Coils for ingestible capsules: Near-field magnetic induction link



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2IILFLDO85/ 

an author's

https://oatao.univ-toulouse.fr/24012

https://doi.org/10.1016/j.crhy.2015.07.009

Grzeskowiak, Marjorie and El Hatmi, Fatiha and Diet, Antoine and Benamara, Megdouda and Delcroix, David and

Alves, Thierry and Protat, Stéphane and Mostarshedi, Shermila and Picon, Odile and Le Bihan, Yann and Lissorgues,

Gaëlle Coils for ingestible capsules: Near-field magnetic induction link. (2015) Comptes Rendus Physique, 16 (9).

819-835. ISSN 1631-0705

Radio science for connecting humans to information systems / L’homme connecté

Coils

for

ingestible

capsules:

Near-field

magnetic

induction

link

Boucles

pour

gélules

ingérables :

bilan

de

liaison

en

champ

proche

Marjorie Grzeskowiak

a

,

∗

,

Fatiha El Hatmi

a

,

Antoine Diet

b

,

Megdouda Benamara

a

,

David Delcroix

a

,

Thierry Alves

a

,

Stéphane Protat

a

,

Shermila Mostarshedi

a

,

Odile Picon

a

,

Yann Le Bihan

b

,

1

,

Gaelle Lissorgues

a a_{ESYCOM(EA2552),UPEM,ESIEE-Paris,CNAM,77454Marne-la-Valléecedex,France}

b_{GEEPS(UMR8507),UniversitéParisSaclay,CNRS,Centrale-Supélec,UniversitéParis-Sud,UPMC,rueJuliot-Curie,plateaudeMoulon,}

91192 Gif-sur-Yvette,France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Keywords:

Mutualinductance

Magneticinductionlinkbudget Monolayerandmultilayerstackedcoil On-bodyspiralcoil

Wirelessingestiblecapsule

Mots-clés :

Inductancemutuelle

Bilandeliaisonparinductionmagnétique Bouclemonocoucheetmulticouche Bouclespiralesurlecorps Géluleingérablesansfil

Monolayer- andcompact-multilayer-stackedingestibleTXcoilsareinvestigatedfor ingesti-blecapsulesystems.Theinductivelinkthroughthehumanbodyismodeled.Theefficiency ofthenear-fieldmagneticinductionlinkbudgetisevaluatedintheair,inthehomogeneous humanbodyand inthethree-layerhumanbody.Thevariations oftheposition andthe orientationoftheTXcapsulecoilaretakenintoaccounttoevaluatethecouplingresponse betweentheTXingestibleandRXon-bodycoils.

r

é

s

u

m

é

Desbouclesmonocouchesetmulticouchescompactessontétudiéespourdessystèmesde gélulesingérables.Lelieninductifàtraverslecorpshumainestmodélisé.L’efficacitédu bilan de liaisonen champ proche par induction magnétiqueest évalué dans l’air, dans le corpshumain homogène etdans lecorpshumain trois couches. Lesvariations de la position etdel’orientationdelaboucledanslagéluled’émission sontprisesencompte pourévaluerlaréponsedecouplageentrelesbouclesd’émissiondanslagéluleingérable etderéceptionsurlecorpshumain.

1. Introduction

Wirelessingestible capsule devicesget moreandmoreused asa tool forcancerdiagnosisandmedicaltreatments of diseasesofthegastrointestinal(GI)tract[1].Thesesystemswereproposedtoovercomethelimitedtestcoverage,because

*

Correspondingauthor.

E-mailaddresses:Marjorie.Grzeskowiak-lucas@u-pem.fr,marjorie.grzeskowiak@univ-mlv.fr,grzeskow@univ-mlv.fr(M. Grzeskowiak).

Table 1

Specificationsofantennasforingestiblesystems.

Workingfrequency Sizeφxh (inλ0) Pathloss(capsuleinsidehuman

body,5 cmawayfromthe externalsurface)

Bandwidth(in%) Reference

460MHz–535MHz 4mm×10mm (0.0066λ0×0.0166λ0) Ø 75MHz(15%) [2] 32MHz 40MHz 1mm×1mm×0.5mm (0.00013λ0×0.000013λ0×0.00006λ0) Attenuationfrom1dBto5dB andnochannelfading

Ø LF1[4] LF2[4] 868MHz 869MHz 7mm×0.5mm (0.020λ0×0.0028λ0) 27mm×13mm×1.5mm (0.078λ0×0.037λ0×0.0043λ0) Attenuationfrom15dBto26dB andchannelfadingupto52dB

Ø HF1[4]

HF2[4]

1.4GHz Antennaconformedon10mm×26 mm

(0.046λ0×0.1214λ0)

Peak gain= −28.4dBi;

η=0.05% Ø [6] and[8] 404.5MHz 2.387GHz 10mm×32.1mm (0.013λ0×0.043λ0) 10mm×32.1mm (0.08λ0×0.2568λ0)

Peak gain= −28.8dBi;

η=0.058%

Peak gain= −18.5dBi;

η=0.9% 401–406MHz (2.3%) 2.4–2.5GHz(6%) [14] 2.4GHz 11mm×3.81mm (0.088λ0×0.030λ0)

Peak gain= −26.7dBi 26% [17]

offinitelengthofwiredsystems,andthusreducethediscomfortandpainofthepatients[2–4].Awirelesscapsuleusually includes sensors, IC chip with an antenna, battery and is used to transmit physiological information from the GI tract, such astemperature[9],pressure, pH, andoxygen concentration [5,6], to provide drugto the patient [7,8], andto send real-timeGItractimagesathighdatarateforvideoendoscopiccapsule[5–9].Capsulesystemsrequirehavingasmallsize to beswallowedbypatients.Thedesignofelectricallysmallcoilsthatlimit efficiencyisachallenging task:efficiencycan affectpower consumption,whichmustbe reducedsincethe small-sizebatterycontainslimitedenergy.The wirelesslink is considered in the presence of a “hostile” environment such as the human body.Inductive linksfor providing energy to ingestible capsule [11–14] are investigated in the laboratory. In the present study,the system seems to be suited to ambulatory, i.e., out-of-the-laboratory devices: batteries inserted in capsule could transfer energy(and potential data of sensor)toareceivedbeltorcoverandthesystemcouldpermitcapturingphysiologicaldatainrealtimeandasconveniently aspossible,witharelativecheapambulatorysystemofsensorsandwearables.

