Development of Flexible Parylene-Based Microtechnologies for Retinal and Spinal Cord Stimulation and Recording

Abstract

The problems of outer retinal degeneration and spinal cord injury affect millions of people worldwide, often resulting in devastating blindness and para- or quadriplegia that strongly impair a person’s activities of daily living and impact their level of happiness. To help thwart the effects of these diseases, novel flexible parylene-based microtechnologies have been developed for functional electrical stimulation and recording in retinal and spinal cord prosthetics. Microelectrode arrays have been microfabricated according to a single-metal-layer process and a revolutionary dual-metal-layer process that promises to meet the needs of extremely high-density stimulation applications. Arrays have been fabricated of thin-film platinum, electroplated platinum, and iridium, all on parylene substrates, some electrodes surviving for more than 430 million pulses without failing. In addition, a new annealing and heat-molding process has been implemented to improve parylene to parylene adhesion and conform electrode arrays to approximate the curvature of canine retinas. A chronic implantation study of the mechanical effects of parylene-based electrode arrays on the retina over a six month follow-up period has provided excellent results. Both retinal and spinal stimulation and recording from such arrays have been demonstrated. The first packaging technology for high lead-count prostheses capable of fully scalable interconnection of a high-density electrode array, radiofrequency telemetry coils, and other discrete components such as chip capacitors, with prefabricated, stand-alone driver circuitry is also presented, combining the best features of chip-level and wafer-level packaging technologies. This parylene-based drop-chip technology enables application-specific integrated circuits (ASICs) to be directly integrated into the fabrication process of the other system components, such that the resulting device is flexible, facilitating surgical implantation. The ASIC-to-electrode interconnects are patterned using standard photolithography and standard microfabrication techniques, enabling the density of interconnects to scale to the limits of the lithographic equipment used to define the etch holes over the on-chip pads. Electrical test results verify the efficacy of this cost-effective packaging scheme, and pave the way for a monolithic implantable parylene-based prosthesis system, which has been designed. Surgical tests of monolithic geometries for all-intraocular retinal prostheses have been conducted, and an exciting new configuration for such a device has been discovered.