Characterization of In-Body to On-Body Wireless Radio Frequency Link for Upper Limb Prostheses

19 Pages·2016·2.28 MB·English

RESEARCHARTICLE
Characterization of In-Body to On-Body
Wireless Radio Frequency Link for Upper
Limb Prostheses
AntoniettaStango1,KamyaYekehYazdandoost2,FrancescoNegro3,DarioFarina1*
1 InstituteofNeurorehabilitationSystems,BernsteinFocusNeurotechnologyGoettingen,BernsteinCenter
forComputationalNeuroscience,UniversityMedicalCenterGoettingen,Georg-AugustUniversity,37075,
Goettingen,Germany,2 CentreforWirelessCommunications,DepartmentofCommunications
Engineering,UniversityofOulu,P.O.Box4500,90014,Oulu,Finland,3 DepartmentofClinicaland
ExperimentalSciences,UniversityofBrescia,25123,Brescia,Italy
*[email protected]
a11111
Abstract
Wirelessimplanteddevicescanbeusedtointerfacepatientswithdisabilitieswiththeaim
ofrestoringimpairedmotorfunctions.Implanteddevicesthatrecordandtransmitelectro-
OPENACCESS myographic(EMG)signalshavebeenappliedforthecontrolofactiveprostheses.Thissim-
Citation:StangoA,YazdandoostKY,NegroF, ulationstudyinvestigatesthepropagationlossesandtheabsorptionrateofawirelessradio
FarinaD(2016)CharacterizationofIn-BodytoOn- frequencylinkforin-to-onbodycommunicationinthemedicalimplantcommunicationser-
BodyWirelessRadioFrequencyLinkforUpper
vice(MICS)frequencybandtocontrolmyoelectricupperlimbprostheses.Theimplanted
LimbProstheses.PLoSONE11(10):e0164987.
antennaisselectedandasuitableexternalantennaisdesigned.Thecharacterizationof
doi:10.1371/journal.pone.0164987
bothantennasisdonebynumericalsimulations.Aheterogeneous3Dbodymodelanda
Editor:DingguoZhang,ShanghaiJiaoTong
3Delectromagneticsolverhavebeenusedtomodelthepathlossandtocharacterizethe
University,CHINA
specificabsorptionrate(SAR).ThepathlossparameterswereextractedandtheSARwas
Received:February1,2016
characterized,verifyingthecompliancewiththeguidelinelimits.Thepathlossmodelhas
Accepted:October4,2016
beenalsousedforapreliminarylinkbudgetanalysistodeterminethefeasibilityofsuch
Published:October20,2016 systemcompliantwiththeIEEE802.15.6standard.Theresultinglinkmarginof11dBcon-
Copyright:©2016Stangoetal.Thisisanopen firmsthefeasibilityofthesystemproposed.
accessarticledistributedunderthetermsofthe
CreativeCommonsAttributionLicense,which
permitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginal
authorandsourcearecredited.
DataAvailabilityStatement:Allrelevant Introduction
parametersandsimulationresultswillbeavailable
athttps://dx.doi.org/10.6084/m9.figshare. Inthelastdecade,therehasbeenanincreasingneedofacquiringbiometricsignalsformoni-
3408223.v1. toringvitalsignsandsupportingchronicallyillpatients.Inparticular,implanteddevicescan
beusedtomonitoranddiagnosecardiacpathologies,cancer,asthmaandneurologicaldisor-
Funding:ThisworkwassupportedbytheGerman
MinistryforEducationandResearch(BMBF)via ders[1].Morerecentapplicationsofwirelessimplantedsystemsfocusalsoonhelpingpeople
theBernsteinFocusNeurotechnology(BFNT) withphysicaldisabilities[2],e.g.cochlearandretinaimplants,oractivelimbprostheses[3].
Go¨ttingenundergrantnumber1GQ0810. Thisstudyinvestigatesthepossibilitytouseimplanteddevicesabletorecordandtransmit
CompetingInterests:Theauthorshavedeclared electromyogram(EMG)signalsinordertoallowamputeestocontrolactivemyoelectric
thatnocompetinginterestsexist. prostheses.
