An oxidative metal ions-free lignin-catalyzed multifunctional hydrogel bioelectronics for codable eye communication

To meet the stringent requirements of wearable and flexible electronics for functionality and comfort, it is urgent to develop green conductive, self-adhesive, and stretchable functional hydrogels. The chelates of transition metal ions and lignosulfonate sodium (LS) can impart multi-functionality to the hydrogel and significantly improve the hydrogel's gelation speed. However, the presence of metal ions may weaken the adhesiveness of hydrogels by shielding the functional adhesive groups. Here, an oxidative metal ions-free lignin-catalyzed multifunctional polyacrylic acid (PAA) hydrogel is proposed. LS itself can undergo a redox reaction with the initiator to generate many free radicals, thereby catalyzing the rapid polymerization of polymer monomers at room temperature and subsequent gelation. Furthermore, LS can easily improve the hydrogels' softness (compressive modulus: ∼7 kPa) and stretchability (maximum ∼2700 %). Interestingly, LS can simultaneously promote the hydrogel's conductivity, adhesion, and UV blocking. Notably, the hydrogel integrating these advantageous features is suitable as non-invasive electronics in the human epidermis. We explored its ability to act as adhesive bioelectrodes to collect electrooculographic signals in patients with physical and language impairments. Bioelectrodes can recognize the patient's eye movements. The displayed electrical signal can be output in 6 languages after being encoded. This provides a valuable case for LS-doped functional hydrogels in the medical field.