Sensors and Actuators A 130131 (2006) 4853

Integrating wireless ECG monitJohan Coosemans , Bart Hermans,

ICAS,elgiumber 20er 200

Abstract

This pape oringpowering. T g ofrisk of Sudd adeflexible circ e in tintegration i . 2005 Else

Keywords: E

1. Introdu

Previoubiomedical monitoring systems can greatly benefit from theintegration of electronics in textile materials [18]. This syn-ergy between textiles and electronics mainly originates from thefact that clothing is our most natural interface to the outsideworld. The higher the level of integration of the sensors andcircuits insystem becuser.

This papsystem, prito Sudden Ian increaseof the electsive gel elehinder bothterm use ofand allergithat overco

CorresponE-mail ad

robert.puers@

s, inin th1.

2. ECG measurement

As an alternative to conventional ECG electrodes, both knit-

0924-4247/$doi:10.1016/jthe clothing, the more unnoticeable the monitoringomes to its user and thus the higher the comfort of the

er presents a garment embedded patient monitoringmarily intended for the monitoring of babies pronenfant Death Syndrome (SIDS). As these babies haved risk of cardiac arrest, a continuous measurementrocardiogram (ECG) is necessary. Up to now adhe-ctrodes are used, wired to an alarm unit. These wiresthe baby and the nursing staff. Furthermore, the long-conventional gel electrodes can cause skin irritation

c contact reactions. This paper presents a solutionmes these inconveniences by the integration of the

ding author. Tel.: +32 16321716; fax: +32 16321975.dresses: johan.coosemans@esat.kuleuven.be (J. Coosemans),esat.kuleuven.ac.be (R. Puers).

ted and woven stainless steel electrodes (called Textrodes) weredeveloped in collaboration with Bekintex [8]. These dry Tex-trodes (Fig. 2) are less irritating than gel electrodes. In addition,stainless steel has a low toxicity, the yarns can be manipu-lated as a textile material, and can be washed without losingtheir properties. Disadvantages of the Textrodes however, arethe increased sensitivity to motion artifacts, as well as the poorskinelectrode contact. Therefore, the analog ECG front-endhad to be redesigned.

Because the Textrodes are of bad quality, a three-electrodeECG configuration circuit is preferred over a two-electrode ECGcircuit. In a first attempt, a driven right leg (DRL) topology(see Fig. 3) was implemented, because of its known high com-mon mode rejection ratio (CMRR) [9]. Fig. 4 (left) shows theequivalent circuit, for the calculation of the common mode inter-ference.

In this equivalence, it is supposed that all electrodeimpedances Ze are equal and that the amplifiers poles do notlie in the aimed frequency range (0.5150 Hz). The common

see front matter 2005 Elsevier B.V. All rights reserved..sna.2005.10.052Katholieke Universiteit Leuven, Department ESAT-MB-3001 Leuven-Heverlee, B

Received 20 June 2005; received in revised form 13 OctoAvailable online 20 Decemb

r reports on the full realization of a garment embedded patient monithe developed system is primarily intended for the continuous monitorinen Infant Death Syndrome (SIDS). The sensors and the antenna are muit to facilitate integration in the babys pajamas. A significant increasn textiles. A prototype baby suit was fabricated and successfully testedvier B.V. All rights reserved.

CG; Intelligent textiles; Inductive powering; Telemetry

ction

s work on so-called intelligent textiles shows that

sensor

cuitryin Fig.oring in textilesRobert PuersKasteelpark Arenberg 10,

05; accepted 20 October 20055

system, including wireless communication and inductivethe electrocardiogram (ECG) of children with an increasedout of textile materials. All electronics are mounted on ahe comfort of patient and nursing staff is achieved by this

terconnects and the processing and transmission cir-e babys suit. A general system overview is depicted

J. Coosemans et al. / Sensors and Actuators A 130131 (2006) 4853 49

Fig. 1. General overview of the system.

mode voltage Vc is then given by:

Vc = s Zg K (Ze + Zi)Zi(1 + A) + Ze + Zg + s Zg + Zg Cx (Ze + Zi) Vp

(1)

Fig. 2. Belt with Textrodes.

