Electrodeposited Biocompatible Coatings for 3D Electrodes on Retinal Prostheses

Abstract

Photovoltaic subretinal prosthesis, PRIMA, provides central vision to patients blinded by age-related macular degeneration, with acuity matching the 100 m pixel size. Further miniaturization requires pillar electrodes to position the stimulating surfaces closer to the inner retinal neurons. While such structures can be electroplated in gold and coated on their tops with SIROF, the exposed gold sidewalls are not biocompatible. Sputtering or atomic layer deposition of protective coatings are unsuitable for selectively passivating the pillar structures without also coating the photosensitive regions and return electrodes of the implant. Here, we present a strategy for biocompatible coating of pillar sidewalls while preserving surrounding implant functionality. The approach combines non-critical photoresist lithography to protect planar return electrodes with electrodeposition of TiO2; or Pt onto gold pillar sidewalls. In-vivo studies demonstrated that both TiO2 and Pt coatings are biocompatible and prevent adverse reactions of the retinal tissue to gold. Since specific capacitance of electroplated TiO2 (25 F/cm2) is much lower than that of Pt (240 F/cm2), the former better limits the current from the side walls and ensures that charge injection occurs predominantly through the pillar tops coated with SIROF (~6 mF/cm2). Electrodeposition, combined with noncritical photolithography provides a scalable wafer-level solution for fabrication of biocompatible three-dimensional electro-neural interfaces, addressing a critical bottleneck in bioelectronics.