Transparent polypyrrole-nanocellulose hydrogel electrodes with durable skin adhesion for wearable ECG monitoring

Skin-adhered electrocardiography (ECG) demands electrodes that maintain low and stable skin-electrode coupling during motion while remaining soft, adhesive, and, ideally, transparent for accurate placement. However, conductive hydrogels rarely combine optical transparency with electronic conductivity because conductive polymers such as polypyrrole (PPy) aggregate in water. Herein, we develop a transparent, self-adhesive hydrogel bioelectrode enabled by incorporating APS-pretreated polypyrrole-phosphorylated cellulose nanofibers (PPy-PCNF) into a crosslinked poly(acrylic acid)/gelatin (PAA/Gel) network. PCNF serves as a charged nanofibrous scaffold that stabilizes PPy in water, while APS-assisted radical pretreatment reduces the apparent size of PPy-rich aggregates and yields an optically clearer conductive dispersion. The PAA/Gel matrix integrates covalent crosslinking with reversible hydrogen bonding and electrostatic interactions, enabling mechanical compliance and durable skin adhesion. The optimized hydrogel achieves >700% stretchability, 21.8 kPa skin adhesion and 2.26 S·m^-1 conductivity; optical transmittance at 660 nm (2 mm thickness) decreases from 56.8% to 22.6% as PPy-PCNF loading increases. When applied as a self-adhered ECG electrode, it captures clear cardiac waveforms with signal quality comparable to commercial Ag/AgCl electrodes, enabling continuous heart-rate readout under rest and motion. Overall, this work provides a scalable dispersion-and-network strategy to unify transparency, adhesion, and electronic conduction in wearable hydrogel bioelectrodes for stable electrophysiological monitoring.