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Yasuo Terasawa, Hiroyuki Tashiro, Yukari Nakano, Koji Osawa, Motoki Ozawa; In vivo chronological observation of electrochemical properties of porous electrodes with chronic suprachoroidal-transretinal stimulation. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1054.
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© ARVO (1962-2015); The Authors (2016-present)
Stabilizing electrochemical properties of stimulating electrodes in vivo for long term is a key issue for visual prostheses. We previously proposed stimulating electrodes with porous surface [Terasawa et al., Invest Ophthalmol Vis Sci 2012;53: E-Abstract 5538]. The purpose of this study is to confirm the stability of electrochemical properties of stimulating electrodes with chronic suprachoroidal-transretinal stimulation in vivo.
An electrode array with two porous electrodes was developed for this study. The diameter and height of an electrode were 0.5mm and 0.3mm respectively. The electrode array was inserted into a scleral pocket of a rabbit eye and sutured onto the sclera. Charge-balanced biphasic pulses were applied to one of two electrodes on the array for eight hour per day for one month. Electrode impedance spectra were obtained periodically. In addition, charge injection capacities (CIC) of the electrodes were determined both in saline and in vivo. All in vivo experiments were conducted in accordance with the Association for Research in Vision and Ophthalmology statement for the use of animals in ophthalmic and vision research, and institutional guidelines for the care and use of laboratory animals.
Data analysis using equivalent circuit fitting suggested that double-layer capacitances in vivo were two orders of magnitude lower than that in vitro. No significant changes were observed in impedance spectra during chronic electrical stimulation. However increase of electrode impedance was observed at sixth day after cessation of stimulation. Double-layer capacitances decreased by half after cessation of stimulation. The CIC decreased from 225μC/cm2 to 78μC/cm2 after one month stimulation. Inter pulse potential (electrode potential before onset of each pulses) decreased from 0.217V (vs. Ag/AgCl) to 0.005V after stimulation. The decrease of inter pulse potential moved electrode potential closer to the lower limit of water window, which resulted in the decrease of the CIC of stimulating electrodes.
Impedance spectra in vivo kept unchanged during one-month chronic stimulation. The increase of impedance after cessation of stimulation suggested that electrical stimulation significantly affects the electrochemical properties of stimulating electrodes. Studies with longer period of stimulation are ongoing.
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