We have been detailing the progress of advances in artificial vision. Past advances consisted of the successful implantation of arrays of electrodes, which took the place of dead rods and cones and could stimulate the optic nerve to "see" patterns. Such efforts could be seen as a stepping stone to what these researchers hope to accomplish. The University of Illinois and Northwestern University have taken artificial vision to the next level by designing a fully artificial eyeball, which could one day "plug in" to the optic nerve for a vision replacement or enhancement.
The project began with Yonggang Huang, Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern University's McCormick School of Engineering and Applied Science, and John Rogers, the Flory-Founder Chair Professor of Materials Science and Engineering at the University of Illinois at Urbana-Champaign. They teamed up to create a naturally curved array of silicon detectors and electronics that can make in essence a curved camera sensor, which mimics the human eye's design. The curved surface, like in the eye would act as the focal plane of the camera and capture an image.
The research was the cover story of the August 7 edition of the journal Nature and can be read here.
Optics engineers have long known that the natural solution was the optimal one. Where cameras have to bounce light through a series of lenses to get it to form an image on a flat surface, a curved sensor could accept light passed through a single lens covered aperture akin to the lens and pupil of the eye.
Professor Rogers took over the design of curved surface to print the electronics on creating a thin elastomeric membrane -- basically rubber -- that could be stretched flat. After printing electronics on the membrane, it was unstretched, popping back to a hemispherical configuration.
One critical challenge is that brittle semiconductor materials typically crack under the stress of curving. To overcome this Professor Rogers and Professor Huang created an array of electronics so tiny it was unaffected by the curvature. The array's photodetectors and circuits comprised a 100 micrometer square, comprising a pixel of the device. Similar to buildings on a curved Earth, the scale of the curvature versus the tiny size of the array was enough that the silicon went unharmed.
Multiple pixels are connected together via thin metal wires on plastic, which the pair call "pop-up bridges" as they pop off the rubber surface when the device snaps back in place. These bridges help to relieve stress when the substrate returns a spherical shape. They were able to further relieve stress by sandwiching the silicon devices between two curved layers in the so-called natural mechanical plane, which minimizes stress.
The method works quite well. When tested after returning to a spherical shape, 99 percent of the devices still worked. Better yet, the silicon was only compressed .002 percent -- well below the 1 percent where silicon devices typically fail and break.
The researchers took initial images from the electronic eye-type camera and found them to be startlingly clear. When compared to planar camera images with a similar sensor, the eyeball camera easily triumphed. Professor Huang stated, "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."
To make the final design, the hemispherical membrane plus electronics was mounted on a hemispherical piece of glass. Then a lens and further components are added. The end result is a camera roughly the size and shape of a human eye. The current version is limited to 256 pixels, but researchers are quickly increasing this number.
This is the first device of this nature that could potentially be used as a full replacement to the human eyeball. As imaging electronics improve, the image sensors of the device will only improve, yielding the possibility of better than human vision resolutions in years to come.
The new device, which has transformed a long-standing science fiction staple into reality, could eventually see full implant if an interface to the optic nerve is developed. Before that, it will likely see action in the next generation of ocular implants. It could also see use in the next generation of war robots, many of which contain image processing capabilities.
With either application the revolutionary curved sensor is the key to it all, as it produces a better image. As Professor Rogers said, "Optics simulations and imaging studies show that these systems provide a much broader field of view, improved illumination uniformity and fewer aberrations than flat cameras with similar imaging lenses. Hemispherical detector arrays are also much better suited for use as retinal implants than flat detectors. The ability to wrap high quality silicon devices onto complex surfaces and biological tissues adds very interesting and powerful capabilities to electronic and optoelectronic device design, with many new application possibilities."
The research was funded by the National Science Foundation and the U.S. Department of Energy.