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TheactiveprosthesescurrentlyavailableinthemarkethavemultipleDegreesofFreedom
(DoFs)andareusuallydrivenbysurfaceEMGsignals.SurfaceEMGcanbeeasilydetected
non-invasivelywitharelativelylargepickuparea[4].However,duetothedonning/doffingof
theprosthesis,thesurfaceEMGelectrodesinthesocketmaychangepositionwithrespectto
theunderlyingmusclescompromisingthereliabilityofthesystem.Moreover,surfaceEMG
canonlyberecordedfromsuperficialmusclesandtheycanbepronetochangesinskinimped-
anceandbreakageofthewires[5].Intramuscularelectrodesmaylimittheseproblemswhen
coupledwithwirelesstransmission.Therefore,recently,theuseofimplanteddeviceswhich
canwirelesslytransmittheEMGsignalstoacontrollerembeddedinthesocketoftheprosthe-
sishasbeenproposed[5,6].ImplantedEMGsensorscanprovideinformationfromdeepmus-
clesthatarenoteasilyaccessiblewithstandardsurfaceEMGelectrodes,andthereforeitcan
helptoimprovethecontroloftheprosthesis.
In[6]theauthorsproposeasystemwithtwoimplantedelectrodes,whichuseatransmission
protocoltailoredforrecordingphysiologicalsignals.TheRFlinkisbasedonIEEE802.15.4[7]
inthe2.4GHzfrequencyband,whilethepowertransferisdonebyaninductivelink.Inthe
mostdetailedstudyinthisfield[5],theauthorspresentamultichannelsystemthatcanreceive
andprocesssignalsfromEMGimplantedsensors.Thesystemalsocomprisesalargeexternal
powercoil,placedaroundthelimb,areceivingantenna,andatelemetrydevicewhichpasses
thedatatotheprosthesiscontroller.Thissystemhasbeenrecentlyimplantedinapatient[8]
andimproved[9],butithassomelimitations:thelargeinductivepowerfieldlowersitsoverall
efficiencyandthereisnoflexibilityinthelocationoftheimplantssincetheymustbewithin
theinductiveareaandparalleltotheexternalcoil.Furthermorethepresenceofacircularcoil
limitstheuseofsuchsystemtoamputeeswitharoundstump[10].
Theradiocommunicationbetweenimplantedbiosensorandexternalbiomedicalsystems
hasspecificissuesduetothefactthatthehumanbodyisaheterogeneouspropagationenviron-
ment.Thehumanbodyischaracterizedbymultiplelayersoftissueswithdifferentthicknesses
anddielectricproperties.Furthermore,everyapplicationdemandsdeviceswithappropriate
shapesandrequirements,dependingontheplacewheretheyhavetobeimplantedandonthe
tissuesinvolvedinthetransmission[11].Theantennashapeanddimensionsdependonthe
applicationandonthebandofoperation[11].ThebandassignedbytheFederalCommunica-
tionCommission(FCC)andacceptedinmostCountriesforimplantedcommunicationis
402–405MHz[12,13].Thisbandiscalledmedicalimplantcommunicationservice(MICS).It
allowslowpowertransmission(outputpowermaximum25μWEIRP)satisfyingantennaper-
formancecompatiblewiththehumanbody,assuringnointerferencewithotherradiooperat-
inginthesamefrequencyband.Forthesereasons,industriesareveryinterestedtodevelop
implanteddevicesoperatinginthisband[14].CompaniessuchasMedtronic,Biotronicand
St.Jude,havealreadymadecommerciallyavailabledevicesworkinginthisband[14].The
MICSbandhasalsobeenadoptedbythestandardIEEE802.15.6[15]forimplantedapplica-
tions.ThestandardIEEE802.15.6supportsmedicalcommunicationbetweennon-invasive
devicesplacedonthebodyandimplanteddevicesplacedinthebody,aswellasexternaldevices
thatarearoundthebody.However,thisstandarddoesnotprovideinformationontheabsolute
performanceofthechannelmodels.Aspecifictaskgrouprecommendeddifferentchannel
models,basedonsevenrepresentativescenarios[16],threeofthemrelatedtoimplantedappli-
cations.However,in[16]ithasbeenstatedthatthechannelmodelsdescribedinthedocument
arenotintendedtoprovideinformationofabsoluteperformanceandthateachapplicationcan
havespecificrequirements.
