| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 99 === In this thesis, new retinal stimulation chips have been designed, analyzed, and fabricated to improve current stimulation efficiency of the subretinal prosthesis. Three types of stimulation chip are proposed, one is single solar cell array stimulation chip; another is two cascode solar cell array stimulation chip and still another is four-block divisional power-supply stimulation chip.
The single solar cell array stimulation chip with local surrounding return electrode has been designed and verified after the in vitro experiment. The silicon retina with solar cell array can successfully trigger the ganglion cells with threshold charge lower than 2.8nC with 75μm x 75μm stimulating electrode. Probed retinal stimulation current of solar cell array with local surrounding return electrode is 1.08μA, which is 10 times of remote surrounding return and 3 times of remote single return. Probed retinal stimulation current of two cascode solar cell array is 2.09μA, which is 2 times more current efficient than single solar cell array. Retina interface impedance has been measured by the multi-size microelectrode chip. Retina interface impedance decreases with increasing frequency, increasing electrode size and decreasing center-to-center distance of electrodes. At low frequency, retina interface impedance is over 1MΩ which is difficult for stimulation current generated from solar cell array to flow into retina. High potential of two cascode structure is one of the solutions to this problem. With local surrounding return electrode, optimum photo-sensing regions of each pixel of single and two cascode solar cell array stimulation chips are 14400μm2 and 19200μm2 and current stimulation efficiency with the optimum photo-sensing region of the latter is 1.65 times larger than the former.
In the work of four-block divisional power-supply stimulation chip, the capability of on-chip solar cell supply system integrated with circuit system in CMOS technology has been proved, and the feasibility of using power supply system in turn to elevate power management efficiency has also been verified. Power control unit with original 1066 solar cells and extra 2540 solar cells is able to generate four power control signals, which are nonoverlapping and on different phases, to control each block of pixel array. Clock frequency is 1.5kHz under 15.8mW/cm2 incident light intensity with only 24.8nW power consumption. The output stimulating current of each pixel is approximately 1.1μA under same light intensity. Comparing with 16-pixel conventional solar cell array, which can only generate 0.31μ A at 15.8mW/cm2 for total 5003 solar cells, output stimulation current of four-block divisional power supply stimulation chip is 3 times higher and power efficiency is 4 times higher. The proposed power management structure could be considered as one of the highly integrated solutions for the efficient current stimulation CMOS chips of subretinal prosthesis.
Multi-size microelectrode chip and single solar cell array stimulation chip are fabricated with standard 0.35μm and tsmc process; two cascode solar cell array stimulation chip and four-block divisional power-supply stimulation chip are fabricated with standard 0.18μm tsmc CMOS process. The proposed efficient current stimulation chips all have contribution to the subretinal prosthesis.
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