All-optical measurements of the bending rigidity of lipid-vesicle membranes across structural phase transitions

By exploiting the nanometer sensitivity of the confocal response to the position of an in-focus reflecting surface, we measured the bending rigidity of lipid-bilayer vesicles with a noninvasive all-optical method. The vesicles were weakly deformed with femtonewton optical force, and the bending rigidity was measured continuously from the L-alpha through the P-beta, to the L-beta' phases on the same specimen for the first time. The bending modulus is found to increase by an order of magnitude from the L-alpha phase to the L-beta' phase, as a result of the increasing area-compressibility modulus and bilayer thickness. The dips of bending modulus give precisely the main-transition and pretransition temperatures, which supports the recently proposed chain-melting model of pretransition.