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SEMINAR REPORT
Submitted in partial fulfillment of the requirements for the award of the Degree of
BACHELOR OF TECHNOLOGY
In
By
APRIL 2011
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING MAHATMA GANDHI UNIVERSITY COLLEGE OF ENGINEERING Muttom P. O, Thodupuzha, Kerala 685 587
CERTIFICATE This is to certify that the Seminar Report titled BIONIC EYE: A LOOK INTO CURRENT RESEARCH AND FUTURE PROSPECTS, submitted in partial fulfillment of the requirements for the award of the Degree of Bachelor of Technology in Electronics and Communication Engineering of Mahatma Gandhi University, is a record of the seminar presented by the candidate VISHNU NARAYANA PANICKER, Department of Electronics and
Communication Engineering, Mahatma Gandhi University College of Engineering, Thodupuzha, Kerala under my guidance and supervision and the report has not formed the basis for award of any other Degree, Diploma or other studies previously.
Mr. VAISAKHAN K R
Seminar Co-ordinator Dept. of ECE MGUCE,Thodupuzha
Internal Examiner
ACKNOWLEDGEMENT
A seminar presentation is indeed an experience and helps a lot in framing up a person in his carrier. First and foremost, I express my illimitable gratitude to our Principal, Prof.K.T.Subramanian, Mahatma Gandhi University College of Engineering, for his valuable cooperation and advices. I express my profound thanks to Dr. Lethakumari B, HOD, Electronics and Communication Department, for providing various facilities to instigate this seminar. I convey my gratitude to Mr. Vaisakhan K R, Seminar Co-ordinator of our Department. I also express my sincere thanks to Ms.Vrinda P and Mrs.Renju R, faculty of our department for their support and encouragement. I thank my friends for their cooperation and inspiration. Above all, my gratitude has no bounds before the compassion and love of the Almighty God, without whose help this seminar would not have attained a successful conclusion.
CONTENTS
List of Figures Abstract 1. Introduction 1.1 VISUAL SYSTEM 1.2 THE EYE 1.3 ANATOMY OF EYE 1.4 HOW ARE WE ABLE TO SEE? 1.5 RETINA 1.6 RETINAL DISEASES 2. NEED FOR BIONIC EYE 2.1 WHAT IS A BIONIC EYE? 2.2 THE BIONIC EYE SYSTEM 2.3 RETINAL IMPLANT SYSTEMS 2.4 WORKING 3. OCULAR IMPLANTS 3.1 EPI-RETINAL IMPLANTS 3.2 SUB RETINAL IMPLANTS 4. MULTIPLE UNIT ARTIFICIAL RETINA CHIPSET (MARC) 4.1 WORKING 4.1 (a) MARC SYSTEM BLOCK DIAGRAM 4.1 (b) BLOCK DIAGRAM OF IMAGE ACQUISITION SYSTEM 5. ON GOING DEVELOPMENTS 5.1 ARGUS III 5.2 MITS RETINAL IMPLANT 5.3 OPTOELECTRONIC RETINAL PROSTHESIS (STANFORD IMPLANT) 6. CHALLENGES 7. CONCLUSION 8. BIBLIOGRAPHY 1 2 3 4 5 6 7 8 10 10 11 12 14 16 17 20 24 26 26 27 28 28 29 30 32 34 35
LIST OF FIGURES
1. Block Diagram Of Visual System 2. Eye-Camera Similarity 3. Anatomy Of Eye 4. The Eye 5. Retina 6. Argus II 7. Block Diagram Of The Epi-Ret System 8. Subretinal Implant 9. Ultra Thin Microphotodiode Array 10. MARC System 11. Rf Coil Configuration Of MARC System 12. MARC System Block Diagram 13. Block Diagram Of Image Acquisition System 14. Argus III Implant 15. Transistor Electrode Array For The Argus III
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ABSTRACT
A visual prosthesis often referred to as a bionic eye or retinal implant, is an experimental visual device intended to restore functional vision. A visual prosthetic or bionic eye is a form of neural prosthesis intended to partially restore lost vision or amplify existing vision. It usually takes the form of an externally-worn camera that is attached to a stimulator on the retina, optic nerve, or in the visual cortex, in order to produce perceptions in the visual cortex. Bionic eye restores the vision lost due to damage of retinal cells. 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. In damaged or dysfunctional retina, the photoreceptors stop working, causing blindness. By some estimates, 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 Retinis pigmentosa (RP) and Age-related macular degeneration (AMD) has motivated the development of experimental strategies to restore some degree of functional vision 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.
