Effect of Pressure on Skin-Electrode Impedance in Wearable Biomedical Measurement Devices

Objective: This paper investigates the effect of applied pressure on the skin-electrode impedance. Applied pressure, which affects the skin-electrode impedance, can fluctuate in many acquisition setups, particularly in wearable devices. The skin-electrode impedance, in turn, impacts the quality of the recorded signal in biomedical monitoring devices. Methods: Three types of electrodes were examined: Ag/AgCl electrodes, conductive textile electrodes, and dry electrodes with surface microfeatures (Orbital Research Inc.). Impedance measurements were conducted as pressure was repeatedly applied ( $P = 4$ kPa) and removed ( $P = 0$ kPa) over several trials. A Cole–Cole impedance model was utilized to model the skin-electrode interface. Significance and Results: Results demonstrated large decreases in the skin-electrode impedance of dry electrodes (conductive textile and orbital electrodes), especially with the initial application of the pressure. Model parameters also proved to be highly dependent on the level of pressure in dry electrodes but less dependent and more stable in wet electrodes. Decreases in skin-electrode impedance associated with applied pressure were thought to be caused by an increased effective electrode contact area. Changes in skin-electrode impedance were irreversible, lasting even after the applied pressure was released. Differences skin-electrode impedance associated with changes in applied pressure, decreased as the number of trials increased. Conclusion: Applied pressure has larger effect on dry electrodes than wet electrodes. Wearable devices that employ dry electrodes may have poorer biomedical signal quality when initially donned; however, the advantage of wet electrodes with their lower sensitivity to applied pressure is diminished in long-term monitoring applications.