ieee transaction on antennas and … · 2017-04-03 · a wideband circularly polarized conformal...

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 1 A Wideband Circularly Polarized Conformal Endoscopic Antenna System for High-Speed Data Transfer Rupam Das and Hyoungsuk Yoo Abstract—In this study, a conformal wideband circularly polarized (CP) antenna is presented for endoscopic capsule application over the 915 MHz Industrial, Scientific and Medical (ISM) (902–928 MHz) band. The thickness of the antenna is only 0.2 mm, which can be wrapped inside a capsule’s inner wall. By cutting meandered slots on the patch, using open-end slots on the ground, and utilizing two long arms, the proposed antenna obtains a significant size reduction. In the conformal form, the antenna volume measures only 66.7 mm 3 . A single- layer homogeneous muscle phantom box is used for the initial design and optimization with parametric studies. The effect of the internal components inside a capsule is discussed in analyzing the antenna’s performance, and to realize a more practical scenario. In addition, a realistic human body model in a Remcom XFdtd simulation environment is considered to evaluate the antenna characteristics, CP purity, and to specify the Specific Absorption Rate (SAR) limit in different organs along the gastrointestinal (GI) tract. The performance of the proposed antenna is experimentally validated by using a minced pork muscle phantom, and by using an American Society for Testing and Materials (ASTM) phantom immersed in a liquid solution. For measurements, a new technique applying a printed 3D capsule is devised. From simulations and measurements, we found that the impedance bandwidth of the proposed antenna is more than 20%, and with a maximum simulated axial ratio (AR) bandwidth of around 29.2% in homogeneous tissue. Finally, a wireless communications link at a data rate of 78 Mbps is calculated by employing link-budget analysis. Index Terms—capsule endoscope, circular polarization, in- gestible devices, ISM band, link-budget. I. I NTRODUCTION T HE endoscopic diagnosis system is essential in the medical field for its ability to properly diagnose and treat conditions in the digestive tract. In the traditional wired endoscopic procedure, only the upper 120 cm of the stomach and small intestine can be imaged, not the remaining parts of the colon and rectum. To examine these remaining parts, there is another diagnosis procedure called colonoscopy. Moreover, these types of system can induce discomfort or pain and can cause an infection (unless sterilized), because they are used multiple times on different patients. In addition, it is difficult to access regions of the twisted large intestine by means of wired endoscopy. Hence, capsule endoscope (CE) systems This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2016R1D1A1A09918140). R. Das and H. Yoo, Department of Biomedical Engineering, School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea. (Correspondence: [email protected]). Liver Stomach Large intestine Small intestine Capsule entry 1. Sensor electronics 2. Spacer 3. Batteries 4. Spacer 5. Antenna 6. CMOS camera 7. Lens 8. LED light 9. Lens holder 10. Optical dome Capsule exit 1 2 3 4 5 6 7 8 9 10 Fig. 1. Overview of capsule endoscopy and capsule details. were invented to overcome these restrictions [1-4], and they can perform endoscopy and colonoscopy simultaneously. Generally, a CE contains a small camera, a wireless IC transceiver, LEDs, batteries, an optical dome, and an antenna at dimensions of 26 mm × 11 mm [5]. The antenna system in the CE is used for telemetry [6]. Designing a suitable antenna for a CE is of a great challenge due to the small capsule size and the variable properties of the digestive organs. Wide bandwidth is also a required feature for high-resolution data transmission. Therefore, designing an implantable or ingestible antenna is currently attracting a lot of attention [7]-[13]. In the previously used capsule endoscopy products, the data rate of the images was a few megabits per second (Mbps) because of the limitations of the technology in this field [14]. However, recently developed High-Definition (HD) Complementary Metal-Oxide Semiconductor (CMOS) image sensors provide up to 30 fps at 1920×1080 pixels per frame and a data rate of 78 Mbps [15]. Furthermore, the selection of a proper operating frequency has received significant attention, as there are some popular bands available. For instance, the 402 MHz Medical Implant Communication Service (MICS) is a global license-free service that has small bandwidth, is improper for CE application. On the other hand, the 2.45 GHz Industrial, Scientific and Medical (ISM) band offers a larger bandwidth [16]. An endoscopic capsule benefits from a circularly polarized

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Page 1: IEEE TRANSACTION ON ANTENNAS AND … · 2017-04-03 · A Wideband Circularly Polarized Conformal ... By cutting meandered slots on the patch, using open-end slots on the ground,

IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 1

A Wideband Circularly Polarized ConformalEndoscopic Antenna System for High-Speed Data

