Locking of poly(sodium acrylate) hydrogels for underwater ultra-stretchability but overwater non-flexibility

Although water is an essential component of hydrogels, developing hydrogels that show distinct deformability depending on the surroundings remains interesting and challenging. Herein, a structural remolding strategy of hydrophilic poly(sodium acrylate) is proposed to develop anti-swelling hydrogels, which interestingly show a significant and reversible mechanical switching of underwater ultra-stretchability and overwater non-flexibility. The super anti-swelling hydrogels are derived based on super-swelling poly(sodium acrylate) hydrogels by simply immersing into 0.2-0.8 mol L^-1 CaCl₂ solutions to form strong Ca²⁺-carboxyl coordination. The strong Ca²⁺-carboxyl coordination in the hydrogels serves as a lock, which bundles neighboring poly(sodium acrylate) chains to resist water intrusion into the polymer network for structural maintenance in underwater conditions, as well as constructs loose porous structures exposing the inside water to the surroundings. The obtained hydrogels with coordination-locked networks show long-term stability, self-healing, and ultra-high stretchability of ∼30 with an equilibrium-water-content of ∼70 wt% in versatile underwater conditions, and achieve rapid dehydration and loss of flexibility within 30 min in overwater conditions. Owing to their unique switchable characteristics, the hydrogels demonstrate multiple concealed functionalities that are activated only in underwater conditions. The work here deepens the understanding of poly(sodium acrylate) hydrogels and paves a new way for the design and remolding of hydrogel topology tailored for underwater-only functions.