Indentation Rate Sensitive Relaxation of Soft Hydrogels

Mechanical behavior of hydrogels is strongly time-dependent, often characterized as a combination of viscoelastic and poroelastic relaxation. Load-relaxation of hydrogels largely depends on gel composition. Here, we further demonstrate that mechanical loading conditions also influence load relaxation of hydrogels. Spherical indentation experiments are performed for agar, gelatin and polyacrylamide gels with a range of indentation rates. As expected, faster indentation results in more pronounced relaxation in all three gels. The rate-sensitivity is also indentation depth-dependent, such that the degree of relaxation decreases with depth for a constant rate. We interpret the findings within both viscoelastic and poroelastic frameworks. The theoretical analysis demonstrates that viscoelastic relaxation is more rate sensitive compared to poroelastic relaxation. Hydrogels become more viscid at higher indentation rates, indicating significant network reorganization at short time-scales. On the contrary, intrinsic permeability is observed to be largely indentation rate-independent, meaning solvent migration is not affected significantly. Overall, the findings emphasize the importance of loading conditions during mechanical characterization of hydrogels and hydrated biological materials.