Electrical characterization of 2D and 3D microelectrodes for achieving high-resolution sensing in retinal prostheses with in vitro animal experimental results

In order to provide high-quality visual information to patients with implanted retinal prosthetic devices, the number of microelectrode should be large. As the number of microelectrodes is increased, the dimensions of the microelectrode are also decreased, which in turn results in increased interface impedance of the microelectrodes and decreased dynamic range of injection current. In addition, the reduced maximum limit of injection current may not be sufficiently large to stimulate the ganglion cells in the retina. In order to improve the trade-off between the number of microelectrodes and the current injection limit, a 3D microelectrode structure can be used as an alternative. Due to the advancement of microfabrication technology, the fabrication of highly-accurate 3D structures with small dimensions is possible. This paper presents a comprehensive electrical characterization of 2D and 3D microelectrodes. Microelectrodes that differ in shape and diameter are compared to evaluate the feasibility of use in high-resolution retinal prostheses. Their electrode–electrolyte interface impedances and charge injection limits are quantitatively analyzed. Also, in vitro animal experiments using rd1 mice are performed to observe the evoked neural responses by increasing the stimulation current amplitude from 10 to 100 μA. This research can be used to define requirements for further retinal prosthetic device research.