A bio-based Janus hydrogel from cellulose and lignin with bilayer structure and asymmetric adhesion for accurate and sensitive human motion monitoring.

Janus hydrogels for human motion monitoring are thriving due to their conductivity, flexibility, anisotropy and self-adhesion, etc. However, most of them face challenges such as complex processes, interlayer detachment, and surface contamination, which degrade their sensing accuracy and sensitivity. Hence, this study proposes a facile strategy using the cellulose and lignin as building blocks to construct a Janus hydrogel for accurate and sensitive sensing. The proposed process involves sequential pouring and in-situ thermal-induced polymerization. Initially, the cellulose dissolved in ZnCl2 solution, along with acrylic acid (Cel/ZnCl2-AA), forms the antifouling and conductive precursors in the top layer, while lignosulfonate and acrylamide (LS-AM) function as the adhesive precursor in the bottom layer. The viscosity difference of the two precursors allows local diffusion and polymerization at the interlayer interface, thereby developing a bilayer structure with strong interface bonds. Consequently, the Janus hydrogel exhibits high conductivity (1.2 S/m), excellent asymmetric adhesion, good stretchability (520 %) and compressive strain (∼70 %). These properties enable the hydrogel to accurately monitor both large (elbow and wrist bending) and small (swallowing and frowning) human movements with accurate sensitivity (gauge factor up to 2). This work offers new insight and synthesis strategy to design the bilayer hydrogels for practical applications in electronic skin and flexible sensing.