Thenear-fieldsystememploysMI(magneticinduction)betweentheTXandRXcoils.Becauseofthemutualinductance, the coilsactasamagneticallycoupledtransformer.The magneticfieldpenetrateshigh-permittivitymaterials,such asthe humanbody,betterthanelectromagneticwavesthatsufferfromhighpowerabsorption[15]andleadtolowantenna radia-tionefficiency[10–21].Table 1synthesizesthespecificationofexistingcapsuleantennasintermofresonantfrequency,size, pathlossandfrequencybandwidthandconfirms inmeasurement[4]thatmagneticallycouplednear-fieldcommunication systems presentan attractive optionforfuture applicationswiththeadvantages ofingestiblecapsulesminiaturization (at oneeighthofthevolumeofthe868-MHzscheme),nochannelfading,andallowsfourtimesthebatterylifeofthe868-MHz ingestiblesystems.

In this paper,a near-field MI ischaracterized between theTX capsule coil andthe RX spiralcoil outsidethe human body. Inthe first part,thenear-field MI linkbudget is expressedfrom thecoils’ parameters andan equivalentelectrical circuitismodeledusingADSsoftware[22].Then,themutualinductancebetweenTXandRXcoilsiscalculatedversusthe size ofthe RX coilssize, thelateral misalignmentandthe angularmisalignment ofthe TXcoil. Ina following section,a TX capsule one-layercoil isdesignedat40.68 MHz. The efficiencyofthe linkis evaluatedfrom simulatedresultsin the homogeneous humanbodyversustheorientation andpositionofthecapsulecoilandinathree-layeredmodelofhuman body. Inthe last part, toimprove theperformances ofthe inductive link, thesize of the RX spiralcoilis increasedand the TXcapsule becomesa five-layerstructure.The efficiencyofthelinkis simulatedandmeasured inthe airandin the tissue-simulatingliquidversusthefrequency,versus thedistancebetweencoaxialcoilsandversusthelateralandangular misalignment.

2. Near-fieldMIlink

TheTXandRXcoilsaresupposedtobeplacedrespectivelyInandOnBody.Theimpactofthemedia(humanbodyorair) isseeninthemodificationoftheRFwave’sattenuation,whichistherealpartofthecomplexpropagationconstant(1).The relative permittivity andtheconductivityofthetissue-simulatingmuscle arecalculated thanksto the4-Cole–Cole model

[23,29],andarerespectivelyequalto83and0.67 S/m.Intheair,theseparametersareconsideredtobeequalto1forthe relativepermittivityandto0fortheconductivity.

γ

=

α

+

j

β

=

j

ω

√

με

0

ε

r



1

−

j

σ

ωε

0

ε

r

(1)

The average attenuationis equal to 0.78 dB/cmin thehuman body.The signal transmission at5 cm fromthe airto thehuman bodycould be decreasedby 3.92 dB.In anon-conductive medium, suchasair,orthe humanbody withnull conductivity,theattenuationissimilarandequaltozero:at40.68 MHz,themediumattenuationdependsessentiallyonits conductivity.Bytakingintoaccountthedistanceof5 cmbetweentheTXandRXcoils,theRXcoilisplacedinnon-radiative near-fieldoftheTXcoilandsotheMInearfieldphenomenonisstudiedinthefollowingpart.Themagneticfieldstrength on the (z) axis,generated by a current-carrying coil in XY plane is quantified by the mathematical expression (2) and dependsontheintensityofthecurrent(I),theloopradius(R),thenumberoftheloopturns(N)andthedistance(z)from thecenterofthecoilinthenormaldirection[24,25]:

H

(

z

)

=

I

×

N

×

R 2

2

× (

R2

₊

_z2

₎

3/2 (2)

The classic very-near-fieldzone D islimitedto D

<<

λ/

2

π

forthe operatingcentral frequency f .At 40.68 MHz,the wavelengthisreduced from750 cmintheairto 53 cmin thehumanbody,while considering

ε

r

=

83 for thedielectric

characteristics ofthemuscle.Since theaverage distancebetweenthesmallintestine andtheon-body surface issuperior to5 cm,thelimit D of 8.5 cm(

= λ/

2

π

=

53cm

/

2

π

) isacceptableforingestible systemsinthiscase. Thedimensionsof theTXcoil,operatinginsidetheGI tract,arelimitedbythesize ofthecapsule inwhichitwillbeencased.The diameter ofthe capsule is about10.5 mmandits length is about25 mm [2]. So,the in-body antennadiameter mustbe smaller than10.1 mm. Thecoilsin near-fieldare characterizedby thefollowingparameters:the couplingcoefficient(k) between thetransmitterandthereceiver,theinductance L ofthecoils,andtheloadedquality Q factorofthecoils.Inafirststep, thecoils aredesignedwiththeoptimizedparameters:the geometryanddimensionsofthe coils,thesection ofthewire andthenumberofturnsbyempiricalformulas.ThecoilresistanceR increaseswiththenumberofturns.Alow-quality Q

factorwillresultinaweaker energytransfer.Incontrast, Q ,whichisinverselyproportional tothebandwidth,allows, in thiscase,highdatatransfer.Inordertoestimate empiricallythevoltagelevel VR (3)receivedby theRXcoil, anear-field

MIlinkbudgethasbeencarriedoutusing:

V_R2

(

ω

)

=

V_T2

η

T

η

RQTQRk2

(

d

)

(3)

k2

(

d

)

=

r 3

T

×

rR3

×

π

2

(

r2_T

+

d2

₎

3 (4)

where

η

T and

η

R are theresistiveefficiencies QT,and QR arethe loadedqualityfactors, QuT,and QuR are theunloaded

qualityfactorsandrTandrRaretheradiiofthetransmitterandofthereceivercoils.Theformula(4)ofk isempiricaland

isusedwhenthecoilsarecoaxiallypositionedinfree-space,where

η

T,

η

R, QT,QR, QuT, QuRandk canbegivenas:

η

T

=

RS RS

+

RLT

;

η

R

=

RL RL

+

RLR (5) QT

=

ω

0LT RS

+

RLT

;

QR

=

ω

0LR RL

+

RLR

;

QuT

=

ω

0LT RLT

;

QuR

=

ω

0LR RLR (6) k

=

√

M LTLR (7)

Thecouplingcoefficient k in(7)dependson themediumconductivityandonthemutualinductance, whosevalue varies asafunctionofthepositionandorientationofbothcoils.Thecouplingcoefficient,calculatedin(7)by asimulationwith an electromagnetic softwareandmeasured witha VNA (Vector NetworkAnalyzer) ismore accurate than theone deter-minedby theempiricalformula(4). RLT and RLR aretheresistancesofthetransmitterandthereceivercoils,respectively,

RS andRL arethesourceandtheloadresistances, LT and LR theinductancesofthetransmitterandofthereceivercoils

and M is the mutualinductance. The near-field systemcan be modeled withthe Keysight-ADSsoftware. The equivalent circuit model,includingthe TXcapsule andRX on-body coils is presentedin Fig. 1 [22]. L-match circuits,using lumped capacitances C ,formthe matchingnetworks, inorderto matchtheimpedance betweenthesourceandthe capsule coil, andbetweentheon-body coilandtheload.The sourceandloadimpedancescorrespondto50



inastandard measure-ment.

