Towards gel-free electrodes: A systematic study of electrode-skin impedance

Emerging real-world EEG applications require gel-free electrodes, which have to break through technical bottlenecks and achieve satisfactory electrode-skin impedance. It is crucial to understand the electrical properties of the electrode-skin interface. In this work, the electrode-skin impedance of bioelectrodes (wet, semi-dry, and dry) has been studied systemically, concerning not only magnitude but stability. Various factors have been investigated including types of electrodes, skin locations, pressure, skin abrasion, and electrode contact area. The electrode-skin impedance always decreases in the following order: forearm, scalp and forehead for all electrodes, Compared with the impedances of wet electrodes and semi-dry electrodes, the dry electrode impedances are significantly higher (58.50 +/- 64.16 k Omega cm(2)) and unstable (impedance variation 31.2 +/- 31.3 k Omega/10 min). Even worse, the dry electrode impedance variation between six subjects is considerably large (57.5-540.0 k Omega). As a result, no satisfactory EEG signals could be obtained. Moreover, the dry electrode impedances are lowered significantly under pressure or after skin abrasion. Accordingly, alpha rhythms from the dry electrodes appeared with the assistance of pressure or skin abrasion. These findings provide insights for the development of new gel-free electrodes to complement the emerging new EEG applications, such as brain-computer interfaces and wearable EEGs. (C) 2016 Elsevier B.V. All rights reserved.