Measurement of membrane rigidity on trapped unilamellar phospholipid vesicles by using differential confocal microscopy.

We use optical tweezers to trap a unilamellar phospholipid vesicle and measure the out-of-plane thermal fluctuations by using differential confocal microscopy. Bending moduli of the lipid membranes are calculated directly from the mean-square values of the fluctuation amplitudes. Owing to the refractive index contrast between the inner and outer solutions of the vesicle, optical tweezers trap the vesicle laterally and improve the reliability of the measured fluctuation amplitudes along the optical axis. Bending moduli of membranes in gel or fluid phases obtained by the combination of differential confocal microscopy and optical tweezers are close to those reported previously. We also obtain the bending modulus of sphingomyelin membranes in the gel phase, which was not reported previously.