Ultrasound Cavitation Enables Rapid, Initiator-Free Fabrication of Tough Anti-Freezing Hydrogels

Hydrogels are often synthesized with thermal or photo-initiated gelation, leaving alternative energy sources less explored. While ultrasound has been used for polymer synthesis and mechanochemistry, its application through cavitation for hydrogel synthesis as a constructive force is rare, and the underlying sonochemical mechanisms are poorly understood. Here, the application and mechanism of ultrasound cavitation for rapid, initiator-free, and oxygen-tolerant fabrication of tough anti-freezing hydrogels is reported. By incorporating polyol solvents and interpenetrating polymers into the gelling solution, radical generation is amplified and network formation is enhanced. Using tough polyacrylamide-alginate hydrogels as a model system, rapid gelation (as fast as 2 minutes) and high fracture toughness (up to 600 J m^{- 2}) is demonstrated. By varying ultrasound intensity, crosslinker-to-monomer ratio, and glycerol concentration, the synthesis-structure-property relation is established for the resulting sonogels and the underlying mechanism is uncovered using combined molecular, optical, and mechanical testing techniques. The coupling of gelation and convection under ultrasound results in sonogels with unique structural and mechanical properties. Additionally, the fabrication of hydrogel constructs is demonstrated using both non-focused and high-intensity focused ultrasound. This work establishes a foundation for ultrasound-driven sono-fabrication and highlights new avenues in soft materials, advanced manufacturing, bioadhesives, and tissue engineering.