Freezing-induced ice-polymer structuring enables tough, additive-free hydrogels under extreme cold conditions

Designing hydrogels that retain mechanical robustness under subzero conditions remains a major challenge for applications in bioengineering, soft robotics, and extreme-environment systems. In this study, we introduce a fundamentally new strategy for creating tough frozen hydrogels by transforming ice - traditionally regarded as a brittle and damaging phase - into a functional, load-bearing component. Through the in situ formation of ice crystals within a fully physically crosslinked double-network (DN) hydrogel, ice is redefined from a transient structural template to an intrinsic reinforcing phase. The resulting frozen hydrogels exhibit remarkable stretchability, flexibility, and fracture resistance, even at cryogenic temperatures as low as -196 °C. Unlike conventional approaches that rely on nanofillers or cryogel templating followed by ice removal, this additive-free method harnesses directional freezing to achieve uniform ice-polymer structuring. Mechanistically, embedded ice crystals act as sacrificial energy-dissipating domains, enhancing toughness via interfacial debonding, microcrack deflection, and crack path redirection. This approach circumvents the dispersion and interfacial limitations of nanocomposites and establishes a scalable design paradigm for next-generation ice-reinforced soft composites capable of operating under extreme thermal conditions.