Evaluation of the capacitive behavior of 3D carbon electrodes for sub-retinal photovoltaic prosthesis

Here, we evaluate if microfabricated 3D pyrolytic carbon electrodes are suitable for application in sub-retinal photovoltaic prosthesis. This is done by measuring the charge storage capacity (CSC) and the maximum injectable charge, which indicate if the electrodes allow accumulation of sufficiently high charges in the charge cycle and are able to provide sufficiently fast discharge to stimulate neurons, respectively. The CSC was determined to 10.9 mC/cm2 for carbon pillars and 6.4 mC/cm2 for planar carbon electrodes. These values are comparable with values obtained for state-of-the-art electrode materials applied for retinal stimulation such as iridium oxide (IrOx). The maximum injectable charge was determined from cyclic voltammograms (CV) with values of 1.0 and 1.7 mC/cm2 for planar and pillar carbon electrodes, respectively. The measured contact resistance between carbon and n + doped Si confirms that pyrolytic carbon is a possible candidate for integration as a 3D electrode material on photovoltaic silicon retinal implants. The elemental composition of the fabricated pyrolytic carbon pillars was analyzed by X-ray photoelectron spectroscopy (XPS). The analysis showed that the Al2O3 passivated sample with fabricated pyrolytic carbon pillars only contained aluminum, oxygen and carbon, indicating a successful pyrolysis process without any unwanted elements. The study shows promising potential for pyrolytic carbon as a material for 3D electrodes in retinal, photovoltaic implants.