In thefollowing section, we focus on themutual inductanceto studytheoretically the coupling efficiencyversus the relativesize,lateralandangularmisalignmentbetweenthecoilsintheair.

Fig. 1. (Color online.) Inductively coupled equivalent circuit model in ADS software.

3. Mutualinductanceconsiderations

Totheoreticallystudythemagneticcouplingintheair,themutualinductancebetweenthecoilsiscalculated.For sim-plicity,wefocusonthecaseofaone-turncoil,representedbyacircularloop1 cmindiameterfortheTAG,andbyasquare loopoflengthL forthereader.Thesquareloopcandefineasurfaceonwhichthemagneticfluxisintegratedeasily[26,27]. Asthemutualinductanceisreciprocal,thecircularloopisconsideredgeneratingthe B-fieldbyacurrentcirculationi1 on

the loop.The magneticfield

(

Br

,

Bθ

,

By

)

evaluated inthe

(

x

,

y

,

z

)

position[27], generatedby acircularloop ofradius R

positionedinthe

(

Y 0Z

)

plane(Fig. 2),centeredin

(

0

,

0

,

0

)

isexpressedbyequation(8):

⎧

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎨

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎩

Br

=

i1

μ

0y 2

π

[(

R

+

r

)

2

₊

_y2

_]

1/2_r



R2

₊

_r2

₊

_y2

[(

R

−

r

)

2

₊

_y2

_]

E

(

p

)

−

K

(

p

)



Bθ

=

0 By

=

i1

μ

0 2

π

[(

R

+

r

)

2

₊

_y2

_]

1/2_r



R2

₋

_r2

₋

_y2

[(

R

−

r

)

2

₊

_y2

_]

E

(

p

)

+

K

(

p

)



p2

=

4Rr

[(

R

+

r

)

2

₊

_y2

_]

⎫

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎬

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎭

(8)

wherer correspondstothedistancebetweenthecenterofthecircularloopsandthefunctionsK

(

p

)

andE

(

p

)

arethe com-pleteellipticintegraloffirstandsecondkinds,respectively.Bydiscretizing(1),witha500-μmpace,themutualinductance canbecalculatedwithMATLAB.

3.1. TAGparallelconfiguration

Whenthecoilsareperfectlyalignedandspacedwithadistanced,asseeninFig. 2,themutualinductancebetweenthe coilsisreportedversusthelengthofthesquareloop,fordifferentdistancesd.Themutualinductance(notedM)depends on thelength ofthe squareloop fora givendistance.When thedistanceincreases, thelength ofthe square loophasto beincreasedtooptimizethemutualinductance.However,byconsideringtheoptimumvalueofM foreachvalueofd,the maximumvaluedecreaseswhenthelengthL increases.Forawirelessingestiblecapsuleinthesmallintestine,weconsider a distanceof5 cmbetweenthecoils.Inthiscase, increasingthesquare loopside from4to7 cmallowsmultiplying the mutualinductanceby2.15.

3.2. TAGnormalconfiguration

Furthermore,whentheplanesofthecoilsaretiltedtoforma90◦ angle(Fig. 3),anddistancedby5cmoverthex-axis,

themagneticcouplingisnullwhenthecoilsareperfectlyaligned[25].Thecentersofthecoilsaredisplacedbyadistance d y, moving from0 to 10 cm over the y-axis. Forthe single square loop, the optimum value ofthe mutual inductance is obtainedfora d y misalignmentof3.5 cm,i.e.underthe wire,whichgenerates amagnetic fieldaround thewire. The optimumvalueisalmostdividedby3comparedtotheTAGparallelconfiguration.

Fig. 2. Distanced intheTAGparallelconfiguration:a)squareloopforthereadercoilandcircularloopfortheTAG;b)Mutualinductanceversusthelength ofthesquareloop.

Fig. 3. Misalignmentd y intheTAGnormalconfiguration;a) squareloopforthereadercoilandcircularloopfortheTAG;b) mutualinductanceversusthe d y misalignment.

3.3. TAGorientationandpositionversusthecenterofthereader’splane

Thegeneralcase,wherebothlateralandangularmisalignmentsofthecapsulearepresent,isconsideredinHL(horizontal linkinFig. 4a)andinVL(verticallinkinFig. 4b)configurationsforRXsquareloopswiththepreviousTXcircularloop.We report 10Log10

(

M

)

versus theangularmisalignment d

θ

fordifferent lateralmisalignments d y inparallel andorthogonal

configurationfortheRX squareloop (4cm

×

4cm and7cm

×

7cm) andtheTXcircularloop. As S21 isproportionalto

M2 _{accordingto(3)and(7),thecomparisonbetweenthemutualinductanceandthesimulatedormeasuredLog}

10

(

S21

)

is

moreeasilymade.Inordertolimitthedataprocessing,thestudiedd

θ

rangeiscomprisedonlyintherangebetween0and

±

90◦,whereinterestingphenomenaareobserved.ItisimportanttonotethattheevolutionofLog₁₀

(

M

)

versusthe[

±

90◦; 0◦]rangeisnotsymmetrical:thereisonlyasymmetryforthelateralmisalignmentd y

=

0cm.

Bothconfigurationscouldbeassociatedwitheachotherinanarraytoguaranteeaneffectivemutualinductance. For HL (solid line), the mutual inductance is optimum versus the lateral misalignment d y (in cm) and the angular misalignment d

θ

(in degrees). For instance, with a 4 cm

×

4 cm loop and in the [0◦; 90◦] range (Fig. 5), the mutual inductance, versustheangularrotation ata distancedx

=

5cm andamisalignment d y from0to 3cm,ismaximumfor

(

d y

,

d

θ )

equalto(0,50◦);(1,35◦);(2,30◦);(3,20◦),andminimumfor(0,90◦);(1,

+

75◦);(2,59◦);(3,44◦).

