Tribological Characterization of Gradient-density Polyacrylamide Hydrogel Surfaces

Background: The contact and frictional response of a hydrogel is dependent on the polymer structure at the gel surface. Recent work has shown that different mold materials in contact with the gel during polymerization will affect the resulting polymer density. Objective: The tribological response of a gel with a ‘brushy’ less-dense polymer surface has not been thoroughly studied. Our goal was to perform a suite of tribological experiments to better understand the response of the less-dense layer. Methods: In this work, we conducted indentation, creep, and sliding experiments with various loads, speeds, and probe materials to determine the impact of the less-dense layer on the contact and frictional behavior of polyacrylamide hydrogels. We additionally used micro-fluorescent particle exclusion to measure the contact areas throughout each experiment. Results: Indentation revealed a non-Hertzian regime for the first 13–29 µm after first contact that has a weaker force response for a given indentation depth. Creep experiments showed that the surface layer relaxes poroelastically, with water exudation occurring within the gradient layer despite the low contact pressures. Friction was highly speed-dependent, with faster sliding speeds decreasing friction to values as low as 0.01; transient behavior was not seen for most of the experiments, suggesting that the surface layer is capable of quick water re-uptake when out of contact. Conclusions: We have provided a deeper understanding of the contact and frictional response of this gradient-density surface layer, which will prove useful for hydrogel designs requiring ultra-low friction in a dynamic application.