Multiple Physical Bonds to Realize Highly Tough and Self-Adhesive Double-Network Hydrogels

Tough and adhesive hydrogels have shown great potential in wearable devices, flexible electronics, and human–machine interfaces. However, most existing tough hydrogels have weak surface adhesion, or highly adhesive hydrogels have low mechanical strength. It remains a long-standing challenge to develop the hydrogels with both highly mechanical and adhesive properties. Here, we design and fabricate a fully physical double network hydrogel of Agar/pHEAA-AAc-Fe3+, which consisted of the first Agar network by hydrogen bonds and the second poly(N-hydroxyethyl acrylamide) (pHEAA)-AAc-Fe3+ network by multiple hydrogen and ionic bonds. Because of the special interpenetrating network structures and all reversible physical cross-linking effects, the resulting optimal hydrogels achieved high mechanical performances (breaking strength of 960 kPa, fracture strain of 11, tearing energy of 1306 J/m3, and energy dissipation of 1644 kJ/m3), high stiffness/toughness recovery of 75/51.9% at 80 °C after 30 min resting, and certain degrees of self-healing property. Meanwhile, Agar/pHEAA-AAc-Fe3+ hydrogels also exhibited strong surface adhesion to various untreated solid surfaces, with high interfacial toughness of 2619–4414 J/m2. Both high bulk and interfacial toughness are mainly contributed to synergetic cooperation of multiple noncovalent interactions and double-network structure. This work introduces a set of design concept, hydrogel system, and synthesis approach that could be applicable to other polymers, elastomers, and soft materials to meet different demands (toughness, extensibility, healing, and adhesion) in different bench-to-bedside applications.