Flexible and biomimicking hydrogels are highly attractive engineering materials of biosensors, soft robotics and flexible electronics. However, the practical application of hydrogels is currently limited by a series of challenges, including inherently fragile networks, trade-offs between mechanical and electrical properties, and limited environmental adaptability. In contrast, natural materials such as tendons and skins, are renowned for their excellent combination of high strength and toughness, owing to their hierarchical structures with synergistic reinforcement mechanisms ranging from nano to macro scales. Moreover, due to their nature as biological hydrogels, these bio-tissues have high sensitivity and exceptional environmental adaptability that are difficult to achieve in artificial materials.

To replicate the hierarchical structures and excellent properties of natural materials, we devised an intricate and versatile processing route for preparing strong and tough hydrogels by a freeze casting-assisted solution substitution strategy (FASS). FASS enables the formation of organo-hydrogels in one step with tissue-like hierarchical structures including anisotropic honeycomb microstructures, entangled and aggregated fiber bundles and fibers, high-functional nanocrystalline domains, and strong molecular bonding. This optimized hierarchical structure endows the organo-hydrogel with outstanding tensile strength (20.8 MPa), stretchability (2227%) and crack insensitivity with a record-high fracture toughness (260 MJ/m3). In addition, the organo-hydrogel is endowed with broad-spectrum multi-functions, including strain and stress sensitive, fatigue- and fracture-resistant, as well as even long-term stability, tolerant to both high temperature and freezing that are difficult to achieve with conventional hydrogels, making it an all-purpose material that can overcome the challenges in real-life applications of flexible electronics. More importantly, combining multi-scale simulation and experimental modelling, our work establishes a systematic roadmap for strong and tough functional hydrogels by unraveling the processing-structure-property relationships in our organo-hydrogel through its multiple levels of structure and functionality. The roadmap also offers an all-around approach that can inspire the development of other advanced functional materials for a multitude of applications.

References

[1] X. Dong, X. Guo, Q. Liu, et al. Advanced Functional Materials, 2022, 32(31), 2203610.

[2] X. Guo, X. Dong, et al. Science Advances, 2023, 9 (2), eadf7075.

[3] X. Dong, X. Guo, Q. Liu, et al. Materials Today, 2023, accepted.