04-01-2011, 04:17 PM
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Submitted by
VEERENDRA SHETTY T P
2nd Semester
In partial fulfillment of
MASTER OF TECHNOLOGY IN VLSI & EMBEDDED SYSTEMS
ABSTRACT
In damaged or dysfunctional retina, the photoreceptors stop working, causing blindness. By some estimation, there are more than 10 million people worldwide affected by retinal diseases that lead to loss of vision.
The absence of effective therapeutic remedies for retinitis pigmentosa (RP) and age-related macular degeneration (AMD) has motivated the development of experimental strategies to restore some degree of visual function to affected patients. Because the remaining retinal layers are anatomically spared, several approaches have been designed to artificially activate this residual retina and thereby the visual system.
At present, two general strategies have been pursued. The “Epiretinal” approach involves a semiconductor-based device placed above the retina, close to or in contact with the nerve fiber layer retinal ganglion cells. The information in this approach must be captured by a camera system before transmitting data and energy to the implant. The “Sub retinal” approach involves the electrical stimulation of the inner retina from the sub retinal space by implantation of a semiconductor-based micro photodiode array (MPA) into this location. The concept of the sub retinal approach is that electrical charge generated by the MPA in response to a light stimulus may be used to artificially alter the membrane potential of neurons in the remaining retinal layers in a manner to produce formed images.
Some researchers have developed an implant system where a video camera captures images, a chip processes the images, and an electrode array transmits the images to the brain. It’s called Cortical Implants.
INTRODUCTION
The retina is a thin layer of neural tissue that lines the back wall inside the eye. Some of these cells act to receive light, while others interpret the information and send messages to the brain through the optic nerve. This is part of the process that enables us to see. The device consists of a silicon chip inserted into the eye, which is designed to act like a retina, receiving images captured by a pair of glasses worn by the user. It will restore vision to people who have lost sight during their lifetime. Intelligent eye work by stimulating nerves, which are activated by electrical impulses. In this case the patient has a small device implanted into the body that can receive radio signals and transmit those signals to nerves. In the case of the intelligent eye the device is a circle about the size of a five-cent piece, inserted into the eye where the retina sits.
It is a silicon chip which decodes the radio signals and delivers the stimulations. The chip sends messages to the retinal ganglion cells through small wires. The camera feeds the visual information into a separate image-processing unit, which makes ‘sense’ of the image by extracting certain features. It might find a door, for example, by contrasting the bright open door with a dark room.
The intelligent eye is a “Bio-electronic eye” which is also called as bionic eye. It is an electronic device used to replace the functionality of the eye.
The unit then breaks down the image into pixels and sends the information, one pixel at a time, to the silicon chip, which then reconstructs the image.
Future 'Intelligent eye' technology would be people with eye diseases like retinitis pigmentosa and age-related macular degeneration, who were born with vision and therefore have the necessary brain pathways established for processing visual information, unlike those who were born blind.
Recently in London, doctors have fitted a blind man with a Intelligent eye that has given the 73-year-old virtual eyesight.
TECHNOLOGY OVERVIEW
Ocular implants are those which are placed inside the retina. It aims at the electrical excitation of two dimensional layers of neurons within partly degenerated retinas for restoring vision in blind people. The implantation can be done using standard techniques from ophthalmic surgery. Neural signals farther down the pathway are processed and modified in ways not really understood therefore the earlier the electronic input is fed into the nerves the better.
There are two types of ocular implants are there epi-retinal implants and sub retinal implants. Figure3.1 shows the major difference between two approaches.
IMPLANT DESIGN AND FABRICATION
The current micro photodiode array (MPA) is comprised of a regular array of individual photodiode subunits, each approximately 20×20-µm square and separated by 10-µm channel stops which is shown in the figure-4.1. Across the different generations examined, the implants have decreased in thickness, from ~250 µm for the earlier devices, to approximately 50 µm for the devices that are currently being used. Because implants are designed to be powered solely by incident light, there are no connections to an external power supply or other device. In their final form, devices generate current in response to a wavelength range of 500 to 1100 nm.
