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Optical Prosthetics Technology Mimicking the Eye
Optical Prosthetics
Technology Mimicking the Eye
Parts of the Eye
The parts of the eye we are covering are:The CorneaThe IrisThe Lens
Index of Refraction of the Eye
To be used as a reference.
Cornea—1.376Aqueous humor—1.336Lens—Ranges from 1.386 in the outer layers to 1.406 at the center.Vitreous body—1.337
The Cornea and its Functions
The cornea is the outermost layer of the eye and provides the main curvature.Properties of the cornea
Index of Refraction of 1.376Consists of three layers:
Epithelium—Similar to the epidermis of our skin.Stroma—Highly organized collagen. Endothelium—Maintains water balance and transparency.
How the Cornea is Lost
Two main ways a person can lose their cornea.
Damage caused by disease (affects 10 million people worldwide).Physical injury.
Currently today, most cases are solved by sugically implanting a donor’s cornea.However, there are some cases where implants are rejected and won’t suffice…
Growing a Cornea
In 2003, the University of Ottawa managed to grow a pig’s cornea in partially blind pigs by implanting a “scaffold” of plastic and protein where the cornea will grow.
Artificial Corneas
Rather than abusing pig eyes, artificial corneas are being developed from hydrogels, polymers that consist of ~70% water.Desirable properties of hydrogels:
Wear resistantFracture resistantOptically clearGlucose-permeable
Artificial Corneas
The current product on the market is an artificial cornea called AlphaCor. It is developed by Argus Biomedical Pty Ltd in Perth, Australia.
Details on AlphaCor
Under clinical investigation since 1998.FDA-approved in 2002.AlphaCor is made from a hydrogel called PHEMA (poly 2[hydroxyethyl methacrylate]).Its diameter in 7.0 mm, its width is 600 µm.
The Two Layers
Outer skirtSpongy, porous, and opaque, allowing natural corneal tissue to grow and help hold the implant in place.
Inner LayerTransparent with an index of refraction of 1.43.
Implanting an Artificial Cornea
Surgery is nearly identical to that of a regular cornea.
Open up a Gunderson’s flap that the implant will go under.Dissect enough corneal tissue to form a lamellar corneal pocket.Insert the implant into the pocket.
Implanting an Artificial Cornea
(continued from above)Sew a mattress suture to prevent the implant from riding up.Close the pocket with nylon sutures.After 12 weeks, remove the Gunderson’s flap to expose the corneal implant.
Complications
There are two things that can go wrong with implanting an artificial cornea like AlphaCor.
Stromal melting is a fairly common side effect occuring in 30% of patients, where the patient’s stroma layer loses structural integrity.Optic deposits (i.e. calcium phosphate) may form on the center of the device.
Future Plans for AlphaCor
Enhance the optical zone (the transparent part).Color the outer skirt to match the patient’s iris color.
Double Network Hydrogels
Currently being developed by the University of Stanford.Tensile strength is 20 times greater than its single hydrogel components.However…
Working prototypes still need to be developed.Cellular response still needs to be tested.
The Iris and its Functions
By contracting and dilating, it determines the f/# number of the eye.Can either be born without one (aniridia, affects 1 out of 50,000 people), or lose function by physical injury.In addition, iris patterns are unique enough to be used for personal identification.
Artificial Irises
Most notable of artificial irises is the Morcher Iris Diaphragm Ring.
Invented by Dr. Volker Rasch from Potsdam, Germany and Dr. Kenneth J. Rosenthal in 1996.In addition, Dr. Rosenthal also performed the first surgery with the device in 1997.
Picture of Artificial Iris
Realistic-looking Artificial Irises
It is possible to make very realistic looking irises by using computer rendering with 30 textured layers.In addition to CG, can also be used in prosthetic design.
The Lens and its Function
By adjusting its angle of curvature, the focal length of the eye can be adjusted (to certain limits).The index of refraction varies in the lens from 1.386 at the outer layers to 1.406 at the inner layers.Due to its position, shape of lens less a factor than that of the cornea.
Losing the Lens
Generally, lenses need to be replaced after cataract surgery.Most artificial lenses are made from plastic, silicone, or acrylic and last for life.Sorry, not the place of the eye with great advances in science.
References
Chu, W. Ralph. “Peer Review: AlphaCor”CRSToday.com. Nov/Dec 2004. Cataract and Refractive Surgery Today. 27 February 2006 <http://www.crstoday.com/PDF%20Articles/1104/crst1104_pr.pdf>Lefohn, Aaron et al. “An Ocularist’s Approach to Human Iris Synthesis” IEEE Computer Graphics and Applications. Nov/Dec 2003: 70-75.
References
Peterson, Elizabeth. “Cataract Surgery: Lens Extraction and Prosthetic Lens Insertion.”Aurorahealthcare.org. November 2003. Aurora Health Care. 29 February 2006 <http://www.aurorahealthcare.org/yourhealth/healthgate/getcontent.asp?URLhealthgate="45991.html”>Rosenthal, Kenneth J. “The Aniridia Page.“Accessed 28 February 2006 <http://www.eyesurgery.org/THE%20ANIRIDIA%20PAGE.htm>
References
“Artificial Cornea.” Stanford.edu. 2006. Stanford School of Medicine. 28 February 2006http://ophthalmology.stanford.edu/basic_research/projects/artificial_cornea.html“Implanted Artificial Iris Improves Patient's Vision and Appearance.” Nyee.edu. 2003. New York Eye and Ear Infirmary. 29 February 2006 <http://www.nyee.edu/highlights-2003h.html>
References
Cooper Vision Surgical. 2006. Cooper Vision Surgical. February 30 2006 <http://www.coopervisionsurgical.com/coopervisionsurgical/cvs_alphacor.htm>Ophtec.com. 2006. Ophtec B.V. 29 February 2006 <http://www.ophtec.com>
