  A method for determining permeability of phospholipid bilayer based on the
osmotic swelling of micrometer-sized giant unilamellar vesicles (GUVs) is
presented as an alternative to the two established techniques, dynamic light
scattering on liposome suspension, and electrical measurements on planar lipid
bilayers. In the described technique, an individual GUV is transferred using a
micropipette from a sucrose/glucose solution into an isomolar solution
containing the solute under investigation. Throughout the experiment, vesicle
cross-section is monitored and recorded using a digital camera mounted on a
phase-contrast microscope. Using a least-squares procedure for circle fitting,
vesicle radius R is computed from the recorded images of vesicle cross-section.
Two methods for determining membrane permeability from the obtained R(t)
dependence are described: the first one uses the slope of R(t) for a spherical
GUV, and the second one the R(t) dependence around the transition point at
which a flaccid vesicle transforms into a spherical one. We demonstrate that
both methods give consistent estimates for membrane permeability.
