biosmartsensors (1)

8/4/2019 BioSmartSensors (1)

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A

PAPER PRESENTATION

ON

(FOR THE VISUALLY IMPAIRED)

SUBMITTED

TO

AVISHKAR-2K11

ELLENKI COLLEGE OF ENGINEERING &TECHNOLOGY

PRESENTED

BY

B.Ranjith kumar Reddy N.Amarnath

[email protected] [email protected]+918125550605 +919490204455

III-I B.TECH III-I B.TECH

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Dr. K.V .Subba Reddy Institute Of Technology, (approved by AICTE & affliated toJNTU-A),lakshnipuram(PO),Kurnool (dist)

Abstract In this paper, the current version ofthe artificial retina prosthesis and


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cortical implant that is beingdeveloped will be described. A multidisciplinary team of researchers inOphthalmology, Neurosurgery,Computer Networking, VLSI, andSensors has been assembled to

develop the novel solutions needed tomake artificial vision for the visuallyimpaired a reality. This paperdescribes the novel approach that hasbeen adopted for providing a completesystem for restoring vision to visuallyImpaired persons from the signalsgenerated by an external camera toan array of sensor that electricallystimulate the retina via a wirelessinterface.

IntroductionIn this paper, we describe the currentversion of the artificial retinaprosthesis and cortical implant that isbeing developed. This research is amultidisciplinary project involvingresearchers in Ophthalmology,Neurosurgery, Computer Networking,Sensors, and VLSI. Restoring vision tothe blind and visually impaired ispossible only through significantprogress in all these research areas.

In the future, artificial retinaprostheses may be used to restorevisual perception to persons sufferingfrom retinitis pig mentosa, maculadegeneration, or other diseases of theretina. In patients with these diseases,most of the rods and cones aredestroyed, but the other cells of theretina are largely intact. It is wellknown that the application of electricalcharges to the retina can elicit theperception of spots of light. Bycoupling novel sensing materials withthe recent advances in VLSItechnology and wirelesscommunication, it is now feasible todevelop biomedical smart sensors thatcan support chronic implantation of asignificant number of stimulationpoints. Although the development anduse of artificial retina prostheses is

still in the early stages, the potentialbenefits of such technology areimmense.

Similarly, the use of corticalimplants has promise for the visuallyimpaired. Unlike the retina prosthesis,

a cortical implant bypasses most ofthe visual system, including the eyeand the optic nerve, and directlystimulates the visual cortex, whereinformation from the eyes isprocessed. Therefore, in addition toovercoming the effects of diseased ordamaged retina tissue, a corticalimplant could circumvent many otherproblems in the visual system,including the loss of an eye.

The smart sensor package is

created through the backside bondingof an array of sensing elements, eachof which is a set of microbumps thatoperate at an extremely low voltage,to a integrated circuit for acorresponding multiplexed grid oftransistors that allows individualvoltage control of each micro bumpsensor. The next generation designsupports a 16 array of sensors and isbeing fabricated by MOSIS based onthe circuit design created in our Smart

Sensors and Integrated Devices (SSID)research lab. Our earlier circuit design,which has been fabricated and tested,supports a 10 array of sensors. Thepackage is encapsulated in inertmaterial except for the micro bumps,which must be in contact with theretina. The longterm operation of thedevice, as well as the difficulty ofphysically accessing a biomedicaldevice implanted in the eye, precludesthe use of a batterypowered smartsensor. Because of the high volume ofdata that must be transmitted, thepower consumption of an implantedretinal chip is much greater than, forexample, a pacemaker. Instead, weplan to power the device using RFinductance. Because of the difficultiesof aligning the two coils one being


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within the body and the other oneoutside the body for RF powertransmission, a low frequency isrequired to tolerate misalignment ofthe coils. On the other hand, arelatively high frequency is required to

operate in the unlicensed ISM band.For this reason, we have adopted thenovel approach of using twofrequencies: RF inductance using afrequency of 5 MHz and RF datatransmission using a frequency in therange of either 900 MHz or 2.4 GHz.