ForHL(solidline)andVL(dottedline)inthe[

−

90◦;0◦]rangewitha7cm

×

7cm loop(Fig. 6),themutualinductance versustheangularrotationatdistancedx

=

5cm andthemisalignmentd y from0to3 cmisoptimumversus thelateral misalignmentd y (incm)andangularmisalignmentd

θ

(indegree):themutualinductanceinHLismaximumfor(d y,d

θ

) equalto(0,

−

45◦);(1,

−

54◦);(2,

−

64◦);(3,

−

70◦)andminimuminVLfor(0,0◦);(1,

−

9◦);(2,

−

18◦);(3,

−

26◦).

WhenthelengthL oftheRXsquarecoilincreasesfrom4cmto7cminHL(solidline)andVL(dottedline)configurations (Fig. 7), the maximumvalue of mutualinductance, versus the angular rotationat distancedx

=

5cm and misalignment d y

=

3cm,ishigher,andtheminimumofM isobtainedwhentheangled

θ

decreases.ThelengthL,increasingfrom4cm to7 cm,multipliesthemutualinductanceby2.15whenthecoilsareco-axial(Fig. 2b),whichcorrespondsto10Log₁₀

(

M

)

=

3

.

3dB.ToavoidVLconfigurationandassurethecompactnessofRXarray,itseemseasiertofabricateanRXprototypewith alengthof7 cm.

Fig. 4. HF and VL configurations.

Fig. 6. Mutual inductance in HL (solid line) and VL (dotted line) configuration (7 cm×7 cm square loop).

Fig. 7. Mutual inductance in HL (solid line) and VL (dotted line) configuration for L=4 cm and 7 cm.

In the following section, the design of the coils and the inductive link was carried out using HFSS (High-Frequency Structural Simulator)[28],whichisa commercialfull-wavesoftwarebasedontheFinite-ElementMethodto evaluatethe inductivelinkperformancesthroughthehumanbodyandtheair.

Fig. 8. (a)SpiralcoilTXantennageometry(antenna1);(b)MIlinkthroughthehomogeneoushumanbody;(c)squarecoilRXantennageometry(antenna 2).

Table 2

Characteristicsofthesimulatedantennasinthepresenceofthehumanbodyandintheairat40.68 MHz.

Antenna characteristics RLT() RLR() XLT() XLR() LT(μH) LR(μH) QT QR ηT ηR k

Human body (εr=83;σ=0.67 S/m) 0.8 7 74.5 514 0.3 2 1.5 9 0.98 0.88 0.04

Air (εr=1;σ=0 S/m) 0.8 4 74.5 512 0.3 2 1.5 9.5 0.98 0.92 0.05

4. MonolayercoilforTXcapsule:simulatedMIcharacteristics

Inthisstudy,thelengthofthesquarecoilcanbeconsideredequalto4 cm,tobecomparedwiththepreviousstudy(in

Fig. 2)intheparallelTAGconfiguration.Thecapsulediameterisofthesameorderasthatofthecircularcoil.

4.1. Simulatedcharacteristicsandelectricalequivalentmodel

Giventhedielectricparameters(

εr

=

83 and

σ

=

0

.

67S/m at40.68 MHz)oftheaveragehumanmuscle,thetransmission parameterisinvestigatedintwodifferentmedia,airandhumanmuscleina10

×

10

×

10cm3 cubicboxasillustratedin

Fig. 8b.Thein-bodytransmittercapsulecoil[30]isinsertedintoacylinderhavingthesizeofavitaminpillfilledwithair. The capsuleisplaced atthecenterofthecubicbox:it consistsofa6-turncoil, withanouter diameterof10.1 mm,and line widthandspacebetweenthe lines equalrespectively to0.25 mm and0.2 mm, asdepictedin Fig. 8a. The on-body coilispositioned coaxiallywiththein-bodycoilat5 cmbeneaththesideofthecubicbox(Fig. 8b).AsshowninFig. 8c, thedimensionsofthefour-turnon-body antennaare 41

×

41

×

1

.

5mm3_{,withalinewidthandspacingbetweenlinesof}

0.3 mm. RogersRT/duroid 5880substrate(

εr

=

2

.

2; tan

δ

=

0

.

0009) isused todesign both thein-body andtheon-body antennas.

Amatchingsystem,composed oftwocapacitors,isdesignedforeachcoiltomatchthecoilimpedancescloseto 50



, when thecoilsare consideredseparately.Accordingto(3),theempiricaltransmission S21can beexpressedfrom QT, QR,

ηT

,

η

Rand k.TheresistancesRSandRLaresetto50



andPTto1 W.RLT,RLR,LTandLRarededucedfromthesimulated

resultsoftheTXandRXcoils.FromTable 2,themodificationofthemediaaffectstheRXcoilresistanceRLR.

Theempiricaltransmissioncoefficientiscalculatedusingequation(7)forthecouplingcoefficient,inwhichthevaluesof inductancesareextractedfromthesimulationusingZi j parametersandisupdatedin(9):

k

=

√

Im

(

Z12)

Im

(

Z11

)

×

Im

(

Z22

)

(9)

where port 1 feeds theTX coiland thereceived power of theRX coilis evaluated atport 2. Z11, Z22 and Z12 are the

impedancesoftheTXcoil,RXcoilandthemutualimpedancebetweenbothcoils,respectively.TheimaginarypartsofZ11,

Z22 and Z12 correspond respectively tothe self-inductance ofthe RX coil, of theTX coil, andto the mutualinductance

betweenbothcoils.Thecouplingcoefficientisequalto0.04inthehumanbodyand0.05intheair.Thelowvaluesofk are

essentiallyduetothesmalldimensionsofthecoils,particularlytheTXcapsulecoil.FromTable 3,wecanconcludethatthe empiricalvaluesarenotinagoodagreementwiththesimulatedresults.Nevertheless,themodificationofthemediaaffects theinductivelinkbecauseoftheincreaseoftheRXon-bodyspiralcoilresistance,inthepresenceofthehumanbody.The previous simulations areperformedinthehomogeneous humanbody.We havetoperformsome ofthemina multilayer humanbodymodel,inordertoevaluatetheimpactontheMIlink.

Table 3

Comparisonbetweenthecouplingresponsefoundbyempiricalformulaand HFSSsimulationinthehumanbodyandintheairaround40.68 MHz.

Human body Air

S21empirical (dB) −17.3 −15

S21simulation HFSS (dB) −22.1 −20.7

Fig. 9. MI link through a three-layer human body (muscle/fat/skin).

Table 4

Dielectricpropertiesofhumantissuesemployedinthethree-layerhumanbodyat40.68 MHzand RFattenuation(dB/cm).