TransferRupam Das and Hyoungsuk Yoo

Abstract—In this study, a conformal wideband circularlypolarized (CP) antenna is presented for endoscopic capsuleapplication over the 915 MHz Industrial, Scientific and Medical(ISM) (902–928 MHz) band. The thickness of the antenna isonly 0.2 mm, which can be wrapped inside a capsule’s innerwall. By cutting meandered slots on the patch, using open-endslots on the ground, and utilizing two long arms, the proposedantenna obtains a significant size reduction. In the conformalform, the antenna volume measures only 66.7 mm3. A single-layer homogeneous muscle phantom box is used for the initialdesign and optimization with parametric studies. The effectof the internal components inside a capsule is discussed inanalyzing the antenna’s performance, and to realize a morepractical scenario. In addition, a realistic human body modelin a Remcom XFdtd simulation environment is considered toevaluate the antenna characteristics, CP purity, and to specifythe Specific Absorption Rate (SAR) limit in different organsalong the gastrointestinal (GI) tract. The performance of theproposed antenna is experimentally validated by using a mincedpork muscle phantom, and by using an American Society forTesting and Materials (ASTM) phantom immersed in a liquidsolution. For measurements, a new technique applying a printed3D capsule is devised. From simulations and measurements, wefound that the impedance bandwidth of the proposed antennais more than 20%, and with a maximum simulated axial ratio(AR) bandwidth of around 29.2% in homogeneous tissue. Finally,a wireless communications link at a data rate of 78 Mbps iscalculated by employing link-budget analysis.

Index Terms—capsule endoscope, circular polarization, in-gestible devices, ISM band, link-budget.

I. INTRODUCTION

THE endoscopic diagnosis system is essential in themedical field for its ability to properly diagnose and

treat conditions in the digestive tract. In the traditional wiredendoscopic procedure, only the upper 120 cm of the stomachand small intestine can be imaged, not the remaining parts ofthe colon and rectum. To examine these remaining parts, thereis another diagnosis procedure called colonoscopy. Moreover,these types of system can induce discomfort or pain and cancause an infection (unless sterilized), because they are usedmultiple times on different patients. In addition, it is difficultto access regions of the twisted large intestine by means ofwired endoscopy. Hence, capsule endoscope (CE) systems

This work was supported by the Basic Science Research Program throughthe National Research Foundation of Korea (NRF) funded by the Ministry ofEducation, Science and Technology (2016R1D1A1A09918140).

R. Das and H. Yoo, Department of Biomedical Engineering, Schoolof Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea.(Correspondence: [email protected]).

LiverStomach

Large intestine

Small intestine

Capsule entry

1. Sensor electronics2. Spacer3. Batteries4. Spacer5. Antenna6. CMOS camera7. Lens8. LED light9. Lens holder10. Optical dome

Capsule exit 1 2 3 45

6 7 8 9 10

Fig. 1. Overview of capsule endoscopy and capsule details.

were invented to overcome these restrictions [1-4], and theycan perform endoscopy and colonoscopy simultaneously.

Generally, a CE contains a small camera, a wireless ICtransceiver, LEDs, batteries, an optical dome, and an antennaat dimensions of 26 mm × 11 mm [5]. The antenna systemin the CE is used for telemetry [6]. Designing a suitableantenna for a CE is of a great challenge due to the smallcapsule size and the variable properties of the digestive organs.Wide bandwidth is also a required feature for high-resolutiondata transmission. Therefore, designing an implantable oringestible antenna is currently attracting a lot of attention[7]-[13]. In the previously used capsule endoscopy products,the data rate of the images was a few megabits per second(Mbps) because of the limitations of the technology in thisfield [14]. However, recently developed High-Definition (HD)Complementary Metal-Oxide Semiconductor (CMOS) imagesensors provide up to 30 fps at 1920×1080 pixels per frameand a data rate of 78 Mbps [15]. Furthermore, the selection ofa proper operating frequency has received significant attention,as there are some popular bands available. For instance, the402 MHz Medical Implant Communication Service (MICS)is a global license-free service that has small bandwidth, isimproper for CE application. On the other hand, the 2.45 GHzIndustrial, Scientific and Medical (ISM) band offers a largerbandwidth [16].

An endoscopic capsule benefits from a circularly polarized

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 2

(CP) antenna due to the reduction of multipath distortionand improvement in bit error rate. There are a few studiesrelated to circular polarization of implantable antennas [5][17]-[19]. In most of these studies, high-frequency bands (1.4 GHzor 2.45 GHz band) are used for circular polarization. Liuet al. [17] discussed a CP microstrip patch antenna in theISM band, but it has a poor axial ratio (AR) and impedancebandwidth. The same research group furthered their own work,and a much improved AR and impedance bandwidth wereobtained by employing a helical antenna [18]. They alsodiscussed CP purity, and evaluated the communications linkby using an omnidirectional CP external antenna. Nonethe-less, higher frequencies (above 1 GHz), may cause increasedradiation loss due to liquid body tissues [20]. In addition,present leading wireless systems like ZigBee (IEEE 802.15.4),wireless local area networks (WLANs), and Bluetooth (IEEE802.15.1) operate in the 2.4 GHz ISM band and cause stronginterference with each other when located in the same place[21]. Besides these, the free space path loss increases withthe high frequencies, which can leave inadequate separationbetween the transmitter antenna and external receiver. Tomaintain safety in such exposures, Specific Absorption Rate(SAR) is also an issue at high frequencies. Hence, a frequencyband that can offer a small size, and a reasonable AR andimpedance bandwidth while adhering to the SAR safety limitis desired [16].

