supported human erythrocyte membranes are almost free
from local defects and expose the cytoplasmic domain to the bulk
aqueous phase. The success of this deposition suggests that cellu-
lose cushions fulfill the conditions of complete wetting of cellular
membranes and finely tune the cell-surface interactions. However,
FRAP experiments showed no sign of lateral diffusion, probably
due to the presence of spectrine cytoskeletons.
Usually, polymer cushions are anchored to supports such as
glass, silica and mica, by using polymers derivatized with alkyl
silanes
20
or triethoxysilane for covalent linkage to silanols at the
surface of the silicate substrates.
225
More rarely, they are anchored
to gold or to semiconductors such as GaAs via thiolated polymers.
Covalent coupling of polymers to silane or thiol anchors can be
achieved through epoxy groups, photoactive groups and so-called
active esters.
217
A variety of polymeric materials have been em-
ployed. These include polyacrylamide,
218
polyethyleneimine, dex-
tran,
219
trimethylsilylcellulose,
220
chitosan
221
and hyaluronic ac-
id
222
.
Wong et al.
223,224
characterized lipid bilayers supported by a
polyethyleneimine cushion on a quartz substrate by neutron reflec-
tometry and found that thickness, surface roughness and coverage
of the polymer depend strongly on the method of preparation.
Kühner et al.
218
prepared 30-40 Pm thick polyacrylamide gels on
glass and coated them with monolayers and bilayers, using Lang-
muir-Blodgett techniques. When examined by epifluorescence
microscopy, these bilayers exhibited residual domain structure.
Lateral mobility of a lipid-linked 20-residue peptide antigen was
verified in these bilayers. In an extension of this work, the electro-
phoretic mobility of charged lipids was determined in monolayers
that were supported by agarose gels;
225
bilayers did not form on
these substrates. In another study, bilayers were formed on a poly-
vinyl substrate that had functionalized diethylene amino groups for
reaction with lipid head groups.
226
Although homogeneous bilayers
with high lateral mobility were obtained under some conditions,
the hydrophilic linkers were too short to accommodate membrane
proteins in this system. Elender et al.
219
were able to form uniform-
ly fluorescent bilayers of DMPC and 20 mol% cholesterol on 600-
800 nm cushions of dextran supported by Si/SiO
2
and glass. The
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