Hydrogel electrodes with conductive and substrate-adhesive layers for noninvasive long-term EEG acquisition

Abstract

Noninvasive brain-computer interfaces (BCIs) show great potential in applications including sleep monitoring, fatigue alerts, neurofeedback training, etc. While noninvasive BCIs do not impose any procedural risk to users (as opposed to invasive BCIs), the acquisition of high-quality electroencephalograms (EEGs) in the long term has been challenging due to the limitations of current electrodes. Herein, we developed a semidry double-layer hydrogel electrode that not only records EEG signals at a resolution comparable to that of wet electrodes but is also able to withstand up to 12 h of continuous EEG acquisition. The electrode comprises dual hydrogel layers: a conductive layer that features high conductivity, low skin-contact impedance, and high robustness; and an adhesive layer that can bond to glass or plastic substrates to reduce motion artifacts in wearing conditions. Water retention in the hydrogel is stable, and the measured skin-contact impedance of the hydrogel electrode is comparable to that of wet electrodes (conductive paste) and drastically lower than that of dry electrodes (metal pin). Cytotoxicity and skin irritation tests show that the hydrogel electrode has excellent biocompatibility. Finally, the developed hydrogel electrode was evaluated in both N170 and P300 event-related potential (ERP) tests on human volunteers. The hydrogel electrode captured the expected ERP waveforms in both the N170 and P300 tests, showing similarities in the waveforms generated by wet electrodes. In contrast, dry electrodes fail to detect the triggered potential due to low signal quality. In addition, our hydrogel electrode can acquire EEG for up to 12 h and is ready for recycled use (7-day tests). Altogether, the results suggest that our semidry double-layer hydrogel electrodes are able to detect ERPs in the long term in an easy-to-use fashion, potentially opening up numerous applications in real-life scenarios for noninvasive BCI.

Keywords: Bionanoelectronics; Engineering.

Fig. 1. Design strategy for the double-layer hydrogel electrodes.

a The structure of the conductive-adhesive double-layer hydrogel. The conductive hydrogel acquires bioelectric signals on the gel-tissue interface via mobile chloride ions through micro channels on the epidermis. b SEM image of the conductive hydrogel. c Casted hydrogel electrodes with different ergonomic designs to maximize contact with the scalp. d Assembly of the hydrogel electrode with a conductive layer, an adhesive layer, supports, and Ag/AgCl

Fig. 2. Mechanical and bonding properties of the conductive and adhesive hydrogel.

a Compressive modulus of conductive hydrogels with different MBAA contents and different SA: AM ratios. b Stress‒strain curves of conductive hydrogels with different KCl concentrations. c Stress‒strain curves of the conductive hydrogel with 200 cycles of compression. d Images of the conductive hydrogel after 50 cycles and 200 cycles of compression. e Contact angle of solid substrates before and after plasma cleaning. f Contact angle images of glass, PVC, and PA before (left) and after plasma cleaning (right). g Schematic diagram of lap-shear tests. h Equations of shear strength calculation. i Tested shear strength of the adhesive hydrogel on glass, PA, Rubber, PVA, and PP. j Shear strength of the adhesive hydrogel on three solid substrates without chemical treatment at 1, 7, and 14 days. k Shear strength of the adhesive hydrogel on three solid substrates treated by TMSPMA at 1, 7, and 14 days. l Shear strength of the adhesive hydrogel on solid substrates treated with BP at 1, 7, and 14 days. m Adhesion-conductive bilayer hydrogels bonded to TMSPMA-treated glass. n The interface between the adhesive and the conductive layers

Fig. 3. Water retention ability and electrode-skin contact impedance of the hydrogel.

a Water retention of the conductive hydrogel with and without glycerol over 10 days. b The appearance change of the conductive hydrogel after 7 days of exposure to air. c Setup of the measurement of the electrode–skin contact impedance using model animal skin. d Skin contact impedance of dry electrode, double-layer hydrogel electrode, and wet electrode under different operating voltages. e Skin contact impedance of conductive hydrogels with different KCl concentrations. f Skin contact impedance of the wet electrode and double-layer hydrogel electrode for 12 h of continuous monitoring. g Electrochemical impedance spectroscopy of the conductive hydrogel with different KCl contents across a wide range of frequencies. h Electrical impedance and phase angle of the conductive hydrogels across different frequencies

Fig. 4. Biocompatibility test of the conductive hydrogel.

a Cell viability results of conducting hydrogel in DMEM complete medium for 14 days (n = 6). b Cell viability results of the conductive hydrogel with different SA:AM ratios placed in DMEM complete medium for 7 days (n = 4). c Skin irritation test of the conductive hydrogel. d Images of hematoxylin and eosin (H&E)-stained tissues of the skin without conductive hydrogel (left) and after 7 days of conductive hydrogel application (right)

Fig. 5. N170 test using wet, dry, and hydrogel electrodes by an EEG cap.

a A photograph of a subject being tested during EEG monitoring to evoke an N170 response. b Location of two working electrodes and two reference electrodes for EEG measurements in the EEG cap. c Schematic diagram of the picture stimulation flow. d The experimental function diagram of the N170 test. e EEG raw signals of wet, conductive hydrogel, and dry electrodes in Experiment 1. f N170 waves at O1 and P3 for dry, wet, and double-layer hydrogel electrodes (n = 3)

Fig. 6. P300 test using dry, wet, and hydrogel electrodes by an EEG cap.

a A photograph of a subject being tested during EEG monitoring to evoke a P300 response. b Location of four working electrodes and two reference electrodes for EEG measurements. c Schematic diagram of the picture stimulation flow of Experiment 2. d Visual stimulus pictures with target items. e P300 waves at O1 and P3 for dry, wet, and double-layer hydrogel electrodes (n = 4)

Fig. 7. Long-term stable semidry hydrogel electrodes with conductive and substrate-adhesive layers for multiple EEG recordings.

a Semidry hydrogel electrodes with conductive and substrate-adhesive layers mounted on EEG caps. b Pictures of wet electrodes removed after 12 h of continuous wear. c The condition of pig skin after the double-layer hydrogel electrodes and dry electrodes were placed for 12 h (under 300 g weight). d N170 waves at the O_Z for double-layer hydrogel electrodes that were worn for 12 h. e PSD of hydrogel and wet electrodes that were worn for 12 h. f Real-time impedance of the wet electrodes and hydrogel electrodes detected by the EEG headset with 12 h of continuous recording. g N170 waves at the O_Z for conductive hydrogel electrodes that were used for 14 days (wore for 2 h a day). h PSD of conductive hydrogel electrodes that were used for 14 days (wore for 2 h a day)