Conclusions and Future Work

5.1 C

Based on the previous work on ‘ON brisk transient’ ganglion cell set, we developed a novel retinal chip that imitates the ON sluggish sustain ganglion cell set in the retina of rabbits. The model-building approach of the design methodology is to incorporate the available concerning morphology, electro-physiology, and pharmacology and by only using elementary building blocks.

An OFF bipolar cell block features low DC current variation is proposed to solve the DC level variation problem in the previous work. The simulation result of the original structure shows that the variation of output current level is 8µA while the variation of the modified circuit is less than 0.04µA. The reduction of DC current variation is verified by the measurement results indicating that the maximum inter-pixel variations of DC current output is less than 0.26µA.

Besides the difference of the OFF bipolar cell, the ON bipolar cell block is also different in its structure. In the previous work, the ON bipolar cell performs bandpass-filtering on the signals, whereas the ON bipolar cell of this work performs large time delay and performs lateral diffusion in space. The structures are dissimilar.

The chip contains 32x32 pixels where each pixel imitates one cell set of the ON sluggish sustain ganglion cell. The chip is manufactured in TSMC 0.35µm double-poly quadruple-metal standard CMOS technology. The area of a basic retina cell is a square of 100µm, and the total area of the whole chip is approximately 4.3 mm x 4.2 mm. The fill factor is 9.14% and the maximum inter-pixel variation is less than 0.26mA. Most parts of the chip deal with current-mode signals. The total power consumption is 1.15W under light stimuli.

An experimental chip which imitate the ON sluggish sustain ganglion cell set of rabbits’ retinas is fabricated and measured to verify the design methodology. Through HSPICE simulation and chip measurement, the functions of the chip are verified. Such consistency strongly suggests that the chip,

in extracting the features of the visual world, behaves in a way which is similar to that of real retinal cells. Therefore, the verifications of the implemented chip establish the success of the proposed design methodology. The tunable parameter associated with the CNN model is the space constant of the horizontal. Varying bias voltage, the desired space constant could be obtained. The functions of designed chip has been verified by the measured spatiotemporal responses under flashing light stimulus which shows the function consistence between the designed chip and the rabbit retina.

5.2 F

Though the function of the chip is verified to be correct, there is still room to improve. First, reducing the power consumption of the designed chip and shrinking the size of the cell. The cell area of the proposed retina is still too large with compared to that of a real retina cell. Shrinking the size of the cell may make some potential applications such as implantation of artificial retina in human body more practical.

Secondly, the chip contains 32x32 pixels where each pixel imitates one cell set of the ON sluggish sustain ganglion cell. In the practical application, the output signal of each pixel should transmit the ganglion spiking to an electrode to stimulate and imitate the ON sluggish sustain ganglion cell set of rabbits’retina, respectively.

Third, based on the ‘ON brisk transient’ and the ‘ON sluggish sustain’ ganglion cell set of rabbits’ retina, every kind of ganglion cell sets can be implemented and integrated in the same way.

Based on the results, the full ganglion cell sets of retina can be designed and realized. Thus the research will help the blind to restore vision.

Finally, with the design methodology proposed in this thesis, many potential applications of retinal chips including motion sensors, computer vision, retinal prosthesis, and biomedical devices are feasible.

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