We present a multi-chip electric stimulator for a retinal prosthesis. The stimulator consists of small silicon devices (unit chips) molded in a thin film. It has an advantage over the conventional devices in physical flexibility and extendibility. The smart unit chip (600 μm square, in this work) is an integrated circuit (IC) that includes digital serial interface circuits, analog switch circuits and on-chip stimulus electrodes. In contrast to conventional stimulators, the present stimulator can be driven with only four wires. The multi-chip configuration enables to make the stimulator flexible and durable to bending stress. The device can be bended to place the stimulation electrodes in good contact with retinal tissue. In this paper, we present the design of the stimulator device with 0.35-μm complementary metal-oxide semiconductor (CMOS) technology. We also report a thin, flexible packaging technique for the stimulator and preliminary experimental results of a sputtered iridium oxide (IrOx) film that can be used for chronic stimulation.
KEYWORDS: Digital image processing, Digital electronics, Image processing, Digital electronic circuits, Amplifiers, Sensors, Cameras, Imaging systems, Digital photography, Electronic imaging
We are exploring the application of pulse-frequency-modulation (PFM) photosensor to retinal prosthesis for the blind because behavior of PFM photosensors is similar to retinal ganglion cells, from which visual data are transmitted from the retina toward the brain. We have developed retinal-prosthesis vision chips that reshape the output pulses of the PFM photosensor to biphasic current pulses suitable for electric stimulation of retinal cells. In this paper, we introduce image-processing functions to the pixel circuits. We have designed a 16x16-pixel retinal-prosthesis vision chip with several kinds of in-pixel digital image processing such as edge enhancement, edge detection, and low-pass filtering. This chip is a prototype demonstrator of the retinal prosthesis vision chip applicable to in-vitro experiments. By utilizing the feature of PFM photosensor, we propose a new scheme to implement the above image processing in a frequency domain by digital circuitry. Intensity of incident light is converted to a 1-bit data stream by a PFM photosensor, and then image processing is executed by a 1-bit image processor based on joint and annihilation of pulses.
The retinal prosthesis vision chip is composed of four blocks: a pixels array block, a row-parallel stimulation current amplifiers array block, a decoder block, and a base current generators block. All blocks except PFM photosensors and stimulation current amplifiers are embodied as digital circuitry. This fact contributes to robustness against noises and fluctuation of power lines. With our vision chip, we can control photosensitivity and intensity and durations of stimulus biphasic currents, which are necessary for retinal prosthesis vision chip. The designed dynamic range is more than 100 dB. The amplitude of the stimulus current is given by a base current, which is common for all pixels, multiplied by a value in an amplitude memory of pixel. Base currents of the negative and positive pulses are common for the all pixels, and they are set in a linear manner. Otherwise, the value in the amplitude memory of the pixel is presented in an exponential manner to cover the wide range. The stimulus currents are put out column by column by scanning. The pixel size is 240um x 240um. Each pixel has a bonding pad on which stimulus electrode is to be formed. We will show the experimental results of the test chip.
In this paper, we propose and demonstrate a new pixel structure of a vision chip for visual recovery of the blind by means of electric stimulation against the retina. We introduce output frequency range control to a conventional pulse frequency modulation (PFM) photosensor. To configure the lower and upper limits of the output pulse frequency suitable for electric stimulation, we added nMOS and pMOS leak transistors and a switched-capacitor filter to an original PFM pixel circuit. We fabricated a test circuit of the improved PFM pixel using a standard 0.6 μm CMOS process. The pulse frequency range was successfully fixed, for example, from 100 Hz to 1 kHz.
We have developed a CMOS vision chip, an image sensor with pixel-level signal processing, to replace photoreceptor cells in the retina. In this paper, we describe a pixel-level signal processing, which is to control on the stimulus waveform and the amount of the electrical injection charge.
Our CMOS vision chip is an array of a pixel, which consists of a photo detector, a pulse shaper, and a current stimulus circuit. The photo detector circuit generates a pulse frequency modulated (PFM) pulse, which frequency is proportional to the intensity of the incoming light. The PFM photo detector is also modified to restrict the maximum frequency of PFM pulse signal for safety neural stimulation.
The PFM pulse signal should be converted into suitable waveform for efficient neural stimulation. We have employed a pulse shaper to generate one stimulus pulse from one PFM pulse. The pulse parameters (i.e., pulse duration, polarity, etc) of the output pulse signal are controlled by the external signal.
For the electrical neural stimulus, the stimulus intensity is given by the amount of the electrical injection charge. The amount of the injection charge should be enough to evoke a phosphene but should be low to avoid the damage of the retinal tissue caused by the excess charge injection. In our prototyped CMOS vision chip, the stimulus current amplitude is used to control the amount of charge. The 6-bit binary-weighted digital-to-analog converter (DAC) with 2μA resolution is used to control the stimulus current amplitude.
KEYWORDS: Image processing, Photodiodes, Analog electronics, Modulation, Frequency modulation, Logic, Transistors, Power supplies, Digital electronics, Very large scale integration
This paper proposes and demonstrates a novel type of a vision chip that utilizes pulse trains for image processing. The chip is based on a pulse frequency modulation (PFM) technique, which is used in neurobiological systems. Two types of chips are designed; one is a pixel TEG (test element group) chip for testing availability of PFM for image acquisition using 0.35 micrometers triple-metal double-poly CMOS process and the other is for a vision chip with inhibitory interconnections using 1.2 micrometers double-metal double-poly CMOS process. The TEG chip works well in the power supply voltage of 0.7 V and has a dynamic range of 20 dB with a power consumption of less than 1 (mu) W. The operation of the mutual inhibition in the vision chip is confirmed by simulation. Also the comparison with the other pulse modulation technique, pulse width modulation is discussed.
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