The Effect of Tissues in Galvanic CouplingIntrabody Communication

Behailu Kibret, MirHojjat Seyedi, Daniel T. H. Lai, Mike Faulkner

Faculty of Health, Engineering and Science, Victoria UniversityBallarat Road, Footscray, VIC 3011, Australia

behailu.kibret@live.vu.edu.aumirhojjat.seyedi@live.vu.edu.au

daniel.lai@vu.edu.aumike.faulkner@vu.edu.au

AbstractIntrabody Communication (IBC) is a technique thatuses the human body as a transmission medium for electricalsignals to connect wearable electronic sensors and devices. Un-derstanding the contributions of tissues for signal transmission inIBC, which requires a good understanding of dielectric propertiesof biological tissues, paves a way for practical implementationof IBC in Body Sensor Networks. Presently, there is a lackof accurate and clear analysis on how the different tissuesaffect signal propagation through human body. In this work,we introduce a simple and efcient approach to understandthe inuence of different tissues on signal propagation throughhuman body. In this approach, we propose a single Cole-Coledispersion for the dielectric spectrum of four tissues (skin, fat,muscle, and cortical bone) for the frequency range of 100 KHz to10 MHz. We measured gain and phase shift of galvanic couplingtype IBC on the upper arm of three subjects. It was found thatfor the given frequency range, the impedance magnitude of skindecreases quickly whereas impedances of the other tissues wereless affected. In a similar manner, the impedance phase angleof skin is up to 60 degrees larger than that of the other tissues.From the measurement, we observed that these characteristicsof skin highly affect the measured gain and phase shift. As aresult, we infer skin mainly affects signal propagation throughthe human body in galvanic coupling.

I. INTRODUCTION

The accomplishments in technology of telemedicine andbody sensor networks contribute to the fulllment of thevisions in pervasive healthcare systems [1]. In body sensornetworks, the short range wireless communication betweenbiomedical sensors is mainly obtained using the commonlyused radio frequency based wireless links, like Bluetooth andZigbee. These protocols are designed for communications atdistances of several tens of meters by radiating electromagneticenergy into the air; hence, they intrinsically require excessivepower [2], [3]. As an alternative, a new method of wirelessdata transmission that uses the human body as transmissionmedium, or Intrabody Communication (IBC), was rst pro-posed for Personal Area Network (PAN) by Zimmerman [4].This technique uses near-eld and electrostatic coupling ofsignals; consequently, low frequency communication withoutelectromagnetic radiation can be achieved that potentiallyleads to reduced power consumption.

Generally, there are two approaches of IBC, namely, capac-itive coupling and galvanic coupling. In capacitive coupling,the signal is transmitted through the human body; and areturn path is formed by the capacitive coupling between thetransmitter and receiver ground electrodes through the externalenvironment. In this approach, the transmission quality isaffected by the external environment and size of the receiverground planes [5]. In galvanic coupling, the signal is applieddifferentially between two transmitter electrodes and receiveddifferentially by two receiver electrodes [6]. The signal in thegalvanic coupling approach is conned within the body as itis transmitted from a pair of transmitter electrodes to a pairof receiver electrodes, and therefore, is not affected by theexternal environment [7].

Many studies have been devoted to the development ofIBC test modules and simulations, rather than characterisingthe human body as a signal communication channel. Char-acterization of the human body as transmission channel wasattempted by [8] using the Finite Element Time Difference(FETD) approach; but the body impedance components wererepresented with capacitor elements for reasons of simplicity.In their study, the dielectric property of the whole humanbody, based on the assumption that it is composed of ahomogenous material, was considered; hence, the inuence ofdifferent tissues was not analysed in detail. In a similar studyby the same authors [7], a cylinderical human arm phantomwas built from elongated insulator holding conductive liquid(0.9% physiological saline), which has a similar chemicalcomposition as body uid, for the purpose of characterisingsignal propagation through the human body. The complexityof polarization mechanisms in human body, which give riseto the intricate dielectric properties of tissues (dispersion),was modeled by the saline solution due to the similarity ofthe gain prole between the human body and the phantom.Unfortunately, this does not give sufcient information onhow the signal is affected by different tissues of the body.In a recent study [9], attenuation and dispersion of signalsin IBC was investigated based on the assumption that signalpropagates through the skin; however, the assumption was notjustied with further analysis. A more comprehensive study

