Implantable neural stimulators have recently attracted attention because of their potential applicability to the treatment of sensory neural disorders such as hereditary hearing loss and retinitis pigmentosa. However, the requirements for stimulation electrodes tend to be contradictory in some applications that require transmission of complex information to the nervous system, such as cochlear implants and retinal prostheses. They have to be sufficiently small to realize fine interfacing with the nervous system while remaining sufficiently large to inject enough charge to stimulate neurons without causing an irreversible electrochemical reaction. One solution to these requirements is to employ three-dimensional (3D) instead of planar electrodes. However, in conventional photolithography, the available material and size for fabricating electrodes are greatly limited. To overcome these limitations, we propose a novel fabrication process for stimulation electrodes, using mechanical micromachining. Using 3D bullet-shaped electrode increased the surface area by 3.6 times compared to conventional planar electrode with the same diameter. The proposed electrode, which was developed for retinal prostheses, showed sufficient charge injection capacity (CIC) to evoke light perception (phosphene) for blind patients. Furthermore, the CIC and electrode surface morphology remained stable during a six-month period of current pulsing in phosphate-buffered saline, which suggests suitability for chronic neural stimulation. The cause of the variance in the measured CIC and future applications of the proposed 3D electrodes were also considered.
2016 Japanese Society for Medical and Biological Engineering