Fig. 3. SimplDRL; full line

with Ze1 =the DRL fe

A = 2 RfRa

K = CCg +

Vp is the cinterferenc

Eq. (1)are much sZg Zi) ainterest. Eq

Vc = s Z1 +We can melectrode cthe calculaThe calcula

Vc =Zi +

Simplificat

Vc = s ZgEqs.mpaon m

dbacFromtion cocomm

the feeified model for thee-electrode ECG configuration. Dotted line:: grounded configuration.

the commo

Fig. 4. EquivconfigurationZe2 = 2Ze, the input impedances and Zi1 = Zi2 = 2Zi,edback gain A:

, Cx = Cc (Cg + Cb)Cg + Cb + Cc and

c CbCb + Cc .

ommon mode source, e.g. due to 50 Hz power-linee.can be further simplified: the electrode impedancesmaller than the input impedance of the amplifier (Ze,nd the pole in (1) lies out the frequency range of. (1) then becomes:

g KA

Vp (2)

ake the same calculations for a grounded three-onfiguration. In this case, the equivalent circuit fortion of the common mode is shown in Fig. 4 (right).tions then lead to:

s Zg K (Ze + Zi)Ze + Zg + s Zg Cx (Ze + Zi) Vp (3)

ion yields:

K Vp (4)(2) and (4), the advantage of the DRL configura-

red to the grounded configuration becomes clear. Theode is reduced with a factor A, which is the gain ofk amplifier in the DRL configuration. To minimize

n mode interference, the gain A can be increased.

alent circuit for the common mode. Left: DRL; right: grounded.

50 J. Coosemans et al. / Sensors and Actuators A 130131 (2006) 4853

Nonetheless, saturation of the output of the feedback amplifiermust be avoided.

In the presented design, the common mode signal does notonly consist of 50 Hz power-line interference, but mainly origi-nates from the electro-magnetic field that is used for the inductivepowering of the circuit (as described in next paragraph). Mea-surements at the input of the ECG amplifier learned that theresulting common mode signal almost forces the opamp intosaturation, due to the large RF field. Eq. (2) illustrates that thecommon mode of a DRL circuit can only be reduced if thefeedback amplifier has a large gain. However, under the abovementioned common mode conditions, this gain must be limitedto A 1. Consequently the DRL circuit has only little outcomeon the common mode rejection and becomes ineffective. For thisreason, a grounded three-electrode configuration is implementedinstead.

The deshamperedlevels of cois implemeobtain a hithe amplifiration due tis followedquency of 1The filter c

3. Power a

Power ibatteries caof the volukey benefitof the inducpower constance Rload

Startingcalculated.frequency

r = 1Ls

Fig. 5. Top: scoupled circu

FlexpCu; N

axim

max

k isural

Ls2Ls1

ternith

.g. dng ce cir 10timid aciatedm E. A larmer ratio n. On the other hand, a large N also meanse coil windings become thin, resulting in a large Rs2low quality factor Q. The inductive link was simulatedTHENRY [10], for a mutual coil distance of 9 cm and500 . Fig. 7 shows the simulated link gain as a func-the number of turns, with a maximum at N = 27. Table 1

ments and simulated values of the inductive link properties

Simulated Measured

) 21.2 21.1) 82.4 82.732 kHz) 24.9 21.2

4.63 4.70.52a 0.47

printed circuit coil.(6).ign of this grounded configuration ECG amplifier isby the presence of the RF field as well. For normalmmon mode interference, a large amplification factornted in the differential to single ended amplifier, togh signal-to-noise ratio. In this application however,cation in the first stages must be limited to avoid satu-o the large common mode signal. The ECG amplifierby a third-order low pass filter, with a cut-off fre-

50 Hz, to eliminate the remaining RF common mode.ontains further distributed amplification.

nd data transmission

s provided trough an inductive link at 132 kHz, son be left out. Continuous measurements, reductionme and weight, and ease of package sealing are thes of this approach. Fig. 5 shows a schematic overviewtive link and the equivalent non-coupled circuit. Theumption of the system is represented by a load resis-.

form this equivalence, the link gain Vsec/V1 can beThis link gain reaches a maximum at the resonancer, with

2 C= 132 kHz (5)

chematic overview of the inductive link. Bottom: equivalent non-it.

