Evaluation of a MEMS-Based Dual Metal-Layer Thin-Film Microelectrode Array for Suprachoroidal Electrical Stimulation

A double metal-layer thin-film platinum microelectrode array was fabricated for implantation between sclera and choroid based on MEMS processing techniques and photosensitive polyimide material. The array was composed of 60 stimulating sites (6 × 10) and four selectable returning electrodes. The diameter of each stimulating electrode was 350 μm with a center-to-center spacing of 750 μm. The transient voltage responses of the electrode to current pulse stimulation indicated a charge-injection capacity greater than 52.1 μC/cm². Acute in vivo animal experiments showed that the implicit time of electrically evoked potentials (EEPs) was 17.09 ± 1.45 ms at a threshold current of 25.55 ± 5.43 μA for a full-row of simultaneously stimulated electrodes (i.e. current applied simultaneously to each of the 10 electrodes). Individual electrode stimulation threshold was 48.57 ± 6.90 μA. The corresponding threshold charge densities were 13.28 ± 2.82 μC/cm² and 25.24 ± 3.59 μC/cm², respectively. The spatial spread of the maximally recorded P1 response in the EEPs indicated a correspondence between the retinal stimulation site and the focal response location in the cortex. This method of array fabrication is suitable for acute suprachoroidal stimulation, and has a potential use for the fabrication of a visual prosthesis.