Surface lubrication mechanisms in hydrogels mimicking articular cartilage

Articular cartilage, which lines the surfaces of the major joints, is a remarkably well lubricated surface, unmatched by any man-made material for its low coefficient of friction and life span. Understanding how such self-renewing biological lubrication works, and utilizing it to create artificial surfaces with low friction, is still a major challenge.

Hydration lubrication is one of a number of lubrication mechanisms proposed to provide the low friction properties of cartilage, and biologically-ubiquitous phosphatidylcholine (PC) lipids have been proposed as highly efficient lubrication elements via this mechanism. Their highly-hydrated phosphocholine headgroups form a hydration shell with water molecules that interchange easily with unbound molecules, but are not squeezed out under high pressures, retaining low friction. An alternative popular theory is biphasic lubrication. In brief, this theory proposes that the fluid component of cartilage (60-80%) provides significant load support, reducing the pressure on the solid component at the surface, sufficiently reducing the surface friction.

This work aimed to establish how biphasic and hydration lubrication co-exist, and in which loading regime each becomes significant. Double-network hydrogels with high-water content (90%) and tough bulk mechanical properties, in compression and shear, were used to mimic articular cartilage. Novel lipid-based lubricants were added, and interfacial properties were determined using a reciprocating tribometer, while bulk properties of the gels were determined with an oscillating rheometry system. The microstructure of the gel-lipid combination was studied via fluorescent confocal microscopy. The effect of the addition of lipids on surface lubrication provided the cartilage model with exceptional interfacial lubricity approaching that of natural cartilage.