Fig. 6.(70m

The mVsecV1

wherethe nat

n =

The excope wcoil, ereceiviflexibl10 cmwas oparea an

the radFro

minedtransfothat thand ain FASRload =tion of

Table 1Measure

Ls1 (HLs2 (HQs2 (at 1k (%)Vsec/V1

Flexiblea Eq.rint with integrated circuitry and secondary coil; 10 cm 10 cm= 27).

um gain equals:

k n [

1Ls2nRload

+ Rs2Ls2n

](for Rload Rs2) (6)

the coupling factor, n the transformer ratio and n isfrequency, with

and n = 1Ls2 C

1 + Rs2Rload

.

al coil is made large (26.5 cm 26.5 cm 2 cm), tomisalignments between the primary and secondaryue to movement of the baby. In a first prototype, theoil is implemented together with all electronics on acuit (Fig. 6). The available area for this coil is aboutcm. The number of turns N of this secondary coilzed in software to maximally exploit the availablehieve a maximum link gain. This allows minimizingmagnetic field strength.

q. (6), the optimal number of turns N can be deter-rge N results in a large Ls2 and consequently a large

J. Coosemans et al. / Sensors and Actuators A 130131 (2006) 4853 51

Fig. 7. Link gain as a function of the number of turns of the secondary coil.

illustrates tsurements

Wirelesthrough thetransmissiorithm to theof the drivedata and bymented basampled at

After thextends upcoil that crange is noimplementpurpose, thage regulatThis voltagIn case thee.g. due tothe transmthe class-E

Fig. 9. Detail of the backside of the baby suit, showing the embroidered coiland the flexprint.

totype fabrication

elecss tra). T

nate0ms. Aknitthe

ive pe priesult

inothinn thto te ele

d.seco

uit (sbabyd frohe good agreement between the simulations and mea-of the inductive link properties.s bi-directional data communication is provided

same inductive link as the power transfer. The datan to the implant is used to adapt the measuring algo-patients specific needs and is done by on/off keyingr. The uplink to the receiver transmits the measuredload modulation. This passive telemetry is imple-

sed on the proven techniques in [11]. The ECG is300 Hz. The transmission data rate is at 16.5 kbit/s.e inductive link optimization, the telemetric rangeto an axial distance of 18 cm from the external

an be placed under the mattress. As the maximalt always demanded, an automatic power control is

ed to reduce unnecessary power dissipation. For thise induced voltage is measured at the input of the volt-or that regulates the supply voltage to 5 V (see Fig. 1).e is sampled at 10 Hz and sent to the external unit.induced voltage drops below 6 V or rises above 8 V,movement of the baby, the external unit will correctitted power level. To this end the supply voltage ofdriver can be digitally controlled from 2 to 24 V.

4. Pro

Allwirele(Fig. 6a lamifoil (7procescan be

Forinductflexibllayer rknittedmal clbetweenectedway, thwashe

In ababy sof thereduce

sists of 19

Fig. 8. Baby suit prototype. Left: inside showing the Textronics for the sensor interface, data processing andnsmission are mounted on a flexible printed circuit

he substrate material (Dupont LF9210) consists ofstructure of Kapton polyimide (25m) and copper) and is patterned by a standard photolithographic

s the flexible printed circuit is highly compliant, itted in clothing without a reduction in comfort.ease of fabrication, in a first prototype, the coil forowering and data transmission is integrated on thented circuit as well (see Fig. 6). The thick coppers in a high quality coil (Table 1). The Textrodes arean elastic belt, which can be worn under the nor-g. The elasticity of the belt assures a good contact

e body and the electrodes. The flexible circuit is con-he electrodes by means of three press-studs. In thisctronics can easily be removed when the belt is to be

nd prototype, the secondary coil is embroidered on aee Figs. 8 and 9). This further enhances the comfort, as the size of the non-breathing Kapton film can bem 10 cm 10 cm to 5 cm 5 cm. The coil wire con-m ( 79m) stainless steel yarns with a coppertrodes; right: backside.

52 J. Coosemans et al. / Sensors and Actuators A 130131 (2006) 4853

ight: baby suit prototype, worn by a 21 weeks old baby.

Table 2Measurement

Ls2 (H)Qs2 (at 132 kHk (%)Vsec/V1

Embroidereda Eq. (6).

core and hthe coil wir

The condered in ainterconnecelectrodes.

For thisnot feasibldering techlated valueprototype.

