Dimensionally Stable and Mechanically Adaptive Polyelectrolyte Hydrogel

Mechanically adaptive hydrogels with reversible cross-links can change their mechanical characteristics to adapt to the external environment. However, inevitable swelling/shrinkage occurs with the mechanical property change, which impedes the applications of these hydrogels. In this study, mechanical adaptivity with high dimensional stability is achieved in alginate-based polyelectrolyte hydrogels by introducing an opposite swelling mechanism. The dually crosslinked alginate-polystyrene sulfonate (Alg-PSS) hydrogels are constructed through the copolymerization of alginate-methacrylate (Alg-MA) and sodium p-styrene sulfonate (NaSS), as well as Ca²⁺ crosslinking. In the Alg-PSS hydrogel network, the reversible Ca²⁺ -carboxylate and Ca²⁺ -sulfonate cross-links can be disrupted by Na⁺ and soften the hydrogels. Moreover, the PSS chains crosslinked in the hydrogel network undergo the coil-globule transition in concentrated NaCl solutions to suppress hydrogel swelling during softening. The optimized Alg-PSS hydrogel (Alg₅ -PSS_0.75 -MBAA_2.5 ) shows a dramatic tensile modulus change from 191.3 kPa in deionized water (DIW) to 15.1 kPa in 2.0 mol L^-1 NaCl solution with a negligible volume increase ratio of only 0.6%. The Alg-PSS hydrogels may find applications in artificial valves or soft robotics, where high dimensional stability and invariable volume are required for smart hydrogels.