Semi-dry EEG electrode solution

Category: Brain-computer interface

Time：2024-12-21

　　The semi-dry electrode technology is a novel electrode approach that lies between dry electrodes and wet electrodes; it is also referred to as the glue-free electrode. The working principle of the semi-dry electrode is as follows: the electrode can slowly release a small amount of electrolyte (such as an NaCl solution) onto the skin surface, forming a stable ion-conducting pathway and thereby achieving a relatively low and stable electrode-skin impedance. Compared with wet electrodes, semi-dry electrodes offer advantages such as rapid assembly, ease of cleaning, and enhanced comfort, since they do not require conductive gel or prior skin preparation. In contrast to dry electrodes, semi-dry electrodes maintain a small amount of electrolyte between the electrode and the skin, resulting in a relatively low and stable electrode-skin impedance and high-quality EEG signals. In summary, semi-dry electrodes combine the advantages of both dry and wet electrodes while simultaneously addressing their respective shortcomings.

The research group led by Professor Duan Yanwen, founder of the GreenTech team and based at Wuhan University, has innovatively proposed the concept of “semi-dry electrodes” and developed a passive semi-dry electrode based on porous ceramics. This porous ceramic semi-dry electrode features a “reservoir” containing an NaCl solution, with an Ag/AgCl powder electrode placed inside the reservoir. The NaCl solution in the reservoir can be slowly released onto the skin surface, moistening the scalp and establishing a favorable ion-conducting pathway. The research results show that this semi-dry electrode exhibits low and stable electrode-skin impedance: for a pair of electrodes, the impedance at 10 Hz is 44.4 ± 16.9 kΩ (n=10), and after 8 hours, the impedance increases by only 20 kΩ—meaning the impedance of a single electrode is 22.2 ± 8.5 kΩ (n=10), with an increase of 10 kΩ over 8 hours. To validate the signal quality of the semi-dry electrode, EEG signals were simultaneously recorded using both semi-dry and wet electrodes during睁闭眼 (open-and-close-eye) tasks. The correlation coefficients between the semi-dry and wet electrodes for the睁闭眼 task and the steady-state visual evoked potential (SSVEP) task were 0.938 ± 0.037 and 0.927 ± 0.027, respectively, across 10 subjects.

Figure 1. Physical image (A) and schematic diagram of the electrode structure (B) of the porous ceramic semi-dry electrode developed by Tsinghua University [1].

This project, in collaboration with Professor Dan Zhang from the Department of Psychology at Tsinghua University, employed five classic BCI paradigms—open-eye/closed-eye, steady-state visual evoked potentials, P300 speller, N200 speller, and motor imagery—to verify their feasibility for brain-computer interface (BCI) applications [2]. The experimental results showed that the overall average cross-correlation between the semi-dry and wet electrodes across all nine electrode sites for the 20 subjects was 0.95 ± 0.07. For all frequency bands, the coherence between the semi-dry and wet electrodes exceeded 0.8, and for frequencies above 10 Hz, it exceeded 0.9 (with the exception of the 50-Hz power-line interference). More importantly, the classification accuracies of the five BCI paradigms using semi-dry electrodes were comparable to those obtained with wet electrodes. These findings indicate that semi-dry electrodes can effectively capture electrophysiological responses and can serve as a viable alternative to traditional wet electrodes in both BCI research and practical applications.

Figure 2. Overall similarity of signals from semi-dry and wet electrodes. (A) Time correlation between electrodes; (B) Cross-correlation between semi-dry and wet electrodes at 9 different sites; (C) Total average spectrum; (D) Spectral coherence between the two electrode types.

Figure 3: Glue-free S3 electrode cap and its application in rehabilitation BCI.

Figure 4: Application of hydrogel semi-dry electrodes and helmet device in EEG recording.

For many years, our team has been dedicated to the theoretical research and product development of semi-dry electrodes. In collaboration with the Neural Electrode Laboratory at Wuhan University, GreenTech Technology Co., Ltd. has developed a series of products—including glue-free saline electrodes based on Lincotton technology and hydrogel semi-dry electrodes. We remain committed to providing domestic and international customers with more comfortable, reliable, and convenient EEG electrode solutions.

References

[1] G. Li, D. Zhang, S. Wang, Y.Y. Duan, Novel passive ceramic-based semi-dry electrodes for recording electroencephalography signals from the hairy scalp, Sens. Actuators B, 237 (2016) 167–78.
[2] W. Fei, L. Guangli, C. Jingjing, D. Yanwen, Z. Dan, Novel semi-dry electrodes for brain–computer interface applications, J. Neural Eng., 13(2016) 046021.
[3] L. Guangli, W. Jing-Tao, Review of semi-dry electrodes for EEG recording, J. Neural Eng., 17 (2020) 051004.
[4] L. Guangli, W. Sizhe, D. Yanwen, Z. Dan, Toward gel-free electrodes: A systematic study of electrode-skin impedance, Sensors and Actuators B: Chemical, 241 (2017) 1244-1255.

The previous one

Dry electrode solution

None