Implants are comprised of a doped and ion-implanted silicon substrate disk to produce a PiN (positive-intrinsic-negative) junction. Fabrication begins with a 7.6-cm diameter semiconductor grade N-type silicon wafer. For the MPA device, a photo mask is used to ion-implant shallow P+ doped wells into the front surface of the wafer, separated by channel stops in a pattern of individual micro photodiodes. An intrinsic layer automatically forms at the boundary between the P+ doped wells and the N-type substrate of the wafer. The back of the wafer is then ion-implanted to produce a N+ surface.
Thereafter, an insulating layer of silicon nitrate is deposited on the front of the wafer, covering the entire surface except for the well openings. A thin adhesion layer, of chromium or titanium, is then deposited over the P+ and N+ layers. A transparent electrode layer of gold, iridium/iridium oxide, or platinum, is deposited on the front well side, and on the back ground side. In its simplest form, the photodiode and electrode layers are the same size. However, the current density available at each individual micro photodiode subunit can be increased by increasing the photodiode collector to electrode area ratio.
Implant finishing involves several steps. Smaller square devices are produced by diamond sawing, affixed to a spindle using optical pitch, ground, and then polished to produce the final round devices for implantation. The diameter of these devices has ranged from 2-3 mm (for implantation into the rabbit or cat sub retinal space) to ~0.8 mm (for implantation into the smaller eye of the rat).
ELECTRONICS PACKAGE
The 292 X 512 pixel CCD black and white television camera is powered by a 9 V battery, and connects via a battery powered NTSC page link to a sub-notebook computer in a belt pack. This f 14.5 camera, with a 69° field of view, uses a pinhole aperture, instead of a lens, to minimize size and weight. It also incorporates an electronic "iris" for automatic exposure control.
The sub-notebook computer incorporates a 120 MHz microprocessor with 32 MB of RAM and a 1.5 GB hard drive. It also has an LCD screen and keyboard. It was selected because of its very small size and light weight. The belt pack also contains a second microcontroller, and associated electronics to stimulate the brain. This stimulus generator is connected through a percutaneous pedestal to the electrodes implanted on the visual cortex. The computer and electronics package together are about the size of a dictionary and weigh approximately 10 pounds, including camera, cables, and rechargeable batteries. The battery pack for the computer will operate for approximately 3 hours and the battery pack for the other electronics will operate for approximately 6 hours.
This general architecture, in which one computer interfaces with the camera and a second computer controls the stimulating electronics, has been used by us in this, and four other substantially equivalent systems, since 1969. (8) The software involves approximately 25,000 lines of code in addition to the sub-notebooks' operating system. Most of the code is written in C++, while some is written in C. The second microcontroller is programmed in assembly language.
ADVANTAGES
Implementation of an Intelligent Eye has certain advantages. An electronic eye is more precise and enduring than a biological eye and we cannot altogether say that this would be used only to benefit the human race. In short successful implementation of a bioelectronics eye would solve many of the visual abnormalities suffered by human’s to date.
To be honest, the final visual outcome of a patient cannot be predicted. However, before implantation several tests have to be performed with which the potential postoperative function can be estimated. With this recognition of large objects and the restoration of the day-night cycle are the primary goals of the prototype implant.
• Compact Size – 6x6 mm
• Reduction of stress upon retina
CONCLUSION AND ENHANCEMENTS
The application of the research work done is directed towards the people who are visually impaired. People suffering from low vision to, people who are completely blind will benefit from this project. The findings regarding biocompatibility of implant materials will aid in other similar attempts for in human machine interface. Congenital defects in the body, which cannot be fully corrected through surgery, can then be corrected.
There has been marked increase in research and clinical work aimed at understanding low vision. Future work has to be focused on the optimization and further miniaturization of the implant modules. Commercially available systems have started emerging that integrates video technology, image processing and low vision research.
