Addressing Impedance and Comfort Challenges in Through-Hair Electroencephalogram with Pyramid-Based Temperature-Responsive Ionic-Biogel Electrodes

Several invasive treatments for epilepsy have been studied to help drug-resistant patients manage or even prevent seizures. However, long-term, non-invasive electroencephalogram (EEG)-triggered transcranial alternating current stimulation (tACS) for preventing and controlling epileptic seizures is still in its infancy. Current EEG and tACS devices are uncomfortable for long-term usage, suffer from signal distortion, and degrade over time, limiting their use to a few hours. Therefore, biocompatible, adhesive, wearable, and scalp-conforming closed-loop electrodes for continuous EEG recording and tACS are crucial.<br/>Long-term EEG recordings using dry electrodes are often hindered by high skin-contact impedance, whereas wet-gel-based electrodes face comfortability issues due to hair. Achieving direct scalp contact necessitates small electrode areas to bypass hair obstruction. However, reducing the electrode contact area inherently increases skin contact impedance, creating a trade-off between impedance and comfort. To address this, we developed a novel hollow-based pyramid electrode design capable of buckling through congested hair to achieve direct scalp contact. The electrodes use a temperature-responsive conductive gel that flows through hair guided by these pyramids, increasing the contact area only when contacting the scalp. This design leverages the buckling mechanism and conductive material to achieve low skin-contact impedance without hair interference.<br/>Our approach combines PEDOT:PSS and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) based ionic liquid with a gelatin-glycerol matrix to form a synergistic ionogel-hydrogel composite. The composite maintains high ionic conductivity, electrochemical stability, and anti-dehydration properties of the ionogel while retaining the inherent biocompatibility of the hydrogel. The ionic-biogel exhibits a high storage modulus (1.5 kPa) and loss modulus (0.3 kPa) at scalp temperature (35°C), which transitions to a low modulus of 0.4 kPa at the transition temperature of 45°C. The skin-contact impedance of the ionic-biogel remains consistent (3 kΩ @ 4kHz) across the applied temperature range (40-50°C), with an electrical impedance of 200 Ω in the thickness direction. The high electrochemical stability of the ionogel-hydrogel composite enables long-term, high charge injection capacity compared to non-ionic biogels. The ionic biogel retains almost 99% of its content at 50°C, whereas non-ionic biogels retain only 93%, inhibiting long-term use. Additionally, the signal-to-noise ratio of the ionic biogel increases with applied temperature, showing a consistent signal-to-noise ratio of 36 dB at room temperature.<br/>The improved electrode design and material combination offer a promising solution to the impedance-comfort trade-off in long-term EEG recordings.