ThereareonlyfewstudiesonimplantedapplicationswithintheMICSband,sinceoftenthe
ISM(IndustrialScientificandMedical)bandwasusedinstead.Infact,thedesignofanantenna
whichoperatesintheMICSbandischallenging,mainlyduetolimitationsinthedimensions.
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IntheMICSbandthewavelengthinsidethehumanbodyis~9cm,whileintheairis~74cm.
Nevertheless,theISMbandisusedforalmostallthedevicesthatweuseineverydaylife,such
asradio-frequencyidentifications(RFID),BluetoothandZigBee,thereforetheriskofinterfer-
enceisveryhigh.OnthecontrarytheMICSbandhasbeenreservedforhumanimplantable
devices,sharingthefrequencyonlywithweatherballoons(400–406MHz).In[17,18]artificial
cardiacpacemakerswithimplantedantennasoperatingintheMICSbandareconsidered.In
bothstudiestheperformanceoftheantennashasbeenanalyzedbysimulationswithsimplified
phantoms.In[18]anexperimentonporcinetissueispresentedwhereitwasobservedthatthe
performancewasdifferentwithrespecttothesimulatedmodelbecauseofthethickfatlayer
andtheabsenceofbonesinthepigtissues.Additionalstudieshavebeenconductedoningested
implants,suchasacapsuleendoscope[19,20],anddemonstratedthattheMICSbandisideal
forwirelessimplant[17].In[20]thepathlossmodelandtheSARlevelhavebeeninvestigated
foraspiralantennainMICSfrequencieswithnumericalsimulationsina3Dhumanmodel.It
hasbeendemonstratedthatforanendoscopycapsulethepathlossexponentcanvarydepend-
ingonthesubject(adultorchild)andonthedeepnessinthebody,i.e.,onthelayersoftissues
betweenthetransmittingandthereceivingantenna.Therefore,itisnotpossibletoprovidea
singlepathlossmodelforwirelesscommunicationforin-bodytoon-bodyandeachapplica-
tionneedstobespecificallyinvestigated.InthecaseofwirelessimplantsforrecordingEMG
signalstodrivehandprostheses,therearenostudiesonpathloss,exceptourpreliminary
report[21].
Inthepresentwork,weproposeasystembasedontheIEEE802.15.6standard[15]com-
posedbytwoimplanteddevicesthatrecordandtransmitwirelessEMGsignalstoanon-body
devicepositionedinsidethesocketofahandprosthesis.Thesystemcanbeusedforeachtype
ofamputee,includingsubjectwithtargetmusclereinnervation(TMR),sincetherearenocon-
straintsfortheplacementoftheimplants.Moreoveritusesafrequencybanddedicatedto
implanteddevices.
ThepathlossmodelhasbeeninvestigatedbyFDTD(FiniteDifferenceTimeDomain)sim-
ulations,witha3Dhumanbodymodelforthesystemproposed.Furthermore,theSARvalues
areinvestigatedtodemonstratethatthesystemisnotexceedingthelimitationsimposedby
regulations,andconsequentlyisnotharmfultohumans.Finally,apreliminaryevaluationof
thelinkbudgethasbeendonetakingintoaccounttheresultsobtained.
MaterialsandMethods
Theproposedsystem(Fig1)consistsoftwoEMGsensorsimplantedintheforearmofan
amputeeandanexternaldevicethatcontrolstheprosthesis.Theimplantedsensorsrecordand
processtheEMGsignals,whicharewirelesslytransmittedtoanexternalantenna,placedon
thesocketprosthesis.Theprocessedsignalsarethenusedtodrivethehandprosthesis.The
communicationlinkbetweenthedevicesfollowsthestandardforWirelessBodyAreaNetwork
(WBAN)[15].
HumanModelandElectromagneticSimulationTool
Thedesignofwirelessimplantcommunicationsystemsrequiresknowledgeonthemultiple
layersoftissuesofthehumanbodyandtherespectivedielectricproperties.Propagationlosses
andSARmeasurementsofimplanteddevicescannotbeinvestigatedwithin-vivoexperiments.
