jason lewis

8/4/2019 Jason Lewis

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ECE994:AntennasPatchAntennaImplantedInsideofaHumanBody

JasonLewis


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TableofContents

1.Introduction..........................................................................................................................31.1OverviewofImplantableMedicalSystems........................................................................3

F1.1MaterialCharacteristicsat403.5MHz..........................................................................................3F1.2PenetrationDepthofTissuesat403.5MHz ..............................................................................4

1.2UsesofImplantableCommunicationSystems..................................................................41.3RadioFrequenciesofInterestforImplantableMedicalSystems ..............................41.4ImplantCommunicationSystemSpecifications ..............................................................5

2.AntennaDesignforISMBand .........................................................................................52.1DesignConsiderations..............................................................................................................5

F2.1CalculationsforRectangularPatchAntenna.............................................................................5F2.2LayoutofthePatchAntennausedfortheISMBand..............................................................6F2.3EquivalentCircuitModelandEquations.....................................................................................7

2.2AnalysisofISMBandAntenna ...............................................................................................7F2.4ElectricFieldPatterninFreeSpace...............................................................................................8F2.5AntennaGainvs.Angle........................................................................................................................8F2.6ImpedancePlots.....................................................................................................................................9F2.7ElectricFieldStrengthInsideofPhantom...............................................................................10F2.8GainvsAngleInsideofPhantom.................................................................................................10

3.AntennaDesignforMICSBand.................................................................................... 113.1DesignConsiderations...........................................................................................................11

F3.1PIFASchematic....................................................................................................................................11F3.2XFDTDLayoutofPIFA......................................................................................................................11

3.2AnalysisofMICSBandAntenna.......................................................................................... 12

F3.3ElectricFieldPatterninFreeSpace............................................................................................12F2.4AntennaGainvs.Angle.....................................................................................................................13F3.5ImpedancePlots..................................................................................................................................13F3.6ElectricFieldStrengthInsideofPhantom...............................................................................14F3.7GainvsAngleInsideofPhantom.................................................................................................15

Conclusion............................................................................................................................... 15

WorksCited............................................................................................................................ 17


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1.Introduction

1.1OverviewofImplantableMedicalSystems

Thepurposeofthispaperistoinvestigatethevariousaspectspertainingtotheuseofimplantablemedicaldevices,specificallytheantennaportionofthedevice.The

examplesthatarepresentedwithinthispaperareverifiedusingtheFDTDapproach

viaRemconsXFTDsoftware.Asourpopulationisagingandpeoplearewishingto

liveontheirownatolderagesaneedforremotemonitoringofindividualsarises.Devicesarebeingimplantedintothetissuesofhumanbeingstomonitorvitalsigns

andtocontrolotherimplanteddevices.

Typical implantedcommunication systems involvethreemaincomponents.These

componentsareatransceiver,anantenna,andapowersupply.Thetransceiverismadeupof the circuitry thatwill send,receiveandprocessdata toand fromthe

antenna. The power supply will be a battery that is generally encased withinproximitytothetransceiver.Thetransceiverandtheantennawillneedtoconsume

as small of an amount of power as possible during the data transfer cycles to

prolongtheuseofthedevice.Toreplacea batterythat ispowering animplanted

medicaldevicewillrequireinvasivesurgery.

Designingantennasthataretobeimplantedwithinthehumanbodyarechallenging.

Sizelimitationsandtheharshenvironmentofthehumanbodycreateextraworkfor

theantennadesigner.Precisionneedstobepresentatsuchsmalldevicesizes.Body

tissueshaveamuchhigherrelativepermittivitythantheair.Thisinturnwillleadtoaslowerpropagationvelocityandthereforeasmallerwavelength.F1.1showsthe

differencesinpermittivityandwavelengthfordifferentbodilytissuesoperatingat403.5MHz(Richerd).

F1.1MaterialCharacteristicsat403.5MHz


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Thistableleadstoaninterestingconclusion,thehighrelativepermittivityofbodily

tissues will create shorter wavelengths and antennas can be designed for these

shorter wavelengths. In other words the antenna implanted within the body is

electrically larger. This will help designers create the antenna to be implanted

withinthebody,butthisalsomeansthatanantennadesignedinforimplantusewill

notoperatethesameinthefreespacerealm.Thepenetrationdepthofawaveisalsoeffectivethroughthebodytissues.Lower

frequencieswillpenetratefurtherthanthehigherones,aswouldbeexpected.For

the 403.5MHz the penetrationdepth for various tissues are summarized inF1.2(Yazdandoost).