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1. INTRODUCTION
Technology has done wonders for the mankind. We have seen prosthetics that helped overcome handicaps. Bio medical engineers play a vital role in shaping the course of these prosthetics. Now it is the turn of Artificial Vision through Bionic Eyes. Chips are designed specifically to imitate the characteristics of the damaged retina, and the cones and rods of the organ of sight are implanted with a microsurgery. Whether it be Bio medical, Computer, Electrical, or Mechanical Engineers all of them have a role to play in the personification of Bionic Eyes. This multidisciplinary nature of the new technology has inspired me to present this paper. There is hope for the blind in the form of Bionic Eyes. This technology can add life to their vision less eyes! Today, we talk of artificial intelligence that has created waves of interest in the field of robotics. When this has been possible, why not artificial vision? It is with this dream that I present this paper on Bionic Eyes. Sooner or later, this shall create a revolution in the field of medicine. It is important to know few facts about the organ of sight i.e, the Eye before we proceed towards the technicalities involved.
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The nerve system can achieve this type of high volume data transfer by confining such capability to just part of the retina surface, whereas the centre of the retina has a 1:1 ratio between the photoreceptors and the transmitting elements, the far periphery has a ratio of 300:1. This results in gradual shift in resolution and other system parameters. At the brains highest level, the visual cortex, an impressive array of feature extraction mechanisms can rapidly adjust the eyes position to sudden movements in the peripherals filed of objects too small to see when stationary. The visual system can resolve spatial depth differences by combining signals from both eyes with a precision less than one tenth the size of a single photoreceptor.
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Our ability to see is the result of a process similar to that of a camera. In a camera, light passes through a series of lenses that focus images onto film or an imaging chip. The eye performs a similar function in that light passes through the cornea and crystalline lens, which together focus images onto the retinathe layer of light sensing cells that lines the back of the eye. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. Once stimulated by light, the cells within the retina process the images by converting their analog light signals into digital electro-chemical pulses that are sent via the optic nerve to the brain. A disruption or malfunction of any of these processes can result in loss of vision.
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Fig. Anatomy of eye The 1 million fibres in the retina form the optic nerve and transmit visual information to the visual cortex and its various areas in the back of the brain. The area of the retina that receives and processes the detailed imagesand then sends them via the optic nerve to the brainis referred to as the macula. The macula is of significant importance in that this area provides the highest resolution for the images we see. The macula is comprised of multiple layers of cells which process the initial analog light energy entering the eye into digital electro-chemical impulses.
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Fig. The Eye Having seen the anatomical part of human eye, lets try to know as to how we are able to see how is an image being formed? For vision to occur, 2 conditions need to be met: An image must be formed on the retina to stimulate its receptors (rods and cones). Resulting nerve impulses must be conducted to the visual areas of the cerebral cortex for interpretation. Four processes focus light rays, so that they form a clear image on the retina: 1. Refraction of light rays 2. Accommodation of the lens 3. Constriction of the pupil 4. Convergence of the eyes
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1.5 RETINA
The retina is the innermost layer of the wall of the eyeball. Millions of lightsensitive cells there absorb light rays and convert them to electrical signals. Light first enters the optic (or nerve) fibre layer and the ganglion cell layer, under which most of the nourishing blood vessels of the retina are located. This is where the nerves begin, picking up the impulses from the retina and transmitting them to the brain.
Light
Fig. Retina
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Fig. The retinal layers The light is received by photoreceptor cells called rods (responsible for peripheral and dim light vision) and cones (providing central, bright light, fine detail, and colour vision). The photoreceptors convert light into nerve impulses, which are then processed by the retina and sent through nerve fibres to the brain. The nerve fibres exit the eyeball at the optic disk and reach the brain through the optic nerve. Directly beneath the photoreceptor cells is a single layer of retinal pigment epithelium (RPE) cells, which nourish the photoreceptors. These cells are fed by the blood vessels in the choroids.
