Biomedical Engineering Reference
In-Depth Information
then formed in the pores by the painting method.
113,114
The stability
of these free-standing bilayers was found to depend on the nature
of the lipid. Bilayers formed from the cylindrically shaped phos-
phatidylethanolamine exhibit a membrane resistance higher than 1
G: for more than one week, while those formed from DPhyPC
remain above this threshold value for two days. Bilayers of the
naturally occurring soy PC mixtures are stable for only about 2 h,
but a reduction of the pore size from 800 to 200 nm increases their
stability by a factor of 30. Membrane sheets with a well-defined
orientation were transferred onto a regular array of holes, 50-600
nm in diameter, in a silicon nitride chip, 100-500 nm thick.
115
By
pressing the chip against a supported layer of cultured cells and by
then removing it, the cells rupture and membrane fragments re-
main attached to the chip, spanning the holes. The chip was inte-
grated into a microfluidic chamber suitable for optical imaging.
Tien et al.
116
devised a biomimetic membrane formed at the
end of a salt-bridge tubing. Briefly, a freshly cut salt-bridge tubing
attached to the end of a Ag/AgCl(KCl) reference electrode is first-
ly dipped into an alkane solution of the lipid and then transferred
into an aqueous solution, so as to form a lipid bilayer. In this case,
the hydrophilic surface required for bilayer formation is provided
by the freshly cut agar gel containing KCl. The resistance of these
supported BLMs is usually irreproducible from one membrane to
another. More recently, replacing water by a hydrogel as a hydro-
philic medium has permitted to achieve a notable increase in sta-
bility of lipid bilayers in micropores. Thus, for instance, a lipid
bilayer formed across a Teflon aperture was protected with a layer
of photopolymerized poly(ethylene glycol) dimethacrylate, and
proteins were incorporated by allowing them to diffuse slowly
through the previously formed gel.
117
Covalent conjugation of the
lipid bilayer to the encapsulating hydrogel was found to increase
preservation of the lipid-containing solvent reservoir (the Plateau-
Gibbs border) surrounding the membrane, thereby extending the
membrane lifetime. To further reduce the flow of organic solvent
away from the Plateau-Gibbs border, the Teflon septum was re-
placed by a glass cover slide where a 500 Pm hole was drilled.
118
The hydrophilic glass surface was made hydrophobic by function-
alizing it with 3methacryloxypropyltrimethoxysilane, to provide
cross-linkable vinyl groups on the glass surface. In this way, the
hydrogel precursor could covalently bind to the vinyl groups of the
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