In this study, a wideband, thin (0.2 mm), conformal patchantenna with a volume of 66.7 mm3 is proposed for wire-less CE application in the 915 MHz ISM band. To achieveminiaturization of the proposed antenna, an angled meanderedslot on the patch and an open end ground slot are used.Circular polarization is obtained by wrapping the antennaaround an ingestible capsule, and by creating a helical shape[22]. The paper is composed as follows. In Section II, the flatgeometry of the proposed antenna is discussed, as well as theantenna’s conformal shape by using a standard 11 mm × 26mm capsule. Initially, the capsule antenna was simulated insidea 50 mm deep homogeneous muscle phantom using ANSYS,Inc.’s HFSS software without defining any inner capsulecomponents. Then, antenna performance in terms of flat andconformal versions is discussed, and circular polarization ofthe conformal antenna is analyzed by using vector currentdistributions. In addition, variations in AR performance withrespect to conformal antenna orientation are inculded. Theparametric study and optimized design of the proposed antennaare explored. Detuning of the proposed conformal antenna isalso observed by placing a metallic battery and other auxiliaryparts inside the capsule. Next, the CP purity and SAR of theproposed antenna are investigated by placing it in a realistichuman model in a Remcom XFdtd simulation environment. InSection III, to keep the antennas conformal shape, a specialarrangement inside a printed 3D capsule is considered. Theperformance of the conformal antenna is measured by usingminced pork, and by applying it in an American Society forTesting and Materials (ASTM) phantom containing a liquidsolution. Finally, approximation of a 78 Mbps communicationslink is calculated by using antenna link-budget analysis.

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 3

100× 100× 100 mm3

Antenna with capsule

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Muscle phantom at 915 MHz (εr = 55, σ = 0.948 S/m)

Capsule orientation yx

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Fig. 3. Homogeneous muscle phantom simulation setup for numericalcalculations in HFSS.

II. METHODOLOGY

A. Layout of the proposed antenna

The geometry of the proposed two-layer conformal antennais depicted in Fig. 2. This antenna has two long arms, andis generally a microstrip patch antenna with a coaxial feedof 0.3 mm in diameter located at the edge of one arm, asshown in Fig. 2(a). On the other arm, one slot is placed inthe ground, which introduces an additional parasitic element.In addition to this slot, there are two more open-end slots onthe ground surface near the two arms, which help in antennaminiaturization.

The radiating patch has a serpentine structure that is madeby placing different lengths of angled meandered slots at a49.5o angle against the direction of the arms. The detailsof slots configuration in the radiating patch are illustrated inFig. 2(b). To give the conformal shape, Rogers ULTRALAM3850HT (εr = 3.14, tanδ = 0.002) liquid crystalline polymermaterial is used as substrate and superstrate. A 0.1 mmthickness for this material was chosen. In the flat form, theantenna occupies a volume of 53.5 mm × 10 mm × 0.2 mm(107 mm3), and in the conformal shape the proposed antennaeasily fits into an 11 mm × 26 mm standard endoscopiccapsule with a volume of 66.7 mm3, or π × 10 mm × (5.42 -5.22) mm2, as seen in Figs. 2(c) and (d). The thickness of thecapsule is 0.1 mm, and the material is defined as biocompatiblepolyethylene (εr = 2.25, tanδ = 0.001) [18]. To tune theantenna, ground slot position P1 and the dimensions of S1,S2, and S3 can be varied. In addition, meandered slot (S4,S5, S6, and S7) dimensions as well as the slot angle (θ) canbe adjusted for tuning. The parametric study section discussesthe effect of P1 and slot angle θ on antenna performance.

B. Simulation environment

The proposed antenna arrangement was simulated by plac-ing it at a 50 mm depth inside a homogeneous muscle phantomwith dimensions of 100 mm ×100 mm ×100 mm, as visual-ized in Fig. 3. The muscle electrical properties were εr = 55,

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Fig. 4. Simulated reflection coefficient for the flat and conformal versions ofthe proposed antenna.

σ = 0.948 S/m at 915 MHz [23]. Primarily, this homogeneousmuscle phantom was considered for easy optimization of theantenna. For simplicity, the associated electronic componentswere initially ignored, and only the antenna was placed insidethe capsule, as seen in Fig. 3. The inside of the biocompatiblepolyethylene capsule is considered to be empty. However, theeffect of the electronic circuitry and batteries on the proposedantenna will be discussed in a later section. HFSS was usedfor homogeneous phantom simulations.

