BY DANIEL F. BALDWIN AND DOUGLAS SHIRE
Chronically implantable retinal prostheses currently are under development to restore useful vision to blind patients with degenerative retinal diseases such as age-related macular degeneration and retinitis pigmentosa. They provide a nice case study illustrating the significant advances being made in bioelectronics. These devices electrically stimulate the remaining healthy ganglion cells in the retina in response to wirelessly transmitted video data from outside the body, thereby effectively bypassing the dying photoreceptor cells. While these devices cannot be expected to provide the high acuity vision to which we are accustomed, they should restore the independence of some blind patients in performing many of the activities of daily living.
Vision is produced when light enters the eye through the cornea, lens and humors, and ultimately reaches the retina. In the retina, the incoming light is turned into action potentials on optic nerve fibers through a complex mechanism that begins with two types of photosensitive cells: rods and/or cones. Blindness results when the signal pathway is interrupted or when light entering the eye is not properly converted into electrical impulses due to a malfunction in the rods and/or cones. In retinal diseases such as age-related macular degeneration and retinitis pigmentosa, the photosensitive cells gradually die over a period of years, yet the nerve pathway to the brain remains intact. These diseases are the leading cause of inherited blindness. Together, millions of patients could benefit from a prosthesis of the type being developed.
Chronically Implantable Retinal Prosthesis
The chronically implantable retinal prosthesis consists of components mounted within and outside of the patient's eye, as shown in Figure 1. The patient wears glasses mounted with a miniature digital video camera that captures images within the line of sight, based on the head's position. The digital images are sent to a visual processing unit that converts the pictures into electronic signals compatible with those produced by the retina. To transmit the data to the unit in the eye, a high-speed RF wireless technique is used. The glasses have a coil mounted near the lens that transmits the image signal to a receiving coil implanted just outside the eye. The image data is then transmitted to the retinal implanted just outside the eye. The image data is then transmitted to the retinal implant, causing the implant to stimulate the retinal cells in turn. These cells send the image along the optic nerve to the brain, resulting in a crude form of vision.
 Figure 1. The internal and external components of the implantable retina prosthesis. |
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Packaging and Assembly
The miniaturized, high-density form factor of the implantable retina modules necessitates the use of flex circuitry with a dense circuit design. While the device count is relatively low, the device size must be minimized to reduce the likelihood of implant rejection by the body's immune system. For biocompatibility, the implanted device uses a parylene passivation coating over the flex circuit. Vias are etched in the parylene coating to open the bond pads for the component assembly. Post assembly, the entire implanted device is encapsulated in an additional parylene layer to further enhance biocompatibility. The primary ASIC is mounted using a flip chip interconnect. Chip capacitors and resistors comprise both 0603 and 0402 body sizes. A first-generation prototype assembly is shown in Figure 2. Secondary receiving power and data coils are mounted on the mother flex circuit above the flex-to-flex connection of the stimulating electrode array. To ensure high-yielding assemblies, a unique fixturing platform was developed to maintain planarity of the flex circuit during solder paste print, component placement, solder reflow and interconnect encapsulation.
 Figure 2. Prototype assembly of the implantable retina prosthesis. |
The processor is mounted to the high-density flex circuit using a flip chip configuration to achieve the smallest possible footprint and enhanced electrical performance. In this early stage of product development, the ASIC devices are not available in a wafer form and typically are singulated, which precludes low-cost solder bumping at the wafer level. Hence, the current application lends itself well to flip chip stud bumping that can be done on the singulated device or at the wafer level.
The prototype assembly process flow centers around an isotropic conductive adhesive interconnect technique because of the temperature limits associated with parylene passivation. Stud-bumped flip chip devices are assembled to the flex substrate by dipping the bumps in a precision film of silver-filled epoxy and mounting the flip chip onto the substrate bond pads. The flip chip interconnects are cured, the underfill is applied and then cured to minimize the impact of surface mount component placement. Isotropic conductive adhesive is precision-dispensed onto the flex substrate using a high-speed automated dispenser. Surface mount components are placed at high speed. The stimulating electrode array flex circuit, secondary receiving power and data coils are hand-assembled to the mother flex circuit, and the interconnects are cured. As the design matures, automated assembly of the coil and stimulating electrode array may be implemented, based on product demand.
Implantable retina prosthesis development signifies an intriguing advancement in medical electronics. The resulting module is miniaturized, emphasizing biocompatibility and high functionality. The high-density form factor of the modules necessitates the use of flex circuitry with a dense circuit pattern. Flip chip technology is used to mount the primary ASIC and enables scalability for future higher-resolution designs.
DANIEL F. BALDWIN, president, may be contacted at Engent Inc., 3140 Northwoods Parkway, Suite 300A, Norcross, GA 30071. DOUGLAS SHIRE, visiting scientist, may be contacted at the Boston Retinal Project, VA Center for Innovative Visual Rehabilitation, 119 Phillips Hall, Cornell University, Ithaca, NY 14853.