Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 2

capturing image data, (2) implanted microelectronic for converting image data
into a stimulus pattern, and (3) a microelectrode array interface for delivering the
stimulus current to the retina. The implementation of these functions is system
dependent; however, the potential for harmful interaction between the device
and the eye is considerable in all cases. The following issues must be considered
for any of these implants.
• Toxicity : The device should either be made of (or hermetically coated with)
materials that are not toxic to the retina or any other ocular tissue.
• Degradation : Even if the outer coating is not toxic, over time, the degradation
of the device coating can expose the retina to toxic materials which are part
of the implant electronics.
• Mechanical damage : The electrode array should also properly fit over the area
of the retina that is being implanted. The retina is a delicate neural tissue, and
rigid electrodes and microelectronic chips can easily cut the retina, without
careful mechanical design. Additionally, limited space in the eye and orbit
will constrain the device size and shape.
• Electrical damage : An electrical stimulus beyond safe limits may damage the
retina.
• Thermal damage : The retinal prosthesis electronics produce heat which can
be damaging to the retina and other ocular structures, if beyond a given
threshold.
• Reversibility : Extreme measures in fabricating a robust prosthesis and preop-
erative testing of the device reduce the chance of its becoming dysfunctional;
however, despite best efforts, some prostheses may break down, and it is
important to be able to remove the faulty device.
Background
Anatomy and Physiology of the Eye
The eyeball is classically divided into two segments: the anterior segment,
composed of the cornea, iris, lens, and ciliary body; and the posterior segment,
composed of the sclera, choroid, retina, vitreous, and optic nerve (Figure 1.1).
The eyeball is located within a bony socket called the orbit (Figure 1.2). In
addition to housing the eyeball, the orbital cavity contains extraocular muscles,
vessels, nerves, connective tissue, and orbital fat.
Although it may be possible to insert a prosthetic device around the eyeball,
the device size and shape are important limiting factors. For instance, a poorly
shaped, rigid object inside the orbital cavity may damage vessels and nerves,
interfere with eye movement, and directly damage the eyeball. On the other
hand, a bulky device may displace the eyeball, jeopardize the blood supply to the
eye, or raise the intraocular pressure (IOP), thereby resulting in nerve fiber layer
damage. These constraints will obviously limit the capability of an electronic
device because of necessary size restrictions.