Understanding the Lubrication Mechanism of Poly(vinyl alcohol) Hydrogels using Infrared Nanospectroscopy

Poly(vinyl alcohol) (PVA) hydrogels are promising contemporary candidates for artificial cartilage owing to their excellent biocompatibility and tribological properties. The origin of their low coefficient of friction, however, is contentious, with contradictory results surrounding biphasic lubrication and fluid load support (FLS) mechanisms. PVA hydrogels consist of cross-linked polymer chains presenting a hydrophilic environment, that yields high water absorption. Their surface water environment, however, has not yet been understood, warranting further investigation. The present work utilizes Attenuated Total Reflection–Fourier Transform Infrared (ATR–FTIR) and Atomic Force Microscopy–Infrared (AFM–IR) spectroscopies to selectively probe the O–H stretching and bending regions of the hydrogel surface statically and dynamically under increasing loads and shear forces. Analysis of donor–acceptor H-bonding environments revealed migration of interstitial water to the surface on increasing compression, supporting the FLS model. However, AFM–IR results showed that shear forces applied under sliding conditions resulted in further water migration, supporting a complementary, replenishing, self-lubrication mechanism that is independent of FLS.