A Fully Flexible Hydrogel Electrode for Daily EEG Monitoring
The electroencephalogram (EEG) is a test designed to monitor the electrical activity in the brain by utilizing small metal discs, or electrodes, attached to the scalp. This method provides valuable insights into the neural functions within the brain.
Traditionally, healthcare professionals have relied on Ag/AgCl wet electrodes for EEG signal monitoring. However, the use of these electrodes has its challenges. The conductive gel used in these electrodes adversely affects signal quality over time, requires hair washing for removal, and causes inconvenience. In response to the limitations of wet electrodes, alternative dry electrodes, including active electrodes, micro-needle electrodes, and conductive polymer electrodes, have emerged. Yet, these alternatives have drawbacks, such as infection risks and susceptibility to motion artifacts. Semi-dry electrodes with liquid reservoirs, while employing capillary force for electrolyte flow, are also prone to volatilization.
The paper explores the fully flexible hydrogel electrodes, showcasing their capability to capture high-quality EEG signals. These electrodes prove superior to their rigid counterparts in being flexible to adapt to the skin curves, ensuring comfort during prolonged monitoring. The installation process is quite convenient when compared to the traditional wet electrodes. The water vapor in the hydrogel moisturizes and enhances the skin's electrical conductivity. Adding glycerol to hydrogel further improves its moisture-retaining quality since it can form strong hydrogen bonds with water.
The electrode design integrates biopotential monitoring advantages, incorporating hydrogel and conductive silver fabrics within a Polydimethylsiloxane (PDMS) shell. The synthesis of the n-acryloyl glycinamide (NAGA) hydrogel involves a meticulous process utilizing various reagents, including Diethyl ether, potassium carbonate, ammonium persulfate, and glycerol. Additional materials such as Glycinamide hydrochloride, Sylgard 184 PDMS prepolymer, adhesive PDMS (aPDMS), N,N,N’,N-Tetramethylethylenediamine (TEMED) are further packaged with conductive silver thread and fabrics.
The NAGA Hydrogel prepolymer undergoes synthesis by chilling in an ice-water bath with a 3D printing mold, ensuring size suitability. In the subsequent step, the PDMS and curing agent mixture is poured into a 3D printed mold at 11:1, incorporating chlorinated conductive silver fabric stitched with conductive silver thread. The hydrogel is then placed within the PDMS shell and secured with aPDMS.
Following the international 10/20 system for EEG electrode placement, both the traditional Ag/AgCl wet electrode and the NAGA hydrogel electrode are installed near FP1, utilizing Ten20® conductive gel. Scalps of the subjects are prepared through abrasion.
Material confirmation involves observing the absence of the characteristic peak at 1626 cm−1, which marks the successful synthesis of the hydrogel. A Tensile and compressive deformation of 70% is achieved, with no noticeable changes in strength after a 90-day period.
The fully flexible hydrogel electrode demonstrates stable EEG signals with acceptable potential drift and noise amplitude. Its performance in open/closed paradigms and coherence with wet electrodes establishes its suitability for daily EEG monitoring. Its design is best optimized for daily EEG monitoring, effectively addressing the limitations associated with traditional electrodes. The comfort, ease of installation, and robust performance position it as a promising solution for various applications, including Brain-Computer Interfaces based on EEG data analysis algorithms.