Lubrication mechanisms and frictional behaviour of hydrogels

Abstract

Damaged articular cartilage due to osteoarthritis or sports injuries cannot be cured and the only treatment for severely damaged cartilage is a total joint replacement. Tissue replacement can potentially improve treatment and reduce costs. Hydrogels are considered as cartilage replacements because they have similar physical properties – they are bi-phasic, porous, and have appropriate stiffness – but their clinical success is limited due to poor tribological performance. Our understanding of hydrogel tribology relies on fluid load support theory, which was established for cartilage. The theory holds pressurisation of the interstitial fluid to be crucial for low friction by assuming the load is shared between the solid and liquid phase of the material. However, evidence for this mechanism in hydrogels is limited; a hydrogel-specific understanding of lubrication mechanisms and frictional behaviour is required to successfully optimise hydrogels as cartilage replacement materials. In this study, the frictional behaviour of hydrogels was assessed by measuring the friction forces in reciprocating sliding tests, using photoelastic and fluorescent imaging techniques to identify the lubrication mechanisms. The results showed that fluid load support was not the governing lubrication mechanism and that a new theory is required to explain the frictional behaviour. A new ‘replenishment’ theory was established based on the observation that the motion of the contact played a crucial role in providing lubricant and lowering friction. The major contribution of this thesis is the replenishment theory, highlighting contact motion as an important parameter in biotribological assessments, and which can be used for optimisation of hydrogels. Better hydrogels can be developed by studying the effect of material choices on interfacial shear under non-replenishment conditions. Another contribution is the introduction of photoelastic imaging methods to the study of hydrogel tribology, which can be further applied to better understand hydrogel behaviour and improve the design of hydrogels.