Feasibility of Direct Current stimulation through hair using a dry electrode: potential for transcranial Direct Current Stimulation (tDCS) application

Conventional transcranial direct current stimulation (tDCS) protocols typically deliver 2 mA for 20-30 minutes. The most common administration uses a wet electrode approach which dries out in similar to 60 minutes at room temperature. This restricts its application to limited duration electrode-scalp contact use cases unless additional conductive media (saline, gel, or paste) is re-applied. This problem is further compounded by the subject's hair which not only presents administration challenges (interferes with electrode attachment and adhesion) but also acts as a conduit of current flow into the scalp resulting in current hotspots. This non-uniform current injection results in increased skin sensation. The aim of this study was to determine suitability of a commercially available hydrogel for DC delivery through hair. Experiments involved both non-clinical testing on an agar block and clinical testing on subjects' forearms. Electrodes were positioned on the posterior side of the forearm that has hair for the clinical testing. Typical dose as used in tDCS was delivered and pain scores were collected. Results indicate suitable current delivery performance and all subjects tolerated delivery with pain scores ranging between 0-6. Our study paves the way for future testing on the scalp for tDCS application.