978-1-4673-5501-8/13/$31.00 2013 IEEE IEEE ISSNIP 2013318

of the effect of tissues in galvanic coupling IBC was reportedin [10]; a nite-element model was developed to simulate thepotential distributions in concentric cylinderical representationof the human upper arm. The inuence of tissues was studiedby observing the output response of the simulation by varyinginput parameters representing dielectric properties of sometissues.

II. MODEL FOR DIELECTRIC SPECTRUM OF TISSUESThe dielectric properties of biological tissues and cell sus-

pensions determine the pathways of current ow through thebody; for this reason, they are very important in the analysisof IBC. The response of a tissue to an applied electricalstimulation is analysed based on its specic conductivity andrelative permittivity. Since there is a variety of cell types andcell distributions inside a tissue, the microscopic description ofthe response is highly complicated to model. Consequently, weused a macroscopic approach to characterize eld distributionin the human body.

In this work, the frequency range selected for the investi-gation of the transmission medium in galvanic coupled IBCis from 100 kHz to 10 MHz, which is mainly -dispersion inbiological tissue dielectric spectrum. We set the lower boundof the frequency range, which is well above the spectrum ofbiological signals, based on the low frequency limitation of themeasuring device we used. The upper frequency bound wasselected due to a restriction of using a static circuit modelat higher frequencies as the wavelength of the signals getscomparable to the dimension of human body. Moreover, athigher frequencies, signals are not conned within humanbody due to the human body antenna effect [11]. In our study,we investigated signal propagation in human upper arm, whichhas smaller dimension compared to the whole body; thus,we assumed the human body antenna effect at the highestfrequency is negligible.

For the given frequency range, the contributions of -dispersion and -dispersion in the dielectric spectrum arevery small; therefore, we propose a model for the dielectricspectrum of tissues using a single Cole-Cole dispersion withparameters given by Gabriel dispersion relation [12] as thesecond Cole-Cole dispersion. From the proposed model, theexpression for complex relative permittivity (r) as a functionof angular frequency () is given as

r() = r() j

r ()

= +n

1 + (jn)1n +

ij0

(1)

where n = 2 represents the second dispersion region inthe Gabriel dispersion relation,

r and

r are the real and

imaginary parts of r(), n refers to the strength of thedispersion, is permittivity at innite frequency, n is therelaxation time constant, n is distribution parameter thatcontrols the width of the dispersion, i is the static ionicconductivity, and 0 is permittivity of vacuum.

The complex conductivity can be calculated from (1) as

() = () + j

() = j0

r() (2)

where and

are the real and imaginary parts of ().

Due to the simple geometry of the upper arm, we choseit as the medium of transmission to study galvanic currentcoupling. We assumed the major tissues of the upper arm tobe skin, subcutaneous fat, muscle, and cortical bone. In thisstudy, we investigated these tissues based on their dielectricand anatomical characteristics to calculate their impedance andthus their contributions to the pathways of current ow ingalvanic current coupling.

In addition, we took into consideration the anisotropic prop-erties of muscle tissue, which was not previously consideredin other IBC analysis. The anisotropy of muscle tissue ismore pronounced in the -dispersion [13], which is out ofthe frequency range we are interested in. Even though thedielectric data on muscle tissue are the most abundant inthe literature, most of them are limited to lower frequencies.Moreover, due to frequency dependence of anisotropy, forfrequencies in MHz range, the anisotropic properties of muscletend to become insignicant [13]. Consequently, we used theCole-Cole dispersion parameters in [12] for both longitudinaland transversal dielectric property of muscle tissue.

Since the fundamental processes of charge build up andconduction in tissue occur in parallel [13], we propose asimple two-component equivalent circuit that represents theadmittance Y of tissues, which is a parallel combination ofconductance G and susceptance B, as shown in (3). For thismodel, we assumed homogenous dielectric properties of tissue;thus, the admittance can be represented in terms of specicconductivity and relative permittivity, which are estimated by(1) and (2), respectively.

