Biomedical Engineering Reference
In-Depth Information
membrane potential to be measured and varied at will. Other wide-
ly employed biomembrane models are unilamellar lipid vesicles
(also called liposomes). Due to their small size, they cannot be
studied by electrochemical techniques. Rather, they can be investi-
gated by spectroscopic techniques using fluorescent or spin labeled
molecular probes. Electrical methods are used to apply square
wave electric field pulses to cell suspensions, for the investigation
of pore formation in biomembranes induced by electric fields
(electroporation).
123
In an effort to overcome the drawbacks of BLMs, more robust
biomimetic membranes have been fabricated, by immobilizing
lipid bilayers on solid surfaces. They are normally obtained by
self-assembly, which can be driven exclusively by noncovalent,
hydrophobic interactions or with the additional contribution of
covalent linkages. In spite of notable improvements in this direc-
tion, some major problems of solid supported membranes arise
when attempting to use them for analytical applications; the very
small space between the bilayer and the support (from 2 to 10 nm)
is rapidly saturated by the transported species. This makes quanti-
tative dynamic measurements difficult.
4. Solid Supported Bilayer Lipid Membranes (sBLMs)
The term
solid supported bilayer lipid membrane
(sBLM) or,
simply,
solid supported membrane
, is commonly used to denote a
biomimetic membrane consisting of a lipid bilayer in direct contact
with a solid support. These biomimatic membranes are typically
formed on a hydrophilic solid support by immersing it in an aque-
ous dispersion of small unilamellar vesicles (SUVs), which slowly
rupture and spread on the surface of the support. Alternatively,
they can be formed by Langmuir-Blodgett and Langmuir-Schaefer
transfers. When appropriately formed, these sBLMs are separated
from the support surface through a water layer, estimated between
6 and 15 Å thick.
124,125
Several theoretical and experimental inves-
tigations of water near polar hydrophilic surfaces suggest that it is
more ordered than bulk water, with higher viscosity and lower
dielectric constant.
126,127
In accordance with these predictions, the
characteristics of the lipid bilayer have been found to be structural-
ly coupled with the support properties, such as its charge
128
, wetta-
bility
129
and topography
130
. Frictional or electrostatic coupling be-
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