An Optically Addressable Retinal Stimulator in Distributed Microchip Architecture

Abstract

Purpose: : To realize over thousand stimulus electrodes in retinal prosthesis, we have proposed distributed microchip architecture, in which a number of Silicon-microelectronics-based microchips are placed on a flexible polyimide substrate, and demonstrated the fundamental operation by in vivo experiment using a rabbit. In this time, we introduce an optically addressable function into the stimulator device. The aim of this research is to apply an optically addressable retinal stimulating to our distributed microchip architecture.

Methods: : A microchip is employed with a function of a light-triggered stimulus current, where a stimulus current is switched by light impinged on a microchip. When the light intensity reaches a threshold value, the stimulus current control switch is turned on, and the stimulus current flows into retinal cells. The threshold value can be changed through an external control signal into a microchip. The microchip was designed to be compatible with the previously developed microchip except for integrating with light detection and decision circuits. After the chip fabrication, nine bump Pt metals were formed on Al metal pads of the chip for stimulus electrodes. The four microchips were assembled on a flexible polyimide substrate using a flip-chip technology specially developed for this distributed architecture. The fabricated stimulator was implanted into a pocket formed in the sclera of a rabbit eye. The NIR (near infrared light) around 950 nm by using an LED array was illuminated on the eye implanted the stimulator. EEP signal was measured through the screw electrodes set in a visual cortex. The stimulus was monophasic pulse current with the duration of 1000 msec. The amplitude was varied.

Results: : We tested the function of the fabricated microchip before in vivo experiment. The output signal was confirmed when the light is illuminated on the chip and its threshold was controlled externally. After implanting the stimulator, we confirmed that VEP signal was not measured by NIR light used in this experiment before the measurement of EEP signal. When NIR light incidents on the eye, a clear EEP signal was obtained. The threshold of the stimulus current was about 100 µA, which is the same as in the previous experiment.

Conclusions: : An optically addressable retinal stimulator based on distributed microchip architecture was demonstrated in vivo experiment using a rabbit.

Keywords: retina