Investigation of friction and surface degradation of innovative boundary lubricant functionalized hydrogel material for use as artificial articular cartilage

Hydrogels have been proposed as a synthetic bearing material to replace locally damaged articular cartilage in order to delay a joint replacement. A crucial limitation is that hydrogel tribological properties do not compare to natural cartilage. In this study, the well-known hydrogel polymer, polyvinyl alcohol, was functionalized with organic fatty acid derivative boundary lubricants to create boundary lubricant functionalized hydrogels prepared by freeze-thaw methods. The tribological behavior of the boundary lubricant functionalized hydrogels was investigated and compared against polyvinyl alcohol hydrogels by friction and polymer loss measurements from a pin-on-flat reciprocating sliding apparatus. The wear behavior of the hydrogels was compared while articulated against steel pins in deionized water and bovine cartilage pins in phosphate buffer saline solution. Also the effect of increasing the hydrocarbon chain length of boundary lubricant was investigated. Polymer loss results revealed that at high wear cycles neat polyvinyl alcohol hydrogels displayed the highest polymer loss, while the boundary lubricant functionalized hydrogels exhibited greater wear resistance. Optical microscopy qualitatively showed the polyvinyl alcohol hydrogels exhibited a predominant abrasive wear mechanism, while the boundary lubricant functionalized hydrogel exhibited an adhesive wear mechanism with reduced wear, with the hydrogels having the lowest hydrocarbon chain length boundary lubricant displaying the most improved wear performance with barely noticeable wear track formation. Sliding against cartilage, all hydrogels displayed low polymer loss, and qualitative microscopy showed virtually no wear track development on the boundary lubricant functionalized hydrogel surface for all wear cycles and minimal damage of the cartilage pins. This is the first study reporting the tribological properties of boundary lubricant functionalized hydrogels and established that the boundary lubricant functionalized hydrogels exhibited improved tribological performance as compared to neat polyvinyl alcohol hydrogel.