This chapter describes retinal prosthetic devices that are used with complementary metal oxide semiconductor (CMOS) technologies or large-scale integration (LSI) circuit technologies. The introduction of CMOS technologies has made retinal prosthesis systems compact and versatile. Moreover, CMOS technology is particularly effective for increasing the number of stimulus electrodes with limited wiring. The remainder of this chapter is organized as follows. In Section 25.1, the principle of the retinal prosthesis and its basic components are discussed. In Section 25.2, various types of retinal prostheses are described. In Section 25.3, multiple microchip architectures that can realize a large number of stimulus electrodes are introduced and demonstrated in detail. In Section 25.4 integration of a photosensing function in a retinal stimulator is discussed. Finally, Section 25.5 proposes a smart electrode as an avenue for future research in retinal prostheses. A preliminary demonstration is described.

Principle and basic components of the retinal prosthesis

The front end of visual information is the retina. The human retina is a thin, layered tissue with a thickness ranging from 0.1 to 0.4 mm attached to the inner surface of the eyeball [1], as shown in Figure 25.1. The retina has a layered structure with photoreceptor cells for light detection in the bottom layer and ganglion cells for output in the top layer. The retina plays an important role in visual information collection and processing, and so its dysfunction can result in blindness. Among blindness diseases, retinitis pigmentosa (RP) and age-related macular degeneration (AMD) have no effective remedies at present. In both cases, the photoreceptors gradually become dysfunctional, so that the patient eventually becomes blind. However, some portion of ganglion cells remains alive [2]. Consequently, by stimulating the remaining retinal cells, visual sensation or phosphenes can be evoked. This is the principle of the retinal prosthesis or artificial vision. Based on this principle, a retinal prosthetic device stimulates retinal cells with a patterned electrical signal so that a blind patient can sense a phosphene pattern, or something like an image. Stimulating the optic nerve and the visual cortex can also restore visual sensation, but this would require more complicated surgical operations.