A Microfabricated Penetrating Electrode Array for a Subretinal Prosthesis

Abstract

Purpose: : This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. The specific purpose of this presentation is to describe our efforts to develop microfabricated penetrating electrode array on polyimide-based flexible carriers that will place the stimulating electrodes nearer the target cells and thereby potentially decrease the threshold current. The hypothesis was that such penetrating structures could be realized using microfabrication technology.

Methods: : A base polyimide layer was spin-coated and cured on a Si substrate followed by the evaporation deposition of a patterned conductor metal stack (Ti/Au/Ti). A second layer of polyimide was spin-coated over the top, pad/site openings were dry-etched through the polyimide to the metal by reactive ion etching (RIE) and the arrays were singulated by dry-etching through the full polyimide thickness. Next SU-8 posts were patterned and high-temperature cured and then sputtered iridium oxide film (SIROF) was patterned over top to encompass the entire post. A coating of Parylene-C was conformally deposited, lithographically patterned and RIE dry-etched from the periphery of the flexible polyimide carrier and the top of the posts to expose the underlying SIROF electrode. The completed arrays were removed from the carrier substrate. Sample penetrating electrode arrays were electrochemically evaluated in an inorganic interstitial fluid.

Results: : A set of microfabrication processes was successfully engineered to integrate SU-8 posts to elevate the SIROF electrodes above the flexible polyimide carrier. The in vitro characterization of the SIROF electrodes on the top of the posts demonstrated that the performance compared favorably with results previously reported for planar SIROF electrode arrays.

Conclusions: : A means of fabricating a flexible polyimide-based penetrating electrode array has been developed. The technology was shown to demonstrate excellent SIROF electrode performance. This microfabrication technology may provide the basis for a future in vivo component of the Boston retinal prosthesis.

Keywords: retina • age-related macular degeneration • retinitis