Viscoelasticity and wearability of hyaluronate solutions

This work systematically studied the viscoelastic properties of hyaluronic acid (HA) solution, a major component of synovial fluid, under various testing conditions. The optimum relaxation time of HA solution is around 4.5 s at pH 6.8, in the absence of salt at room temperature, indicating that synovial fluid is viscous when the shear rate is less than 1/4.5 s^-1 but elastic when the shear rate exceeds this critical value. HA viscosity declines markedly as the following factors are increased in their order, salt concentration > pH level > temperature, demonstrating that these factors weaken the intermolecular attraction among HA molecules. The non-thixotropic behavior of the HA solution suggests that the breakdown and recovery of the HA structure proceed through the same intermediate states, reconfirming the strong performance of HA as a main component of synovial fluid. The wear results reveal that when the shear rate exceeds a critical value of around 20 s^-1, the drop in viscosity leveled out independent of any further increase in shear rate. However, in a broad range of wear rates (20-300 s^-1), the HA viscosity sustains for wearing time less than 20 min but declines without leveling off as the wear duration increases thereafter. Finally, experimental results verify that bovine albumin (BA), in HA solution, acts as both hydrodynamic and boundary lubricant, substantially improving the wearability of HA.