LED implant gives hope to opaque-cornea sufferers

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.The IOVA idea is this: the blind patient wears a pair of spectacles that have 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.

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.

IOVA consortium members Wilhelm Stork and Klaus Müller-Glaser at Karlsruhe University have worked on the optical and physiological constraints that must be overcome in the implanted system. They identified two potential problems. First, they measured the pixel size of the LED array, the divergence of the display and the size of the eye to find the ideal optical design to focus an image on the retina.

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." IOVA project