The complex admittance is given as

Y = G+ jB (3)

and frequency dependant conductance and susceptance are

G() = K() = K0

r () (4)

B() = K() = K0

r() (5)

where K is a value that depends on the geometry of tissues andlocation of measurement electrodes. For simple homogenouesgeometrical volumes, K is ratio of cross-sectional area tolength.

III. IMPEDANCE CHARACTERISTICS OF TISSUES

Galvanic signal coupling follows the method of couplingalternating current into the human body. A simplied rep-resentation of this method is shown in Fig. 1. The signalis applied differentially over the two transmitter electrodes.The majority of the current ows through the shortest pathbetween the transmitter electrodes; and this current is depictedas primary ow. The remaining small part of current, whichis the secondary ow, propagates farther into the body due tothe conductive nature of human body. It suffers attenuationand delay due to the lossy dielectric nature of human body.This current contributes to the alternating potential difference

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picked up by the receiver electrodes. The potential differencegets smaller when the receiver electrodes are located fartheraway from the transmitter.

Fig. 1. Current ow paths in galvanic coupling. The primary ow is themajority of current; and the secondary current is small part of the current thatpropagates farther through the conductive medium of human body; and thiscurrent contributes to the potential difference detected by receiver.

We abstracted the human upper arm with concentric layersof four tissues to skin, subcutaneous fat, muscle, and corticalbone. Hence, each of the four tissues in the upper arm canbe represented by four distinctive impedances characterising aunique galvanic coupled set-up as shown in Fig. 2. The set-up depends on the geometry and location of the electrodes,namely, the inter-electrode and electrode pair separation dis-tance. The four impedances represent the primary and sec-ondary ow paths for a given set-up. In other words, theimpedances represent the loss and delay the current suffersowing through the two paths. The rst impedance, Zt1, is thetransverse impedance at the transmitting side; and it representsthe primary current ow that has wider current distributiondue to the majority of current owing on this path. Thesecond and third impedances, Zl1 and Zl2, are impedancesof the longitudinal forward and return path of the secondarycurrent, respectively. Assuming the locations of the electrodesare symmetrical along the longitudinal axis of the arm, Zl1 andZl2 are equal. The fourth impedance, Zt2, is the transverseimpedance between the receiver electrodes. It impedes thetransverse ow of the secondary current and the voltage dropacross this impedance is detected as potential difference bythe receiver.

The impedances are calculated using,

Z() =1

Y ()=

1

K0(r () + j

r())

(6)

where Y () is the complex admittance characterising a singlehomogenous tissue and calculated based on (3); and the valueof K is adjusted based on the geometrical dimensions ofa given set-up, width and depth of current distribution, andsubject-specic anatomical parameters like tissue thickness.

In order to investigate the effect of tissues in signal prop-agation, we studied the major tissues in the upper arm basedon their dielectric and anatomical properties. Consequently,the impedance characteristics of each tissue was compared inorder to see their contribution to current ow in the upper arm.

Fig. 2. The four impedance representation of primary and secondary currentow paths of single tissue. Zt1 represents the impedance of the primaryow path; Zl1 and Zl2 refer to the impedance of the longitudinal forwardand return path of the secondary ow, respectively; and Zt2 refers to thetransverse path of the secondary ow.

For simplicity and reason of comparison, we considered a unitcell volume (i.e., cube of side 1 cm) of each tissue; and as aresult, the value of K is reduced to the ratio of cross-sectionalarea to length, which is equal to 1 cm. Moreover, we assumeda uniform current distribution thoughout the unit cell. Fig. 3and Fig. 4 show impedance modulus and the correspondingphase angle, respectively, of each tissue calculated using (6).

0.1 2 4 6 8 10102

103

104

Frequency (MHz)

Impe

danc

e |Z

| (O

hm)

SkinMuscleFatBone

Fig. 3. Impedance magnitude |Z|, in , of tissues in the upper arm. Forreason of comparison, a unit cell of cube with side 1 cm is used for eachtissue. And the frequency range is between 100 kHz to 10 MHz.

