Mechanics and tribology of a zwitterionic polymer blend: Impact of molecular weight

Despite decades of biomimetic materials development, the tribological properties of articular cartilage remain unrivalled. This manuscript presents the design and material properties of a polymer blend composed of poly (vinyl alcohol) (PVA) and a zwitterionic polysulfobetaine (PMEDSAH) prepared into hydrogel form using a cyclic freeze-thaw method. The PVA hydrogel matrix provides mechanical strength while the zwitterionic polymer, PMEDSAH, is intended to act as a boundary lubricant. The formation of PVA-PMEDAH hydrogel blends was found to result in unique biomimetic system where the boundary lubricant elutes from the bulk material to the surface in response to applied pressure. This behavior is attributed to the high-water content of the PVA hydrogel matrix and the solubility of PMEDSAH in aqueous solution. In addition to characterizing the effects of boundary lubricant molecular weight on the diffusive properties of the hydrogel blend, we report the coefficient of friction, mu, versus sliding speed for the hydrogel/glass interface. Consistent with our prior findings, PMEDSAH was found to engender lubricious behavior and the dependence of mu on sliding velocity indicated a repulsive interaction with glass rather than an attractive one. This result agrees with the hydration lubrication hypothesis. Contact mechanics analyzed within the context of Hertzian biphasic theory were also investigated, revealing that the introduction of PMEDSAH enhances the hydrogel's ability to provide interstitial fluid load support.