Influence of the Electrolyte concentration and amount on the Performance of Textile Electrodes in Electrostimulation: a systematic study

Abstract

Background: Textile-based stimulation electrodes are a fast-growing research area. With their advantages including 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 3x3 cm2. The electrodes were wetted step-wise 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 system wetted with saline 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. Conclusion: 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.