Human tissue Relative permittivity Conductivity (S/m) α(dB/cm)

Muscle 82.6 0.67 0.78

Fat 7.3 0.034 0.16

Skin 124.3 0.38 0.48

Table 5

S21parameteroftheinductivelinksthroughtheair,thethree-layerhumanbodyandthe

homogeneousmodelat40.68 MHz.

Air Three-layer model Homogeneous model

S21 (dB) −20.7 −21.1 −22.1

4.2. Morerealisticmodel:3-layerhumanbodyandbatteries

Athree-layerhumanbodyisrepresentedinFig. 9.Thedielectricpropertiesofeachlayerandattenuation

α

calculated thanks to formula(1) are reportedin Table 4. The very low conductivityof fat isclose to that ofthe free-space,which reducesthedissipated energybythelossymedia.Byconsidering thethicknessandattenuationofeachlayer, the3-layer humanbodyimprovesby0.9dBthemediumlosses;thefieldmagnitudeisimprovedfrom3.92dBwithamuscleto3.02 dB. According to Table 5, the inductive link can be improved in the three-layer human body, which is a more realistic model,incomparisonwiththe homogeneoushumanbody.By takinginto accountthe tissue’sthicknessandattenuation, thediscrepancyofalmost 1 dBbetweenthethree-layermodelandthehomogeneousone isverifiedwiththeattenuation formula(1)andwiththeS21parameter(Table 5):thelossesareproportionaltothevolumeandtheconductivityofbody tissues.

Theingestiblecapsuleispoweredbyatwocoin-shapedsilver-oxidebattery,whichcangenerate20 mW.Themaximum specificabsorptionrate(SAR)ofthehumanbodyisrecommendedunderthebasicrestrictionof0.4W/kg,averagedoverthe wholebody,bytheInternationalCommissionofNon-IonizingRadiationProtection(ICNIRP)[31].SotheSARvaluerespects thestandards.

Themeasurementofbodytemperatureby aningestiblesensor[9],fed bytwosilver-oxidebatteries,at40.68 MHz, has beeninvestigated.Thereceiverwasplacedlessthan50cmawayfromthetransmitter,thedatatransmittedevery15 swith adatarateof80 bpsandabatterylifeof10 days.

Thevolumeallocatedtothebatteriesisfilledwithametallicmaterial.A coppercylinderisincludedintheTXingestible capsule tomimicthepresenceofthepowersupply.Intheworstcase, i.e.whenthemedium isthehomogeneoushuman body, a copper battery (

μr

=

0

.

99 et

σ

=

58

·

108 S/m), whose diameter andthickness are about 10.5 mm and4 mm, respectively,isplaced 3 mmawayfromthecoilsubstrate.The transmissionparameter isreducedby 0.5 dB.Toconclude, theperformancesconfirmtheadequatecoildesignandthebatteriesaffectslightlytheinductivelink.

Fig. 10. TX coil angle and position variation according to the OY axis in the HL and VL.

Fig. 11. Simulated transmission parameter at different TX antenna y-positions as a function of the angleθat 40.68 MHz in the HL and in the VL.

4.3. WithaRX-coilarray

The ingestiblecapsuleslidesdownintothegastro-intestinal(GI),sotherelativepositionandorientation oftheTXcoil versus theRX spiralcoilmustbe takenintoaccountto evaluatetheinductivelink. TheTXcoilorientationisreportedas a functionofthe angle

θ

,

θ

=

0◦ corresponding tothe horizontallink(HLlink) and

θ

=

90◦ totheverticallink (VLlink) andtheTXcoilpositionisreferencedbytheOYaxis(Fig. 10).Therelativepositionisdefinedasthespacingbetweenthe capsulecoilandthespiralcoilinthe y-direction.ThetransmissionparametersoftheHLandVLlinkshavebeenperformed separately. Another possibility is to realize a multi-coil capsule. Three coils, which are geometrically orthogonal to one another,areinsertedinthecapsuletoensureanadequatecouplingwiththereadercoil[11].

Foreachrelativeposition,comprisedbetween

−

3 cm and3 cm,theinductivelink’sefficiencyat40.68 MHzisacceptable foranycoilangleorientation,subjecttoconsideringbothcomplementaryVLandHLlinks.Infact,whiletheTXcapsulecoil slides in theGI tract, theRX-received powercan be optimizedby the positionandthe orientation. The evolutionofthe

S21 parameter according tothe angle indifferent y-positions at 40.68 MHz isdepicted inthe four sub-plots ofFig. 11,

whereeveryportioncorrespondstoaTXantennay-position( y

=

0

,

−

1

,

−

2 and

−

3 cm).Wecanconcludefromtheseplots that forall selected y-positions, the maximumoccurs when

θ

=

0◦.This configuration representsthe optimum position where there are maximumH-field lines perpendicular to theon-body antenna surface. Over the d

θ

variation range[0◦; 90◦],thebehaviorof10Log₁₀S21(Fig. 11)isalmostsimilartotheLog10

(

M

)

function(Fig. 5),whichisoptimumversusthe

angularvariation:intheHLconfiguration,theangle,wheretheminimumvalueisobtained,decreasesaccordingtothedx misalignment rise, butthemaximumis always obtainedinsimulation whend

θ

is equalto zero,whilethe curve profile is different under MATLAB, where the angularposition, allowing one to obtain a maximum level ofmutual inductance, changesanddependsonthelateralmisalignment(Fig. 5).TheoptimumanglesareslightlydifferentunderHFSSsimulation andMATLABanalysis, probablybecauseofthemulti-turnstructureinsimulationandtheone-turncoilunderMATLABfor

Fig. 12. (a)FabricatedTXmultilayercapsulecoil;(b)fabricatedRX3-turnreceiverspiralcoil;measurementset-up:(c)sideview;(d)topview;(e)pictures ofthemeasurementset-up.

Table 6

Capacitances valuesofthe transmitterand thereceiver antennas,whenthe transmission channeliseitherthehumanbodyortheair.

Series capacitance (pF) Parallel capacitance (pF)

In free-space TX coil 180 800

RX coil 8.2 213

In the human body TX coil 180 800

RX coil 7.6 56

the TXandRX coils andalsobecause ofthedifferent media:we considered air(

εr

=

1

;

σ

=

0 S/m) asthemedium on MATLABandthehumanbody(

εr

=

83

;

σ

=

0

.

67S/m)forsimulatedresults.

Duetothenon-compactness oftheRXarray,withthecoilsorientedperpendicularly tothesurface,we proposeto in-creasethedimensionsoftheRXcoilsfrom4cmto7cm(toincreasethemutualinductance,likereportedinFig. 6)andthe layers oftheTXcapsule coil.Toperceive dissimilarities,the fabricatedprototypes willbe positionedinan human-tissue-equivalentfluid.

