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Electroencephalography (EEG) is a non-invasive technique used to measure the electrical activity of the brain. The use of EEG is very important for the diagnosis of traumatic brain events and mental states such as injury, stroke, depression and many others including the COVID-19 brain fog syndrome. The quality of EEG signals largely depends on the nature of the interface between the surface of the electrode material and the surface of the scalp from where the electrical brain signals are acquired. The scalp surface is composed of an epidermic substrate with hair, grease, dirt, dandruff, skin peels and eventually many different hair products. The electrodes must combine several properties including electrical conductivity, mechanical strength, biocompatibility and corrosion resistance. They also must be manufactured with shapes designed to overcome the inherently complex nature of the scalp–electrode interface. This review reports the latest advances in the design of materials, surface coatings, conductive gels and information and communication technologies being developed to increase the quality of measurement of brain electrical signals in EEG protocols.

In bioelectrical potential measurement with restricted skin-electrode contact area, such as in dense array EEG measurement where the electrolyte bridging effect¹ is a major concern for signal reliability, an enhanced electrolyte solution is required for the skin-electrode impedance to reach the sufficiently low level within the minimum time interval. In this study, an electrolyte gel with its skin permeation ability enhanced by ethanol or propylene glycol has been investigated. The standard skin-electrode impedance measurement was carried out on the forehead in an area of 6 mm in diameter using standard Ag/AgCl EEG electrodes. The gel solutions with 0%, 7%, 18% and 28% of enhancers by volume are compared. The results show that both ethanol and propylene glycol reduce the permeation barrier of the stratum corneum so that ions in the electrolyte gel can penetrate more readily into the skin, enhancing the skin-electrode conductivity in reaching the steady value at a faster rate. It was further found that for the gel with higher percentage of ethanol, lower minimum skin-electrode impedance value was obtained. However, as the percentage of propylene glycol increased, it fails to attain low steady impedance values in the skin-electrode impedance measurements.