Stretchable and Biocompatible Transparent Electrodes for Multimodal Biosignal Sensing from Exposed Skin

Real-time health monitoring technology in daily life requires mechanically robust and transparent electrodes for multimodal biosignal sensing from exposed human epidermis. Here, highly stretchable transparent electrodes comprising a water-dispersed conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and a protic ionic liquid (IL), 3-methylimidazolium:bis(trifluoromethylsulfonyl)amide (p-MIM:TFSI) are reported. Owing to the high water miscibility of p-MIM:TFSI and its favorable ion exchange capability with PEDOT:PSS, PEDOT:PSS/p-MIM:TFSI transparent electrodes show enhanced electrical conductivity (σ = 450 S cm−1) and thin-film stretchability represented by crack onset strain (εc) exceeding 50%. These electrodes outperform other PEDOT:PSS electrodes processed with an aprotic counterpart, 1-ethyl-3-methylimidazolium(EMIM):TFSI, or a traditional ionic salt, Li:TFSI. The PEDOT:PSS/p-MIM:TFSI thin-film electrodes are also biocompatible and conformally adhere to human skin; therefore, multimodal biosignals including electrocardiogram, electrooculogram, and electromyogram with high signal-to-noise ratios from exposed epidermis on human faces and arms under various measurement conditions mimicking daily activities are collected. Considering the importance of light penetration through human skin for stable biological activity during biosignal monitoring, the results can broaden the applicability of daily-use wearable biosignal sensors by applying them to exposed human skin.