Anomalous mechanical strengthening of nanocomposite hydrogels upon swelling

Synthetic hydrogels usually exhibit swelling-induced weakening upon water uptake, which greatly hampers their applications in tissue engineering, soft robotics, and load-bearing devices. Inspired by swelling-strengthening feature of hyperaemic muscles in biological soft tissues, we designed a new type of swelling-strengthening nanocomposite hydrogel through the controlled additional cross-linking of ionic coordination in polymer network. More impressively, the swelling nanocomposite hydrogel exhibits a remarkable increment of mechanical properties including tensile stress by 78 %, Young's modulus by 152 %, fracture energy by 33 % and compressive stress by 136 %. The unprecedented performance is attributed to the swelling-mediated additional cross-linking of ionic coordination originating from the decreased acidity in the resultant hydrogel. Furthermore, the self-strengthening hydrogel upon swelling can be designed into a strong turgor actuator, which could break a glass slide within 17 min and lift a 1 kg weight to twice the initial height within 80 min. This bioinspired strategy of using the additional cross-linking for swelling-strengthening of hydrogels may enable broad application potentials in artificial cartilage and soft machine in aquatic environment.