Forthisreason,numericalsimulationsandflatphantomsfilledwithmuscletissuesimulating
fluidarecommonlyused.Phantomsarenon-standardizedyetandtheyrepresentasimplified
versionofthehumanbody[3]andnotalltheinterfacesbetweenthetissuelayerscanbe
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Fig1.Systemoverview.
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represented.Moreovermimickingthelimbsitismorechallengingthanotherpartsofthe
humanbody[14].Forthesereasonsinthisstudyanumericalsimulationhasbeenpreferred.
TheEMnumericalevaluationshavebeencarriedoutbyFDTDsimulationswiththe3D
solverSEMCAD-X[22]andwitha3Dhumanbodymodel.Thehumanbodymodelhasbeen
obtainedfrommagneticresonanceimages(MRI)ofhealthyvolunteersandisprovidedby
SPEAG[23].Themodelrepresentsamanof34yearsold(denotedDuke)anditispartofthe
VirtualFamily,whichhasfouranatomicalmodels(twoadultsandtwochildren).Thesemodels
include80bodytissueswithdielectricpropertiesbasedonthedatabasegeneratedbyGabriel
[24].Themaximumgridstepofthehumanmodelis2mm.
ImplantedAntennas
TheimplantedantennaselectedinthisworkisthesameusedbytheIEEE802.15.TG6commit-
teetodefinethechannelmodelsforthestandardIEEE802.15.6andisextensivelydescribedin
[16,25],and[26].Thedimensionsandshapeofthisantennafitwellinadeviceimplanted
insideaphantomlimbofatransradialamputee.Theantennaiscomposedofasinglemetallic
layerofcopper.ThemetalliclayerisprintedonasideofaD51(NTK)substratewithdielectric
constantε =30,losstangenttanθ=3.8e-05,thicknessof1mm,andcoveredbyRH-5sub-
r
stratewithdielectricconstantε =1.0006,losstangenttanθ=0andthicknessof1mm(Fig
r
2a).TheantennahasbeendesignedtooperateinthehumanbodywithintheMICSfrequency
band(402–405MHz)[25].Twoantennashavebeenimplantedintheleftarmofthehuman
model,approximatelyinthewristextensorandflexormuscles.Fig2bshowstheimplanted
antennasinsidethearmofDuke.AsectionofthearmwiththeimplantisdepictedinFig2c.
To verifythattheoperatingfrequencyinthehumanbodyisintherangeofMICSband,the
reflectioncoefficient(S )oftheantennahasbeensimulated.Thereturnlosshasbeencalcu-
11
latedafterpositioningtheantennainsidethehumanbodyfixingthecentralfrequencyto403.5
MHz,withtheFDTDsimulationplatformSEMCADX[22].TheS valuewasabout-10dBat
11
403.5MHzasshowninFig3,whichconfirmsagoodimpedancematch.Fig4ashowsa3D
polarplotofthegainoftheimplantedantenna.Thegainisnotisotropicbutvarieswiththe
direction.Themaximumgain,takingintoaccountalsothelossesofthebodyphantom,is
-55.37dBi.Fig4bshowsthesimulatedradiationpatternnormalizedto1V/m.Theradiation
patternhasbeencomputedtakingintoaccounttheimpactofthehumanbody,asrecom-
mendedby[27].Theoriginofthecoordinatesystemisplacedontheborderoftheimplanted
Fig2.MICSimplantedantennaandantennapositioning.a)frontviewoftheimplantedantenna;b)humanmodelwiththeimplantedantennasinthe
redsquaresandtheexternaldipole;c)crosssectionofthearmwiththeimplantedantennas.
doi:10.1371/journal.pone.0164987.g002
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Fig3.ReturnLossoftheimplantedantenna.S isabout-10dBat403.5MHz.
11
doi:10.1371/journal.pone.0164987.g003
antenna.Themaximumdirectivityisobservedinthedirectionoppositetothexaxis(intheXZ
plane),butitispossibletoobserve(Fig4c)thattheE andE componentsaresimilar.
