Novel passive ceramic based semi-dry electrodes for recording electroencephalography signals from the hairy scalp

This study reports on a novel passive ceramic-based semi-dry electrode prototype for electroencephalography (EEG) applications. With the help of capillary forces of the porous ceramics pillars, the semi-dry electrodes build a stable electrode/scalp interface by penetrating hair and releasing a small amount saline in a controlled and sustained manner. The semi-dry electrode/scalp impedances were low and stable (44.4 ± 16.9 kΩ, n = 10), and the variation between nine different positions was less 5 kΩ. The semi-dry electrodes have shown non-polarization characteristics and the maximum difference of equilibrium potential between eight electrodes was 579 μV. The semi-dry electrodes demonstrated long-term stability, and the impedance only increased by 20 kΩ within 8 h. EEG signals were simultaneously recorded using a 9-channel gel-based electrode and semi-dry electrode arrays setup on ten subjects. The average temporal cross-correlation between them in the eyes open/closed and the steady state visually evoked potentials (SSVEPs) paradigm were 0.938 ± 0.037 and 0.937 ± 0.027 respectively. Spectral analyses revealed similar response patterns with expected functional responses. Together with the advantages of quick setup, self-application and cleanliness, the result suggests the semi-dry electrode is suitable for emerging real-world EEG applications, such as brain-computer interfaces and wearable EEGs.