The reflection coefficient (|S11|) of the antenna in the flatform and in the conformal form is depicted in Fig. 4. Fromthis plot, we can see that, the conformal shape of the antennaimproves the performance as well as the bandwidth. This isbecause, in the folded version of the antenna, the two longarms introduce parasitic capacitance between themselves andthe radiating patch element as indicated in Fig. 4. These par-asitic elements are not present in the flat form of the antenna.The proposed conformal antenna has a -10 dB impedancebandwidth of 185 MHz (780–965 MHz).

Antennas produce circularly polarized waves when twoorthogonal field components with equal amplitude, but inphase quadrature, are radiated. The circular polarization ofthe proposed antenna can be realized by analyzing the surfacecurrent distributions in different phases, as shown in Fig. 5.For θ = 0o and 180o, the bulk of the surface current flowsin the two arms of the antenna but in an opposite directionin each phase, as shown in Figs. 5(a) and (b). In contrast,for θ = 90o and 270o, most of the surface currents followthe slots of the serpentine patch structure, and this patchlies in an orthogonal direction to the arm in the conformalform. Figs. 5(c) and (d) demonstrate the current distributionsat θ = 90o and 270o, respectively. The cumulative effects ofthese surface currents in different phases generate the circularpolarization. The axial ratio (AR) is shown in Fig. 6. CPradiation is usually characterized at an AR of 3-dB. Therefore,for simplicity, the y-axis ticks are normalized (multiples of3) to 3-dB for all AR calculations. From Fig. 6, we found

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 4

θ = 0o

θ = 180o

θ = 90o

θ = 270o

(a)

(b)

(c)

(d)

Arm

Patch

Fig. 5. Simulated vectors of surface current distributions of the conformalantenna at 915 MHz in HFSS.

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Fig. 6. Simulated normalized 3-dB axial ratio (in the main radiation direction)for flat and conformal versions of the proposed antenna in a homogeneousmuscle phantom.

that, the antenna has a 3-dB AR bandwidth of 222 MHz(900–1122 MHz) in the conformal shape, however, in theflat shape the antenna is linearly polarized. The proposedantenna has a peak gain of -19.4 dBi in a homogeneousphantom at 915 MHz. A closer look at Figs. 5(a) and (c)helps us to realize, the direction of the surface current thatflows from the lower arm at θ = 0o, and then see that theradiating patch at θ = 90o is counter-clockwise. Therefore, thesuggested antenna has left-hand circular polarization (LHCP)with cross-polarization discrimination (XPD) of -17 dB in themain radiation direction.

For further explanation of the circular polarization, thereare two cases to be considered. One is helix (in conformalform), and another one is a slot. It is well-known that when

the helix circumference is about one wavelength it radiatescircularly polarized wave. This mode of radiation called theaxial or beam mode, which may be dominant over a widefrequency range with the desirable pattern, impedance, andpolarization characteristics. On the other hand, by introducingthe perturbations along the diagonal directions of a squarepatch using arbitrarily shaped slots with a feed location alongthe orthogonal axis, two orthogonal modes with 90o phase shift(circular polarization) can be achieved [24]. Furthermore, slotscan be used for antenna miniaturization.

For the proposed antenna, at Ly = Lx = 10 mm, a square-patch is formed as shown in Fig. 2, and the patch has arbitrarydiagonal slots along with a feed in the orthogonal directions(i.e., in one of the arms). However, in the flat form, theseslots are not enough to produce the circular polarization (Fig.6). Hence, a helix form is introduced by giving a conformalshape of the proposed antenna, where, two long arms act astwo turns of the helix. This helix has a circumference of≈ 33 mm (π×diameter of the conformal antenna). As theproposed antenna placed inside a different dielectric materials(such as tissue, substrate/superstrate material), the effectivewavelength decreases and it can be estimated around 25∼30mm. Since, the circumference of the conformal antenna isalmost equal to the effective wavelength, therefore, it canradiates circularly polarized wave in the direction of maximumor main radiation. This maximum radiation direction is alwaysnormal to the surface of the slot, as visualized in 3D polar plotof Fig. 6. It is obvious from the Fig. 6 that the theta = 0o

is the main radiation direction for this particular orientationof the proposed antenna. According to the study [24], it isalso found that orthogonal modes are typically excited if slotsare placed at θ = 45o to the axis of the feed location (Fig.2). Therefore, parametric studies of the proposed antenna interms of different slot angle will be discussed in the followingsection.

C. Parametric Studies of the Proposed Antenna

Parametric studies of an antenna play an important role inoptimization, as well as for adapting the antenna to differentscenarios. For the proposed antenna, the important parametersare considered to be the ground slots, the radiating patch slotangle (θ), and the length of the two arms, as well as theradiating patch. A detailed study of each parameter is beyondthe scope of this paper. Nevertheless, variation of slot angle θ,which controls the antenna bandwidth, along with variation ofslot position P1 and patch dimensions Ly are discussed below.In addition, a dielectric permittivity variations also carriedout to extend the range of the antenna for different flexibledielectric substrates.

