17 Jun 2002
A German research consortium is developing an intraocular microdisplay system that could restore some degree of vision to people who are blind because their corneas have been severely damaged, writes Michael Hatcher.
From Opto & Laser Europe October 2001
Restoring sight to the blind is an emotive topic that is the subject of a number of research programmes. Most of these efforts are focused on fixing problems with the retina - the eye's photodetector. And understandably so, since one of the main causes of blindness is retinitis pigmentosa, a disease that debilitates the retina's performance.
However, a less common cause of blindness is damage to the cornea caused by a chemical burn or an explosion, and this presents a different problem. In essence, the retina of the patient works, but the damaged corneal tissue scatters rather than transmits light, which cannot get through the cornea to be focused on the retina.
A German research consortium - comprising teams from the universities of Duisburg and Karlsruhe and two hospital-based groups in Cologne and Tübingen - was set up in March 1999 to develop a concept dubbed Intraocular Vision Aid (IOVA) to restore sight to those with what is known as an opaque, or "blurred", cornea.
An
opaque cornea is usually treated by transplanting a donor cornea in its place. However, transplanted
organs of any type often suffer immune reactions, or rejection, by the new host body. Other research
groups are working on "replacement" corneas made from materials such as
silicone. Rüdiger Buss, Dieter
Jäger and colleagues at Duisburg University, Germany, have concentrated on making the critical
microdisplay element: "To our knowledge we are the only group working on an intraocular aid based on
a microdisplay," said Buss. So far, the consortium has built 5 ¥ 5 and
8 ¥ 8 LED arrays, encapsulated them in silicone and implanted them into
rabbits' eyes. The encapsulation was carried out by German firm Acritec and the implantation was
performed at the University of Cologne, Germany. The results showed that the eyes did not overheat and
the rabbits were not blinded. "The idea is to build a proof-of-principle system that works," Buss told
OLE. Now, Buss and his colleagues are working on a 32 ¥
32 device to enhance the spatial resolution of the projected image. "The crucial thing is to give [the
patient] an idea of motion, so that they can 'see' a car coming, for example," said Buss. The LED
microdisplay is based on a gallium phosphide substrate. This high-energy bandgap material is chosen so
that wavelengths in the green-orange spectrum can be generated - this is the region that the retina
detects most readily. The ideal wavelength for retinal detection is between 555 (brightness) and 590 nm
(contrast). Complications have emerged throughout the development of the device: one intriguing
problem arises from the fact that an implanted display system would cast a fixed image onto the
retina. In 1952, British scientist Robert Ditchburn discovered that fixed images on the retina
disappear and cannot be recognized. For the image to reappear, either its brightness must change or its
position on the retina must move. Having found this, the consortium is working on a way to introduce an
eye tracking system that continuously moves the image on the microdisplay. According to
their calculations, a conventional lens would have to be fixed about halfway between the implanted
microdisplay and the retina to focus light from the 10 to 15 µm LED pixels to generate a reasonable
image. Unfortunately, surgeons would be unable to fix such a lens in place without it moving inside the
eyeball. To get round the problem, Stork says that a microlens array fixed directly onto the implanted
microdisplay is needed. Creating this microlens array remains a tough obstacle, predicts Buss:
"One of the most challenging tasks that remains is the design and manufacture of focusing micro-optics.
That, and the integration of all of the component devices," he said. The Karlsruhe team also
calculated the thermal energy expected to flow into the eye from the microdisplay - if this is too high the
eyeball will overheat. Stork and Müller-Glaser calculated that for a green display 0.1 nW of optical
power per receptor is needed. This equates to 5 µW over 50 000 receptors and an in-eye electrical
power of about 5 mW. According to Stork the average thermal energy dissipation of the human body is
1 W/kg, so, provided that the eye disperses heat in the same way as the rest of the body, natural energy
dissipation should be sufficient to keep the eye from overheating. The current project runs until
2003, and Buss is confident about its outcome: "What we are expecting to deliver in 2003 is a wireless,
fully-encapsulated prototype, implanted into an animal subject's eye. We are sure that we will
succeed."
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