Tough and Stiff Biobased Hydrogels Reinforced by Strain-Induced Ordering of Hierarchical Structure

Biobased hydrogels that exhibit high stiffness and notable toughness are highly sought after for mimicking load-bearing biological tissues due to their inherent biocompatibility. However, hydrogels sourced from natural polymers often display fragility and brittleness. Inspired by the intricate hierarchical structure of biological polymers found in tissues and muscles, we implemented a strategy to replicate these organized structures, resulting in a biobased hydrogel that combines toughness and stiffness, effectively addressing the mechanical limitations associated with traditional biobased hydrogels. Herein, we introduce chemical cross-linking to enhance the stretchability of carrageenan-based hydrogel as a model biobased polymer, alongside the formation of bundles made of double helix aggregates, facilitated by the synergistic effects of multivalent ions. The existence of double helix bundles from physical cross-links and the enhanced stretchability due to chemical cross-linking result in strain-induced ordering and crystalline-like characteristics under considerable deformation, introducing a toughening mechanism for biobased hydrogels. This effect leads to strain-stiffening, which boosts toughness while also demonstrating antifatigue properties. The resultant biobased hydrogels showcased remarkable properties with a Young's modulus of up to 17 MPa, fracture toughness reaching up to 7.15 kJ/m2, and high breaking energy approaching 7.0 MJ/m3, all while maintaining stretchability. Therefore, this method, which takes advantage of the combined effects of chemical cross-linking and extensive aggregation driven by multivalent ions, establishes a foundation for developing tough biobased hydrogels suitable for groundbreaking applications.