Biomedical Engineering Reference
In-Depth Information
is explained by electrostatic attraction between the charge of the
mixed thiols and the oppositely charged end of the dipole of the
zwitterionic lipid molecules.
As distinct from solid supports, such as gold or silver, mercu-
ry imparts lateral mobility to lipid monolayers directly self-
assembled on its surface, thanks to its liquid nature. This is
demonstrated by rapid spontaneous phase separation, with micro-
domain formation, in a lipid monolayer self-assembled on top of a
DPTL thiolipid monolayer tethered to a mercury microelectrode.
39
The presence of microdomains was directly verified from the im-
ages of the distal lipid monolayer by using two-photon fluores-
cence lifetime imaging microscopy (2P-FLIM).
V. THE VARIOUS TYPES OF BIOMIMETIC
MEMBRANES
In examining the various types of biomimetic membranes, we will
first consider simple experimental models consisting of a single
lipid monolayer, self-assembled either on mercury or on an al-
kanethiol monolayer tethered to a metal electrode. Subsequently,
biomimetic membranes comprising a lipid bilayer will be exam-
ined, in the order of approximately increasing complexity.
1.
Mercury Supported Lipid Monolayers
A simple and easily prepared biomembrane model obtained by
noncovalent self-assembly consists of a phospholipid monolayer
supported by a hanging mercury drop electrode. The use of a lipid-
coated mercury electrode as a biomimetic membrane was intro-
duced by Miller
40
and subsequently adopted in a modified version
by Nelson.
41
The lipid coating is obtained by spreading a solution
of the lipid in pentane on the surface of an aqueous electrolyte,
allowing the pentane to evaporate and immersing a hanging mer-
cury drop electrode in the electrolyte. This procedure gives rise to
a lipid monolayer, with the hydrocarbon tails directed toward the
hydrophobic mercury surface and the polar heads directed toward
the solution. The defect-free support provided by liquid mercury to
the lipid monolayer and the complete absence of pentane in the
film impart high mechanical stability, resistance to electric fields
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