  The outstanding lubrication of articular cartilage in the major synovial
joints such as hips and knees, essential for the joint well-being, has been
attributed to boundary layers of lipids at the outer cartilage surfaces, which
have very low friction mediated by the hydration lubrication mechanism at their
highly hydrated exposed headgroups. However, the role of spontaneously present
lipid splays, lipids with an acyl tail in each of the opposing bilayers, in
modulating the frictional force between lipid bilayers has not, to date, been
considered. In this study, we perform all-atom molecular dynamics simulations
to quantitatively assess the significance of splayed molecules within the
framework of lubricating lipid bilayers. We demonstrate that, although
transient, splayed molecules significantly increase the inter-membrane friction
until their retraction back into the lamellar phase, with this effect more
steadily occurring at lower sliding velocities that are comparable to the
physiological velocities of sliding articular cartilage.
