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Cowley, Leonie

Mechanical Engineering

2011

M.S.

The objective of this work is to investigate the elastic response of ananisotropically compressed lipid bilayer membrane in the context of synovial joint lubrication. It is our hypothesis that the multilamellar arrangement of lipid memhranes and water layers found to coat cartilaginous surfaces in human synovial joints has a fundamental mechanical function in the exceptional lubrication of these joints. In particular the remarkable ability of synovial fluid to resist anisotropic compression sustained during normal joint function. We carried out a series of molecular dynamic (MD) simulations of a model phospholipid (DPPC) bilayer with various levels of hydration, i.e. number of waters per DPPC molecule, and anisotropic compression, i.e. pressure maintained in parallel direction while a larger pressure is applied normal to the bilayer.
The lipid membrane deformed elastically to an equilibrium state, up to a critical pressure, at which the bilayer ruptures and the fluid drains from the system. The critical pressure sustained significantly exceeded normal gait pressure and was found to be affected by changes in the hydration levels, with higher pressures sustained in systems of lower hydration. Examination of the change in area per lipid per change in pressure, normalized for a proposed scaling considering lipid packing and volumetric vacancy, yielded a universal response across the range of simulated hydration levels and this universal response indicates the existence of two distinct regimes of modulus of area compressibility. Finally, we illustrate how this outcome is a significant step in the development of continuum models of lipid membrane elastic behavior.