Biomedical Engineering Reference
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silane group (for silica and glass supports). The monolayer inter-
posed between the support surface and the lipid bilayer should
have a well-defined composition and geometrical arrangement, as
distinct from polymer-cushioned bilayer lipid membranes. TBLMs
will be classified on the basis of the nature of the molecules com-
posing the tethered monolayer, as follows:
(1) spacer-based tBLMs;
(2) thiolipid-based tBLMs;
(3) (thiolipid-spacer)-based tBLMs.
We will examine these three types separately.
( i ) Spacer-Based tBLMs
The spacer may consist of an alkanethiol, functionalized with
a hydrophilic group (e.g., a hydroxyl, carboxyl or amino group) at
the apposite end of the alkyl chain with respect to the sulfhydryl
group. More frequently, it consists of a thiolated or disulfidated
hydrophilic chain. The spacer is tethered to a support via its
sulfhydryl or disulfide group and a lipid bilayer is self-assembled
on top of it, often by vesicle fusion. The interior of Z-
functionalized alkanethiol monolayers is hydrophobic, as opposed
to that of thiolated hydrophilic chains. However, even the latter
spacers do not necessarily ensure a satisfactory hydration if they
are too closed packed. In both cases, a thin water layer, about 1 nm
thick, is normally interposed between the spacer monolayer and
the lipid bilayer, as in the case of sBLMs; this provides a small
ionic reservoir for channel-forming peptides incorporated in the
lipid bilayer moiety. Since the lipid bilayer is not covalently linked
to the support, it is expected to be sufficiently fluid to accommo-
date relatively bulky membrane proteins, unless they have large
extramembrane domains on both the cytosolic and the extracellular
side. The lateral mobility of lipid molecules on a hydrophilic spac-
er was verified by FRAP in about 10 min on very smooth gold
surfaces 6 , but a sufficient fluidity may probably be verified at
longer times even on rougher surfaces. The capacitance of the lipid
bilayer in these tBLMs is close to 1 PF cm -2 , but its resistance as-
sumes relatively low values, of the order of 0.5 M: cm 2 .
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