Evaluation of friction properties of hydrogels based on a biphasic cartilage model

Characterizing hydrogels using a biphasic cartilage model, which can predict their behavior based on structural properties, such as permeability and aggregate modulus, may be useful for comparing active lubrication modes of cartilage and hydrogels for the design of articular cartilage implants. The effects of interstitial fluid pressurization, inherent matrix viscoelasticity and tension–compression nonlinearity on mechanical properties of the biphasic material were evaluated by linear biphasic (KLM), biphasic poroviscoelastic (BPVE) and linear biphasic with anisotropy cartilage models, respectively. The BPVE model yielded the lowest root mean square error and highest coefficient of determination when predicting confined and unconfined compression stress–relaxation response of hydrogels (n=15): 0.220±0.316 MPa and 0.93±0.08; and 0.017±0.008 MPa and 0.98±0.01 respectively. Since the differences in error between models were not statistically significant, the simplest model we considered, KLM model, was sufficient to predict the mechanical response of this family of hydrogels. The coefficient of friction (COF) of a hydrogel–ceramic articulation was measured at varying loads and pressures to explore the full range of lubrication behavior of hydrogel. Material parameters obtained by biphasic models correlated with COF. Based on the linear biphasic model, COF correlated positively with aggregate modulus (spearmanʹs rho=0.5; p<0.001) and velocity (rho=0.3; p<0.001), and negatively with permeability (rho=−0.3; p<0.001) and load (rho=−0.6; p<0.001). Negative correlation of COF with load and positive correlation with velocity indicated that hydrogel–ceramic articulation was separated by a fluid film. These results together suggested that interstitial fluid pressurization was dominant in the viscoelasticity and lubrication properties of this biphasic material.