3D Printing of Ultrastretchable and Tough Double‐Network Hydrogel for Strain Sensor

Stretchable conductive hydrogels have garnered considerable recognition due to their uses in strain sensors, electronic skins, soft robotics, and actuators. However, many hydrogels have poor mechanical properties limiting widespread implementation. While the development of ultrastretchable and mechanically robust hydrogels remains a challenge, the fabrication of these materials with customized designs is also highly desirable. Herein, a direct‐ink write 3D printable double‐network (DN) hydrogel is reported by integrating a physically cross‐linked κ‐carrageenan and a chemically cross‐linked poly(acrylamide‐co‐hydroxyethyl acrylate‐co‐Pluronic F127‐bisurethane methacrylate) with an ionically cross‐linked coordination between κ‐carrageenan and Fe3+ ions in water–glycerol binary solvent. The DN hydrogel demonstrates excellent stretchability (1770% strain), remarkable toughness (6.24 MJ m−3), high ionic conductivity (1.55 S m−1), biocompatibility, and nondrying behavior. A variety of 3D printed constructs including auxetic structures are fabricated and used as a strain sensor. The sensor exhibited real‐time electrical response to strain to detect human motions demonstrating the practicality of this system. These 3D printable DN hydrogels show great potential for on‐demand fabrication of flexible health‐monitoring devices.