LED implant gives hope to opaque-cornea sufferers

OCTOBER 22--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 by a chemical burn or an explosion. 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.

The German research consortium--comprising teams from the universities of Duisburg and Karlsruhe and two hospital-based groups in Cologne and T�bingen--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.

The IOVA concept has the blind patient wear a pair of glasses that has a tiny CMOS detector attached. The electronic image collected is sent through a wireless telemetry unit to an LED microdisplay that is implanted behind the damaged cornea. The display replicates this crude image and projects it onto the person's healthy retina.

R�diger Buss, Dieter J�r, and colleagues at Duisburg University 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 x 5 and 8 x 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. 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 said.

Now, Buss and his colleagues are working on a 32 x 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.

For more information on the IOVA project: www.iova.net.