Doctors and engineers have developed a wide variety of implantable lenses that improve eyesight. But, the ultimate ophthalmic device is an artificial retina. Believe it or not, the bionic eye is more fact than fiction.
A device called Argus II is currently being developed by engineers at five U.S. Department of Energy laboratories and four universities. A private company, Second Sight Medial Products Inc., is marketing the bionic eye. The high-density microelectronic-tissue hybrid device aims to restore sight to people blinded by diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP).
People with AMD and RP are blind because retinal photoreceptors called rods and cones degenerate and lose function. According to Mark Humayun, M.D., an ophthalmologist at the Doheny Eye Institute at the University of Southern California, rods and cons are cells that capture light and transmit it into electrical signals. The signals are passed through underlying retinal cells and down the optic nerve to the brain, where visual images are formed.
The U.S. Department of Energy has spent $63 million on the artificial retina project since 2001. Funding is scheduled to end in 2010. The goal of the program is to replace the lost light-gathering rods and cones with a video camera and to use the information captured by the camera to electrically stimulate the part of the retina not destroyed by disease.
“Argus II is a three-part system designed to transmit information about the physical environment directly to an individual’s retina, thus bypassing the photoreceptors that have been damaged,” says Humayun. It consists of an array of 60 electrodes that are surgically implanted and attached to the retina.
“The electrodes conduct information acquired from an external camera mounted on a pair of eyeglasses,” Humayun points out. A battery pack worn on a belt powers the system. “The implant has been designed to last many years, but can be safely removed if necessary,” explains Humayun.
The metal traces forming the electrodes are less than 10 micrometer thick. The electrode array is embedded in a soft biocompatible polymer to allow it to conform to the curvature of the retina.
Stimulation is done with a thin, flexible metal electrode array that has been patterned on soft plastic material similar to a contact lens. The delicate, electrical stimulation of the retina needs to be performed in the eye’s saltwater environment without shorting out any electronic components. Another challenge engineers face is finding a bioadhesive that can be used to attach the microelectrode array to the surface of the retina.
Engineers at Lawrence Livermore National Laboratory are addressing those challenges and developing an advanced ocular surgical tool that allows ophthalmologists to implant microelectrode arrays with minimal tissue damage.
The 60-electrode Argus II device has already been implanted in 29 patients around the world. A newer, higher resolution model will be available within the next few years. The third-generation device will feature more than 200 electrodes. However, the long-term goal of the research project is to develop a bionic eye equipped with more than 1,000 electrodes, which would allow facial recognition.
Because that density is beyond conventional packaging technology, it creates a wide variety of engineering challenges. For instance, the compact size of the artificial retina’s electronics package makes it difficult to mechanically and electrically interconnect the microelectronics inside.
Researchers at Sandia National Laboratories are developing state-of-the-art packaging technology to assemble and integrate the microelectronic components with the thin-film electrode array. Biocompatibility issues are driving much of this effort, requiring the high-density interconnects to be insulated with a nonconductive film to prevent moisture and ionic and biological contamination from causing device failure.
The artificial retina’s custom-designed integrated circuit (IC) is the system’s brain. Its job is to take signals from the external camera and convert them into stimuli that are transferred to the electrode array. The IC performs this function via a series of interconnected, nanosize nodes.
“The current method for achieving higher electrode currents involves assembly with a lot of bond wires and other interconnects,” says Sean Pearson, an IC design engineer at Sandia. “This makes the device tedious to build and very difficult to yield full functionality.”
Pearson and his colleagues are developing a dual-sided IC to simplify how data are routed and to better integrate the electronics package with the electrode array. “We’re using one side to bring the signals in and the other side to put them out,” Pearson points out.
For the electronics substrate, the engineers are using a Sandia-patented MEMS technique to selectively etch away parts of the silicon chip or add new structural layers to create tiny features that cannot be made any other way. This micromachining process allows wiring of the electrical connections through the chip for access to both sides.
“By using that bottom surface, which adds interconnect space instead of eliminating it, we’re able to get higher interconnect densities,” thereby allowing the number of electrodes on the array to be increased without making the device bigger, says Murat Okandan, a microsystems engineer at Sandia.