RETINITIS PIGMENTOSA (RP) is a general term for a number of diseases that predominately affect the photoreceptor layer or light sensing cells of the retina. These diseases are usually hereditary and affect individuals earlier in life. Injury to the photoreceptor cell layer, in particular, reduces the retinas ability to sense an initial light signal. Despite this damage, however, the remainder of the retinal processing cells in other layers usually continues to function. RP affects the mid-peripheral vision first and sometimes progresses to affect the far-periphery and the central areas of vision. The narrowing of the field of vision into tunnel vision can sometimes result in complete blindness. AGE-RELATED MACULAR DEGENERATION (AMD) refers to a degenerative condition that occurs most frequently in the elderly. AMD is a disease that progressively decreases the function of specific cellular layers of the retinas macula. The affected areas within the macula are the outer retina and inner retina photoreceptor layer. As for macular degeneration, it is also genetically related, it degenerates cones in macula region, causing damage to central vision but spares peripheral retina, which affects their ability to read and perform visually demanding tasks. Although macular degeneration is associated with aging, the exact cause is still unknown. Together, AMD and RP affect at least 30 million people in the world. They are the most common causes of untreatable blindness in developed countries and, currently, there is no effective means of restoring vision.
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optic nerve to the brain, which is able to perceive patterns of light and dark spots corresponding to which electrodes have been stimulated. The device receives signals from a pair of glasses worn by the patient, which are fitted with a camera. The camera feeds the visual information into a separate image-processing unit, which makes 'sense' of the image by extracting certain features. 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. Data is broadcasted into the body using radio waves. It's like a radio station that only has a range of 25 millimetres. Currently the technology is only able to transmit a 10 x 10 pixel. Participants must be profoundly blind to be eligible those with even partial vision are excluded due to the potential risk of visual damage. The most recent version of the implant features an array of 60 pixels, allowing users to distinguish between light and dark, and see certain distinct objects. The ultimate goal, according to the research team, is to allow for reading and face recognition by increasing the number of pixels to 1,000.
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Now, a company called Second Sight has received FDA approval to begin U.S. trials of a retinal implant system that gives blind people a limited degree of vision. Second Sights first generation Argus 16 implant consists of a 16 electrode array and a relatively large implanted receiver implanted behind the ear. The second generation Argus II is designed with a 60 electrode array and a much smaller receiver that is implanted around the eye. It (Argus II) is an array of electrodes that is surgically implanted onto the retina the layer of specialised cells that normally respond to light found at the back of the eye. This array of electrodes is able to send signals to the brain that the persons biological retina is unable to send. Of course, the electrode array is not very useful unless it is receiving visual data to send to the brain. To solve this problem the patient is fitted with a pair of glasses that contain a tiny video camera that continuously records footage of what is in front of the patient. This video signal is sent wirelessly to a wearable computer that first filters and processes the video signal and then feeds this formatted data to the electrode array. A picture of the entire setup can be seen below:
Fig. Argus II
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The Argus II Retinal Prosthesis System can provide sight -- the detection of light -- to people who have gone blind from degenerative eye diseases like macular degeneration and retinitis pigmentosa. Both diseases damage the eyes' photoreceptors, the cells at the back of the retina that perceive light patterns and pass them on to the brain in the form of nerve impulses, where the impulse patterns are then interpreted as images. The Argus II system takes the place of these photoreceptors.
