Verification of threshold parameters and tissue compatibility of IrOx film–electrodes on epiretinal electrical stimulation of retina in mini pigs

Abstract

Abstract: : Purpose: IrOx–film–electrodes are a possible choice for stimulation microelectrodes of a projected retina implant and are placed on retinal surface for conducting electrical charges. The aim of this study was to examine parameters of epiretinal electrical stimulation and their tissue compatibility. Methods: A polymide based microcontact film (MCF) of 1 mm width and eleven IrOx electrodes was used. In a randomized set up 12 Mini–Pigs were allocated to 4 groups. Each pig was performed by vitrectomy in general anaesthesia at the right eye and the MCF was inserted through a scleral port, placed epiretinal, fixed by perfluordecaline (PFCL) and connected by cable to an external electrical stimulator. The eyes were stimulated with standardized programs for 4 hours with different charges (Group (Gr) 1–4 with 0, 0.5, 1 and 4 mC/ cm²). Possible dislocation of the MCF was excluded by indirect ophthalmoscopy. After removal of the MCF and PFCL the animals were observed for a period of 14 days. Postperiodic examination to observe retinal changes due to insertion and/or stimulation followed by comparison to each untreated left eye and to Gr 1. After sacrificing the eyes were enucleated and worked off histologically. Results: 1 pig died of heart failure directly after stimulation. No retinal detachments or ocular infections have been observed. We noticed sporadic reversible punctiform bleeding of retina during imlantation. Post–stimulation examination showed in 2 eyes of Gr 4 strong edema in the stimulated area. Gr 1–3 did not have any retinal changes. After 2 weeks we found in each Gr 1–3 sporadic pigment coating in stimulated area. 2 eyes of Gr 4 showed local pale retina. Histological evaluation of the stimulated area revealed no structural changes in any treatment Gr 1–3. In 1 eye of Gr 4 there was an arcuated structure of retina which is correlated to the observed edema. Conclusions: The performed procedure was safe and effective, but nonvisual MCF–implantation causes problems as lens touch and retinal punct bleeding. To optimize implantation a handheld instrument has been developed. Charges of up to 1 mC/ cm² are applicable to retinal tissue in a time range of 4 hours without expecting a significant damage of retina, if administered by IrOx–electrodes. Our stimulation parameters and MCF with IrOx electrodes are suitable for electrical stimulation of human retina so that this study represents an important preparatory work for a further development of the retina implant.

Keywords: nerve fiber layer • retina • retinal degenerations: hereditary