A Wide-Field Suprachoroidal Retinal Prosthesis Reliably Elicits Cortical Activity after Chronic Implantation

Abstract

Purpose: : The research goal is to develop a clinically viable wide-field retinal stimulating array. In this study the aim was to evaluate the efficacy of the electrode array in evoking cortical responses after long term suprachoroidal implantation.

Methods: : A silicone based electrode array (8×19mm) was implanted into the suprachoroidal space in 6 normal-sighted cats. Each array consisted of 21 platinum stimulating electrodes (600µm diameter) located on the distal end and 2 return electrodes. A transcleral lead wire terminated in an implantable subcutaneous connector. The animals were assessed for 3 months. After this period they were anaesthetized, the connector exposed and the array stimulated with constant current biphasic pulses (180µC/cm² maximum). Electrode impedance was measured from electrode voltage transients using 500µs, 1mA pulses. Evoked potentials were measured over the epidural surface of V1. Animals were then perfused and the eyes collected for histopathology assessment.

Results: : The implant was well tolerated in all animals. Evoked potentials were recorded from monopolar stimulation for 80% of the electrodes. The median threshold for all electrodes was 225nC per phase (charge density of 80µC/cm²), while the lowest threshold had a median of 150nC across animals.The electrode impedance measured in vivo at 3 months had a median of 13.7kΩ (10-90 percentile 10.9-19.6kΩ). This was significantly higher than the pre-implantation median impedance in saline of 3.3 kΩ (2.9-3.9kΩ) (Mann-Whitney P<0.001). Pathology assessment revealed a thin fibrous capsule and mild foreign body response around the implant.

Conclusions: : Stimulation thresholds were similar to those reported for acute suprachoroidal stimulation, whereas higher electrode impedances indicated the need for increased stimulator voltage. The wide-field suprachoroidal array reliably activated the retina and visual cortex well within safe charge limits after chronic implantation.

Keywords: electrophysiology: non-clinical • retina • visual cortex