Ultra-stretchable hydrogels with hierarchical hydrogen bonds.

Hydrogels are attractive for applications in intelligent soft materials and flexible electronics. Herein, we report a new hydrogel with a hierarchical hydrogen bond system consisting of (1) weak hydrogen bonds between N,N-dimethylacrylamides (DMAA) and acrylic acids (AAc) and (2) strong multiple hydrogen bonds between 2-ureido-4[1H]-pyrimidinone units. By optimizing the ratios of DMAA and AAc and the balance of weak and strong hydrogen bonds, the hydrogels have unique properties. The transparent hydrogels have tunable Young’s modulus (70–1,250 kPa) and are highly stretchable (up to 4,340% strain), tough (fracture energies of 10.8 kJ/m2, matching natural rubber) and insensitive to notches when it is highly stretched (λ = 19.6).