F1.2PenetrationDepthofTissuesat403.5MHz

Tissue PenetrationDepthIncm

DrySkin 5.51

WetSkin 5.81

Fat 30.85Muscle 5.25

1.2UsesofImplantableCommunicationSystems

As statedabove implantable communication systems can beused for biomedical

applications.Inthispaperwewillassumethatthisistheprimaryfocus.Someofthe

otherapplicationsforimplantablecommunicationsystemswillbepresentedinthissection.Implantablecommunicationsystemscanbeusedfortrackingbothhuman

andanimals.Thisiscurrentlydoneonsomesmallpets.RFIDtaggingisbecominga

popularwaytocontroltheflowofpeoplesegressanddeegressinabuilding.The

sameproximitydevicesusedtoaccessdoorscanbeimplantedintohumantissue.Although this is not used currently it will be interesting to see if implantedcommunicationsystemswillbeutilizedinbuildingsecurityandautomation.

1.3RadioFrequenciesofInterestforImplantableMedicalSystems

There are two main bands of interest with implantablemedical communicationsystems.ThesebandsareknowastheIndustrial,ScientificandMedical(ISM)band,

and theMedical ImplantCommunicationService (MICS)band.This ISMbandhas

typicallyusedthe2.42.5GHzregionforoperatingmedicalequipment.Thebandis

limitedto1Wradiatedpower.Thisisaneasierbandtodesignantennasatdueto

thehigherfrequencyofoperation(ISMBand).

ThesecondbandutilizedforimplantedmedicalcommunicationsiscalledtheMICS

band. This band has typically used the frequency rage of 402 405 MHz. The

maximumbandwidthforcommunicationsis300kHz.TheMICSbandisalsolimits

theEffectiveRadiatedPower(EFP)to25Woutsideofthebody.Thislimitationwill

hinder other equipment from interferingwhile operating on the same frequency

band(Medical ImplantCommunicationService).Thisbandis sharedwithweather

balloonsand satellite telemetry.Methods such asultrasound communication and


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opticalcommunicationhavealsobeenpurposedfortheMICSband.Theselatertwo

methodswillrequirethereceivertobeveryclosetosurfaceoftheimplantlocation.

SincethispaperisbasedonantennatheorytheRFcommunicationwillbeobserved.

Thesetwobandswillbepracticalinthispaperandtwosubsequentantennaswillbe

designedaroundthem.

1.4ImplantCommunicationSystemSpecifications

Antennasthatwillpotentiallybeusedinsideofimplantedcommunicationsystems

willneedtoconsumeverysmallamountsofpower.Systemswilltypicallyonlybe

consumingpowerduringdatatransmissionandperiodicpollingforsignalsfroma

transmittingunit.Tofacilitatethisthesystemwillsleepwhenitisnottransferring

datatoremotestations.Awakeupsignalwillbesentfromtheremotetransceiver

when it needs to communicate with the implanted device. To further conserve

powerdataissentinshortbursts.Theseshortburstswillconsumelesspowerthanasteadydataflow.

Remote stations that will receive data from the implanted medical device arecapableofreceivingdatafromveryweaksignals.Typicalreceiverscanoperateat

99dBm(.1pW)(dBm).Therangeoftheremotestationsislimited;thisismostlikely

duetothefactthattheyarecapableofoperatingataverylowpower.Typicalranges

areapproximatelyaround2mfromtheimplantedantenna(Bradley).

2.AntennaDesignforISMBand

2.1DesignConsiderations

FortheISMbandarectangularpatchantennawascreated.Thistypeofantennais

attractive because the bandwidth is very narrow. For the ISM band a centerfrequency of 2.45 GHz was chosen. The design was created using the following

equationsshowninfigureF2.1(Balanis).Fromtheseequationsslightmodificationswerecreatedtomaximizeperformance.