The second incarnation of Second Sight's retinal prosthesis consists of five main parts: A digital camera that's built into a pair of glasses. It captures images in real time and sends images to a microchip. A video-processing microchip that's built into a handheld unit. It processes images into electrical pulses representing patterns of light and dark and sends the pulses to a radio transmitter in the glasses. A radio transmitter that wirelessly transmits pulses to a receiver implanted above the ear or under the eye. A radio receiver that sends pulses to the retinal implant by a hair-thin implanted wire. A retinal implant with an array of 60 electrodes on a chip measuring 1 mm by 1 mm. The entire system runs on a battery pack that's housed with the video processing unit. When the camera captures an image -- of, say, a tree -- the image is in the form of light and dark pixels. It sends this image to the video processor, which converts the tree-shaped pattern of pixels into a series of electrical pulses that represent "light" and "dark." The processor sends these pulses to a radio transmitter on the glasses, which then transmits the pulses in radio form to a receiver implanted underneath the subject's skin. The receiver is directly connected via a wire to the electrode array implanted at the back of the eye, and it sends the pulses down the wire. When the pulses reach the retinal implant, they excite the electrode array. The array acts as the artificial equivalent of the retina's photoreceptors. The electrodes are stimulated in accordance with the encoded pattern of light and dark that represents the tree, as the retina's photoreceptors would be if
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they were working (except that the pattern wouldn't be digitally encoded). The electrical signals generated by the stimulated electrodes then travel as neural signals to the visual center of the brain by way of the normal pathways used by healthy eyes -- the optic nerves. In macular degeneration and retinitis pigmentosa, the optical neural pathways aren't damaged. The brain, in turn, interprets these signals as a tree and tells the subject, "You're seeing a tree."
2.4 WORKING
Normal vision begins when light enters and moves through the eye to strike specialized photoreceptor (light-receiving) cells in the retina called rods and cones. These cells convert light signals to electric impulses that are sent to the optic nerve and the brain. Retinal diseases like age-related macular degeneration and retinitis pigmentosa destroy vision by annihilating these cells. With the artificial retina device, a miniature camera mounted in eyeglasses captures images and wirelessly sends the information to a microprocessor (worn on a belt) that converts the data to an electronic signal and transmits it to a receiver on the eye. The receiver sends the signals through a tiny, thin cable to the microelectrode array, stimulating it to emit pulses. The artificial retina device thus bypasses defunct photoreceptor cells and transmits electrical signals directly to the retinas remaining viable cells. The pulses travel to the optic nerve and, ultimately, to the brain, which perceives patterns of light and dark spots corresponding to the electrodes stimulated. Patients learn to interpret these visual patterns. It takes some training for subjects to actually see a tree. At first, they see mostly light and dark spots. But after a while, they learn to interpret what the brain is showing them, and they eventually perceive that pattern of light and dark as a tree. Researchers are already planning a third version that has a 1000 electrodes on the retinal implant, which they believe could allow for reading, facialrecognition capabilities etc.
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1: Camera on glasses views image 2: Signals are sent to hand-held device 3: Processed information is sent back to glasses and wirelessly transmitted to receiver under surface of eye 4: Receiver sends information to electrodes in retinal implant 5: Electrodes stimulate retina to send information to brain.
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3. OCULAR IMPLANTS
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: Epi-retinal implants and Subretinal implants.
Fig. Section of the eye showing the retina and its layers. In conditions such as retinitis pigmentosa and macular degeneration, the light sensing rod and cone cells ("photoreceptors") no longer function. A retinal prosthesis can be placed either on the retinal surface ("epi-retinal") or below the retina in the area of damaged photoreceptors ("sub-retinal") to try to stimulate the remaining cells.
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The issues involved in the design of the retinal encoder are: CHIP DEVELOPMENT BIOCOMPATIBILITY RF TELEMETRY AND POWER SYSTEMS
CHIP DEVELOPMENT:
EPI RETINAL ENCODER The design of an epiretinal encoder is more complicated than the sub retinal encoder, because it has to feed the ganglion cells. Here, a retina encoder (RE) outside the eye replaces the information processing of the retina. A retina stimulator (RS), implanted adjacent to the retinal ganglion cell layer at the retinal 'output', contacts a sufficient number of retinal ganglion cells/fibers for electrical stimulation. A wireless (Radio Frequency) signaland energy transmission system provides the communication between RE and RS. The RE, then, maps visual patterns onto impulse sequences for a
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number of contacted ganglion cells by means of adaptive dynamic spatial filters. This is done by a digital signal processor, which, handles the incoming light stimuli with the master processor, implements various adaptive, antagonistic, receptive field filters with the other four parallel processors, and generates asynchronous pulse trains for each simulated ganglion cell output individually. These spatial filters as biology-inspired neural networks can be 'tuned' to various spatial and temporal receptive field properties of ganglion cells in the primate retina.
