Zinc-ion engineered Plant-based multifunctional hydrogels for flexible wearable strain Sensors, Bio-electrodes and Zinc-ion hybrid capacitors

Multifunctional hydrogels, particularly with superior mechanical properties and using green/sustainable approaches, have attracted increasing attention because of their many applications and consistency with the green chemistry principle; unfortunately, their effective fabrication process is challenging. In this work, inspired by unique functions in plant-derived components, a multifunctional polyacrylic acid (PAA) hydrogel was innovatively developed using ZnCl2 and biomass-derived materials. Ingeniously, ZnCl2 has multiple functions, in combination with cellulose, lignin, and citric acid (CA), delivering intriguing properties to the hydrogel system. As a solvent system, the ZnCl2 solution has extraordinary H-bonding-donating ability, dissolving cellulose macromolecular chains; subsequently, solubilized cellulose as green reinforced-fillers are added to hydrogel, improving the strength properties, with compression strength of ∼ 4.1 MPa, tensile strength of ∼ 795.4 kPa, and elongation at break of ∼ 620 %. The lignin-ZnCl2 catalysis system imparts a fast PAA gelation process, and the resultant hydrogel has strong/long-lasting/repeatable adhesive strength of 25.5 kPa (on Zn-carbon cloth) in 28 days. Additionally, ZnCl2 can form reversible ionic/electrostatic interactions with both lignin and CA, further improving the adhesive and mechanical properties. As anti-freezing agents, water-retaining agents and conducting medium, ZnCl2 also endows the hydrogel with desirable environmental compatibility and good conductivity (14.2 mS·cm−1). Furthermore, the presence of plant-based components endows the hydrogel with ultraviolet (UV) blocking, biocompatibility and antibacterial attribute. Significantly, the as-prepared hydrogels are suited for such diverse applications as flexible wearable strain sensors, bio-electrodes and zinc-ion hybrid capacitors (ZHCs), even can efficiently work at −65 °C. This study provides a new strategy of engineering multifunctional hydrogels using plant-based functional components, and these hydrogels show promising applications in many emerging areas including electronic skin sensors, health monitoring and energy storage devices.