R. Kusche, S. Kaufmann, and M. Ryschka, IOP Biomed. Phys. Eng. Express, 5, 1, 2018; DOI: 10.1088/2057-1976/aaea59; [PDF]

Abstract: Objective: Bioimpedance measurements are mostly performed utilizing gel electrodes to decrease the occurring electrode-skin impedance. Since in many measurement environments this kind of electrode is not appropriate, the usability of dry electrodes is analysed. Approach: The development of five different kinds of dry electrodes, including gold, stainless steel, carbon rubber and metallized textile as contact materials are proposed. All test electrodes are based on a circular printed circuit board as carrier and have the same contact surface dimensions. To compare the electrodes’ characteristics, the occurring electrode-skin impedances are measured under variation of signal frequency, contact duration, contact pressure, placement position and subjects. Additionally, all measurements are performed with silver/silver chloride (Ag/AgCl) hydrogel electrodes for comparison purposes. Main results: The analysed parameters play a significant role regarding the electrode-skin impedance. Choosing a wise setup of these parameters can decrease the electrode-skin impedance of dry electrodes down to ranges of hydrogel electrodes and even below. Significance: The usage of dry electrodes is one of the most difficult challenges when transferring scientific measurement techniques to clinical environments or commercial products but it is indispensable for many applications like body composition measurements or prosthesis control.

Figure 1: Setup and photographs of the developed dry electrodes. All electrodes consist of a circular PCB as carrier and an additional contact material, which is attached via a conductive epoxy adhesive. Photographs of the contact surfaces are depicted as well. In the bottom row, the surfaces are magnified by a factor of 7.5 by a microscope.
Figure 2: Sleeve for equidistant electrode pair positioning. In the upper photograph, the setup for positioning the electrode pairs to the upper-side of the forearm is shown. Below, the setup to place the electrodes at the under-side is depicted. Five pairs of dry electrodes and one pair of Ag/AgCl dry gel electrodes are equipped (E1: Ag/AgCl; E2: Gold; E3: Carbon Rubber (smooth surface); E4: Stainless Steel; E5: Carbon Rubber (textured surface); E6: Metallized Textile). The red circles mark the contact surfaces of the electrodes below the sleeve.
Figure 3: Time dependencies of the investigated total impedances |Ztotal|, which contain two electrode-skin interface impedances and the bioimpedance in-between them. The measurement was performed on the upper-side of the subject’s forearm. For better visualisation, the impedance magnitude is shown in the range from 100 Ω to 30 kΩ. The frequency ranges from 24 kHz to 391 kHz. In the plot of the textile electrode, just the results for f=391 kHz are shown, since the remaining results are not visible within these axes ranges.
Figure 4: Measured total impedances |Ztotal| over contact force in a range from 1 N to 20 N under variation of the signal frequency from 24 kHz to 391 kHz.
Figure 5: Averaged frequency responses of the eight measurements on the forearm, performed on the four subjects in a frequency range from 24 kHz to 391 kHz. As expected the Ag/AgCl electrodes offer the best performance over subjects and frequency. The best dry electrode performance is offered by the gold-plated electrodes.