Sea Cucumber-Inspired Double-Network Biopolymer Hydrogels for Programmable Soft Machines

Engineering biopolymer-based hydrogels with robust load-bearing capacity, reversible mechanical programmability, and adaptive actuation remains difficult due to weak physical crosslinking and porous structure. Inspired by the mutable dermis of sea cucumbers, a collagen-alginate double-network elastomeric (CoDE) hydrogel integrates a covalently crosslinked collagen cryogel scaffold with an ionically crosslinked alginate sacrificial network. This hydrogel system is inherently biocompatible and biodegradable. Compression-induced scaffold densification and ionic locking enable broad tuning of the compressive response, sustaining programmable peak stresses up to 16 MPa. CoDE exhibits reversible lock-unlock states while fully preserving its peak stress, elastic modulus, and energy dissipation after repeated loading to 85% compressive strain. Cyclic compressive loading mechanically "trains" the biopolymer hydrogel by altering its microstructure. Chemically triggering the release of stored elastic energy in the biopolymer network generates actuation forces. Further, CoDE is tested as a bio-inspired, externally driven swimmer that can be subjected to large-scale, repetitive deformation with predictable kinematics that match finite-element simulations at the device level. The CoDE platform has future applications for building soft-material machines for adaptive soft robotics, dynamic biointerfaces, and mechanically instructive biomaterials.