In Vitro Determination of Stimulus-Induced pH Changes in Visual Prostheses

Abstract

Inspired by the success of Cochlear implants, which restores the hearing for the deaf, research efforts worldwide are developing visual prostheses aimed at restoring vision for the blind [1–4]. Several recent developments from research teams and industrial developers working on visual prostheses have raised hopes for a retinal implant and provided other strategies in restoring vision to blind individuals. Intraocular retinal implants developed by Second Sight Medical Products have been chronically implanted in six patients over the past 3 years in an FDA-approved IDE study. Figure 12.1a illustrates part of the design for an intraocular retinal prosthesis [5]. In this model, a small camera that would be housed in the patient’s glasses captures visual information, such as the letter “E.” This information is then relayed to a microprocessor called Visual Processing Unit (VPU) located externally. After processing this information, the VPU wirelessly sends the information to a microelectronic receiver, implanted behind the ear of the patient, underneath the skin of the scalp. This information is then converted into tiny electric impulses and transmitted through a cable across the eye wall to a microelectrode array to stimulate the remaining retinal neurons of the patient. An example of such implanted microelectrode array, developed by Second Sight, is shown in Figure 12.1b. The electrode array is composed of 16 platinum (Pt) disks arranged in a 4 × 4 square array. The array is kept tightly against the retinal surface by a medical tack. Neural prostheses require microelectrodes and stimulation devices that minimize electrochemical damage to surrounding tissue or nerve from chronic use. Electrical stimulation using metallic electrodes in an aqueous electrolyte introduces charges into the environment via electrochemical reactions. At low intensities, charge injection is dominated by capacitive mechanisms [6, 7]. With increasing current intensities, reversible and irreversible Faradaic reactions may occur (Table 12.1) [8]. Almost all Faradic reactions produce or consume