Friction and Lubricant Film Thickness Measurements on Simulated Synovial Fluids

This article is part of a research programme to study fundamental aspects of synovial fluid lubrication of artificial hip joints and focuses on the role of proteins (albumin and γ-globulin) in film formation. Friction and film thickness measurements have been carried out on a series of simple protein solutions (simulating synovial fluid SF) and bovine serum (BS). Protein solutions were prepared with different buffer solutions to simulate healthy and periprosthetic SFs. Simple bench-top tests using a ball-on-disc geometry were used to simulate hip joint articulation. All measurements were made under steady-state load and speed (5—50 mm/s) conditions., The friction results showed that BS and simple protein solutions demonstrate boundary properties by reducing friction in the slow-speed regime. The film thickness results with BS provide evidence of the formation of an adsorbed solid surface layer (<20 nm), which is augmented by a thicker hydrodynamic film. Film thickness was fairly constant over the speed range, but tended to decrease with increasing speed. The choice of buffer solution affected the lubrication properties; much thicker films were recorded for the phosphate-buffered saline compared to 2-amino- 2-(hydroxymethyl)-1,3-propanediol protein solutions. The friction and film thickness results were time and/or rubbing distance dependent, suggesting that the formation of the deposited film was pressure/friction related.