Initiatorless polymerization of mechanically robust hydrogels reinforced by cellulose of wood skeleton as multifunctional sensors

Wood-based hydrogel with a unique anisotropic structure is an attractive soft-and-wet material. However, it remains a challenge to simultaneously achieve robust, multi-functional, and multi-response integrations through a sustainable and green approach. Herein, a bioinspired, additive-free method is reported to fabricate composite hydrogels reinforced by naturally high-strength wood skeleton without using any chemical initiators and crosslinking agents. Specifically, polymers (Polyacrylamide/Polyacrylic acid) are grafted from the surfaces of the aligned cellulose of wood skeleton, forming wood-based hydrogels under UV irradiation. Afterward, Fe³⁺-mediated physical crosslinking is employed further to construct chemically crosslinked poly(acrylamide-co-acrylic acid) networks. Therefore, the resulting initiatorless wood-based hydrogel with a dual-crosslinked network structure exhibits an ultra-high tensile strength of 42 MPa along the longitudinal direction, representing one of the strongest hydrogels ever reported. Furthermore, the wood-based hydrogels with inherent conductive properties appealing versatile sensations on strain, temperature, and light, which could serve as human-motion monitors (detection), thermo-electrochemical sensors, underwater wearable sensors, and smart-home systems. This work offers a green and promising strategy to fabricate robust, anisotropic, flexible, and ionically conductive wood-based hydrogels for multifunctional sensors with excellent performance in complex environments.