Highly strong and flexible composite hydrogel reinforced by aligned wood cellulose skeleton via alkali treatment for muscle-like sensors

Strong and flexible hydrogels have attracted increasing scientific interest due to their soft-and-wetproperties, which are similar to those of human soft tissues. In this study, a bioinspired, facile method to fabricate a composite hydrogel with a naturally high-strength skeleton structure that shows comparable mechanical performance to those of muscle, tendons and ligaments is reported. The method includes extracting an aligned cellulose skeleton directly from wood by delignification and then compositing with polyacrylamide (PAM) by in situ chemical polymerization. During these processes, the natural wood skeleton is well preserved and sufficiently high tensile stress is created along the longitudinal direction (L-direction) of wood cellulose fiber, which leads to a highly anisotropic structure in the prepared PAM/delignified wood (PDW) hydrogels. Although the aligned cellulose skeleton exhibits an efficient strengthening effect, the obtained PDM hydrogels lack the desired flexibility and are easily broken during the harsh deformations. To achieve flexiblility, a simple alkali treatment was applied to the delignified wood which provided the developed PAM/alkali-treated delignified wood (APDM) hydrogels with superflexiblility. As a result, the A-PDM hydrogels show excellent tensile properties with Young's modulus, tensile fracture strength and elongation of 145.52 +/- 2.32 MPa, 16.47 +/- 1.40 MPa and 15.99 +/- 1.81%, respectively. Such favorable mechanical performance is employed to assemble a muscle-like sensor by incorporating the A-PDM hydrogel with ionic Na2SO4 to detect diverse macro-scale human motions. This study provides a facile strategy for designing strong composite hydrogels with superflexibility, opening an effective route for the development of new wood nanotechnology and various functional wood-derived materials.