Figure 1. An application-specific integrated circuit being developed for advanced artificial retinas. Click on image to enlarge.
More advanced artificial retinas are relying on miniaturized electronics for processing incoming images and activating the corresponding electrodes to communicate with retinal cells and ultimately the brain. The goal of these devices, being developed through a U.S. Department of Energy (DOE) collaboration, is to continually improve their visual resolution so that implanted individuals eventually will be able to read large print, recognize faces, and move about without aid. Sandia National Laboratories’ expertise in the development, fabrication, and production of microsystems is helping to make this goal a reality.
Biocompatible electronics packages currently used in medical devices require only a small number of electrical interfaces to operate them. For example, pacemakers at most have four electrical contacts, and cochlear implants for the hearing impaired use 22 or fewer. Additionally, the volume of these packages is typically more than 5 cm3. By comparison, DOE’s artificial retina requires a much smaller electronics package but one to two orders of magnitude more electrical feed-throughs to communicate with retinal cells.
This density is beyond conventional packaging technology. The compact size of the artificial retina’s electronics package makes it difficult to mechanically and electrically interconnect the microelectronics inside. The package also has to withstand the human eye’s harsh saline environment for the lifetime of the patient, so the electronics have to be hermetically sealed, preventing all transfer of moisture and gases between the components inside the package and the human body.
“Essentially, we’re trying to cram more and more things into smaller and smaller spaces,” says Kurt Wessendorf, an analog circuit designer and leader of Sandia’s artificial retina efforts. If more electrodes, and hence more capabilities, can be packed into the system, the images that implanted individuals see will be of higher resolution. This is the area benefitted by Sandia’s expertise in microsystems.
Engineering Tiny Machines
Microsystem devices smaller than a human hair are built on silicon wafers or chips. They contain electrical circuitry and microelectromechanical systems (MEMS), which are miniature machines.
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, whose locations on the chip’s surface are important because they can minimize the wire length along which the signal travels (see figure 1 above).
Figure 2. Three-dimensional model and cross section of a dual-sided integrated circuit. The circuit enables high-density interconnects on both top and bottom surfaces. Click on image to enlarge.
“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.” Consequently, he and his colleagues are developing a novel, dual-sided IC to simplify how data are routed and to better integrate the electronics package with the electrode array (see figure 2 above). “We’re using one side to bring the signals in and the other side to put them out,” Pearson explains.
For the electronics substrate, the researchers 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 on the Sandia team.
Figure 3. High-density hermetic electronics packaging with a dual-sided electronic circuit. Click on image to enlarge.
Additionally, Sandia researchers are developing state-of-the-art packaging technologies 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 (see figure 3).
Sandia has a long history of pioneering microelectronics research, which feeds into several defense-related systems, including sensor technologies and satellite applications. Spinoffs of the Artificial Retina Project—such as the silicon interconnect and higher-density packaging of components—are being evaluated for potential applications in some of these ongoing projects.
“The kind of exposure seen in the eye is not unlike the harsh, corrosive environments in which many defense-related components are required to survive for many years,” Wessendorf says. Moreover, “We’re always looking at miniaturizing and increasing function, and these efforts will help in those directions.”
Sandia National Laboratories is operated by Sandia Corporation, a Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration.
Base URL: http://artificialretina.energy.gov
Last modified: Tuesday, May 15, 2018