A bilinear model for the elastic response of hydrated lipid bilayers under normal pressure difference

The elasticity of phospholipid membranes as a function of hydration was investigated using coarse-grained molecular simulations. Multilamellar membranes consist of two or more lipid bilayers separated by a thin layer of water, a system commonly found in cell membranes that provides surface tension in the alveoli of the lungs and on cartilaginous surfaces of synovial joints. The objective was to quantify the response of such systems to compression in the direction perpendicular to the membranes as a function of the amount of water between the bilayers or hydration of the system. The present study investigated a variety of phospholipids with six levels of hydration found in multilamellar bilayers in biological systems. Our simulations support the existence of a universal behavior of the increase in surface area per lipid as a function of the normal pressure difference, the difference between the pressure applied in the direction normal to the membrane and the pressure applied in the directions parallel to the membrane. Normalizing the surface area per lipid and the pressure difference by their respective values at rupture yields a composite function of two linear regimes for all the hydration levels under investigation. Where possible, a physics-based interpretation of the normalization scales was provided. Although some parameters of the model are determined empirically, the model represents a promising step in continuum modeling of the response of multilamellar lipid membranes as a function of mechanical stress and hydration.