The FCC regulations for low power nonlicensed transmitters are explainedin [1].

Retinal and Cortial implantsProposed retina implants fall into twogeneral categories Epiretinal, which are placed on thesurface of the retina. Subretinal, which are placed underthe surface of the retina.Both approaches have advantages anddisadvantages. The main advantagesof the sub-retinal implant are that theimplant is easily fixed in place, andthe simplified processing that isinvolved, since the signals that aregenerated replace only the rods andcones with other layers of the retinaprocessing the data from the implant.

The main advantage of the epiretinalimplant is the greater ability todissipate heat because it is notembedded under tissue. This is asignificant consideration in the retina.

The normal temperature inside theeye is less than the normal bodytemperature of 98.6o Fahrenheit.Besides the possibility that heatbuildup from the sensor electronicscould jeopardize the chronicimplantation of the sensor, there isalso the concern that the elevatedtemperature produced by the sensorcould lead to infection, especiallysince the implanted device couldbecome a haven for bacteria. Thereare also two options for a cortical

implant. One option is to place thesensors on the surface of the visualcortex. At this time, it is unknownwhether the signals produced by thistype of sensor can produce stimulithat are sufficiently localized to

generate the desired visualperception. The other option is to useelectrodes that extend into the visualcortex. This allows more localizedcontrol of the stimulation, but alsopresents the possibility of long -termdamage to the brain cells duringchronic use. It should be noted,however, that although heatdissipation remains a concern with acortical implant, the natural heatdissipation within the skull is greater

than within the eye.Given the current state of the

research, it is unclear which of thesedisadvantages will be most difficult toovercome for a chronically implanteddevice. Therefore, different researchgroups are investigating differentsolutions. Here we describe ourproposed solution. An implantableversion of the current ex-vivo microsensor array, along with its locationwithin the eye, is shown in Figure 1.

The micro bumps rest on the surfaceof the retina rather than embeddingthemselves into the retina. Unlikesome other systems that have beenproposed, these smart sensors areplaced upon the retina and are smallenough and light enough to be held inplace with relatively little force. Thesesensors produce electrical signals thatare converted by the underlying tissueinto a chemical response, mimickingthe normal operating behavior of theretina from light stimulation. Thechemical response is digital (binary),essentially producing chemical serialcommunication. A similar design isbeing used for a cortical implant,although the spacing between themicro bumps is larger to match the


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increased spacing between ganglia inthe visual cortex.

Figure 1: Location of the SmartSensor within the EyeAs shown in Figure 1, the front side ofthe retina is in contact with the micro

sensor array. This is an example of anepiretinal implant. Transmission intothe eye works as follows. The surfaceof the retina is stimulated electrically,via artificial retina prosthesis, by thesensors on the smart sensor chip.

These electrical signals are convertedinto chemical signals by the gangliaand other underlying tissue structuresand the response is carried via theoptic nerve to the brain. Signaltransmission from the smart sensors

implanted in the eye works in a similarmanner, only in the reverse direction.

The micro sensors pick up theresulting neurological signals from theganglia and the signal and relativeintensity can be transmitted out of thesmart sensor. Eventually, the sensorarray will be used for both reception

and transmission in a feedback systemand chronically implanted within theeye. Although the micro sensor arrayand associated electronics have beendeveloped, they have not yet beentested as a chronic implant. Another

challenge at this point is the wirelessnetworking of these micro sensorswith an external processing unit inorder to process the complex signalsto be transmitted to the array.

Smart sensor chip designFigure 2 shows a close up of thesmart sensor shown in figure1. Eachmicro bump array consists of a clusterof extrusions that will rest on thesurface of the retina. The small size ofthe micro bumps allows them to reston the surface of the retina withoutperforating the retina. In addition, theslight spacing among the extrusions ineach micro bump array provides someadditional heat dissipation capability.Note that the distance betweenadjacent sets of micro bumps isapproximately 70 microns.

