Impedance and Noise of Passive and Active Dry EEG Electrodes: A Review

Abstract

Dry electrodes are a promising solution for prolonged EEG signal acquisition, whereas wet electrodes may lose their signal quality in the same situation and require skin preparation for setup. Here, we review the impedance and noise of passive and active dry EEG electrodes. In addition, we compare noise and input impedance of the EEG amplifiers. As there are multiple definitions of impedance in each EEG system, they are all first defined. Electrodes must be compatible with amplifiers to accurately record EEG signals. This implies that their impedance plays a significant role in amplifier compatibility and affects total input-referred noise. Therefore, we review the impedance and noise of state-of-the-art amplifiers and electrodes. Furthermore, we compare the various structures and materials used and their final impedance to that of wet electrodes. Finally, we compare state-of-the-art electrodes and amplifiers to the standards of the IFCN and IEC80601-2-26. We investigate bottlenecks and propose a guideline for future work on passive and active dry electrodes, as well as EEG amplifiers.

Motion Artifact Removal Techniques for Wearable EEG and PPG Sensor Systems

“…The impedance of the outermost skin, called stratum corneum, is about 1MΩ||10nF when the skin is dry. By applying the conductive gel or saline, the impedance value is reduced to 100kΩ||10nF, so wet electrodes minimize the signal attenuation (Shad et al, 2020). Other than the impedance of the stratum corneum, pressure, electrode structure, and skin environment factors such as sweat and hairs also influence the impedance of ESI since the ESI capacitance easily varies ranging from few pF to hundreds of pF by the contact status (Chi et al, 2010;Yousefi et al, 2020).…”

“…FIGURE 3 | An anatomical graphic of the EEG signal path and the corresponding electrical model (Chi et al, 2010;Shad et al, 2020). V signal : voltage signal from the brain; V in : readout circuit input signal; Z ESI , Z gel , V s.c , and Z E : impedance values of ESI, conductive gel, stratum corneum, and electrode, respectively; Z in.Amp : input impedance of the readout circuit; V hc : half-cell potential; Z c : contact impedance at ESI; C gap : capacitance at the noncontact interface.…”

“…While various articles rigorously review artifact reduction schemes for EEG (Xu et al, 2017;Jiang et al, 2019;Shad et al, 2020) and PPG (Periyasamy et al, 2017;Biswas et al, 2019), this article covers cross-border EEG motion artifact removal schemes that can be utilized at each part of the entire acquisition system: from the recording front-end hardware to the processing algorithm software, emphasizing the necessity of a diversified approach on the artifact issues. Furthermore, in-ear EEG researches that are actively studied in recent years are carefully reviewed from a perspective of the motion artifact removal.…”

Review Dry and Non-contact EEG Electrodes for 2010-2021 years

“…In recent years, studies using non-contact EEG is increasing, which is especially noticeable in review articles [66,67,68]. This is driven by the need to make the measurement process more comfortable for patients.…”

“…Lopez-Gordo, M. et al [4], Molinas et al [75], Xu et al [76] and Chi et al [77] -presented review dry EEG sensors. However, their study did not use a systematic approach so that the complete picture of the applications of the dry electrodes is unclear.…”