Bioinspired mechanical energy storage and non-dissipative release in hierarchically swollen and entangled hydrogels

Living tissues possess synergistic mechanical properties that facilitate specific functions. Unlike toughening allowed by promoted mechanical energy dissipation, by contrast, dissipation is minimized for mechanical energy storage and efficient release, such as facilitating animal jumping. Nature shows such resilient materials, e.g., resilin, and elastin, characteristically involving small strain with high stiffness. Such properties have rarely been achieved in synthetic hydrogels, even if hydrogels are considered analogous to living soft tissue. We show bioinspired resilient hydrogels with high energy storage at small strains and high stiffness upon repeated hydrogel swellings in mixtures of monomers, crosslinkers and water with subsequent photopolymerizations up to 7 hierarchical levels N. Towards higher N, the chains are progressively more coiled and entangled around the stretched chains of lower N, leading to hierarchical swelling. By controlling N, the approach allows to surpass even biological proteins for efficient energy storage. We show generality and application potential for soft robot jumpers.