SMART Silly Putty: Stretchable, Malleable, Adherable, Reusable, and Tear‐Resistible Hydrogels

Cell engineering, soft robotics, and wearable electronics often desire soft materials that are easy to deform, self-heal readily, and can relax stress rapidly. Hydrogels, a type of hydrophilic networks, are such kind of materials that can be made responsive to environmental stimuli. However, conventional hydrogels often suffer from poor stretchability and repairability. Here, hydrogels consisting of boronic ester dynamic covalent bonds in a double network of poly(vinyl alcohol)/boric acid and chitosan are synthesized, which demonstrate extreme stretchability (up to 310 times the original length), instant self-healing (within 5 s), and reusability and inherent adhesion. Their instant stress relaxation stems from a low activation energy of the boronic ester bond exchange (≤20 kJ mol^-1 ) and contributes to the extreme stretchability and self-healing behaviors. Various water-dispersible additives can be readily incorporated in the hydrogels via hand kneading for potential applications such as soft electronics, bio-signal sensing, and soft artificial joints.