Curvature Elasticity of Smectic A Lipid Bilayers and Cell Plasma Membranes

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Abstract
The bending elasticity (in the language of liquid crystal physics called splay elasticity) is an important property of (tension free) monolayers of amphiphiles and lipid bilayers. It plays a role in the formation and stabilization of microemulsionsl, the swelling of lipid bilayers, and the shape transformations of cells and vesicle. The bending stiffness also determines the amplitudes of thermally excited surface undulations and in this way contributes to the interplay of Van-der-Waals attraction dehydration forces, electrostatic and steric repulsion between flaccid interfaces and cells. For the above mentioned reasons, precise measurements of the bending elastic moduli of lipid layers and natural membranes have become an important task of lyotropic liquid crystal research. Such measurements can be performed by classical perturbation methods such as the micropipette technique or the recently developed electric field jump technique, 7. A completely different, non invasive method is based on the Fourier analysis of thermally excited surface undulations of vesicles or cells. In the following, we present measurements of the bending elasticity of giant vesicles and erythrocyte membranes by the latter technique. It is shown that, by determining the mean square amplitudes of the undulations as a function of their wavevector, it is possible to improve the accuracy of the measurement remarkably. Secondly, we address the question of modification of the bending elasticity of lipid bilayers by solutes. Thirdly, we report experiments which provide evidence that the red blood cell controls the membrane elasticity in an active way. This is suggested by quasiperiodic oscillations of the bending stiffness.