1) The Radiating Patch Slot Angle: Parametric studies ofthe proposed antenna in terms of different slot angle areillustrated in Fig. 7. The slot angle was varied from 45o to30o. The |S11| curve in Fig. 7(a) reveals that, the impedancebandwidth (-10 dB) of the antenna decreases as θ changesfrom 45o to 30o. The AR curve in Fig. 7(b) shows that, somepronounced peaks and dips exist when θ decreases from 45o.As a consequence, the AR bandwidth is also reduced with

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 5

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Fig. 7. Parametric studies of the proposed antenna with a different slot angleθ. (a) Reflection coefficients. (b) Normalized 3-dB axial ratio.

a reduction of slot angle θ (see Fig. 2). Therefore, for theproposed antenna, the slot angle has a significant effect onthe antenna bandwidth (impedance and AR) as compared tochanges or shifts in the resonant frequency. Moreover, circularpolarization purity (AR < 3 dB) at the desired resonantfrequency (i.e., 915 MHz) also deteriorates with a decreasingslot angle.

2) Variation in Ground Slot Position P1 and Patch di-mension Ly: Fig. 8 shows the parametric studies of theantenna with different slot positions, P1, as well as withdifferent patch dimensions, Ly . The proposed antenna has tworesonant frequencies, which can be confirmed from |S11| inFig. 8(a). By changing P1, the second resonant frequency ofthe reflection coefficient can be shifted, as seen in Fig. 8(a).However, the resonant frequency of the normalized 3-dB ARremained almost unchanged as we varied P1, as visualizedin Fig. 8(b). Therefore, circular polarization purity can bemaintained while changing P1. On the other hand, changesin patch dimension Ly alter both resonant frequencies of thereflection coefficient, as depicted in Fig. 8(c). There is a subtlechange in the resonant frequency of the AR owing to variationof Ly , as indicated in Fig. 8(d).

3) Variation in antenna dielectric permittivity (εr): RogersULTRALAM 3000 series utilizes highly temperature resistantliquid crystalline polymer (LCP) as dielectric film and εrvaries between 2.9 to 3.14, whereas polyamide substrate εr

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Fig. 8. Parametric studies of the proposed antenna with different (a), (b)ground slot position P1 and (c), (d) patch dimension Ly .

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IEEE TRANSACTION ON ANTENNAS AND PROPAGATIONS, VOL. –, NO. –, – 2016 6

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(b)

Fig. 9. Parametric studies of the proposed antenna with a different εr . (a)Reflection coefficients. (b) Normalized 3-dB axial ratio.

lies between 3 to 4. Hence, most common flexible substrateεr ranges from 2 to 4. Fig. 9 shows the variation of |S11|and AR values with respect to different εr. As shown in Fig.9(a), changes in substrate (as well as superstrate) permittivitysignificantly affects the antenna |S11| characteristics. FromFig. 9(b), it is obvious that the CP purity and AR bandwidthalso changes due to different εr. Hence, proper tuning isrequired based on the substrate used to achieve the optimumantenna performance. In this study, the antenna is optimallytuned for the Rogers ULTRALAM 3850HT material (εr =3.14, tanδ = 0.002).

Therefore, from the parametric studies of the suggestedantenna, we can conclude that, the slot angle (θ) has a sig-nificant effect on impedance and AR bandwidth, and circularpolarization purity degrades for θ < 35o. In addition, alter-ations in ground slot positions and patch dimensions result inchanges in the resonant frequency of the reflection coefficientby maintaining circular polarization purity. The permittivityof the antenna substrate material also has a major influenceon the antenna charateristics. Based on these simulations, weobtained a maximum impedance bandwidth (780–968 MHz)of 198 MHz (21.6%) and an AR bandwidth (900–1168 MHz)of 268 MHz (29.2%) for θ = 45o and P1 = 3.5 mm, and Ly

= 10 mm. The proposed capsule antenna can cover the entire902–928 MHz band, hence, these modified antenna parameterswere considered for the rest of the study. For the ingestible

0.6 0.7 0.8 0.9 1 1.1 1.2-35

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Axi

al R

atio

(dB)

(a)

(b)

d = 0 mmd = 1 mmd = 2 mm

Fig. 10. Performance of the proposed conformal antenna with electricalcomponents. (a) Reflection coefficients. (b) Normalized 3-dB axial ratio.

antenna case, a larger bandwidth than the allowed bandwidth isdesirable. Ingestible devices (such as capsule endoscope) willmove through different tissues while scanning the GI tract. Asthe electrical properties of each tissue differ from each other,therefore, antenna characteristics may also change. In addition,human tissue tends to change properties with age, therefore,having a larger bandwidth is an advantage [25].

