Biomedical Engineering Reference
In-Depth Information
Biological Considerations
for an Intraocular Retinal Prosthesis
Hossein Ameri, James D. Weiland and Mark S. Humayun
Doheny Retina Institute, Doheny Eye Institute,
Keck School of Medicine, University of Southern California
Introduction
The idea of replacing a dysfunctional or missing body part with a prosthesis
is perhaps as old as the history of humankind; yet, it still continues to be
a fascinating theme. While early prostheses were limited to external organs
(e.g. wooden legs), the twentieth century witnessed the implantation of numerous
devices inside the human body, some of which have since become routine
surgical procedures. For external prostheses, both proper fitting and functionality
are needed to make a device acceptable for use; however, internally implanted
prostheses have additional challenges, and the implantation of intelligent devices
which interact with surrounding tissues is even more difficult.
Despite being one of the most delicate organs of the body, the eye has proven
to be a hospitable environment for a variety of materials. Whereas some metals
such as iron and copper are highly toxic to the eye, other substances such
as silicone, polymethyl-metacrylate (PMMA), and acrylic are well tolerated.
Intraocular lenses made of these substances are routinely used to replace the
natural lens during cataract surgery, the most commonly performed intraocular
surgical procedure. Prior to surgical procedures that could be performed within
the back of the eye (vitrectomy), retinal detachment had almost exclusively been
treated with scleral buckling, a procedure whereby silicone rubber is placed
around the eyeball. Even now, a significant number of retinal detachments are
treated this way. Ocular implants are also used in some procedures relating to
orbital, keratorefractive, and glaucoma surgeries. Thus, the use of prosthetic
devices in an ocular environment is well established and a number of materials
are proven to be suitable in the fabrication and use of these devices.
Retinal prostheses are being developed to apply electrical stimulation to the
retina in order to restore vision. Several different configurations have been
proposed and these will be reviewed in detail later in the chapter. However,
common qualities of almost all of the implants are: (1) a light-sensitive device for
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