Development of novel hydrophilic Ag/AgCl dry contact electrodes for mobile EEG

Interest in mobile electroencephalography (EEG) is getting more and more traction in both the scientific and technological world. This technology would in fact allow the Neurosciences community to explore brain responses in a more ecological environment while a product development interest is arising for consumer products including EEG sensors. One of the most challenging part of mobile EEG technologies is the need to use dry contact electrodes. While in standard EEG the operators have the possibility to add conductive gel between the skin and the EEG electrodes surface, this is not possible when we try to bring EEG technology outside of the lab. This causes a worse skin-to-electrode contact and consequently a much higher signal to noise ratio in the EEG recordings. In this project we explored the possibility to improve the skin-to-electrode contact quality of dry contact Ag/AgCl electrodes by making them hydrophilic via a surface modification process. The hydrophilicity is achieved by immersion of the electrode in a solution of 3-mercaptopropionic acid in ethanol, resulting in the formation of a self assembling monolayer (SAM) of short-chain thiols on the electrode surface. This will ensure a better wettability thanks to the presence of carboxile groups on the free end of the chain. The thiol group on the other end instead will bind strongly with the silver in the electrode giving us a long lasting effect. The nanometric thickness of the SAM will also ensure a not to strong increase in the electrode impedance. To verify the positive outcome of our process, and to make sure it gave the desired properties we analyzed the samples with raman spectroscopy, optical contact angle measurements and, in the end, we also measured the impedance of the Ag/AgCl electrode when put in contact with a gel coated electrocardiography electrode, in order to simulate skin contact. Results from raman spectroscopy guaranteed us that the SAM was indeed formed on top of the electrodes and from optical contact angle measurements we could see that the hydrophilicity had indeed increased greatly. Impedance measures showed that the coating did not greatly increase the electrodes impedance but further measures on better skin models will have to be made in the future to assess if and how much the contact between electrode and skin has improved thanks to the higher wettability of the electrode.