  The very low sliding friction of articular cartilage in the major synovial
joints such as hips and knees is crucial for their well-being, and has been
attributed to lubrication by phospholipid boundary layers. While
single-component lipid layers have demonstrated efficient lubricity in model
studies, in living joints there is a large number of different lipids, raising
the question of whether this is natural redundancy, or whether this
multiplicity confers any benefits. Here we examine lubrication by progressively
more complex mixtures of lipids representative of those in joints, using a
surface forces balance at physiologically-relevant salt concentrations and
pressures. We find that different lipid combinations differ very significantly
in their lubricating ability, as manifested by their robustness to hemifusion
under physiological loads, pointing to a clear lubrication synergy arising from
multiple lipid types in the lubricating layers. Insight into the origins of
this synergy is provided by molecular dynamics (MD) simulations of the
different lipid mixtures used in the experiments, which directly reveal how
hemifusion - associated with greatly increased friction - depends on the
detailed lipid composition. Our results provide insight into the role of lipid
type proliferation in healthy synovial joints, and point to new treatment
modalities for osteoarthritis.