IV. MEASUREMENT SET-UPIn vivo measurement of galvanic coupling IBC using the

human upper arm as medium of transmission was carriedout, in order to nd the effect of tissue impedances onsignal propagation. The measurements chosen for purpose ofcomparison are gain and phase shift of the upper arm toapplied sweep frequency from 100 kHz to 10 MHz.

The measurement set-up, shown in Fig. 5, is composed of abattery powered VNA ( miniVNA Pro, output impedance Zo=50 and input impedance Zi= 50 , frequency range 100kHz to 200 MHz, manufactured by Mini Radio Solutions),baluns (Coaxial RF transformers, FTB-1-1+, turns ratio ofone, manufactured by Mini-Circuits), and round pre-gelledself-adhesive Ag/AgCl snap single electrodes (1cm diameter,

320

0.1 2 4 6 8 100

20

40

60

80

Frequency (MHz)

Phas

e A

ngle

(deg

ree)

SkinMuscleFatBone

Fig. 4. Impedance phase angle (degree) as function of frequency for tissuesin the upper arm. The frequency range is between 100 kHz to 10 MHz.

manufactured by Noraxon). The VNA is set to sweep constantinterval frequency of range 100 kHz to 10 MHz in 49 pointswith 0 dBm output power, which is well below the safety limitset by International Commission on Non-Ionizing RadiationProtection (ICNIRP) [14]. The signal is coupled to the upperarm by signal electrode A1 and ground electrode A2 via port 1;and at port 2 the VNA detects the potential difference acrosssignal electrode B1 and ground electrode B2. The VNA isconnected with laptop, where VNA manufacturer providedsoftware runs to calculate the gain in dB (S21) and phase shiftin degrees based on the measured potential difference and theinput voltage. The block diagram of the measurement set-upis shown in Fig. 6.

Fig. 5. Measurement set-up for galvanic coupling IBC using the human upperarm as transmission path. The signal is generated on port 1 of the VNA andcoupled through electrodes at A1 and A2; and it is detected at port 2 via theelectrodes attached at B1 and B2.

For measurement, we used three male volunteers withanatomical and galvanic coupling set-up parameters shown inTable I. All subjects provided verbal consent to participate inthe study. During the measurement, the subjects are allowedto stand in a relaxed manner arms by the side to ensurethe current is conned within the arm by avoiding externalphysical contacts with the arm. The electrode pair A1-A2, seeFig. 5, were attched to location of deltoid muscle and nearthe upper head of bicep brachii, respectively; and electrodepair B1-B2 at location of brachialis and the lower head of

Fig. 6. Block diagram of the measurement set-up. B is half of the insertionloss of the baluns and cables; and H is gain (in dB) of human arm andelectrodes. Vin is the input voltage and Vout is the potential differencedetected.

the triceps brachii, respectivelySince the two ports of the VNA share a common ground,

we need to make sure that the current in the arm does notmake a path through the common ground. As a solution to thisproblem, we used baluns to isolate the current in human bodyso that we can have a galvanic coupled system representedby our simplied equivalent circuit. The baluns are located atthe transmitter and receiver of the VNA, which are port 1 andport 2, as shown in Fig. 5, respectively. Additionally, the turnsratio of the baluns we used is one so that impedance matchingwas obtained. However, the use of baluns in the measurementset-up introduces signal loss. Even though the effect of balunscan be eliminated by calibration of the VNA, the shape of thetest leads does not allow the use of the available known loadsthat are needed for calibration. Therefore, we measured theinsertion loss and phase shift of the baluns and test leads; andin Fig. 6, B is half of the insertion loss (in dB).

Let G be the gain calculated by the VNA; and assumingthe 50 output impedance of the VNA is transformed acrossthe balun (turns ratio is one) and included in H , which is thegain due to human arm and electrodes, then from Fig. 6,

G = 20 log(|Vout||Vin| ) (7)

and it can be written as G = 2B +H.And it follows that the measured gain due to the human

arm, electrodes, and impedances of the VNA is H = G2B,where B is half of insertion loss of baluns and cables; andall are given in dB.In a similar fashion, we substracted thephase shift due to baluns and cables from the measured phaseshift. Fig. 7 shows the measured gain H and Fig. 8 shows thecorresponding phase shifts.