Electrode arrayHybrid with the multi scans

sensing/stimulating electronics


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Fig: illustration of micro bumparray

These sensors are bonded to anintegrated circuit. The integratedcircuit is a multiplexing chip, operatingat 40 KHz, with on chip switches andpads to support a grid of connections.Figure 1 shows a 4 grid for illustrativepurposes, although the nextgeneration of sensor chip has a 16array. The circuit can the ability totransmit and receive, although notsimultaneously. Each connection hasan aluminum probe surface where themicro machined sensor is bonded.

This is accomplished by using atechnique called backside bonding,which places an adhesive on the chipand allows the sensors to be bondedto the chip, with each sensor locatedon a probe surface. Before thebonding is done, the entire IC, exceptthe probe areas, is coated with abiologically inert substanice.

The neural probearray is a user-configured 1:100 demultiplexer/100:1 multiplexer, wherean external switch controls the

configuration. The neural array is amatrix of 100 microelectrodesconstructed as bidirectional switchedprobe units that will stimulate ormonitor the response state of anaggregate of neurons, more

specifically, bipolar cells, which aretwo poled nerve cells. When thearray is configured as a demultiplexer, the switched probe unitsserve to stimulate the correspondingaggregate of neurons thus, the arrayfunctions as a neuro stimulator. Whenthe array is configured as amultiplexer, the units serve to monitorthe evoked response of the aggregateof neurons in the visual cortex thus,the array functions as a neural

response monitor. The array has anadditional bidirectional port called thesignal carrier, where the direction ofthe signal flow to and from this portdepends on the configuration of thearray. As a neuro-response monitor,the neural signals from eachaggregate will be relayed through thesignal carrier port on a single line. As aneuro stimulator, the external signal,whose magnitude will depend on theintensity of the signal required to

revive the degenerate neurons, will beinjected into the circuit through thesignal carrier (bypassing the amplifier)to be distributed to each aggregatethrough the corresponding unit. Eachswitched probe unit consists of aneural probe and two n channelMOSFETs, whose W/L ratio is 6/2. TheW/L ratio defines the behavior of thetransistor, where W is the width thebehavior of the transistor, where W isthe width of the active area of thetransistor and L is the length of thepoly silicon used for the gate channel.For each unit, the probe is a passiveelement that is used to interface eachaggregate of neurons to the electronicsystem and the transistors are theactive elements that are used toactivate the units. The second


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generation prototype adds thedecoder with its outputs connected tothe row and to the column ports of thearray. The addition of the decoderreduces the number of requiredcontact pads from 22 to 5 (Set, master

clock, VDD, VSS, and the signal carrierport) and enhances the reliability ofthe Scanner with the configuration of2 inputs, rather than the externalconnections of 20 inputs. Theconfiguration of the Set and clockcycles will enable the decoder tosequentially activate each unit bysending +5V pulses to thecorresponding row and column ports.

To establish the required Set and clockcycles, further neural analysis on the

periodic stimulation of the bipolar cellsmust be conducted. The Set signalinitiates the scanning of the probefrom left to right and from top tobottom.

Computer communicationIt is not feasible to do the processinginternally using the capabilities of onlythe sensor arrays. Thus, work oninterconnecting these smart sensorswith an external processing system isa fundamental aspect of realizing thepotential of an artificial retina. On-going diagnostic and maintenanceoperations will also requiretransmission of data from the sensorarray to an external host computer.

