A tough semi-dry hydrogel electrode with anti-bacterial properties for long-term repeatable non-invasive EEG acquisition

Abstract

Non-invasive brain-computer interfaces (NI-BCIs) have garnered significant attention due to their safety and wide range of applications. However, developing non-invasive electroencephalogram (EEG) electrodes that are highly sensitive, comfortable to wear, and reusable has been challenging due to the limitations of conventional electrodes. Here, we introduce a simple method for fabricating semi-dry hydrogel EEG electrodes with antibacterial properties, enabling long-term, repeatable acquisition of EEG. By utilizing N-acryloyl glycinamide and hydroxypropyltrimethyl ammonium chloride chitosan, we have prepared electrodes that not only possess good mechanical properties (compression modulus 65 kPa) and anti-fatigue properties but also exhibit superior antibacterial properties. These electrodes effectively inhibit the growth of both Gram-negative (E. coli) and Gram-positive (S. epidermidis) bacteria. Furthermore, the hydrogel maintains stable water retention properties, resulting in an average contact impedance of <400 Ω measured over 12 h, and an ionic conductivity of 0.39 mS cm^-1. Cytotoxicity and skin irritation tests have confirmed the high biocompatibility of the hydrogel electrodes. In an N170 event-related potential (ERP) test on human volunteers, we successfully captured the expected ERP signal waveform and a high signal-to-noise ratio (20.02 dB), comparable to that of conventional wet electrodes. Moreover, contact impedance on the scalps remained below 100 kΩ for 12 h, while wet electrodes became unable to detect signals after 7-8 h due to dehydration. In summary, our hydrogel electrodes are capable of detecting ERPs over extended periods in an easy-to-use manner with antibacterial properties. This reduces the risk of bacterial infection associated with prolonged reuse and expands the potential of NI-BCIs in daily life.

Fig. 1. Design of hydrogel materials and schematic diagram of the electrode structure.

a Design of conductive hydrogel materials with anti-bacterial efficacy with NAGA and HACC. b The drawing of the electrode support and mold assembly was created using Solidworks software (above) and the physical drawing of the hydrogel electrode (below). c SEM images of N₄₀H₃ and N₄₀ are presented in (d)

Fig. 2. The mechanical properties and capacity to absorb moisture of hydrogels.

a Compression modulus of hydrogels with varying NAGA and HACC ratios. b The compressive stress-strain curves of hydrogels with 40 wt% NAGA and varying ratios of HACC. c Stress–strain curves and compression modulus of hydrogels with different KCl contents. d The original shape of the hydrogel and the shape after 500 compressions. e Stress–strain curves and compression modulus of hydrogels for 500 compression cycles. f Stress–strain curves and compression modulus of hydrogels for 10,000 compression cycles. g Changes in the appearance of hydrogels with different humectants added for 24 h under ventilated conditions. h Changes in water retention of hydrogels over 12 h under ventilated conditions. i Changes in long-term water retention of hydrogels under ventilated conditions

Fig. 3. Electrical properties of hydrogel electrodes.

a Schematic diagram of the principle of skin–electrode contact impedance measurement using the four-electrode method. b Contact impedance of hydrogel electrodes with different KCl concentrations. c The contact impedance of the wet and hydrogel electrodes was measured for 12 h consecutively at room temperature. d Electrical impedance of hydrogels with different KCl contents over a wide frequency range. e Electrical impedance and phase angle of hydrogels at different frequencies. f CV cycling curves (1000 cycles) for an exposed area of 1.58 cm². g Calculated CSC during different CV cycles. h CIC curves at the 1st, 1000th, 5000th, and 10,000th cycles of hydrogel electrodes. i CIC curves of anti-bacterial conductive hydrogel interfaces with an exposed area of 1.58 cm². j Conductivity of hydrogels with different KCl contents at different frequencies

Fig. 4. Characterization of hydrogel anti-bacterial properties.

a Anti-bacterial inhibition of E. coli on LB agar plates and S. epidermidis on NA agar plates by hydrogels with different HACC contents. b Diameter of the anti-bacterial circle in which hydrogels inhibit the growth of E. coli. c Diameter of the anti-bacterial circle of hydrogel inhibiting the growth of S. epidermidis. d Changes in OD values of hydrogels acting in E. coli suspensions for 10 h. e Results of coated plates after dilution of hydrogel/E. coli suspension. f Changes in OD values of hydrogels acting in S. epidermidis suspensions for 24 h. g Results of plate coating after dilution of hydrogel/S. epidermidis suspension

Fig. 5. Evaluation of anti-bacterial effect and biosafety of the hydrogel material.

a Comparison of the appearance of hydrogel electrodes with the original hydrogel electrodes after being worn by the subjects for 3 days. b Appearance of bacterial colonies on the surfaces and inside of the worn hydrogel electrodes after being stored for 1 month at room temperature. c Results of the action of the used control and N₄₀H₃ hydrogel electrodes in the LB/NA liquid medium after 24 h. d Changes in OD of the hydrogel electrode suspension in the LB/NA liquid medium with different HACC contents. e Cell proliferation rate of hydrogels with different HACC contents after 3 days of immersion in complete DMEM. f Thickness of granulation tissue after H&E staining of skin tissue for hydrogel action. g Images of hydrogels applied to the dorsal skin of female and male SD rats 1–7 days after application. h Results of H&E staining of skin tissue to which hydrogel was applied (green arrows represent dermis, blue arrows represent nuclei)

Fig. 6. Real-time contact impedance and N170 mood evocation experiments with hydrogel electrodes.

a Pictures of subjects measuring real-time contact impedance in a sonic darkroom. b Pictures of reference and working electrodes used for EEG measurements (GND and CPz are the reference electrodes, and the rest are the working electrodes). c Real-time contact impedance of the wet electrodes and the hydrogel electrodes in the area of less hair. d Real-time contact impedance of the wet electrodes and the water gel electrodes in areas with normal hair. e Schematic of N170 test. f Signal-to-noise ratios of dry, wet, and hydrogel electrodes. g Generation of N170 waves at O1 and O2 using dry, wet, and hydrogel electrodes. h Generation of N170 waves at O1 using hydrogel electrodes for 21 days (worn for 2 h per day). i Variation of signal-to-noise ratios of hydrogel electrodes for 21 consecutive days