Long Term Recording of Charge Injection Capacities of Suprachoroidally implanted Porous Platinum Electrodes

Abstract

Purpose: We have been developing a retinal prosthesis based on Suprachoroidal-Transretinal Stimulation (STS). We previously reported the stability of charge injection capacities (CICs) of electrodes developed for STS in vivo during one month electrical stimulation [Terasawa et al., ARVO E-abstracts 1803, 2014], but little is known about long-term behavior of in vivo CICs. The purpose of this study is to investigate the long-term trend of CICs of electrodes implanted suprachoroidally.

Methods: An electrode array with two stimulating electrodes was developed for this study. The diameter and height of an electrode were 0.5mm and 0.3mm respectively. The in vivo experimental details were described elsewhere [Terasawa, et al, proceedings of IEEE EMBC, pp. 3567-3570, 2013]. In brief, 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 six months. The CICs were recorded every four weeks during six-month chronic stimulation period. 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.

Results: In vivo CICs of active (pulses were applied) and inactive (pulses were NOT applied) electrodes averaged during stimulation period were 108μC/cm² and 89μC/cm² respectively. No apparent trend of increase or decrease in CICs was found in six-month chronic stimulation period. The CICs of active electrodes tended to be greater than those of inactive ones. This difference was statistically significant at second month (p=0.02) and fifth month (p=0.003) of chronic stimulation.

Conclusions: CICs of porous platinum electrodes implanted in suprachoroidal space were generally stable for six months. Chronic electrical stimulation did not affect the stability of CICs. The difference of CICs between active and inactive electrodes suggested that electrochemical environment of electrodes was slightly modified by chronic electrical stimulation.