BIOCOMPATIBILITY: The material used for the chips and stimulating electrodes should satisfy a variety of criterias. They must be corrosion-proof, i.e. bio stable. The electrodes must establish a good contact to the nerve cells within fluids, so that the stimulating electric current can pass from the photo elements into the tissue. It must be possible to manufacture these materials with micro technical methods and, They must be biologically compatible with the nervous system.
RF TELEMETRY: In case of the epiretinal encoder, a wireless RF telemetry system acts as a channel between the Retinal Encoder and the retinal stimulator. Standard semiconductor technology is used to fabricate a power and signum receiving chip, which drives current through an electrode array and stimulate the retinal neurons. The intraocular transceiver processing unit is separated from the stimulator in order to take into account the heat dissipation of the rectification and power transfer processes. Care is taken to avoid direct contact of heat dissipating devices with the retina.
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Fig. Subretinal 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 outer retina or remnants of this structure and thereby activate the visual system. Because the implant is designed to stimulate the retina at an early stage of the visual system, this approach would theoretically allow the normal processing networks of the retina to transmit this signal centrally. In Retinitis pigmentosa disease, the retinal pigment epithelial cells (RPE) begin to die out and the person starts loosing the vision gradually. Since the function of the retina to transduce light into biological signal is weakened, it causes blindness. Subretinal implant is used to substitute the lost RPE cells with the ones of artificial basis to restore the vision. In this implant, a microphotodiode array (MPD), a silicon micromanufactured device, or semiconductor microphotodiode array (SMA) is used. This piece of equipment is placed behind the retina between the sclera and the bipolar cells. The incident light is transformed into electrical potentials that excite the bipolar cells to form an image sensation.
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The arrays can be manufactured by various silicon manufacturing procedures. MPD arrays are manufactured consistently with measurements of each stimulating unit as 20 m X 20 m, and adjacent units separated as 10 m. The elements are produced to be responsive to light corresponding to the visible spectrum (400-700 nm). Several thousands of the devices can be placed on a single structure of diameter of 3 mm, thickness of 100 m and with a density same as the replacing RPE cells. These devices have demonstrated the same electrophysiological behaviours as the healthy RPE cells. The MPDA has to be very thin and flexible enough in order to be able to fit to the curvature of the eye ball. Figure below shows an example of such an ultra thin MPDA having a thickness of 1.5 micron, together with titanium substrate and silicon nitride passivation.
Fig. Ultra thin microphotodiode array In Subretinal implant, the light-sensitive microphotodiodes with microelectrodes of gold and titanium nitride set in arrays is implanted in the subretinal space. The visible light coming from different directions is transformed into small currents by the microphotodiodes at each of hundreds of microelectrodes. These currents are then passed to the retinal network by neurons. The middle and inner retina captures current and then
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processes the part of vision. There are many benefits of using the subretinal prostheses. Such as, the MPD directly replaces the lost or degenerated RPE cells; the retinas remaining network is still capable of processing electrical signals; ease of fixing the high density MPDA in the subretinal position; no need of any external camera or external image processing equipment; and eye movement to locate the objects is not restricted. There are some of the limitations to the subretinal implants as well. The single MPD is not enough to stimulate enough current. So a subretinal implant is supported by an external energy source, such as transpupillary infrared illumination of receivers close to the chip or electromagnetic transfer, is currently under progress. Some of the additional developments in this process are movement to flexible substrates to hold the subtle nature of the retina and to decrease the light intensity. Now, a German firm dubbed Retina Implant has scored a big win for the sub retinal solution with a three-millimeter, 1,500 pixel microchip that gives patients a 12 degree field of view.
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Fig. Shows the major difference between epi-retinal &sub retinal approach
In general, Epiretinal Approach involves a semiconductor based device positioned on the surface of the retina to try to simulate the remaining overlying cells. Subretinal Approach involves implanting the ASR chip behind the retina to simulate the remaining viable cells.
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The schematic of the components of the MARC to be implanted consists of a secondary receiving coil mounted in close proximity to the cornea, a power and signal transceiver and processing chip, a stimulation-current driver, and a proposed electrode array fabricated on a material such as silicone rubber thin silicon or polyimide with ribbon cables connecting the devices.
