A retinal prosthetic device using a CMOS image sensor employed with modified pulse frequency modulation

This paper describes a retinal prosthetic device using a modified pulse frequency modulation (PFM) based photosensor with a standard CMOS technology. The output waveform of the PFM based photosensor is modified to stimulate retinal cells effectively. Two improvements are achieved. First modification is to restrict the maximum output pulse frequency to protect cells and also to shift photosensitivity curve adaptively. By introducing the adaptation, we almost keep an input light dynamic range of 5-Log. Second, biphasic current pulses are introduced to alleviate the fatigue of retinal cells as well as to keep injected charges constant irrespective to impedance change. A 32x32-pixel PFM photosensor array chip is fabricated using 0.6 μm CMOS technology and demonstrated the improved functions.