V. RESULTS AND DISCUSSION

From Fig. 3, it can be seen that bone has large impedance;and it is located deep in the lowest layer covered by othertissues and farther from coupling electrodes. As a result, itscontribution to current ow is very small. Also, due to thehigh impedance of fat, its contribution to the longitudinal andtransverse ow path is negligible compared to muscle and skinthat have smaller impedances. Therefore, the transversal andlongitudinal impedances of fat can be approximated as opencircuit. When considering musscle, due to its low impedance,

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TABLE IANATOMICAL AND MEASUREMENT SET-UP PARAMETERS OF SUBJECTS

A1-A2 (cm) B1-B2 (cm) A-B (cm) PerimeterA (cm) PerimeterB (cm)

Subject 1 13.0 12.0 14.5 28.0 23.0Subject 2 14.5 12.5 11.0 32.0 29.2Subject 3 15.0 13.5 13.0 30.5 28.0

0.1 2 4 6 8 1090

80

70

60

50

40

Frequency (MHz)

Gai

n (d

B)

Subject 2Subject 3Subject 1

Fig. 7. Gain (dB) as a function of frequency. Gain measurement, H , for thethree subjects.

0.1 2 4 6 8 100

20

40

60

80

100

120

140

160

180

Frequency (MHz)

Phas

e Sh

ift (D

egre

e)

Subject 1Subject 2Subject 3

Fig. 8. Phase shift (degree) as a function of frequency. Phase shiftmeasurement for the three subjects.

intermediate location and larger dimension, it remains a po-tential current ow path in both longitudinal and transversedirection. Moreover, skin is located in direct contact withthe coupling electrodes; and its impedance falls as frequencyincreases. Thus, skin is another candidate path for current owin galvanic signal coupling.

For frequencies less than 1 MHz, the slope of measured gaintends to vary much among subjects. Over all, the gain increasesas frequency increases from 1 MHz to 10 MHz. From carefulinvestigation of the gain, we can see that it follows similarcharacteristics to the impedance of skin as shown in Fig. 3,for the given frequency range. Moreover, for frequencies lessthan 1 MHz, the phase shifts show differences as large as 160degrees. Fig. 4 shows that impedance of skin has larger phase

angle compared to that of other tissues.The increase in gain is attributed to the decrease in skin

impedance as shown in Fig 3. The impedances of othercomponents, like muscle and fat, are approximately constantfor the given frequency range; consequently, their contributionto the gain is approximately constant throughout the frequencyrange. At lower frequencies, the dielectric property of skinis dominated by the outermost layer of skin called stratumcorneum (SC), which is composed of dead and at skin cells.The dielectric property of SC depends on the state of thesupercial layers and the water content of the surroundingair in contact with skin [13]. The effect of SC vanishesas frequency increases; this phenomenon is highlighted bythe decreasing skin impedance. The phase angle of skin islarger than that of the other tissues (Fig. 4), which induced ahigher phase shift at the receiver. Therefore, we infer that thedielectric property of skin primarily dictates the characteristicsof the signal transmitted through the human body, in the caseof galvanic coupling IBC.

VI. CONCLUSION

We proposed a model for the dielectric spectrum of tissuesbased on a single Cole-Cole dispersion.We also proposed amacroscopic representation of tissues that characterizes tissuesbased on their impedance. We measured galvanic couplingIBC on the upper arm of three subjects. Finally, we analysedthe effect of tissues on signal propagation based on theirimpedance characteristics and measurement data. We foundout that skin tissue primarily affects signal propagation throughthe human body in the case of galvanic coupling IBC withinthe frequency range of 100 kHz to 10 MHz. A more profoundanalysis can be obtained by developing equivalent circuitmodel of the human body, which is based on the dielectricproperties of tissues that are represented by the dielectricspectrum and impedance proposed in this work.

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