  Duncan Dowson, whom this issue commemorates, was a world leader in the field
of biotribology, with prolific contributions both in fluid-based and boundary
lubrication of biological tissues, in particular articular cartilage, a central
issue in biotribology due to its importance for joint homeostasis. Here we
explore further the issue of cartilage boundary lubrication, which has been
attributed to phospholipid (PL)-exposing layers at the cartilage surface in
part. A surface force balance (SFB) with unique sensitivity is used to
investigate the normal and frictional interactions of the boundary layers
formed by PLs extracted from osteoarthritic (OA) human synovial fluid (hSF).
Our results reveal that vesicles of the OA-hSF lipids rupture spontaneously to
form bilayers on the mica substrate (which, like the in-vivo articular
cartilage surface in synovial joints, is negatively-charged) which then undergo
hemifusion at quite low pressures in the SFB, attributed to the large
heterogeneity of the hSF lipids. Nanometric friction measurements reveal
friction coefficients mu = ca. 0.03 across the hemi-fused bilayer of these
lipids, indicating residual hydration lubrication at the lipid-headgroup vs.
substrate interface. Addition of calcium ions causes an increase in friction to
mu = ca. 0.2, attributed either to calcium-bridging attraction of lipid
headgroups to the negatively-charged substrate, or a shift of the slip plane to
the more dissipative hydrophobic-tail vs. hydrophobic-tail interface. Our
results suggest that the heterogeneity and composition of the OA-hSF lipids may
be associated with higher friction at the cartilage boundary layers, and thus a
connection with greater wear and degradation, due to hemifusion of the exposed
lipid bilayers.
