The effect of force and electrode material on electrode-to-skin impedance

This paper studies the influence of electrode coating material and contact force on the electrical contact impedance between such electrodes and skin. The electrical interface that exists between the electrode and the skin was modeled, and the modeling parameters as a function of force and material were derived. Increasing force from 80g to 350g decreased the overall resistance from 342k to 75k for AgCl and from 9.6MΩ to 0.9MΩ for gold electrodes, while capacitance increased from 2.3μF to 18μF and from 1.1μF to 5.1μF, respectively. By increasing force the variance of estimated resistance and capacitance values is reduced. This demonstrated that the AgCl electrodes are the preferred candidate for EEG recordings due to better properties in the lower end of the frequency range of interest (0.1 - 100Hz), and that with the higher contact force they can lead to stable and reproducible EEG measurements.