D. Electrical Component Consideration for the Capsule En-doscope System

Up to now, the biocompatible polyethylene capsule wasconsidered to be empty. However, in a practical scenario, theinside of this capsule will contain electrical components, asshown in Figs. 1 and 2(c). Therefore, it is necessary to checkfor any coupling issue between the proposed antenna and theelectrical components. To do this, most of the internal partsof Fig. 1, such as the CMOS camera, the lens holder, andthe spacer are considered to be dielectric. The optical domematerial is assigned as a vacuum, and for the batteries PerfectElectric Conductor (PEC) material was selected. After consid-ering all the internal parts, the antenna gain was reduced from-19.4 dBi to -25.2 dBi and a detuning effect was also observed,as shown in Fig. 10. This detuning of the antenna occurredmainly due to the metallic batteries. When the batteries arekept near the antenna, severe reflection has been observed as

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55 cm

Stomach

Colon

Small Intestine

zx

y

Fig. 11. Three diemensional realistic human voxel model used to test theantenna performance.

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2-40

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0

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HFSS muscle phantomRemcom stomachRemcom small intestineRemcom large intestine

Ref

lect

ion

coef

ficie

nt, |

S 11|

(dB

)

Frequency (GHz)

Fig. 12. Simulated |S11| of the proposed antenna in homogeneous musclephantom in HFSS of Fig. 3 and in heterogeneous body model of RemcomXFdtd.

shown in Fig. 10(a). The AR curve in Fig. 10(b) shows that,some pronounced peaks and dips exist when d = 0 mm. Asa consequence, the AR bandwidth as well as the impedancebandwidth are also reduced when d = 0 mm. Nonetheless,if the batteries are placed farther away from the antenna, ata distance d = 1 mm, the effect is neutralized gradually, asdepicted in Fig. 10. Therefore, metallic components such asbatteries should be placed at a minimum distance of 1 mmfrom the antenna ground slot to reduce coupling. However,such a separation (1 mm) in a biomedical device is generallynot acceptable as these devices tend to be very small. Hence,strategic management of the capsule components is required,where conductive components should remain at a distance of1 mm from the ground slot.

E. Analysis of the Proposed Antenna in a Realistic HumanModel

So far, the homogeneous muscle phantom is used to analyzeand optimize the suggested antenna with different capsulecomponents. It should be pointed out that, a homogeneousphantom will also be used for measurements, and measuredresults will be compared with the simulated version for eval-uation. However, the human body is not homogeneous, and arealistic human body consists of various heterogeneous tissues.

0 20 40 60 80 100 120 140 160 180-36

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n (d

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R), dB

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R), dB

Axial ration (A

R), dB

Gai

n (d

Bi)

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n (d

Bi)

Maximum radiation line

6-dB line

(a)

(b)

(c)

2

0

3

5

6

0

3

5

2

0

2

3

6

4

Fig. 13. Simulated normalized 3-dB axial ratio (AR) and radiation patternsof the proposed antenna at 915 MHz in (a) stomach, (b) small intestine, and(c) large intestine.

Antenna locations Gain [dBi] Bandwidth (MHz) AR [dB] XPD [dB] 6-dB

beamwidthStomach -28.7 196.5 3.5 14.0 110o

Small intestine -35.7 184.7 6.3 9.3 138o

Large intestine -29.5 194.2 4.1 13.0 63o

TABLE IPERFORMANCE OF THE PROPOSED ANTENNA IN REMCOM XFDTD ENVIRONMENT AT 915 MHz

Therefore, it is very important to stretch this study into a morepractical human body model in the Remcom XFdtd simulationenvironment. The human model in Remcom has 39 differenttissue types [26]. Fig. 11 shows this model, along with theportion of this body [rectangular box] used for the simulation.The volume chosen for the simulation (60 cm × 34.2 cm × 55cm) is higher than the volume of the homogeneous phantom(Fig. 3), and the performance of the conformal antenna ischecked in the stomach, the small intestine, and the largeintestine (or colon). The depths of the implant are different,relative to the organs.

The simulated reflection coefficient is visualized in Fig.12 for various cases. The results indicate that |S11| of theproposed antenna is not affected due to the different tissueenvironments, and it can cover the 915 MHz ISM band. Theresults also demonstrate the accuracy of the homogeneousmuscle phantom model in Fig. 3, especially for |S11|.

The simulated radiation patterns and AR of the conformal

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Antenna locations Gain [dBi] Bandwidth (MHz) AR [dB] XPD [dB] 6-dB

beamwidthStomach -28.7 196.5 3.45 13.98 110o

Small intestine -35.7 184.7 6.3 9.3 138o

Large intestine -29.5 194.2 4.1 13 63o

TABLE IPERFORMANCE OF THE PROPOSED ANTENNA IN REMCOM XFDTD ENVIRONMENT AT 915 MHz

Antenna locations 1-g SAR power limit (mW)

10-g SAR power limit (mW)