These requirements are in addition tothe normal functioning of the device,which uses wireless communicationfrom a camera embedded in a pair ofeyeglasses into the smart sensorarrays. The research challenges inproviding wireless networkingsolutions for smart sensors isdescribed in[4]. The processing stepsfrom external image reception totransmission to the retina prosthesisare as follows. A camera mounted onan eyeglass frame could direct itsoutput to a real time DSP for datareduction and processing (e.g., Sobel

edge detection). The camera would becombined with a laser pointer forautomatic focusing. The DSP thenencodes the resultant image into acompact format for wirelesstransmission into (or adjacent to) the

eye for subsequent decoding by theimplanted chips. The setup could usea wireless transceiver that is insidethe body, but not within the retina,and a wire to the retina chip. Ourultimate research goal is to support anarray of 1600 smart sensor chips, eachwith a 25 grid of electrodes. The rodsand cones fire at an approximateinterval of 200250ms. Therefore, theprocessing will be performedperiodically in a 200 250ms

processing loop. Hence, data will betransmitted four or five times persecond. Although the actual rods andcones in the eye operate in an analogmanner (variety of possible values),our initial system will operate in astrictly on/off mode. In other words,one bit of data per sensor every 200 250ms. We plan on eventually movingto multiple level stimulation. Theinvestigations into understanding thevisual processing of the brain will

indicate whether or not the sensorarrays will be implanted with uniformdistribution. Functionally, electrodearrays within the center of the macula(the central retina) will have tostimulate the retina differently thanperipherally placed electrode arrays,since the functions of these variousparts of the retina are very different.Centrally, in the macula, we perceiveour high resolution detail vision,while in the periphery, the retina isbetter at detecting motion orillumination transients. (For example,most persons can perceive theircomputer monitor's vertical refreshwhen looking at the monitor usingperipheral vision, since the peripheralretina has better temporal resolution,but poorer spatial resolution than the


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macula.) Thus, a multi electrode arrayvisual prosthesis will have to encodethe visual scene slightly differently,depending upon where on the retinaeach electrode array is placed. Theperipherally placed electrodes need to

generate signals based on lowerspatial resolution with greateremphasis on temporal events, whilecentrally placed sensor arrays uponthe macula need to encode morespatially oriented information. Eacharray will have to transmit somecommon information such as theoverall luminosity of the visual scene.So, each smart sensor will have to becoordinated with other smart sensorsbased on an image processing

algorithm designed to control a set ofsmart sensor arrays, each separate,sending input to functionally differretinal areas.

In order to achieve theenvisioned functionality, two-waycommunication will be neededbetween an external computer andcortical implant so that we can provideinput to the cortical implant anddetermine if the desired image is"seen". We also need two-way

communication with the retinalimplant so that we can determine thatthe sensors in the retina are operatingas expected. Besides input from thecamera, we also need the ability toprovide direct input to the retinalimplant to determine if the patientsees what is expected from that inputpattern. This will validate ourunderstanding of the signalingbetween the camera and the smartsensor array as well as the operationof the wireless communicationprotocols. Our main objective is todesign a communication system thatis energy efficient and performssatisfactorily under interferingsources. For very low powertransmitter applications, reducing thepower consumed in the transmitter

architecture and an ideal modulationtechnique produces the best energyefficiency. Many energy efficienttransmitter architectures have beendeveloped [2] and can be used for lowpower applications. Comparison of

various digital modulation techniqueshave been done in terms of SNR/bitand width efficiency for a known BERand fixed data rate [2][3]. The powermust be carefully controlled to avoiddamage to the retina and surroundingtissue. Each sensor array operateswith less than one microampere ofcurrent. The power can be provided indifferent ways. One option is to usewires to provide the power, althoughwe would still require wireless data

communication to limit the number ofwires. Implanting a battery near theeye could provide the power. A secondoption is to use inductance, providedby RF or IR signals. A third option is aphotodiode array, which converts lightto power. It is important to note thateven if the power source is wired, thedata communication needs to bewireless in order to minimize thenumber of wires and improve theflexibility of the system. After

considering all factors, the decisionhas been made to use radiofrequencies for both power inductanceand data transmission.

