Stretchable, adhesive, antifreezing and 3D printable double-network hydrogel for flexible strain sensors

A multifunctional double-network (DN) hydrogel was produced for flexible strain sensor. It was composed of the brittle physical cross-linked network of agar and the ductile chemical cross-linked network of acrylamide (AAm). It had excellent mechanical properties under the interactions of various noncovalent bonds such as electrostatic force, coordination bonds and hydrogen bonds between nano fillers, polyzwitterions and conductive ions. The DN hydrogel had antifreezing performance and adhesion because of the intervention of zwitterionic proline. Prominent strain sensing ability promoted it as a flexible strain sensor for human motion monitoring and motion feedback. In addition, it was also suitable for 3D printing of complex geometry structure. And all these properties were discussed separately. • A multifunctional double-network (DN) APCP-Ca hydrogel was prepared. • Stretchable, adhesive, antifreezing and 3D printable properties of DN hydrogel were discussed respectively. • DN APCP-Ca hydrogel-based strain sensor and sensing applications were studied. As a new flexible strain sensing material, hydrogels have broad application prospects in the fields of wearable electronic devices, human health monitoring devices and robot aided devices. Therefore, multiple excellent properties of hydrogels were taken into account simultaneously in this work. Double-network (DN) strategy of combining hardness and flexibility was adopted, and the plant materials were adhibited (nanocellulose and proline). The mechanical properties of hydrogels could be significantly improved by combining nanocellulose with conductive ions (stress of 0.293 MPa and strain of 2587%). The excellent antifreezing properties (the conductivity was 0.087 S m −1 at −20 °C) and adhesiveness of hydrogels were endowed with the zwitterionic proline, which was significantly accumulated in plants under low temperature stress. The prepared multifunctional hydrogel was suitable for 3D printing, which provided a new approach for producing complex geometry. Multifunctional hydrogel could be used as a flexible strain sensor to detect human movements, including limb bending, facial expression and vocal vibration because of its outstanding strain sensing characteristics. Hence, we expect it to be an ideal candidate material to flexible electronic detection equipments for human motion detection or machine motion feedback in the future.