Stomach 10.3 48.9Small intestine 7.6 30.5

Large intestine 8.4 46.7

TABLE IIMAXIMUM POWER FOR SATISFYING THE SAR STANDARD IN THE HUMAN BODY AT 915 MHz

antenna for three implant locations are depicted in Figs. 13(a),(b), and (c). AR was chosen in the direction of maximumradiation for each organ. The maximum radiation directionson the YZ-plane in the stomach, small intestine, and colonare 150o, 75o, and 50o, respectively. The maximum 6-dBbeamwidth is obtained in small intestine, which is more than120o. Table I compares the performance of these three implantpositions at 915 MHz. From Table I, it is clear that thepeak gain in the stomach and colon is higher than in thesmall intestine. This could be due to the fact that, the smallintestine (εr = 59, σ = 2.17 S/m) has higher conductivity(lossy components) than the large intestine (εr = 57.9, σ =1.09 S/m) and stomach (εr = 65, σ = 1.19 S/m) at 915 MHz.Because of the higher conductivity, antenna bandwidth andCP purity also deteriorate for the small intestine, as comparedto the colon and stomach. However, the small intestine offersthe highest 6-dB beamwidth of 138o from among the threedigestive organs. The antenna gain in all cases is lower than theHFSS-simulated homogeneous phantom model because of thehigher simulation volume in XFdtd. Nonetheless, the obtainedgains from Remcom XFdtd is more than acceptable [16].

Table II indicates the Specific Absorption Rate (SAR) studyof the proposed antenna inserted in three different humantissues. This study is required to maintain the safety exposurelimit, and to indicate the maximum allowable power to theantenna. The 1-g and 10-g SAR is limited to 1.6 W/Kg and 2W/Kg, respectively for the implantable antenna system [13].In order to meet the regulation, the allowed transmitter poweris 7.6 mW for 1-g SAR, and 30.54 mW in case of 10-g SAR(small intestine, Table II). Similar result also found in thestudy [18]. These values are much higher than the requiredtransmitter power (25 µW), therefore, SAR should not be aproblem for the proposed antenna.

III. EXPERIMENTAL MEASUREMENT

In most previous studies [12]-[19][27]-[28], the designedantenna was placed directly inside the phantom for mea-surement purposes. However, these experimental proceduresdid not completely mimic a practical application scenario. Inreality, the antenna is inside an endoscopic capsule that ishermetically sealed, and this capsule will be in contact withtissue. Therefore, to emulate a more realistic environment,a 3D capsule was designed to properly hold the proposedconformal antenna. The capsule arrangement is depicted inFig. 14. A wheel is used to maintain the conformal shape ofthe antenna. A hole in the capsule (top or bottom) was alsocreated so that coaxial cable can pass through it. The hole’sradius should be equal to the radius of the outer conductor ofthe coaxial cable.

capsule cover wheel

exploded view

antenna

Fig. 14. Design of the 3D capsule for measurement purposes.

ground

patch

superstrateconformal shape

wheel

capsule cover conformal antenna

conformal antenna inside a capsule for measurement

coaxial cable

(a) (b)

(c) (d)

Minced pork

Minced pork

Capsule

(e)

Capsule

ASTM phantom containing liquid solution (water, alcohol, salt)

wire

Fig. 15. (a) Fabricated proposed conformal antenna. (b) Printed 3D capsulealong with the conformal antenna. (c) Reflection coefficient measurementsetup in minced pork. (d) Radiation pattern measurement setup using mincedpork. (e) Experimental setup for reflection coefficient measurement in a liquidASTM phantom.

The fabricated conformal and the fabricated capsule ar-rangement for measurement are shown in Figs. 15(a) and (b),respectively. The capsule material is Acrylonitrile ButadieneStyrene (ABS), which has dielectric properties identical topolyethylene. The reflection coefficient measurement setup isshown in Fig. 15(c). Initially, minced pork was used as aphantom material. The experimental setup for the radiationpattern measurement is shown in Fig. 15(d).

After being swallowed by the patient, an endoscopic capsulewill pass through different tissues having different electricalproperties, as discussed earlier. Furthermore, the permittivityand conductivity of human tissue are frequency-dependent[29]. Therefore, a single phantom property that can be simpli-

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0.6 0.7 0.8 0.9 1 1.1 1.2-25

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Muscle Liquid phantom

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Muscle Liquid phantom

Measured in

Pork Muscle Liquid phantom

Fig. 16. (a) Simulated and measured reflection coefficients. (b) Far-fieldgain radiation patterns of the proposed conformal antenna in a homogeneousmuscle phantom and in the minced pork. Elevation (XZ plane), Azimuthal(XY plane).

fied for the electrical properties of body materials should beconsidered for measurements. The properties of this materialinclude a dielectric constant of 56.7 and a conductivity of 0.94S/m [29]. Consequently, an ASTM phantom [30] containingwater, alcohol (such as methanol), and salt (NaCl) is alsoconsidered for only |S11| measurement. The experimentalsetup is shown in Fig. 15(e).

