Measuring the bending rigidity of giant unilamellar liposomes with differential confocal microscopy

Summary form only given. Giant unilamellar liposomes (diameter>10 /spl mu/m) are not only important model systems for cell-membrane research but also useful in controlled drug-delivery by encapsulating therapeutic components and transporting them into cells. For the understanding and applications of giant liposomes, it is essential to study their mechanical properties under different physical or physiological conditions. A crucial property is the bending rigidity, which is closely related to the activities of liposomes and the gel-liquid phase transition of liposomes bilayer membrane. To measure the bending modulus, one must deform liposome membrane with well-characterized force. Because the measurement requires a small deformation force and sub-micrometer detection of the membrane displacement, it has been a technology challenge. To date, the most popular method to determine the bending rigidity of unilamellar liposomes is micropipette aspiration, in which the bending modulus /spl kappa/ is deduced from surface tension of the sucked projection. However, when working with lipid mixtures, the high curvature in the sucked area can change the lipid composition. In addition, the diameter of the sucked projection, which is necessary for the calculation of /spl kappa/ is difficult to determine accurately. We report an all-optical technique to measure /spl kappa/ of giant unilamellar liposomes directly.