Textile Electrodes: Influence of Knitting Construction and Pressure on the Contact Impedance

Abstract

Textile electrodes, also called textrodes, for biosignal monitoring as well as electrostimulation are central for the emerging research field of smart textiles. However, so far, only the general suitability of textrodes for those areas was investigated, while the influencing parameters on the contact impedance related to the electrode construction and external factors remain rather unknown. Therefore, in this work, six different knitted electrodes, applied both wet and dry, were compared regarding the influence of specific knitting construction parameters on the three-electrode contact impedance measured on a human forearm. Additionally, the influence of applying pressure was investigated in a two-electrode setup using a water-based agar dummy. Further, simulation of an equivalent circuit was used for quantitative evaluation. Indications were found that the preferred electrode construction to achieve the lowest contact impedance includes a square shaped electrode, knitted with a high yarn density and, in the case of dry electrodes, an uneven surface topography consisting of loops, while in wet condition a smooth surface is favorable. Wet electrodes are showing a greatly reduced contact impedance and are therefore to be preferred over dry ones; however, opportunities are seen for improving the electrode performance of dry electrodes by applying pressure to the system, thereby avoiding disadvantages of wet electrodes with fluid administration, drying-out of the electrolyte, and discomfort arising from a “wet feeling”.

A review of textile-based electrodes developed for electrostimulation

“…For example, Zhou et al 50 compared the performance of the same electrode construction in dry and wet conditions. They observed significantly improved stimulation comfort and motor threshold with wet electrodes; similarly, Euler et al 65 found a significantly lower skin-electrode impedance for the same textile electrode construction wetted with tap water.…”

“…As impedance is frequencydependent, values from different frequencies are incomparable. Hunold et al 67 and Euler, Guo and Persson 65 presented impedances at similar frequencies (i.e. 85 and 39 Hz) for which the wet electrodes showed values in a similar range from 10.0 to 15.8 kX, while the electrode from Zhou et al 50 had a much lower presented impedance with 0.95 kX due to presenting the impedance at 1 kHz.…”

“…85 and 39 Hz) for which the wet electrodes showed values in a similar range from 10.0 to 15.8 kX, while the electrode from Zhou et al 50 had a much lower presented impedance with 0.95 kX due to presenting the impedance at 1 kHz. Further, the electrode area has a considerable influence on the skin-electrode impedance, 65 which means that the electrodes from the three compared studies are expected to have different impedances arising from the mismatching electrode sizes. Nevertheless, a general observation can be made that the electrodes in the dry condition showed significantly higher impedances than the electrodes for which electrolytes were used.…”

“…Material research was performed by Ali et al, 44 Gniotek et al, 57 and Frydrysiak et al, 61 Oh et al, 64 Papaiordanidou et al 53 and Stewart et al, 39 who investigated the suitability of various kinds of conductive fabrics to be used as textile stimulation electrodes based on the evaluation of electrical properties as well as stimulation performance aspects. Erdem et al 34 and Euler et al 65 manufactured single textile electrodes, based on different production methods, and evaluated the influence of electrode construction parameters with regard to long-term usage behavior or electrical performance. Further, Zhou et al 50 assessed the electrical properties and stimulation performance of dry and wet multi-layered textile electrodes in comparison to conventional hydrogel electrodes.…”

“…The electrolytes used in electrostimulation varied across studies. Ten studies (presented in 11 publications) used electrodes in a wet condition with either water, saline solution or gel, 21,35,37,39,50,53,54,60,62,65,67 whereas three studies (presented in four articles) used dry electrodes without additional electrolytes. 41,50,52,66 Yet it must be noted that, although D€ olker et al 66 chose dry electrodes, the skin was moistened with a wet towel before use, making the electrodes a between-state of dry or wet electrodes.…”