Evaluation of Electrochemically Treated Bulk Electrodes for STS Retinal Prosthesis by Retinal Optical Imaging

Abstract

Purpose: : We previously developed a novel stimulating electrode for suprachoroidal-transretinal stimulation (STS). It has a higher charge injection capability by enhancing the surface area of platinum electrodes through electrochemical etching (Terasawa et al., ARVO 2010). In this study, we evaluated the activated retinal area by this novel electrode in response to STS using retinal optical imaging.

Methods: : Seven eyes of seven cats were studied. Under general anesthesia, a stimulating electrode array was inserted into the scleral pocket at the posterior pole of the eye. The electrode array comprised two electrochemically treated platinum bulk electrodes (0.5 mm in diameter, 0.3 mm in height). The center-to-center distance between the electrode pair was 1.5 mm. Biphasic pulses (current, 0.03-2.0 mA; duration, 0.5 ms/phase; frequency, 20 Hz) were applied to the retina. Fundus images in the near infrared (800-880 nm) were obtained in every 25 ms for 2 s before, 4 s during, and 20 s after stimulation. By subtracting the pre-stimulus image from the post-stimulus image, a two-dimensional topography of reflectance change was constructed. The impedance of each electrode and optical coherence tomography (OCT) images were also obtained.

Results: : Localized area of reflectance change was observed on and around the stimulating electrode with a threshold current of 0.05 to 1.5 mA. The electrode impedance ranged from 4 to 11 kilo-Ohm. A negative correlation was found between the threshold and the impedance (r = -0.61, p<0.05). In those eyes in which the stimulating electrode attached tightly to the surrounding tissue, as observed by OCT and/or fundus images, the threshold decreased and the impedance increased.

Conclusions: : These results indicate that the novel stimulating electrode is able to localized electrical stimulation to the retina. The threshold current can be decreased if we create a scleral pocket in which the electrodes attach tightly to the surrounding tissue in STS retinal prosthesis.

Keywords: imaging/image analysis: non-clinical • retinal degenerations: hereditary • retina