θ ϕ
On-bodyAntenna
Asafirststep,tosimplifytheanalysisandreducethesimulationtime,ahalf-wavedipole
antennahasbeenselectedasareceivingantenna.Amorerealisticantennahasalsobeeninves-
tigatedasarefinementstep(seebelow).Thehalf-wavedipolehasbeenmodeledandtunedto
operateat403.5MHz.Consideringthatthewavelengthinfreespaceat403.5MHzis~74cm,
thelengthofeacharmofthehalf-wavedipolehasbeensetto180mm,withthicknessof2mm
andagapbetweenthearmsof1mm.Fig2bshowsthelocationoftheexternalantenna,close
tothehumanbody.Thereturnlossofthedipolehasbeensimulatedinfreespace(withoutthe
presenceofthehumanmodel)andat1cmfromthearmsurfaceofthehumanmodel(Fig2b),
inthenearfieldregion[28].Fig5reporttheS11inbothconditions.Thegraphsaresimilarand
showagoodimpedancematchintheMICSbandfrequency(S11at403.5MHzis~-10dBin
freespaceand~-12dBnearthehumanmodel).
Thegainandtheradiationpatternhavebeencomputedfortheexternalantennainthe
samewayandwiththesamecoordinatesystemastheimplantedantenna.The3Dpolarplotof
thegainisshowninFig6aandthemaximumvalueis-3.20dBi.Theradiationpatternnormal-
izedto1V/misdescribedintheFig6band6c.Ithasbeencomputedtakingintoaccountthe
presenceofthehumanbodymodel.Indeed,Fig6cshowsthattheE andE componentshave
φ θ
smallervaluesinthedirectionsofthehumanbody.
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Fig4.3DGainpolarandradiationpatternplotsoftheimplantedantenna.a)3Dgainpolarplot,maximumgainequalto-55.37dBi;b)3D
normalizedradiationpatternpolarplot;c)valuesofE (dB)andE (dB)inthreedifferentplanes.Thecoordinatesystemisshowninfigure.Maximum
φ θ
value-41.04dB.
doi:10.1371/journal.pone.0164987.g004
Ahalf-wavedipoleasdescribedbeforeisveryusefultoanalyzetheelectromagneticcharac-
teristicsofthecommunicationsystembetweentheimplantsandtheexternalcontrollerofthe
prosthetichand.However,consideringitslength,suchantennaisnotagoodcandidatetobe
embeddedinthesocketofahandprosthesis.Forthispurpose,ahelicaldipoleantenna[29]
(Fig7)withconstantradiusandfeedatthemidpointhasbeendesigned.Thehelicalantenna
hasbeenoptimizedwiththeFDTDsimulatorSEMCAD[22]tooperateintheMICSfre-
quencybandwhenpositionednearthehumanbody.Theconductorwirehasaradiusof
1.433mm,thedistancebetweentheturnhasbeenfixedto180mm,andthetotalheightof
theantennais152.4mm(Fig7a).Thediameterofthehelicalis101mm,andthearmofthe
humanmodelhasapproximatelyadiameterof80mm(wheretheimplantsispositioned).
TheantennacanbepositionedaroundthearmasdepictedinFig7b.Thereturnloss(Fig8)
hasbeensimulatedinfreespaceandnearthe3Dhumanbody(asinFig7b).FromFig8,itis
possibletonoticethatthereisade-tuningoftheantennawhenpositionedinfreespace.The
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Fig5.ReturnLossofthedipoleantenna.S is~-12dBat403.5MHznearthehumanbody.
11
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S valueat403.5MHzis-11dBwhentheantennaisaroundthearmofthehumanbody
11
(orangecurve),whileS is~-3dBinfreespaceat403.5MHz(bluecurve).Thisshapeallows
11
theantennatobeembeddedintothesocketofahandprosthesis,withgainandradiationpat-
tern(Fig9)similartothehalf-wavedipole.