5. Results

The proA demo seold baby iin this set-applied. Fia 21 weektotypes arethe same selectrodessured withdrift, andconventiontrodes, thebeat-to-beaapplication

1. ECG measured in set-up of Fig. 10 (left). Raw non-filtered data.Fig. 10. Left: demo set-up with belt prototype, worn by a 12 weeks old baby. R

s and simulated values of the inductive link properties

Simulated Measured

12.7 13.7z) 18.3 17.2

3.33 3.20.34a 0.26

coil.

as a low resistance of 0.20 /m. The extremities ofe are directly soldered to the flexible circuit.nections from the circuit to the electrodes are embroi-similar way. Contact between the electrodes andt wires is achieved by knitting the wire through the

Fig. 1type of coil, the optimal number of turns (N = 33) ise, as the winding density is limited by the embroi-nique. Table 2 compares measurements with simu-s of the inductive link properties for the baby suit

totypes described above were successfully tested.t-up with the belt prototype, worn by a 12 weeks

s depicted in Fig. 10 (left). A raw ECG measuredup is shown in Fig. 11. No signal processing wasg. 10 (right) shows the baby suit prototype, worn bys old baby. The measurement results of both pro-

similar. Fig. 12 depicts an ECG measured withet-up as in Fig. 11, except for using conventionalinstead of the Textrodes. The ECG signals mea-the Textrodes display more low frequent base line

the signal-to-noise ratio is lower than when usingal gel electrodes. Nonetheless, even with the Tex-signal quality is more than adequate for an accuratet detection of the heart rate, as demanded by the.

Fig. 12. ECGconventional

6. Compa

The fielmonitoring[2], the UnMyHeart cborn are unmeasured with the same set-up as in Fig. 11, except for usingelectrodes instead of Textrodes.

rison with other monitoring suits

d of smart clothing is evolving rapidly: ambulatorysuits are being developed by the University of Pisaiversity of Karlsruhe [3], Vivometrics [4] and the

onsortium [5]. More specifically, SIDS suits for new-der development by Sensatex [6] and Verhaert [7].

J. Coosemans et al. / Sensors and Actuators A 130131 (2006) 4853 53

The novelty of the system, presented in this paper, lies in the levelof integration into textiles. Whereas few of the other systems alsouse textile sensors, none of them has an integrated solution forwireless communication and powering. In general, the instru-mentation electronics, together with a battery and a wirelesstransceiver, are packaged in a box that can be plugged to theshirt and that is to be removed before washing. By implement-ing inductive powering over a textile coil, we could downsizethis box to a single flexible circuit that can be rigidly con-nected to tfurther minSo althougmachine w[12] the mi

7. Conclu

The pressensors an

bined withthese develdimension.tors of wireminiaturizavalidation.

Acknowled

Part ofof the Intment of TexGhent Univtion of Inno(IWT). Betheir suppofor Scientifi

Reference

[1] D. MarcKhosla,Kirstein,Electronof the IE

[2] D. DeScilingodevices,

[3] J. Ottenbtion ofof the E(ISWC0

[4] M. Coylthe Seco

on Microtechnologies in Medicine and Biology, Madison, WI, USA,May 24, 2002, pp. 297300.

[5] MyHeart, 2005, http://www.hitech-projects.com/euprojects/myheart/.[6] Sensatex, 2005, http://www.sensatex.com.[7] Verhaert, 2005, http://www.verhaert.com.[8] M. Catrysse, R. Puers, C. Hertleer, L. Van Langenhove, H. Van Egmond,

D. Matthys, Towards the integration of textile sensors in a wirelessmonitoring suit, Sens. Actuators A 114 (2004) 302311.

[9] J. Webster, Medical Instrumentation and Design, Houghton Mifflin Com-pany, Boston, 1978, pp. 273303.

Kamind

hnol.Coosetem,Solid15, 2

Linz,ceediny, Ju

phi

ooseree in2001Leuv(Be

His ms, da

ermaree in

his MCurrathol

His msionems.