F2.1CalculationsforRectangularPatchAntenna


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Initially titania was used to construct the substrate of the patch antenna. Thisseemed like a relatively good idea because the relative permittivity of titania is

around50. This isveryclosetothe relativepermittivitymuscle and blood. Some

researchshows thatmaterialswith a relativepermittivity ofaround 10are veryusefulwhenusedasthesubstrateofapatchantenna.Forthisreasonaluminawas

usedtocreatethesubstrate.Thefeedforthepatchantennawasalsooffsetfromthecenterofthepatchantenna.

Theactualconstructionoftheantennaincludesagroundplaneatthebottomlayerandaconductingplane(microstrip)atthetoplayer.Thetoplayeroftheantenna

hasdimensionsof19.2mmby32mm.Thegroundplaneandtheconductingplane

areseparatedbyanaluminasubstratewithathicknessof2mm.Thevolumeofthe

antennais1.3cm3.Thisisacompactdesignthatwillbeeasilyimplantedwithinthe

tissue of the patient. The simple geometry of this design allows for easy

manufacturing of the equipment. The feed placement of the antenna can be fed

using a 25 coaxial cable connected through a via in the ground plane to the

microstrip.

Thisdesignwentthroughmanydifferentrevisions throughout thesimulations to

increase the effectiveness of the antenna. Most of these revisions were done assmallmodificationstothedesignbasedonworkperformedbyotherengineers.The

antennacreatedisverydifferentfromthemathematicalmodel.F2.2showsthefinal

layoutofthepatchantennausedfortheISMband.Adjustmentstotheterminating

resistanceweremadeusingtheImpedancemeasurementtoolinXFDTD.F2.3showstheequivalentcircuitmodelandequations.

F2.2LayoutofthePatchAntennausedfortheISMBand


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F2.3EquivalentCircuitModelandEquations

where

2.2AnalysisofISMBandAntenna

ThepatchantennausedtooperateontheISMbandwasanalyzedusingtheXFDTD

software. The antennawas placed insideofaphantomtosimulatehumantissue.

Thephantomwascomposedofamaterialmixturethatsimulatesskinandmuscle

components.Theelectricalpropertiesofthephantomareanelectricpermittivityof45.2 and a conductivity of .61S/m. This material makeup was taken from the

researchperformedbyC.M.Leeandassociates(Lee,YoandLuo).Theantennawas

implanted4mmbelowthesurfaceofthephantom.Simulationswereperformedin

theXFDTDenvironmentbothinsideandoutsideofthephantom.

Theinitialsimulationswereperformedoutsideofthephantom.Theantennasystem

wasdesignedtoworkinsideofhumantissuesothefreespaceenvironmentisnot

the ideal test bed for the system. F2.4 shows the electric field pattern for the

antennasysteminfreespace.

Theantennahasaboresightof90andradiates27.8W.F2.5showsthegainoftheantennasysteminfreespace.Theantennahasamaximumgainof11.5dBiat90.

This is relatively good for considering that the antennawas designed tooperatewithinthebody.Thebeamwidthorthe3dBmeasurementoftheantennais160.


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F2.4ElectricFieldPatterninFreeSpace

F2.5AntennaGainvs.Angle

Aftertheseinitialfreespacesimulationswereperformedtheantennasystemwas

placedinaphantom4mmbelowthesurface.Thefirstparameterthatwasobserved

was the input impedance. F2.6 shows the magnitude of the impedance and the

compleximpedanceplots.Thecompleximpedanceisveryconstantoverthebandof

interest(2.4to2.5GHz).Fromtheinformationthevoltagesourcewasterminated

witha25 resistance.Thereforewehaveamatchedsystem.Thiswillreduce the

amountofpowerthatisreflectedbackintothesource.Tuningtheimpedanceofthe


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sourcefortheantennaisaveryimportantprocessthatwillensurethattheantenna

willoutputthecorrectelectricfieldradiation.

F2.6ImpedancePlots

The Electric Field Strength pattern can be seen in F2.7. The bore sight of theantennaisat180andtheantennaisradiating27.7Wofpower.Thisisclosetothe

amount ofpower that should beradiated for the implant device.Within the ISM

bandthepowerlimitationoutsideofthebodyis1W.Theradiatedpowerinthiscase

is very close to the limits on the MICS band. The radiated power is more than

sufficientforwirelesscommunicationwithareceiverthatiswithin2metersofthe

subject.(Remotestationsarecapableofdatatransmissionoperationsat.1pW).


