Integrative Evaluation of Skin–Electrode Interface Based on Graphical Analysis and Circuit Modeling of Electrochemical Impedance Spectra

Electrochemical impedance spectroscopy (EIS) is a powerful tool for characterizing skin–electrode interface of biopotential electrodes. However, large information in the impedance spectra had not been fully utilized and clearly elucidated in an integrative manner, making it challenging to accurately characterize biopotential electrodes in real-world applications. This work presents an integrative evaluation framework that combines graphical analysis and equivalent circuit fitting of EIS with biopotential signal evaluation. The goal is to better understand how factors such as adhesion area, skin hair, and application duration affect the contact quality of biopotential electrodes. Specifically, we introduce a quantitative method for analyzing Bode phase plots, which effectively identifies the electrolyte–electrode interface (EEI) and electrolyte–skin interface (ESI) characteristic frequencies, providing specific information into how different factors influence contact conditions. In addition, a new circuit model is proposed for dry electrodes, which is inspired by the graphical analysis and designed to account for the presence of dielectric bubbles at the contact interface. The results show that the EEI and ESI characteristic frequencies were influenced differently by various factors, indicating distinct contributions to the contact interface under varying conditions. The proposed model demonstrated a better fit to EIS measurements compared to the Randles circuit for dry electrodes, with fit parameters that more accurately capture these effects. The study further emphasized the role of the input impedance of the recording system in interpretating biopotential signal quality under varying contact impedance. In summary, this study highlighted the importance of integrative analysis of the EIS data from graphical and modeling perspectives, alongside biopotential signal quality under a range of conditions, to thoroughly evaluate the real-world performance of different types of electrodes.