Channelmodelingforbodyimplanteddevices
ThecharacteristicsofaRFsignalchangeduringtransmissionbetweentransmittingandreceiv-
ingantennas.Thechangesdependontheenvironmentandthedistancebetweenthetransmit-
terandthereceiver.Inthecasestudyofthiswork,themediumisthehumanbody,whichisa
lossyheterogeneousmediumwithhighpermittivity.In-bodyEMGsensorscontrollingapros-
thetichandshouldrelyonagoodcommunicationlink,whichguaranteeslowlatencyandhigh
reliability.Thefirststepforagoodcommunicationlinkisthedefinitionofthechannelmodel-
ing.Achannelmodelallowstoobtaintheprofileofthetransmittedsignalfromthereceived
signal.
Channelmodelsareoftenobtainedbyphysicalmeasurementsinexperimentalenviron-
ments,butforpracticalreasonsandethicalissuesinthecaseofimplanteddevicesthiscannot
bedone.Thesimulationsareinthiscasethemeansforbuildingchannelmodelssincethis
allowstotakeintoaccountthelossesduetohumanbody.Incaseofbodyareanetworks,the
propagationpathcanbesubjecttofadingduetoshadowingbybodyposture,reflection,dif-
fractionorenergyabsorption.Thepathlossrepresentstheattenuationofthetransmittedsig-
nalandinthecaseofbodyareanetworksitdependsonthedistanceandfrequency[16].For
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Fig6.3DGainpolarandradiationpatternplotsofthedipoleantenna.a)3Dgainpolarplot,maximumgainequalto-3.20dBi;b)3Dnormalized
radiationpatternplot;c)ValuesofE (dB)andE (dB)inthreedifferentplanes.Thecoordinatesystemisshowninfigure.Maximumvalue11.35dB.
φ θ
doi:10.1371/journal.pone.0164987.g006
implantedwirelesscommunications,thepathlossintermsofdistancedisgivenbythefollow-
ingrelation:
G P
PLðdÞ¼ R T ð1Þ
P ðdÞ
R
WhereP isthetransmittedpower,P isthereceivedpower,andG isthegainofthereceiving
T R R
antenna.Thetransmittingantennaisconsideredpartofthechannel[3,26,31].To statistically
modelthepathlossindBasafunctionofthedistance,thefollowingformula,basedonthe
Friisformula[26,30,31],canbeused:
d
PL ðdÞ¼PL þ10(cid:3)n(cid:3)log ð2Þ
dB 0 10d
0
WherePL isthepathlossindBatareferencedistanced expressedinmm,d[mm]isthe
0 0
antennaseparation,andnisthepathlossexponent,whichdependsontheenvironmentwhere
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Fig7.Helicalantenna.a)helicalantennadimensions.b)positionofthehelicalantennanearthehumanbody.
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theRFsignalpropagates.Thepathlossexponentisequalto2infreespace.Eq2doesnottake
intoaccounttheshadowingcomponentduetodifferentbodytissuesandtheantennagainin
differentdirections.Shadowingisdefinedasthevariationofthelocalmeanaroundthepath
loss.Inparticulartheshadowingcomponenttakesintoaccountthefactthattherearedifferent
valuesofthepathlossforsamedistancebetweentransmitterandreceiver.To takeintoaccount
theselosses,thepathlosscanbeexpressedas[30],[26]:
d
PL ðdÞ¼PL þ10(cid:3)n(cid:3)log þS ð3Þ
dB 0 10d
0
whereSisarandomvariablewithnormaldistributionandstandarddeviationσ.Indeedhas
s
beensohwnthatthevariationofthepathlossaroundtheaveragefollowalog-normaldistribu-
tioninmanymeasurements[19].S~N(0,σ 2)representstheshadowingcomponentwhich
S
takeintoaccountthepresenceofthehumantissuesandtheantennagainindiffererent
directions.
SpecificAbsorptionRate(SAR)
ThespecificabsorptionrateistheratethatquantifiestheRFenergyabsorbedinbiologicaltis-
sue.ItisexpressedinWatt perkilogram(W/kg)anditisthemeasureoftheamountofheat
generatedintheantennasurrounding.Sincethisparameterisofextremeimportanceforthe
healthoftheimplantscarrier,therearelimitsandregulationsthatneedtobefulfilled.In
Europeandseveralcountriesintheworld,suchasJapanandKorea,theguidelinesareprovided
byICNIRP(InternationalCommissiononNon-IonizingRadiationProtection)[32]which
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