Background

Textile-based stimulation electrodes are a fast-growing research area. With their advantages including flexibility, reusability and the possibility for integration into garments, textile electrodes open up new possibilities that are not yet feasible today, e.g. various self-administrated treatments and rehabilitation based on neuromuscular electrical stimulation (NMES) or transcutaneous electrical nerve stimulation (TENS). So far, most research has shown that textile-based stimulation electrodes perform more reliable when wetted with an electrolyte. However, there is no systematic investigation about which type and amount of electrolyte to use.

Methods

In this study, double-layered textile electrodes have been produced by machine knitting with a size of 3 × 3 cm2. The electrodes were wetted stepwise with a liquid amount from 5 µL up to 320 µL; four levels of sodium chloride (NaCl) concentrations, i.e. 0.9%, 1.5%, 5% and 35%, plus pure deionized water as a reference liquid were chosen. The study analyzed the behavior of the skin-electrode impedance when changing the moisture content and NaCl concentration. In addition, equivalent circuits were modelled for deeper insights into the mechanisms causing an impedance change.

Results

Results showed that the impedance was greatly influenced by the liquid amount with amounts of 5 µL already significantly reducing the impedance compared to dry electrodes, caused by a substantial reduction in resistance. The reactance, on the other hand, was only partly influenced by the liquid amount showing a reduction upon higher liquid amounts only within a range of 5 – 40 µL. Further, a significant influence on the impedance by the presence of ions was found where the skin-electrode systems wetted with NaCl solution were showing generally lower impedances than systems wetted with deionized water. However, within this, no remarkable influence of the NaCl concentration could be observed. As the impedance was found to be very sensitive to the moisture content in the system, it is recommended to introduce standardizations for impedance testing of wet textile electrodes with precisely controlled electrolyte volumes and liquid migration properties to make independent studies of textile electrodes more comparable.