An anti-swelling, strong and flexible wood-based composite hydrogel as strain sensor

General flexible conductive hydrogels are excellent candidate as strain-sensor. However, they were usually suffered by high water-absorbing and low mechanical properties. Herein, by introducing delignified wood skeleton into polyacrylamide hydrogel, wood-based composite hydrogels (WCH) with low-swelling and high strength were successfully obtained. The fracture tensile strength of the WCH can reach as high as 19.8 MPa along the longitudinal direction of wood and 2.7 MPa along the transverse direction, which originated from the intrinsic structure of wood skeleton. After water-soaking, size and weight of the WCH increased by only 9 % and 42 %, much less than that of the pristine PAM hydrogel (70 % and 383 %). Based on finite element analysis , wood skeleton structure disperses the swelling stress, which was conducive to reduce the water-swelling of hydrogels. The WCH also showed high adhesion to polar substrates (28.55 kPa to paper) and repeatable adhesion properties. Moreover, the WCH sensors demonstrated stable strain sensitivity, which could monitor bending and pressing. The as-prepared WCH will be promising in broaden the application field of flexible conductive hydrogels as strain-sensors. This work also provides a simple method of balsa wood to new multi-functional and smart applications.