Puern eleke U

. He ind Dihnolocours

ulatio. Hisnd stime, acc

packatibleal dag and

thestemsor pahe fabric. In the future, the size of this flex can beimized by integration of the electronic components.h a large effort remains to make the complete suitashable, a solution comes into sight, e.g. by moldingniaturized electronic module to the fabric.

sion

ented system has proven the feasibility of integratingd electronics in textiles for measuring ECG, com-wireless powering and data communication. Throughopments the comfort of the patient has entered a newMoreover, this is one of the first working demonstra-less intelligent textiles. Future work will focus ontion of the electronics, packaging and further clinical

gements

this research was conducted within the frameworkellitex project, in collaboration with the Depart-tiles and the Department of Pediatrics and Genetics,ersity, and sponsored by the Institute for the Promo-vation through Science and Technology in Flanders

kintex NV and ZSK GmbH are acknowledged forrt. J. Coosemans is a research assistant of the Fundc ResearchFlanders (Belgium).

s

ulescu, R. Marculescu, N.H. Zamora, P. Stanley-Marbell, P.K.S. Park, S. Jayaraman, S. Jung, C. Lauterbach, W. Weber, T.D. Cottet, J. Grzyb, G. Troster, M. Jones, T. Martin, Z. Nakad,

ic textiles: a platform for pervasive computing, in: ProceedingsEE, vol. 91, 2003, pp. 19952018.

Rossi, F. Carpi, F. Lorussi, A. Mazzoldi, R. Paradiso, E.P., A. Tognetti, Electroactive fabrics and wearable biomonitoringAUTEX Res. J. 3 (2003) 180185.acher, S. Romer, C. Kunze, U. Gromann, W. Stork, Integra-

a bluetooth based ECG system into clothing, in: Proceedingsighth IEEE International Symposium on Wearable Computers4), Arlington, VA, October 31November 3, 2004, pp. 186187.

e, Ambulatory cardiopulmonary data capture, in: Proceedings ofnd Annual International IEEE-EMBS Special Topic Conference

[10] M.3-DTec

[11] J.syson

12[12] T.

Proma

Biogra

Johan CMS deg2001. InversiteitFlandersa PhD.interface

Bart HBS degreceivedLeuven.of the Kdegree.transmising syst

Robertdegree iKatholiein 1986gium, acuit Tecteachesand stimand ITtoring a(pressurchining;biocompdirectionreviewinshops inmicrosysensorson, M.J. Tsuk, J. White, FASTHENRY: a multipole-accelerateductance extraction program, IEEE Trans. Microwave Theory42 (1994) 17501758.mans, R. Puers, An autonomous bladder pressure monitoring

in: Digest of Technical Papers of the 18th European Conference-State Transducers (Eurosensors XVIII), Rome, Italy, September004, pp. 211212.C. Kallmayer, Integration of microelectronics into textiles, in:ngs of the Avantex Symposium 2005, on CD, Frankfurt, Ger-ne 79, 2005.

es

mans was born in Wilrijk, Belgium, in 1978. He received hismicroelectronics from the Katholieke Universiteit Leuven in

, he joined the ESAT-MICAS Laboratory at the Katholieke Uni-en as a research assistant of the Fund for Scientific Research-

lgium) (F.W.O.-Vlaanderen). He is currently working towardsain research interests are: pressure sensors, low power sensor

ta acquisition systems and biotelemetry.

ns was born in Vilvoorde, Belgium, in 1977. He received hiselectro-mechanics from K.I.H. Denayer in 1999. In 2001, heS degree in electrical engineering at the Katholieke Universiteit

ently, he is a research assistant at the ESAT-MICAS Laboratoriesieke Universiteit Leuven, where he is working towards a PhDain research interests are inductive powering, low power data

for biomedical applications and intelligent implantable monitor-

s was born in Antwerpen, Belgium, in 1953. He received his BSctrical engineering in Ghent in 1974 and his MS degree at theniversiteit Leuven in 1977, where he received his PhD degrees currently professor at the Katholieke Universiteit Leuven, Bel-rector of the Clean Room Facilities for Silicon and Hybrid Cir-gy at the ESAT-MICAS Laboratories of the same University. Hees of microsystems and sensors, biomedical instrumentationn, technology for hospitals and electronics, automatization

main research interests are: biomedical implant systems (moni-ulation), transducer and actuator principles, mechanical sensors

elerometers, strain gauges, temperature, etc.), silicon microma-aging and interconnection techniques (hybrids, hermetic andsealing, etc.) and biotelemetry (inductive power delivery, bi-

ta communication). Prof. Puers is involved in the organization,publishing activities of many conferences, journals and work-

field of biotelemetry, sensors, actuators, micromachining and. He is the author of more then 250 papers on biotelemetry,ckaging in journals or international conferences.

Integrating wireless ECG monitoring in textilesIntroductionECG measurementPower and data transmissionPrototype fabricationResultsComparison with other monitoring suitsConclusionAcknowledgementsReferences