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The stimulating electrode array, an example of which is given in the figure below, is mounted on the retina while the power and signal transceiver is mounted in close proximity to the cornea. An external miniature low-power CMOS camera worn in an eyeglass frame will capture an image and transfer the visual information and power to the intraocular components via RF telemetry. The intraocular prosthesis will decode the signal and electrically stimulate the retinal neurons through the electrodes in a manner that corresponds to the image acquired by the CMOS Camera.
Figure 3: A 5x5 platinum electrode array for retinal stimulation fabricated on silicone rubber and used by doctors at JHU
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4.1 WORKING
The MARC system, pictured in the figures will operate in the following manner. An external camera will acquire an image, whereupon it will be encoded into data stream which will be transmitted via RF telemetry to an intraocular transceiver. A data signal will be transmitted by modulating the amplitude of a higher frequency carrier signal. The signal will be rectified and filtered, and the MARC will be capable of extracting power, data, and a clock signal. The subsequently derived image will then be stimulated upon the patients retina.
Outside Eye: The video input to the marc system block is given through a CCD camera. This image is further processed using a PDA sized image processor & to transmit it, we do pulse width modulation in first stage and then ASK modulation is done. This signal is further amplified using a class E power amplifier and transmitted using RF telemetry coils. Inside Eye: The signal received from the RF telemetry coils is power recovered and then these signal is ASK demodulated and the data and clock is recovered from this signals and these signal are sent to the configuration and control block
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of the chip which from its input decode what information has to be sent to each of the electrodes and sends them this data. And the electrodes in turn stimulate the cells in the eye so as to send this stimulation to the brain through optic nerve and help brain in visualizing the image and while this process is going on the status of each electrode is sent to the marc diagnostics chip outside the eye.
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5. ON GOING DEVELOPMENTS
5.1 ARGUS III THE ARTIFICIAL RETINA IS NEAR!
Fig. Argus III implant Funded by the US Department of Energy and lead by Lawrence Livermore National Labs, Argus seeks to create an epiretinal prosthesis, a device that will take the image from a camera and send it to your brain via your optic nerve. The first two phases of Argus (which we call Argus I and Argus II) have had extraordinary success with implants in more than 30 patients. Now, LLNL is getting ready to launch Argus III the third phase that will expand the number of patients, the quality of vision provided, and ease in which the device is implanted. There are other epiretinal prosthesis in development. The one at MIT is particularly promising. Yet Argus is at the forefront of the field. The Argus III will work by taking the image from a camera and wirelessly transmitting it to an electronics package. That package will stimulate undamaged retinal tissue using a thin film transistor electrode array. In Argus I, it took patients about 15 seconds to recognize objects using the retinal implant. In Argus II that was down to 2-3 seconds. Argus I had implants that provided 16 pixels of resolution. Argus II got up to 60, enough for the edges of doors, or the shape of a building. According to images from the LLNL site, the current implants prepared for Argus III will have 200+
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pixels. The DOE eventually wants 1000 pixels; at that point you could reliably make out someones face.
Fig. Transistor electrode array for the Argus III. Improving the pixel count isnt easy as it depends on the number of electrodes that get attached to the retina. The Argus device works by taking a thin-film electrode array and surgically implanting it onto the retinal tissue. These electrodes communicate wirelessly with an external camera through a biocompatible electronics package that is attached to the array. The entire assembly is smaller than it sounds and fits within the ocular cavity. Increasing the resolution of Argus means placing more electrodes on that array, as well as developing the electronics package that can handle the signal and processing.
