Stretchable, self-adhesive conductive hydrogels based on gelatin and oxidized hydroxyethyl starch as flexible sensors...

Natural polymer based-hydrogels have attracted increasing attention as flexible wearable sensors owing to their excellent biocompatibility. However, hydrogels derived from natural polymers with balanced mechanical and adhesive properties still remain a challenge. To address this issue, a hybrid hydrogel network based on two natural polymers and poly(lipoic acid) (PLA) was fabricated. Specifically, gelatin (Gel) and oxidized hydroxyethyl starch (OHES) were crosslinked via Schiff base linkages to provide mechanical support. The PLA network imparted adhesion through the ring-opening polymerization of lipoic acid (LA). Tannic acid (TA) suppressed PLA depolymerization through Michael addition-mediated interactions, and simultaneously boosted the hydrogel's adhesive performance. Moreover, zinc ions (Zn²⁺) endowed the hydrogel with ionic conductivity and served as dynamic coordination crosslinks within the polymer network, leading to enhanced mechanical robustness and stretchability. Then, the hydrogel was assembled into a strain sensor capable of directly adhering to human joints. It could accurately detect both large-scale motions (e.g., joint and neck bending) and subtle movements (e.g., swallowing and smiling). In addition, the hydrogel functioned as a skin-conformal electrode for high-quality acquisition of physiological bioelectrical signals, including electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG).