Osmotic swelling of chemically responsive hydrogels induced by non-uniform chemical signals

Experiments have shown that hydrogels that couple external chemical stimuli to their constituent polymers offer great potential for controlled shape transformations. For example, rapid release of a chemical stimulus (e.g., copper) trapped within a polyacrylic acid (PAA) hydrogel thin film upon arrival of a second stimulus (acid) can give rise to transient swelling. In contrast with swelling of most gels caused by the osmotic imbalance due to their polymer composition, we hypothesize that the transient swelling of the PAA gels must be driven by the temporary osmotic pressure accumulation upon rapid release of the trapped copper into the gel's fluid phase. Through an augmented 2-dimensional poroelastic theory, we simulate the mechanical response of the copper-complexed gel film to a non-uniform acid front, the subsequent dynamics of the released copper, and the resulting interstitial fluid flow that drives swelling. Our simulations confirm the emergence of traveling swelling fronts at the gel surface, in agreement with experiments. Overall, our theory elucidates the deformation dynamics of the PAA gel films, paving the way for their rational control by using chemical signals.