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The MIT artificial retina may have a superior casing structure, made of titanium. The biggest difference between the two implants is where the electrodes attach. While the Argus array is placed on the retina, the MIT implant will be connected subretinally. This will reduce the risk of tearing during implantation. The MIT team wants the implant to last more than 10 years. In most other ways, the two devices are remarkably similar. The Argus II, as weve said before is being tested in 20 patients with remarkable results. The MIT implant has been proven to be safe in pig eyes for at least 10 months, and the programming algorithms have been thoroughly tested.
invisible. Some patients retinas might still have some photoreceptors that could be stimulated by visible light.
working
Stanford's 3-millimeter-wide chip is configured in three layers that together are 30 micrometers thick. The array is a series of pixels, each formed from a three photovoltaic cells of three different sizes. The purpose of the multiple subpixels, say the Stanford researchers, is to boost the amount of current each pixel sends to the still functional intermediate layers of the retina that perform the eyes natural image processing and data compression. (These layers perform compression so that data from the eyes 130 million photoreceptors can be sent on the 1.2 million axons in the optic nerve). According to Daniel Palanker, a Stanford professor of ophthalmology who worked on the chip, a device with 100 m pixels corresponds to a visual acuity of about 20/200. (That figure, which is the threshold beyond which a person is considered legally blind, means that the person would have to be within 20 feet [6.1 meters] of an object to see it with the level of clarity that a fully sighted person experiences from 200 feet away [61 meters]). In the best-case scenario, a photovoltaic prosthesis is limited to a pixel size of about 50 m, corresponding to visual acuity of 20/100. That should suffice, for face recognition and for reading large fonts.
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6. CHALLENGES
Current retinal implants provide very low resolution--just a few hundred pixels. But several thousand pixels would be required for the restoration of functional sight. A major limiting factor in achieving high resolution concerns the proximity of electrodes to target cells. A pixel density of 2,500 pixels per square millimeter corresponds to a pixel size of only 20 micrometers. But for effective stimulation, the target cell should not be more than 10 micrometers from the electrode. It is practically impossible to place thousands of electrodes so close to cells. With subretinal implants but not epiretinal ones, researchers discovered a phenomenon--retinal migration-that they now rely on to encourage retinal cells to move near electrodes-within 7 to 10 microns. Within three days, cells migrate to fill the spaces between pillars and pores. Development of a high resolution retinal prosthesis faces multiple engineering and biological challenges, such as delivery of information to thousands of pixels at video rate, placement of the electrodes in close proximity to the target cells, avoidance of fibrotic encapsulation of the implant, signal processing that compensates for the partial loss of the retinal neural network, and many others. Biology imposes limitations, such as the needs for a system that will not heat cells by more than 1 degree Celsius and for electrochemical interfaces that aren't corrosive. There are many very many obstacles to be overcome before Bionic Eyes become a success story. Our eyes are perhaps the most sensitive of all organs in the human body. A nano-sized irritant can create havoc in the eye. There are 120 million rods and 6 million cones in the retina of every healthy human eye. Creating an artificial replacement for these is no easy task. Si based photo detectors have been tried in earlier attempts. But Si is toxic to the human body and reacts unfavorably with fluids in the eye. There are many doubts as to how the brain will react to foreign signals generated by artificial light sensors. Infection and negative reaction are the always-feared factors. It is imperative that all precautionary measures need to be ascertained.
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One of the greatest challenges seems to be ensuring that the implant can remain in the eye for decades or more without causing scarring, immune system responses, and general degradation from daily biological wear and tear. These artificial retinas are still years away from becoming widespread because they are too expensive, too clunky, and too fragile to withstand decades of normal wear and tear.
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7. CONCLUSION
This is a revolutionary piece of technology and really has the potential to change people's lives. Artificial Eye is such a revolution in medical science field. Its good news for patients who suffer from retinal diseases. A bionic eye implant that could help restore the sight of millions of blind people could be available to patients within two years. Retinal implants are able to partially restore the vision of people with particular forms of blindness caused by diseases such as macular degeneration or retinitis pigmentosa. About 1.5 million people worldwide have retinitis pigmentosa, and one in 10 people over the age of 55 have agerelated macular degeneration. The invention and implementation of artificial eye could help those people. But whatever be the pro and cons of this system, if this system is fully developed it will change the lives of millions of people around the world. We may not restore the vision fully, but we can help them to least be able to find their way, recognize faces, read books, above all lead an independent life.
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8. BIBLIOGRAPHY
www.spectrum.ieee.org www.stanford.edu www.bionicvision.org.au www.visionaustralia.org www.2-sight.com www.cosmosmagazine.com www.ngm.nationalgeographic.com www.sessionmagazine.com www.health.howstuffworks.com www.wikipedia.org
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