5. MultilayercoilfortheTXcoil:simulationandmeasurement

Inthisstudy,thelengthofthesquarecoilcanbeconsideredtobebetween7 cmand8 cminthetheoreticalstudyon themutualinductance(inFig. 2)fortheTAG parallelconfiguration.Thecapsule diameterisofthesameorderasthatof thecircularcoil.

5.1. RealizedTXandRXcoilsandmeasurementsetup

To increase the magnetic field’s strength, the TX capsule coil is chosen as a five-layer coil and the RX coil surface increasesfrom4

.

1cm

×

4

.

1cm to7cm

×

8cm[32].Theadvantagesofthemultilayerprintedcoilcomparedtotheclassic wired coilare essentiallythe robustness andthe solidity.The structure ofthe capsule antenna(Fig. 12a)consistsoffive differentloops, whichlie ondifferentlayersofFR4 epoxywitha dielectricconstant

ε

r of4.4, adielectric losstangent of

0.02 anda metallicthicknessof 35 μm usingmechanicaletching,separatedby fourFR4 super-strata withathicknessof 0.76 mmtoensurebotha stabledistancebetweenthe coilturnsandamoresolid structure.Theinnerandouterradiiof theloopsarerespectively8.5 mmand9.5 mm.Eachloophastwocircularcatchpads0.7mmindiameter,whichareused toconnectthemetalvialoops.Inordertoincreasethereceivedpowerthroughtheinductivelink,theon-bodyreceivercoil needs tohavelargedimensions,asillustratedinFig. 2.As showninFig. 12(b),thedimensionsofthethree-turnonbody antennaonFR4substrateare70

×

80

×

0

.

76mm3,withalinewidthandspacingbetweenlinesof1.5 mm.

TheL-matching circuit(Fig. 1), usinglumped capacitances(Table 6), hastobe adjustedto matchthe impedancefrom thesource(RS)totheTXcoil(LT,RLT)andfromtheRXcoil(LR,RLR)totheload(RL)andwhentheMIlinkisestablished.

Table 7

Simulatedandmeasuredcoilcharacteristicsinthepresenceofthehumanbodyandinairat40.68 MHz.

Antenna characteristics RLT()in the human body RLR() LT(μH) LR(μH) QT QR ηT ηR k

Simulation in phantom 0.32 14.2 0.12 1.7 0.6 6.7 0.99 0.78 0.11

Measurement in phantom 0.52 26.8 0.12 2.1 0.61 7.1 0.99 0.65

Simulation in air 0.3 2.4 0.12 1.6 0.6 8 0.99 0.95 0.22

Measurement in air 0.57 5.1 0.12 2 0.61 9.1 0.99 0.91

Fig. 13. Simulated and measured reflection coefficients of the transmitter(S11)and the receiver(S22)coils in the air.

Fig. 14. Simulated and measured reflection coefficients of the transmitter(S11)and the receiver(S22)coils through the human body.

The coils were measured by using a homogeneous human-tissue-equivalent fluid available in our laboratory, whose dielectric properties (

εr

=

64 and

σ

=

0

.

78 S/m) are extrapolated from the measured curve with the Agilent Dielectric Probe Kit (200 MHz–20 GHz) andare takeninto account in the simulation toevaluate the inductive link performances. The in-vitromeasurement was carriedout by RohdeSchwarzZNB8, asshowninFig. 12c–e.TheTX coilisplaced inthe rectangularboxwithdimensions29

.

2cm

×

22

.

2cm

×

16

.

5cm (L

,

l

,

h)ath

=

5cm,whichishigherthantheRX external coil, whosespacingof0.4 cmiscomprisedofair,totake intoaccounttheairgapbetweenthereceivercoilandtheback side oftherectangularboxneededtoinserttheSMAconnector.Thecapsuleantennaisisolatedfromthebodyliquidbya Plexiglastube,whosediameterandthicknessarerespectivelyequalto1.4cmand0.5mm[32].Aferritecoreisappliedto thefeedingcoaxialcable,whichisconnectedwiththeRXcoilandtothenetworkanalyzertopreventdisturbancecurrents fromflowinginthecable.

5.2. Coilcharacteristics,matchingandtransmission

ExtractedelectricalcircuitsdeducedfromsimulatedandmeasuredinputimpedancesofTXandRXcoils,whenthemedia ishumanbodyandair,arelistedinTable 7.Theimpedance(Table 7)andthemeasuredbandwidth(Figs. 13 and14)ofthe RX coilimpedancedecrease,whenthetransmissionchannel changesfromthe humanbodyto theair.TheTXcoilseems less sensitive tothe media variation, because ofits insertion in the Plexiglas tubefilled withair: the

−

10 dB matching bandwidth isequal to 1%(around 0.47 MHz)intheair andinthe humanbody.We alsoanalyzethe couplingefficiency

Fig. 15. MeasuredcouplingresponseS21,accordingtothefrequency,betweenthecapsulecoilandthereceivercoilseparatedbyadistanceof5 cmwhen

thepathisthehumanbodyandtheair.

Table 8

Comparisonbetweenthecouplingresponsefoundbyempiricalformula,HFSSsimulation,measurementsandADSsimulationindifferentmediasaround 40.68 MHz.

S21empirical S21simulation (HFSS) S21measurements S21simulation (ADS)

Human body (εr=64;σ=0.78 S/m) −14.2 dB −18.1 dB −20.7 dB −15.9 dB

Air (εr=1;σ=0) −6.6 dB −10.5 dB −16.8 dB −21.3 dB

“Non-conductive” human body (εr=64;σ=0 S/m) Ø −10.4 dB Ø Ø

inthehumanbody andintheairat40.68 MHzwithan empiricalformulabasedontheAgbinya–Masihpourmodel[24], withHFSSelectromagneticsimulation,ADSelectrical circuitsimulationandmeasurement.Moreover, theefficiencyofthe inductivelinkinmeasurement,representedbythe S21parameter,canreach0.85%(

−

20

.

7dB)inthehumanbody.

The followingparagraphs discussthelackof accuracyinthe measurementofthe characteristicsof the human-tissue-equivalentfluid,theattenuationintroducedbythemedia,bymismatchingtheconnectionbetweentheload(orthesource) representedintheexperimentalpartbythecableandtheRXcoil(orTXcoil),byconductorlossesandbytheferritecore encirclingthecable.

