Injectable, tough and adhesive zwitterionic hydrogels for 3D-printed wearable strain sensors

3D printing of high-strength hydrogel remains a great challenge owing to trade-off between strength and printability. Herein, a zwitterionic strategy was proposed through electrostatic reversible interaction to improve the rheology of shear-thinning based on an agarose-based network. Therefore, this mechanism of dual network (DN) ultimately imparts the hydrogel with remarkable strength (with a tensile strength of 0.79 MPa and a compressive strength of 14.18 MPa) and ductility (with a toughness of 766%), while simultaneously possessing multiple functionalities, such as high transparency, excellent biocompatibility, and adhesion. Furthermore, the developed agarose/poly([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium -co-N- hydroxyethylacrylamide) (A/D-H) DN hydrogel exhibited impressive strain sensitivity with a strain gauge factor GF of 4.33 even under large deformations. Various forms of flexible sensors based on this printed gel were successfully utilized in human motion sensing and remote heartbeat monitoring, thereby opening up new perspectives for the further expansion of injectable materials.