Engineers at the University of Washington have added an LED array into a flexible contact lens, ultimately aiming at in-eye head-up displays.

"This is a very small step toward that goal, but I think it's extremely promising," said researcher Dr Babak Parviz.

Medical sensing is another possibility. "There is a large area outside of the transparent part of the eye that we can use for placing instrumentation," he added.

Transparent PCB

The lens, which is also a two layer transparent PCB, is built on a 100µm polyethylene terephthalate (PET) substrate patterned with two layers of fine metal tracks separated by an insulating layer.

320nm diameter flat-bottomed circular self-assembly targets for LEDs are created by forming apertures in the insulating layer.

These disc-shaped holes expose tracks of the lowest metal layer - which will eventually be the LED bottom contact.

Side contacts for the LEDs are formed during the deposition of the second metal layer which drapes down the sides of the apertures where appropriate.

A further insulating layer, with its own lithography step to make circular holes which match the original insulator, completes the self-assembly targets and covers the top metal.

Thermal processing during these steps is kept below the 70°C distortion temperature of PET. Finally the proto-lenses are cut from the substrate and exposed contacts 'tinned' with a low-melting point indium alloy.

Custom fabricated AlGaAs devices

The 320µm diameter disc-shaped LEDs are custom fabricated AlGaAs devices tuned to emit red at 689nm.

These were built by epitaxy with both anode and cathode contacts on top, etched circular, then released by etching through a sacrificial layer in the epitaxial stack.

Shape-based attraction, said Washington, causes the diodes to self-assemble into their targets, the right way up, when the lens/PCBs is washed in a suspension of the diodes.

Low temperature re-flow soldered the diodes in place, and a polymethyl methacrylate (PMMA) top coat sealed the assembly and made it bio-compatible, claimed Washington.

Pressing into a mould at 240°C converts the assembled lens blank into a lens.

For a head-up display, images have to be focussed at infinity, can this be done?

"Constructing a focused image on the surface of cornea is definitely challenging, nobody has tried it or demonstrated it before," Parviz told Electronics Weekly. "We are looking into two ways of doing this: integrated a micro lens with each pixel, or using small ultra-low power lasers for each pixel."

See also: Electronics Weekly's roundup of content related to LEDs, with a special focus on both white LEDs and coloured LEDs:

LED technology - White LEDs

LED technology - Coloured LEDs