Friction, wear, and lubrication of hydrogels as synthetic articular cartilage

In recent years, synthetic hydrogels have been studied not only as possible replacements for articular cartilage but as platforms for growing cartilage in a damaged joint. In the present study, the tribological behavior of synthesized poly(2-hydroxyethyl)methacrylate (polyHEMA) hydrogels was investigated in a four-factor, two-level designed experiment using a device developed for biotribology research. The contact geometry consisted of a 6-mm diameter stainless steel ball on flat polyHEMA disks. The variables in the designed experiment were (a) applied load, (b) lubrication, (c) hydrogel crosslink density, and (d) degree of hydrogel hydration. Linear oscillating sliding contact tests were conducted for 30 min for each test. The results showed several significant main effects and first-order interactions. Increasing the applied load from 6 to 20 N increased average hydrogel wear by 125%. Increased crosslink; density reduced wear by over 60%. And increased hydration resulted in an increase of 130% in wear. The coefficient of friction ranged from a low value of 0.02 to a high of 1.7 while linear wear varied by a factor of over 60. Interactions between (a) hydration and lubrication and (b) hydration and crosslinking on wear were highly significant. The single most important finding from this study was that, as expected, there was no correlation between friction - which is commonly reported in the literature on hydrogels - and wear. Most of the data fell into two groups, namely low wear/low friction and high wear/low friction. These results may be useful in the tribological design of hydrogels for both low wear and low friction. (C) 2000 Elsevier Science S.A. All rights reserved.