Electrodeposited Biocompatible Coatings for 3D Electrodes on Retinal Prostheses

A photovoltaic subretinal prosthesis called PRIMA offers central vision to patients who have lost sight due to age-related macular degeneration. The device achieves this by matching the 100 pixel size of the 100 m resolution. To further miniaturize the prosthesis, pillar electrodes are needed to bring the stimulating surfaces closer to the inner retinal neurons. These structures can be electroplated in gold and coated on their top surfaces with SIROF. However, the exposed gold sidewalls are not biocompatible.

Traditional methods of sputtering or atomic layer deposition for creating protective coatings are not suitable for selectively passivating the pillar structures without also coating the photosensitive areas and return electrodes of the implant. The researchers have now developed a strategy to coat the pillar sidewalls while maintaining the functionality of the surrounding implant.

This involves using non-critical photoresist lithography to protect the planar return electrodes, followed by electrodeposition of either TiO2 or Pt onto the gold pillar sidewalls.

In vivo studies have confirmed that both TiO2 and Pt coatings are biocompatible and prevent any adverse reactions from the retinal tissue to gold. The specific capacitance of the electrodeplated TiO2 (25 F/cm2) is significantly lower than that of Pt (240 F/cm2). This means that TiO2 is better suited for limiting the current from the side walls, ensuring that charge injection occurs primarily through the pillar tops coated with SIROF (~6 mF/cm2).

The combination of electrodeposition with noncritical photolithography provides a scalable wafer-level solution for creating biocompatible three-dimensional electro-neural interfaces. This addresses a significant challenge in the field of bioelectronics, paving the way for further advancements in subretinal photovoltaic prostheses.