The bionic eye: A review of multielectrode arrays

The notion of creating artificial vision using visual prostheses has been well represented though science fiction literature and films. When we think of retinal prostheses, we immediately think of fictional characters like The Terminator scanning across a bar to assess patrons for appropriately fitting clothing, or Star Trek’s Geordi La Forge with his VISOR, a visual instrument and sensory organ replacement placed across his eyes and attached into his temples to provide him with vision. Such devices are no longer farfetched. In the past 20 years, significant research has been undertaken across the globe in the race for a “Bionic Eye”. Advances in Bionic Eye research have come from improvements in the design and fabrication of multielectrode arrays (MEAs) for medical applications. MEAs are already commonplace in medicine with use in applications such as the cochlear device, cardiac pacemakers, and deep brain stimulators where interfacing with neuronal cell populations is required. The use of MEAs for vision prostheses is currently of significant interest. For the most part, retinal prostheses have dominated the research landscape owing to the ease of access and direct contact to the retinal ganglion nerve cells. However, MEAs are also in use for direct stimulation into the optic nerve [1]. Retinal prostheses bypass the damaged photoreceptor cells within the retina and instead replace the degenerate retina with electrical stimulation to the nerve cells. Using electrical stimulation, stimulated retinal ganglion cells have been shown to elicit a percept in the form of a phosphene in blind patients [2–6]. Accordingly, the two diseases commonly linked to the justification for Bionic Eye research are age-related macular degeneration (AMD) and retinitis pigmentosa (RP), diseases which lead to progressive loss of photoreceptor cells and diseases where the patient has had previous vision and thus exhibits prior visual-brain pathways. At present, there has been no reliable cure for any of the retinal diseases that target the photoreceptor cells, and thus the development of prosthetic devices is a viable clinical treatment option [7–9]. The best physical location for a retinal implant remains undecided. Although this chapter concentrates on retinal prostheses, other artificial vision devices have been investigated at sites external to the eye such as the optic nerve [10] and the visual cortex [11]. Referring back to retinal prostheses, at present there have been three sites best identified for potential implant positioning: (i) epiretinal; (ii) subretinal; and (iii) suprachoroidal. Figure 24.1 shows a drawing of an eye, highlighting the three device positions relative to the retinal ganglion nerve cell layer. The implant sites have been selected for various reasons such as distance from the nerve cells, surgical ease and mechanical stability. The majority of clinical groups have opted for the epiretinal implantation of their Bionic Eye devices [3, 12–16]. The epiretinal position offers the advantage that the surgical procedure is relatively well understood and the device is closely aligned with the site it is actively stimulating. In particular, the epiretinal position aligns the device against the intact retinal ganglion cells