Thedielectricproperties(

εr

=

64 and

σ

=

0

.

78S/m)ofthehomogeneoushuman-tissue-equivalentfluidareextrapolated frommeasurementinthe200 MHz–50 GHzbandwiththeAgilentDielectricProbeKit.Astransmissionattenuationdepends onthesquare rootoftheconductivity,thelackofaccuracyinthemeasurementofthecomplexpermittivity canresultin discrepanciesbetweensimulationandmeasurement.

Toevaluate theinfluence ofthe dissipativemedia onthe receivedpower obtainedby magneticcoupling,we plotthe measured couplingresponsewhenthechannel isthehumanbody andtheairinFig. 15.As itisdepictedbytheseplots and in Table 8,the coupling response in the air and in the human body at 40.68 MHz is equal to 7.6 dB in the HFSS simulation andwith empiricalformula (3),3.9 dB in measurement,while media losses are calculated at4.04dB inthe beginningofparagraph 2.Anovercouplingintheairisobservedwiththeelectricalcircuitsimulationanddeterioratesthe transmissionparameterto

−

21

.

3 dB at40.68 MHz.AsitisseeninTable 8,themediaattenuationdependsessentiallyonits conductivity:themodificationoftherelativepermittivityfrom1to64doesnotaffecttheefficiencycouplinginairandin the“non-conductive”humanbody.ThemeasuredfrequencyshiftofTXandRXcoilswithairandwithhumanbodymedia (Figs. 13 and14)canexplainthefactthatthemeasuredMIlinkislesseffectivethanthesimulatedone.Thecriticalfactor seemstobetheL-matchingcircuit,madeoflumpedcapacitors.Wefocusontheserialresistanceandrelativevalueofthis capacitor.

Thereactance X ofthecapacitorsiscomprisedbetween0

.

002



and0

.

2



forcapacitorvaluesvaryingfrom7.6 pFto 800pFat40.68 MHz. Toevaluatetheimpactonthepowertransferwiththecapacitorcircuitsinthehumanbody,ahigh serial resistanceof 0

.

2



is associatedwithboth capacitors inTX andRX coils,and the S21 parameter isreduced from

−

15

.

9dB to

−

16

.

2 dB. A

±

5% variationonthecapacitorswithzeroserialresistanceaffectsthecouplingefficiencyof

±

1 dB.However,thisS21range,withtherelativeerroronthecapacitorandtheserialresistance,stayslessthanthemeasured

one,butcancontributetoincreasethediscrepanciesbetweensimulationandmeasurement.

Inordertoqualifytheeffectoftheconductorlosscomposedofskin-effectloss,proximity-effectlossandDCconduction loss,copperlines intheRX andTXcoilsarereplaced bya PerfectElectric Conductor(PEC). Asthe signaltransmissionis improvedfrom

−

18

.

1 dB to

−

12

.

4 dB withTXcoilmismatching,thereareconductorlosses,whichcannotbeneglected in thesimulation.Inparticular,thesizeofthemeshingcellsintheconductorhastobelessthan10 μmofskinthickness.

Themeasurementofasmallcoilrequirestheconnectionwithacoaxialcable, thatdisruptstheimpedanceofthecoil, duetocurrentontheouter surface ofthe cable.Balunsofferrite [33]forEMI(ElectromagneticInterference)suppression

radia-Fig. 16. Simulated and measured coupling response S21according to the z-distance around 40.68 MHz in the human body.

tive emission, theuseof anelectro-optical transduceranda fibre link,associated withthepost-processing step[36], are investigatedtoavoidtheuseofanRFcable[37].Amethodforexplainingthediscrepancybetweenthesimulationandthe measurementrequirestwodifferentmeasurementsofthe S21parameterwithandwithoutafeedingcoaxialcablein

prox-imity tothedissipativehumanbody,actingasametallicplane.The S21leveliscomparablewithoutcableandwithcable

andde-embedding:alevelshiftof0.2dBontheS21withandwithoutcableisobtainedafteroptimizationofthecapacitor.

ThemeasurementisdoneinthereferencedplaneoftheRXandTXcoilsinputwithSOL(ShortOpenLoad)calibrationwhen theRXcoilisusedwithferriteencirclingthecable.

It maybe concludedthatinvestigations onthediscrepanciesbetweensimulationandmeasurementdemonstratedthat the L-matching circuit, the approximate dielectric characterization of the media, and the conductor losses were critical factors.

5.3. Distanceeffects

Inthe previoussection, theTXcapsule coilisoff-centeredby5cmaway fromtheRX on-bodycoil. Inthisparagraph, theoff-center distancevariesfrom1 cmto8 cminthehomogeneous humanbody.Simulatedandmeasuredtransmission parameters S21 versus theoff-center distanceare reported inFig. 16.A marginal drop from1 dB to 2.6dB is observed

betweenthesimulatedandmeasuredS21,theslopeisalmost10 dB/octaveanddoesnotagreewiththeB-fieldattenuation

function ofd−3 _{(powerattenuation of18dB/octave). B-fieldattenuation dependsond}−3 _{when theradius ofthecoils is}

inferior tothedistancebetweenthecoils: theradius,equalto0.5 cmand4.22 cm(calculatedradiusfromtheequivalent surface)fortheTXandRXcoilsareinthesameorderthanthedistanceof5 cmandformula(4),entailingthesizeofthe coilsandthedistancebetweencoils,cannotbeapplied.Thediscrepanciesbetweensimulationandmeasurementaredueto theL-matchingcircuit(Fig. 1),withafrequencyshift,thevalueofcapacitancenotbeingoptimizedforeachdistance.

5.4. Angularandmisalignmenteffects

Inthisstudy,theTXcapsulecoilis5cmawayfromtheRX coil.Ahomogeneousrectangularcuboidhumanbodywith a smallerheight,i.e.withdimensionsof38

.

5cm

×

33cm

×

8cm (L

,

l

,

h)isconsideredasallowing amaximumvariation ofthe angle,butitis notpossibletoreach the90◦ angle:a measurementsetup isrealizedtoavoidthemechanicalshift betweenthecoilsand, inthisconfiguration,the tissue-equivalentfluid couldflow intotheisolatedtubeinthe 90◦ angle orientation (Fig. 17). Simulatedandmeasuredtransmissioncoefficientsforthefourpositions( y-positionsequal to0,1,2 and 3 cm)aredepictedinFig. 18.Themeasurementisalwaysworsethanthesimulation,butthemaximizedtransmission parameter versus theangle is obtainedin simulation whenthe coils are coaxially positioned, whilethe angle is shifted from0to

−

26◦ whenthecoilsaremisalignedfrom0cmto3 cm, ifweconsiderthat d

θ

inMATLABcorrespondsto

−

d

θ

in theexperimental part:thebehavioris similartothe mutualinductancevariation,whichismaximuminFig. 6 forthe angular risewhen the dx misalignmentpositionbecomes greater. The angle,wherethe maximumis effective, isslightly differentunderHFSSsimulationandMATLABanalysis,becauseofthemultilayerstructure(eachloopcanbeequivalenttoa loopwithadifferentdiameterforaconstantdistance)fortheRXcoilsanddifferentpropagatingmedia.Themeasurement accuracyerrorscanbeincreasedwiththepreviouspartbecauseofamechanicalshiftonthe y-positionandontheangle.

