Poroelastic relaxation of polymer-loaded hydrogels

Recently, Poroelastic Relaxation Indentation (PRI) was developed as a simple indentation-based approach for quantifying solvent diffusion in polymer gel layers. In this work, we extend the PRI approach to study the poroelastic relaxations of polymer-loaded hydrogels. We observe that PRI can quantify the diffusion coefficients of solvent and polymer solutions within the hydrogel separately via control of the indentation depth, i.e., the compressive strain. Specifically, the water diffusion coefficient is measured for strains above a critical value whereas the polymer solution diffusion coefficient is measured for strains below the critical value. With the aid of the Flory-Rehner theory, we show that this strain-dependent diffusion is related to the extent of deformation of the hydrogel network. Beyond a critical compressive strain, this deformation leads to a significant reduction in the water volume fraction relative to that of PEG solution within the hydrogel, thus enabling the measurement of the diffusion coefficients separately by simply adjusting the indentation depth.