Technology inspired by the human eye could be used to produce improved photographic images with a wider field of view.

Researchers from Northwestern University’s McCormick School of Engineering and Mechanical Engineering, teamed up with the University of Illinois, Urbana-Champaign, to create an array of silicon detectors and electronics that can be conformed to a curved surface. Like the human eye, the curved surface can then act as the focal plane array of the camera, which captures the image.

On a normal camera, such electronics must lie on a straight surface, and the camera’s complex system of lenses must reflect an image several times before it can reflect on the right spots on the focal plane.

Yonggang Huang, Northwestern University, explained: “The advantages of curved detector surface imaging have been understood by optics designers for a long time, and by biologists for an even longer time. That’s how the human eye works – using the curved surface at the back of the eye to capture the image.”

But exactly how to place those electronics on a curved surface to yield working cameras has stumped scientists, despite many different attempts over the last 20 years. The electronics lie on silicone wafers, which can only be compressed one per cent before they break and fail.

Huang and fellow researcher John Rogers, University of Illinois, established experimental methods and theoretical foundations respectively, for an effective way to transfer the electrons from a flat surface to a curved one.

Rogers created a hemisphereical transfer element made out of a thin elastomeric membrane that can be stretched out into the shape of a flat drumhead. In this form, planar (flat) electronics can be transferred onto the elastomer. Popping the elastomer back into its hemispheric form enables the transfer of the electronics onto a hemisphereical device substrate. A major challenge is that such a process applied to conventional electronics leads to catastrophic mechanical fracture in the brittle semiconductor materials.

Rogers and Huang got around this by creating an array of photodetectors and circuit elements that are so small (approximately 100 micrometers square) they aren’t as affected when the elastomer pops back into its hemispheric shape.

In addition, each of these devices on the array is connected by thin metal wires on plastic, which form arc-shaped structures that Huang an Rogers call ‘pop-up bridges’. These bridges interconnect the silicon devices, thereby relaxing all of the strain associated with return of the elastomer to its curved shape.

The researchers also designed the array so that the silicon component of each device is sandwiched in the middle of two other layers, the so-called natural mechanical plane. That way, while the top layer is stretched and the bottom layer is compressed, the middle layer experiences very small stress.

When tested, more than 99 per cent of the devices still worked after snapping the elastomer back to its hemisphereical shape. Researchers found that the silicon in the devices was only compressed by .0002 per cent – far below the one per cent compression where silicon fails.

Early images obtained using this curved array in an electronic eye-type camera indicate large-scale pictures that are much clearer than those obtained with similar, but planar, cameras where simple imaging optics are used.

Huang explained: “In a conventional, planar camera, parts of the images that fall at the edges of the fields of view are typically not imaged well using simple optics. The hemisphere layout of the electronic eye eliminates this and other limitations, thereby providing improved imaging characteristics.”