Implanting Electrical Devices Into the Eye: Evaluation of Long–Term Tolerance and Pathological Thresholds of Electrical Stimuli to the Retina

Abstract

Abstract: : Purpose: During recent years electrical devices have been developed that can be implanted into the eye and directly attached to the neuronal retina in an attempt to restore vision to patients suffering from retinal degeneration. A major challenge of this approach is to experimentally determine stimulation paradigms and to find energy thresholds that allow optimal signal transfer from the technical implant to the neuronal network without inducing pathological alterations within the remaining retina. Methods: The high number of experiments to be undertaken in this context do not allow time consuming and costly in vivo implantation. We therefore have developed an organotypic culture that allows us to keep segments of adult rat retina as well as cortex slices over five days in vitro on a micro electrode array (MEA). MEAs are equipped with 60 independently controllable electrodes which can be used as stimulation or as recoding units. To mimic conditions in patients with degenerative diseases we used retinal tissue from Royal College of Surgeon (RCS) rats. Results: For proper tissue nutrition the entire system –stimulation unit and adhering tissue– was kept in a rotating culture system. To prevent tissue loss and to achieve a tight contact between device and tissue during the entire in vitro period tissues were glued onto the MEA with nitrocellulose. To determine thresholds for electrically evoked pathological alterations, electrical stimuli of different intensity and duration were applied to the cultured retinas. Degeneration processes evoked by these stimuli were quantified with cytological and immunohistological techniques and different cell markers were used to determine apoptosis, downfall of specific neuronal types, gliotic/fibrotic reactions, and scar formation in the electrically stimulated retina. Clear signs of degeneration were observed after applying voltage pulses of 2V, 0.5 ms duration and 20 Hz repletion frequency. Conclusions: Organ cultures of neuronal tissue can be cultivated on MEAs up to seven days. This MEA–tissue system can be used to determine long–term tolerance and pathological thresholds of electrical stimuli within the retina.

Keywords: pathobiology • degenerations/dystrophies