  The hydration layer surrounding the phosphocholine headgroups of
single-component phosphatidylcholine (PC) lipids, or of lipid-mixtures,
assembled at an interface greatly modifies the interfacial properties and
interactions. As water molecules within the hydration layer are held tightly by
the headgroup but are nonetheless very fluid upon shear, the boundary lipid
layers, exposing the highly hydrated headgroup arrays, can provide efficient
boundary lubrication when sliding against an opposing surface, at
physiologically-high contact pressures. Additionally, any free lipids in the
surrounding liquid can heal defects which may form during sliding on the
boundary PC layer. Similar boundary lipid layers contribute to the lubricating,
pressure-bearing, and wear-protection functions of healthy articular joints.
This review presents a survey of the relationship between the molecular
composition of the interfacial complex and the lubrication behavior of the
lipid-based boundary layers, which could be beneficial for designing boundary
lubricants for intra-articular injection for the treatment of early OA.