Related workThe goal of artificially stimulating theretina to produce vision is currentlybeing investigated by seven large,multidisciplinary research teamsworldwide, including four groups in theUnited States, two in Germany, andone in Japan. Table 1 describes thelocation of the other six groups andthe design approach used. In addition,to the large groups listed below,efforts toward a visual prosthesis viacortical stimulation are being made byRichard Normann of the Utah VisualProsthesis Project and the NIH VisualProsthesis Project Lab of Neural


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Control/NINDS with Schmidt,Heetderks, and Hambrecht.

Investigators

Location Stimulussite

DeJuan/Humayan

NorthCalorina State

Epiretinal

Rizzo/Wyatt

Mass. eyeand ear

Epiretinal

Chow/Peyman

Optobionics/LSU

Subretinal

Eckmiller Duisburg,Germany

Subretinal

Zrenner Bonn,Germany

Subretinal

Yagi Nagoya,Japan

Unknown

Each group has directed their effortstoward the design of an implantabledevice. The Massachusetts Eye andEar/MIT program, as well as the NorthCarolina State groups have beenindependently working on anepiretinal, electrically based retinalstimulator. Groups designing anelectrical subretinal device includeChow/Peyman, Zrenner, and Eckmiller.

The Yagi group at the University of

Nagoya is attempting to hybridizecultured neurons with asiliconebased stimulator, borrowingfrom work pioneered by the Pine lab atCalTech. The North Carolina Stategroup and the Massachusettss Eyeand Ear/MIT group share commondesign approaches. Both implants areintended to stimulate the retina usingelectrical current, applied to the innerretina by a two dimensional, multipleelectrode array. Although the intended

target cell was believed to be theretinal ganglion cells, the doctoraldissertation of Robert Greenburg, at

Johns Hopkins, demonstrated that theretinal bipolar cells were thepredominant cell populationstimulated. These data were derivedthrough an analysis of the temporaldynamics of neuron responses after

epiretinal electrical stimulation in thefrog. In addition to an epiretinalelectrode array, both groups havedesigned VLSI (very large scaleintegration) chips intended for ocularimplantation. Both of these integrated

circuits are designed to accept anelectrical signal that encodes visualinformation. This signal is formatted asan electrical representation of a visualscene, provided by an external solidstate camera, or computer. The VLSIchip is designed to decode this signaland produce a graded electricalstimulation in the appropriateelectrodes, recreating a spatiallystructured electrical stimulus to theretina. The North Carolina State group

has borrowed from work done with theochlear implant. A radiofrequency (RF)receiver has been integrated into theVLSI design to permit the RFtransmission of the visual signal intothe chip. The MARC IV is animprovement over previous versionsbecause it is capable of measuring theelectrode contact impedance with theretina, automatically compensating forfluctuations by altering the stimulusvoltage. The Massachusettss Eye and

Ear/MIT group in Boston powers theirVLSI implantable silicone chip using aspecially designed silicon photocellarray. The array is capable ofdelivering sufficient energy to powerthe VLSI chip. This photocell array ismounted within a posterior chamberintraocular lens. It consists of an arrayof sixteen parallel sets of twelve linearphotodiodes. A two watt, 820nanometer laser is used to power thephotodiode array. By modulating thelaser output energy according to thepulse stream of a CCD sensor, visualinformation may be transmitted intothe VLSI chip, digitally. Thisinformation is then decoded in asimilar manner to the MARC IV togenerate a stimulus voltage,corresponding to the level of


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illumination for a given pixel at theappropriate stimulus electrode site.Worldwide, three groups are workingon the design of a sub retinalelectrode array implant. Alan Chowand associates at Optobionics

Corporation conducted the first workin this area. Their design has muchinherent strength with some additionaltechnical issues. For patients with hereditode generative retinal disease, theouter retina is most commonlyaffected. Rods and cones aredysfunctional or missing. Thesedisease states often leave the innerretina somewhat less affected.