To conclude, for every capsule antennaposition, a couplingresponse is calculated versus the angleincrease: the S21

response varies between

−

20

.

5 dB and

−

35

.

5 dB. If we consider that for a

−

32 dB response the MI link is effective, the configuration for y-position

=

0 cm and

θ

=

80◦ can be considered like a limit. Moreover, to avoid low coupling responseinsomecapsuleorientations,afabricatedprototypewithmetalized holesallowingaturnnumberincrementand manufacturingfacilitiesforthesametotalTXcoilheightwouldbeanadvantage.

Fig. 17. Measurementsset-upwhenthepositionandtheorientationofthecapsuleantennachange:(a)sideview;(b),(c)picturesofthe measurement-setup.

Fig. 18. Simulated and measured coupling response S21around 40.68 MHz in the tissue-simulating fluid according to the angle at different y-positions.

6. Conclusion

For wireless ingestible capsule systems, modelling and optimization of the complete transmission link in the pres-enceof the human body isnecessary. The FEM method is used tomodel the MI link,which is carried out using HFSS, a commercially-availableFEM pieceof software.Tocapturethe behavior oftheMI inwireless ingestible capsule, the TX coilis insertedina capsule, surroundedby a humanbody.Torepresentthe bodyproperties innumericalmodelling, we consider a homogeneous and a three-layer model. In measurement, we employ a human tissue-equivalent fluid, whose

dielectric propertiesare characterized by adielectric probe kit. TheMI linkis evaluated bytakinginto account the rela-tive positionandorientation oftheTXcoilversusthe RXcoil. Multiple(ideallyorthogonal)loopscould beusedto avoid couplingminimaattributabletotheTXcoilorientation.

Theresultsdiscussedinthisworkare:

(1) analysisofthecoilimpedanceandloadedqualityfactor,

(2) mutualinductancebetweenone-turncircularandsquarecoilsversusthelateralandangularmisalignment, (3) analytical,simulated,measuredMIlinkbudgetandelectricalcircuitsimulation,

(4) observationofthereceivedrelativesignal responseversustheangularandmisalignmentofcapsule’scoilinthesmall intestine,

(5) considerationoftwoMIlinks

– aTXmonolayercoilandaRX4

.

1cm

×

4

.

1cm coil-orthogonalarray

– aTXmultilayercoil

(φ

×

h

=

9

.

5mm

×

2mm

(

0

.

0013

λ

o

×

0

.

0003

λ

o

))

andaRX7cm

×

8cmcoil.

Toconclude,theefficiencyoftheMIlinkasafunctionofvariousenvironmentalandcouplingconditionsisestablished andexperimentallydemonstrated.Ifwecomparethesimulatedresultsforthefirst(Fig. 11)andthesecondcoilstructures (Fig. 18), wecan noticean improvedvalue of S21 duetotheRX coilsize increase,despite themedium conductivityrise

betweensimulationandmeasurement(from0.67 S/mto0.78 S/m):inthesimulation,theS21previouslycomprisedbetween

−

22 and

−

80 dB fora4 cm

×

4cmloopisincreasedto

−

18

.

1 to

−

50 dB fora 7 cm

×

8 cmloop.Forco-axialcoilsin HLconfiguration,the3.9-dBmaximumlevelenhancement,whichvariesfrom

−

22 to

−

18

.

1 dB, isinagreementwiththe theoretical part,where L increasesfrom4cmto7cm(the coilisassimilatedtoasquare,butitisarectangle) multiplies the mutual inductanceby 2.15(Fig. 2b), which corresponds to 10Log10

(

M

)

equal to 3.3 dB.The efficiencyimprovement

dependsontheRXsizeincreasingmainly,butprobablyalsoslightlyontheTXstructure,composedofa6-turnmonolayer coil,anda5-turnmultilayercoil,whosediscrepancyregardingtheoptimizationwasnotevaluatedinthispaper,focusingon mutualinductance.Simulatedinvestigationsonthemeasurementerrorsdemonstratedthatthematchingcharacteristicsof thematchingnetwork,andtheconductorlosseswerecriticalfactorsthatcontrolledmeasurementaccuracy.Lackofaccuracy on thedielectric characterizationofhumanbody equivalent liquidbecauseofthelow frequencylimitat200 MHz ofthe Probeandthemechanicalshiftofthein-vitromeasurementcanbringsomeerrors.

As a perspective,the design of a RX coil array,pressed on the abdomen, to ensure power efficiencyfor an arbitrary capsule’s orientation andto determine the TX coil positionin the small intestine should be considered. To identifythe zone, whereislocatedthecapsuleinthesmallintestine,forinstanceinordertoevaluatethetransittime andfunctional exploration,thepositioniscurrentlydeducedfromthechangesofPHvalues,butthelowsensitivityofPHsensorsandthe PHmodification withdigestive disordersare notaccurate. Witha cheap wearablesystem, wecould focusonthe LFRFID technology to be lesssensitive to the medium’sconductivity. Capsules,including severalsensors, could be swallowed to determinethecapsuleposition:multiplesensorsinthewirelessingestiblecapsulecanprovidedataanddataprocessing al-lowsonetodeterminemoreaccuratelythepositionofthecapsule[39].Inthecapsule,aninclinometer[38],ananticollision function couldbeaddedtomaximizethedetectionbyzone andthebatterylife timethroughlowpowerconsumption.To avoidbatterydischarge,someresearchersfocusonenergyharvestingfromhumanbody:conversionfrombodymechanical energy,suchasbloodflow,breathing. . . intoelectricalenergyfortheimplanteddevices[40–43].Thelossesinthemedium arelowerasfrequencydecreasesandcanbeevaluatednumericallyandexperimentally[44].Infutureworks,thefabrication ofaRXbelttoidentifyandlocalizeLF-RFIDTAGsinthesmallintestineshouldbestudied.

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