Fabrication and evaluation of nanostructured microelectrodes for high-spatial resolution in retinal prostheses

To provide visual information to patients with retinal degenerative disease, retinal prosthetic devices based on multi-channel microelectrode arrays (MEAs) have shown some promising results. For high-quality information, the number of microelectrodes should be increased in the limited area of MEAs, which results in decreased dimensions of the single microelectrode and a limit of dynamic range of injection current. Previously, our research group has presented 3D microelectrodes to overcome the trade-off between high-spatial resolution and injection current range. However, the 3D microelectrode requires a complex fabrication process, including multiple steps in front-side and back-side microfabrication. In this paper, the nanostructured microelectrode with platinum-black is fabricated with a simple process, and its electrical characteristics are evaluated. The nanostructured microelectrode parameters are analyzed by a conventional three-element circuit model. The comprehensive electrical characteristics between planar 2D, 3D, and nanostructured microelectrodes with various base areas are compared by measuring electrode–electrolyte interface impedance and maximum allowable injection current limit. The experimental results show improved interface impedance in nanostructured microelectrodes compared to planar 2D and 3D microelectrodes with same base area. This implies that nanostructured microelectrodes have a large dynamic range for current stimulation, which can be more sufficient in retinal neuron stimulation. This research can be used to estimate the theoretical limit of injection current for high-resolution MEAs.