Therefore, the true goal of a retinalimplant in these cases is to replace

the missing functionality of rods andcones. This involves stimulating thebipolar/ horizontal/amacrine systemsof the retina. Those groups workingwith subretinal electrode arrayshypothesize that these cellpopulations are most accessible fromthe subretinal space. Although thismay be true, the North Carolina Stategroup has shown that the bipolar cellsare stimulated by an epiretinalimplant. The inherent design of the

subretinal electrical implants is vastlydifferent from those groups workingon the epiretinal approach theepiretinal implants are designed toderive their power from a source thatis independent from the electrodearray. The NC State group derivestheir power from an inductivetransformer. Energy from this sourceis then switched to the appropriateelectrodes, according to the visualdata input signal. The MassachusettsEye and Ear/MIT group uses a siliconphotodiode array placed within aposterior chamber intraocular lens forpower. Energy from this array is thenswitched by the VLSI implant to theappropriate epiretinal electrode, basedupon the input of a CCD camera.Subretinal implants are inherently

simpler by design and mimic themodular organization of individualrods and cones. Power is generated atthe site of sub-retinal electricalstimulation, using photosensitivemicro photodiodes (MPDs). When

arranged two-dimensionally, thesemicro photodiode arrays (MPDAs)provide spatially organized electricalstimulation to the retina. Thus, theirdesign is inherently much simpler.Incident light falls upon the MPDA,generating an electrical stimulus withidentical spatial organization. Nocameras or encoding/decodingcircuitry is needed. In addition, thepower supply is integrated within theimplant. Although these are great

benefits in design simplicity, otherfactors may complicate their use.

These include the need for opticallyclear media and also assume that anadequate amount of stimulationcurrent can be generated by an MPDfor each stimulation point. Further,since the implant is within thesubretinal space, metabolic issuesconcerning adequacy of oxygen andnutrient delivery to the outer retinabecome considerations. A report at

ARVO by the Eckmuller group, 1998,noted that microholes had to be madethrough the MPDA to permit prolongedviability within the rabbit retina. Bothepiretinal and subretinal approacheshave thus far been based uponelectrical stimulation of the retina. TheNorth Carolina State group useplatinum electrodes, while theMIT/Mass. Eye and Ear group uses aplatinum/iridium alloy. Using platinumelectrodes, current densities forepiretinal stimulation have beenreported between 2.98 (bullfrog) and11.9 (adult rabbit)microcoulombs/cm2. Reported currentdensities fromthe MIT/Mass. Eye andEar group for 25 micrometer platinumwire is 16 microamperes while a fivemicrometer platinum/iridium electrode


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requires 0.4 micro amperes intheadultrabbit.

Electrical stimulation of theretina through injection of currentdissipates power and heat. In patientswith degenerative retinal disorders,

the choriocapillaris, which normallyprovides heat dissipation in the retina,is markedly pathologic. Therefore, anystudy regarding the energy dissipationrequirements must be performed toaccount for the compromised outerretinal blood flow/heat dissipationsystem. In addition to thethermodynamic issues introduced byelectrical stimulation, ionization ofelectrodes does occur at physiologicpH and temperature within saline

media. Thus, chronically implantedelectrodes oxidize, diminishing theireffectiveness over time.

ConclusionIn this paper, we have described ourinitial approach to an artificial retinaprosthesis and cortical implant, whichwill be refined further as testing anddevelopment continue. The creation ofa smart sensor implant to restorevision to persons with diseased retinasor suffering from other damage to thevisual system has tremendouspotential for improving the quality ofthe life for millions. It also presents anumber of challenging researchproblems that require the involvementof a multidisciplinary research team.

The eventual goal of this research is achronically implanted visual prosthesisthat provides significant visualfunctionality.

REFERENCES: www.smartsensors.co.uk

www.ssim.eng.wayne.edu

www.ee.iitb.ac.in

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