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F2.7ElectricFieldStrengthInsideofPhantom

The gain of the antenna is the next portion of the simulation that will be

investigated. With an antenna so small, made electrically larger via the high

permittivityoftheimplantedtissue,devicessuchastheinputfeedcanhamperthe

measurementofgain(Sivard,BradleyandChadwick).WiththisinmindF2.8shows

thegainfortheantennaimplantedwithinthephantom.Themaximumgainofthe

antenna,whichisseenat180is20dBi.Whengoingforthefreespaceexampleto

the implanted example the gain was lowered 8.5dBi. The beam width for the

antennainsideofthephantomis72.5.

F2.8GainvsAngleInsideofPhantom


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Aplanarsensorwasplacedontheantennafield.Thisallowedforthevisualization

of the electric andmagnetic fields. Reflections that occurred at the source, feed

point,wereminimalbecausetheimpedanceswerematched.

3.AntennaDesignforMICSBand

3.1DesignConsiderations

For theMICS band a rectangular patch antenna would be too large, therefore aPlanarInverterFAntenna(PIFA)wasdesigned.Thistypeofantennaisavariantof

thepatchantenna.FortheMICSbandacenterfrequencyof403.5MHzwaschosen.The design was created using the schematic created by Lee,Yo and Lau. This is

shown in figure F3.1(Lee, Yo and Luo). Layouts for each of the three layers are

showninF3.2.ThexsymbolizesthefeedandtheOsymbolizestheshortingpin.Thefirstlayeractsasagroundplanefortheantennaandthetwootherlayersarethe

conductinglayers.ThisisaverycompactdesignastheradiusofthePIFAantennaisonly7mm.ThisfactalonemakesthePIFAantennaattractiveforimplantuse.The

material Aluminawas again chosen for the substrate and the superstrate of the

PIFA.

F3.1PIFASchematic

F3.2XFDTDLayoutofPIFA


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The ground plane and the two conducting plane are separated by an alumina

substratewithathicknessof0.63mm.ThetotalthicknessofthePIFAis1.9mm.The

antennaisfedusinga20coaxialcableconnectedthroughaviainthegroundplane

to the conducting layers. This feed is in parallel to a 20 shorting pin. The

combinationofthesetwocomponentsmeansthereisa10sourcetermination.Adjustments to the terminating resistance were made using the Impedance

measurementtoolinXFDTD.

3.2AnalysisofMICSBandAntenna

ThePIFAusedtooperateontheMICSbandwasanalyzedusingtheXFDTDsoftware.

Theantennawasplacedinsideofaphantomtosimulatehumantissue.Thephantom

was composedofa materialmixture that simulates skinandmusclecomponents.

Theelectricalpropertiesofthephantomareanelectricpermittivityof45.2anda

conductivity of .61S/m. This material makeup was taken from the researchperformedbyC.M.Leeandassociates(Lee,YoandLuo).Theantennawasimplanted

4mmbelowthesurfaceofthephantom.SimulationswereperformedintheXFDTDenvironmentinsideandoutsideofthephantom.

Theinitialsimulationswereperformedoutsideofthephantom.Theantennasystemwasdesignedtoworkinsideofhumantissuesothefreespaceenvironmentisnot

the ideal test bed for the system. F3.3 shows the electric field pattern for the

antenna system in free space. The antenna has a bore sight of 30 and radiates

22nW.F3.4showsthegainoftheantennasysteminfreespace.Theantennahasa

maximumgainof52dBiat30.Thebeamwidthoftheantennainfreespaceis120.

F3.3ElectricFieldPatterninFreeSpace


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F2.4AntennaGainvs.Angle

After the initial free space simulationswere performed the antenna system was

placedinaphantom4mmbelowthesurface.Thefirstparameterthatwasobserved

was the input impedance. F3.5 shows the magnitude of the impedance and the

compleximpedanceplots.Thecompleximpedanceisveryconstantoverthebandof

interest(402to405MHz).Fromtheinformationthevoltagesourcewasterminated

witha10resistance.Theimpedancesarematchedcreatingamatchednetwork.

Thisreducestheamountofpowerthatisreflectedbackintothesource.Tuningthe

impedanceofthesourcefortheantennaisaveryimportantprocessthatwillensure

thattheantennawilloutputthecorrectelectricfieldradiation.Thisprocessisthesameasthetuningoftherectangularpatchantenna.

