Semi-dry electrode technology is an innovative sensing solution bridging conventional dry and wet electrodes, also known as gel-free electrodes. Its operating mechanism relies on the gradual release of trace electrolyte (e.g., sodium chloride solution) onto the skin to build a stable ionic conduction path, which yields low and steady electrode-skin contact impedance. Unlike wet electrodes that demand conductive gel and pre-abrasion of the skin, semi-dry electrodes allow fast setup, convenient post-use cleaning and superior wearing comfort. Compared with dry electrodes, they retain a thin electrolyte layer at the electrode-skin interface to stabilize interfacial impedance and capture high-fidelity electroencephalogram (EEG) signals. Overall, semi-dry electrodes integrate the merits of dry and wet electrode architectures while overcoming their inherent drawbacks.
Led by Professor Duan Yanwen, the founding leader of GreenTech Research Team affiliated with Wuhan University, the research team pioneered the semi-dry electrode concept and developed a passive porous-ceramic-based semi-dry electrode. The device integrates an internal electrolyte reservoir filled with NaCl solution and an embedded Ag/AgCl powder electrode. Stored saline seeps out slowly to moisten the scalp and form robust ionic conduction channels. Experimental characterization verifies its outstanding impedance performance: tested across ten subjects, the paired-electrode impedance at 10 Hz stands at 44.4 ± 16.9 kΩ, rising merely 20 kΩ after eight consecutive hours of measurement; correspondingly, the single-electrode impedance averages 22.2 ± 8.5 kΩ (n=10), with an 8-hour impedance drift limited to 10 kΩ.
To further verify signal acquisition performance, simultaneous EEG recordings were collected via the developed semi-dry electrodes and standard wet electrodes during eyes-open/eyes-closed paradigms and steady-state visual evoked potential (SSVEP) tests. Statistical results from ten participants reveal high signal consistency: the cross-electrode correlation coefficient reaches 0.938 ± 0.037 for the eye-blink task and 0.927 ± 0.027 for SSVEP acquisition.
Figure 1. Photograph (A) and structural schematic (B) of the porous ceramic semi-dry electrode developed by Tsinghua University [1].
In cooperation with Prof. Yvonne Duan from the Department of Psychology, Tsinghua University, five canonical BCI paradigms were adopted to validate the practicality of the proposed electrodes for brain-computer interface (BCI) implementation, including eyes-open/eyes-closed, steady-state visual evoked potential (SSVEP), P300 speller, N200 speller and motor imagery tasks [2]. Experimental results from 20 test subjects demonstrated that the average cross-correlation coefficient between signals recorded by semi-dry and wet electrodes across nine electrode positions reached 0.95 ± 0.07. The signal coherence between the two electrode types was above 0.8 over the full tested frequency range and exceeded 0.9 at frequencies higher than 10 Hz, apart from the 50 Hz power-frequency interference band. Critically, semi-dry electrodes yielded classification accuracies comparable to conventional wet electrodes across all five BCI paradigms. Collectively, these results prove that the semi-dry electrode is capable of reliably acquiring cerebral electrophysiological signals and constitutes a competent substitute for conventional wet electrodes in fundamental BCI research and real-world deployment.
Figure 2. Overall signal similarity between semi-dry and wet electrodes. (A) Temporal correlation of recorded signals; (B) Cross-correlation at nine electrode positions; (C) Averaged overall power spectrum; (D) Spectral coherence of the two electrode modalities.
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, Greentek Technology Co., Ltd. has developed a series of products—including gel-free saline based electrodes,
it is based on Lincotton technology, and hydrogel semi-dry electrodes. We remain committed to providing 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.
