Boundary lubrication by macromolecular layers and its relevance to synovial joints

We review here two recent works on boundary lubrication by macromolecules with the aim of obtaining an insight into the extremely efficient lubrication of mammalian joints. The first work emulates the structure of the cartilage superficial layer by reconstructing films of hyaluronan (HA) and of HA/aggrecan complexes stabilized by cartilage link protein and studies normal and shear interactions between two atomically smooth mica surfaces coated with such molecules using a surface force balance. These molecules have been conjectured to act as boundary lubricants while sliding past each other at the interface between the articular cartilage surfaces and the synovial fluid. It was discovered that the HA/aggrecan/link protein macromolecular complexes provide better boundary lubrication compared with HA alone, most likely as a result of the higher charge density allowing a more efficient hydration lubrication effect. However, the results suggest that such complexes by themselves cannot be responsible for the very efficient lubrication of synovial joints. On the other hand, in the second work, phosphatidylcholine vesicles were shown to provide extremely efficient lubrication at physiological pressures as long as the robustness of the layer is maintained. Normal and shear interactions between two surfaces bearing different phosphatidylcholine vesicles, having the same head group but varying acyl chain length, were studied systematically. In addition to hydration, robustness of the surface layer plays a major role in the lubrication efficiency of the system. Implications of the results of these studies to the lubrication mechanism of synovial joints are suggested. Copyright (c) 2014 John Wiley & Sons, Ltd.