F3.5ImpedancePlots


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The Electric Field Strength pattern can be seen in F3.6. The bore sight of the

antennaisat30andtheantennaisradiating21nWofpower.TheMICSbandlimits

thepowerradiatedoutsideofthebodyto25W.Theradiatedpowerismorethan

sufficientforwirelesscommunicationwithareceiverthatiswithin2metersofthesubject.(Remotestationsarecapableofdatatransmissionoperationsat.1pW).

F3.6ElectricFieldStrengthInsideofPhantom

The gain of the antenna is the next portion of the simulation that will be

investigated. With an antenna so small, made electrically larger via the high


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permittivityoftheimplantedtissue,devicessuchastheinputfeedcanhamperthe

measurementofgain(Sivard,BradleyandChadwick).WiththisinmindF3.7shows

the gain for the antenna implanted within the phantom.Themaximum gain of

52dBi occurs around 30. The gain is the same for both the free space and the

implantedexample.Thisplotalsoshowsthatthebeamwidthoftheantennais125.

F3.7GainvsAngleInsideofPhantom

Aplanarsensorwasplacedontheantennafield.Thisallowedforthevisualization

of the electric and magnetic fields. Reflections that occurred at the source were

minimalbecausetheimpedanceswerematched.

ConclusionTheresultsfromeachoneoftheantennadesignsshowthattheycanbeutilizedfor

implantedcommunicationsystem.Theresults areverypromising andrewarding.

Initialdesignsfortheantennasweredifficultandagreatdealofreconfiguringwas

neededtocreateanantennathatwouldbesuitableforimplantation.Ifmoretimewas present in the course work would be done to increase the efficiency of theantennas, both antennas were less than 1% efficient according to the XFDTD

software.Thisisthemajorproblemthathasarisenwiththeantennadesignsthat

were chosen. This is also an issue that is plaguing other companies that design

antennas for implanted communication systems. There will always be tradeoffs

betweensize,efficiencyandeffectiverange.


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The interesting portion of this paper is the investigation of the way that the

antennasactwithinaphantom.TheISMbandrectangularpatchantennaiscapable

ofradiating27.3Wandhasabeamwidthof72.5.TheMICSbandPIFAantenna

radiates21nWwithabeamwidthof125 .Thoughthetwodesignsareoperatingat

different frequencies there is a tradeoff between the beam width and radiated

power.Thegainof theantennainbothcasesisverylow.This isdueto themanylossesthatoccurwithinhumantissue.

ImplantedcommunicationsystemsutilizeeithertheMICSorISMbands.Designingantennasforimplantapplicationscanbedifficulttotheharshenvironmentofthe

humanbody.Therearemanytradeoffsthatneedtobeconsideredwhendesigningfor implanted communication systems. These tradeoffs include radiated power,

powerconsumption,efficiencyandsize.Ofthesesizeisthelimitingfactor;larger

antennas are not easily implanted and will encounter different types of tissue.RemconsXFDTDsoftwareisauseful tool for the design and analysisofantenna

systems.Throughtheprojectandwrittenreportaspectofthiscoursealotofnew

informationinantennatheoryaswellasmodelingandsimulatingdevicesinXFDTD.


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WorksCitedBalanis,Constantine.AntennaTheoryAnalysisandDesign.Hoboken:JohnWileyand

Sons,INC,2005.Bradley,Peter.ImplantableultralowpowerRadioChipFacilitatesInBody

Communications.7June2007.23June2009.dBm.22July2009.23July2009.

ISMBand.1July2009.23July2009.

Lee,C.M.,etal."CompactBroadbandStackedImplantableAntennaforBiotelemetry

WithMedicalDevices."7June2007.IEEEXplore.23July2009

.

MedicalImplantCommunicationService.17February2009.15June2009

.

Richerd,JeanDaniel."MedicalDesign:UnderMySkin."EmbeddedSystemsEurope

(2009).Sivard,Ake,etal."TheChallengeofDesigningInBodyCommunications."November

2004.EETimesIndia.23July2009.

Yazdandoost,Kamya."IEEEP802.15WorkingGroupforWirelessPersonalArea

NetworksMedicalImplantCommunicationSystem."July2006.IEEE.21July2009

.

jason lewis

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