In-situ dual-modal detection of electrophysiology (EP) and blood oxygen (BO) during the muscle-vascular activity is crucial for healthcare and human-machine interaction. Achieving this requires an integrated monitoring system comprising a transparent electrode for EP detection directly adhered to skin and a near-infrared spectroscopy (NIRs) layer for BO detection. In particular, the development of multi-channel, highly transparent, stretchable electrodes is of paramount importance. 

While previous studies have utilized stretchable ionogels to achieve single-channel EP monitoring, the accomplishment of in-situ dual-modal monitoring of both EP and BO by a soft and stretchable patch has remained unattained, and there is still a dearth of research on wet-adhesive and multi-channel ionic electrodes in this context. 

A research team led by TIAN Qiong and Prof. LIU Zhiyuan at Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, has resulted in the development of transparent, stretchable, and skin-adherent multi-channel ionotronic electrodes. The electrodes have been seamlessly integrated with NIRs probes to enable in-situ dual-modal monitoring of muscle-vascular activity. 

The study was published in Advanced Functional Materials on Sep. 26. 

In this study, the ionogel is fabricated by UV-crosslinking of functional monomers to form a basic network structure swelling with ionic liquid. The ionogel exhibits excellent softness, high stretchability, strong wet-adhesion (0.15 N cm^-1), stable contact impedance (99.1 kΩ·cm₂ at 100 Hz for 24 hours), and high transparency (~92%). Adhesion to the epidermis is achieved by incorporating N-hydroxysuccinimide (NHS) groups that can establish chemical bonding with the amino group (-NH₂) on skin surface. 

These characteristics endow the electrode with sweat resistance and the capability for prolonged monitoring of electromyography (EMG) and electrocardiography (ECG). Additionally, ionogels find application in the monitoring of EEG signals. In contrast to Ag/AgCl hydrogel electrodes, ionogels maintain conformal adhesion to the skin even in the presence of sweating. This enables successful monitoring of electrooculogram (EOG) signals during eye opening and closing, as well as the monitoring of alpha waves in the forehead. 

The fabrication of highly transparent 16-channel ionotronics involves a process that includes scraping printing and UV curing of a newly developed anhydrous ionogel. "In contrast to thin-film stretchable gold electrodes, our transparent ionotronic electrodes display minimal interference with NIRs sensing, performing comparably to a condition where the skin is bare. This enables precise detection of BO levels. An in-situ negative correlation between the EP intensity and blood oxygen levels over time during muscle fatigue is distinctly observed with the transparent ionotronic electrode." said Prof. LIU. 

These ionogel-based ionotronics serve as an effective tool for monitoring both electrical and optical signals related to muscle-vascular activity. They hold great promise for widespread utilization in human-machine interfaces and clinical applications. 

Schematic illustration of stretchable multi-channel ionotronic electrodes for in-situ dual-modal monitoring of muscle-vascular activity. (Image by SIAT) 

Media Contact:
ZHANG Xiaomin
Email:xm.zhang@siat.ac.cn