Extension and measurements on a phospholipid vesicle by use of dual-beam optical tweezers

A vesicle whose membrane is composed of a phospholipid bilayer is called as a liposome. Since the fundamental structure of the membrane is the same as cell membranes, liposomes have been intensively studied as a simple model of live-cells. Here, we study the mechanical property of a micrometer-sized giant liposome through extending a single liposome from the inside by use of dual-beam optical tweezers. As the liposome is extended, the shape gradually changes from a sphere to a lemon-shape, and finally, a tubular membrane projection is exhibited. The surface tension and the bending rigidity of the lipid membrane are obtained from the measured force-extension curve. The present paper reviews both experimental and theoretical bases of this extension measurement method on the liposome with our experimental data.