Modeling ionic hydrogels swelling: characterization of the non-steady state

Ionic hydrogels can be used as controlled release systems that respond to an external substrate or trigger by swelling or de-swelling. One example is a glucose-sensitive system for insulin-controlled release based on pH-sensitive hydrogel. To enhance understanding of non-steady state swelling, and to facilitate design of specifications (e.g., glucose-sensitivity) of the pH-sensitive ionic hydrogel based on the copolymer poly (2-hydroxyethyl methacrylate-co-N, N-dimethylaminoethyl methacrylate) (poly (HEMA-co-DMAEMA)), we developed a mathematical compartmental model using the software SAAM II. Current analytical and computational methods focus on equilibrium swelling of hydrogels; although for many stimuli-responsive hydrogel applications, the dynamic process is significant. We now report, using a combination of experimental data and kinetic analysis that in the poly (HEMA-co-DMAEMA) the rate of proton entry is governed by a different rate coefficient than water entry rate. The transport coefficient governing water uptake is dependent upon three variables: pH of external media, amine groups incorporated into the polymer, and crosslinking density of the polymer. An additional result is that swelling equilibrium is reached when all the amine groups are protonated. In this study we also demonstrate the predictive capability of the model for both interpolated and extrapolated data, and its use in design of future bench experiments. Uncovering these fundamental properties of pH-sensitive hydrogels with the aid of a kinetic model suggests that the complexities of hydrogel research and development can be overcome by combining experimental and computational approaches.