Fabrication and implantation of hydrogel coated, flexible polyimide electrodes

Within this study, flexible, polyimide based shaft electrodes were fabricated and dip-coated with a PDMAA-co-SSAz solution that formed a thin hydrogel layer on the electrode shanks. This layer stiffened the electrode for insertion when thoroughly dried. The electrodes were successfully implanted into a brain model and into real brain tissue of rodents. The insertion forces during in vitro and in vivo implantation were recorded and compared. Pre-pricking of an insertion channel didn´t simplify the insertion in vitro in all cases. The final insertion forces were in vivo (15 - 58 mN) slightly higher than in vitro (12 - 20 mN). Electrochemical impedance spectra were recorded in vivo up to twelve weeks post implantation, measuring the impedance between two electrode sites through the hydrogel. Three types of impedance spectra were observed during the implantation period: mainly resistive spectra, mainly capacitive spectra and spectra showing resistive and capacitive components. One week after implantation, the amount of spectra with resistive-capacitive components was 90 % and decreased to 50 % in week twelve, while the amount of mainly resistive spectra increased from initially 2 % to 38 %. The amount of mainly capacitive spectra changed from 8 % in week one to 12 % in week twelve.