Fluid load support does not explain tribological performance of PVA hydrogels

Abstract

The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stressrelaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.

A lubrication replenishment theory for hydrogels

“…The graph shows that the 4 mm stroke resulted in a clearly rising friction coefficient over time and the 6 mm stroke resulted in a stable and low friction coefficient over time. This is in agreement with results obtained in previous work 13 with a similar distinction between short and long stroke lengths. The average friction coefficient for the 5 mm stroke lies between the low friction of the 6 mm stroke and the high friction of 4 mm stroke.…”

“…Hydrogels were prepared following the method reported previously. 13 In summary, for the 'base' hydrogel, an optically transparent gel was obtained by dissolving 15 wt% poly(vinyl alcohol) pellets (PVA, 146 000-186 000 M w , 99+% hydrolysed, Sigma Aldrich, UK) in a mixture of 80 wt% di-methyl sulfoxide (DMSO, Z99%, FG, Sigma-Aldrich, UK) and 20 wt% ultra-pure water. 36 The mixture was moulded into a 45 Â 120 Â 8 mm rectangle and frozen at À23 1C for 20 hours.…”

“…The elastic modulus of this hydrogel in unconfined compression was found to be between 0.1-0.25 MPa, depending on the applied strain. 13 A more compliant hydrogel was included in this study to test the general applicability of the replenishment theory. This 'soft' hydrogel had a lower compressive stiffness (0.05-0.08 MPa) compared to the base hydrogel, which was achieved by using a lower polymer content (5%) compared to the base material (15%).…”

“…7,8 However, friction measurements alone do not provide sufficient evidence for fluid load support. In previous work 13 the authors used photoelastic stress measurements of hydrogels during reciprocating frictional testing to show that the observed frictional behaviour could not be related to fluid load support in the material, and it was hypothesized that the replenishment of lubricant in the contact governs the tribological behaviour of the hydrogel. Fig.…”

“…1 illustrates the distinction between the hypothesized replenished and non-replenished regimes, dependent on the contact conditions. 13 When the stroke length is larger than the contact width (42a), the system has a migrating contact condition. This configuration was identified as a replenished state, as the motion likely ensures sufficient lubricant supply in the contact, either by a lubricating film or direct re-exposure to the externally applied lubricant, to maintain low friction.…”