The coaxial (feeding) cable may alter the antenna perfor-mance by giving rise to radiating currents on the outer part ofthe cable. In most of the cases, implantable antennas have onlya superstrate and the ground plane along with the cable are indirect contact with the experimental phantom. Owing to thesignificant losses of the phantom, currents present on the backside of the antenna ground plane are weakened, and restrict thecurrent to flow on the cable. Consequently, the feeding cablehas a negligible effect during measurement [27]. However, ifthe whole implantable antenna is encapsulated by a biocom-patible plastic to mimic subcutaneous implantation (similar toour study), then considerable change in antenna performancecan be found depending on the feeding techniques.

One important finding of the study [27] is, if the electricalsize (as compared the proposed frequency) of a biocompatible-encapsulated implantable antenna is significant, then the feed-ing cable role is not notable. In other words, the coaxialcable effect is of great importance at lower frequency suchas 400 MHz, where antennas are very small as compared towavelengths. In the proposed antenna case, the wavelengthis comparable to the antenna size (as described in sectionII. B), therefore, effect of the feeding techniques (contact or

Transmission (Implant)Polarization LHCP

Transmitter power, Pt (dBm) -4

Antenna gain, Gt (dBi) -28.7/-35.7/-29.5Transmitter antenna AR [dB] 3.5/6.3/4.1

Antenna polarization mismatch loss, PL (dB) 0.3/0.65/0.4Propagation

Distance, d (m) 1-10Free-space loss, Lf Distance dependent

Receiver (Base station)Receiver antenna gain, Gr (dBi) 2

Polarization LHCPReceiver antenna AR [dB] 1

Temperature (K) 273Boltzmann constant, k 1.38×10-23

Noise power density, No (dB/Hz) -203.9Signals quality

Bit rate, Br (Mbps) 78Eb/No (ideal PSK) (dB) 9.6

Bit error rate 1×10-5

Margin (dB) = AP – RP

TABLE IIIPARAMETER OF THE LINK BUDGET CALCULATION AT 915 MHz

1 2 3 4 5 6 7 8 9 1025

30

35

40

45

50

55

60

StomachSmall intestineLarge intestine

Distance, d (m)

Mar

gin

(dB

)

insulation) can be ignored. Nevertheless, to free the cablefrom phantom contact during measurement, additional wireswere inserted, as shown in Fig. 15(b). A comparison betweenthe measured and simulated reflection coefficients is shownin Fig. 16(a). This plot reveals that, the measured result inminced pork is almost identical to that of the simulated result,except for a slight shift in the second resonant frequency.However, a significant alteration in |S11| with the liquidphantom was observed, as compared to the simulated version.This frequency shift and mismatch may be due to the factthat the two layers of the antenna (superstrate and substrate),as shown in Fig. 15(a), were fabricated separately, and bothof these layers were combined by using an adhesive material.Therefore, there can be a thin air gap between these layers,and the properties of the adhesive material should also beaccounted for in the deviation. Moreover, during measurementin the liquid phantom, some liquid may also enter the capsule,which should account for the significant difference in thereflection coefficient. Nonetheless, both measured cases stillmaintain a satisfactory |S11| limit. The radiation pattern isvisualized in Fig. 16(b). In the simulation, the peak gain wasfound to be -19.4 dBi, and in the minced pork, it was around-26 dBi. The reduction in gain is reasonable, as the volumeof the experimental pork-muscle containment box was higherthan the simulated phantom box. In contrast, the shape of theradiation pattern remains identical in both the elevation andazimuthal planes. In addition, the measured gain is higher thanin most of the recent studies [16].

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Transmission (Implant)Polarization LHCP

Transmitter power, Pt (dBm) -4Antenna gain, Gt (dBi) -28.7/-35.7/-29.5

Transmitter antenna AR [dB] 3.45/6.3/4.1Antenna polarization mismatch loss, PL (dB) 0.14/0.53/0.21

PropagationDistance, d (m) 1-10

Free-space loss, Lf Distance dependentReceiver (Base station)

Receiver antenna gain, Gr (dBi) 0Polarization LHCP

Temperature (K) 273Boltzmann constant, k 1.38×10-23

Noise power density, No (dB/Hz) -203.93Signals quality

Bit rate, Br (Mbps) 5Eb/No (ideal PSK) (dB) 9.6

Bit error rate 1×10-5

Margin (dB) = AP – RP

TABLE IIIPARAMETER OF THE LINK BUDGET CALCULATION AT 915 MHz

1 2 3 4 5 6 7 8 9 1025

30

35

40

45

50

55

60

StomachSmall intestineLarge intestine

Distance, d (m)

Mar

gin

(dB

)

Fig. 17. Distance vs. margin in different implant locations at 915 MHz.

A. Wireless Communications Link Study of the Proposed An-tenna

To specify the range of the data telemetry link, a link-budget for wireless communications should be considered.This calculation is associated with different losses, such asfree-space loss (Lf ), antenna mismatch loss, polarization loss[31], antenna base station cable loss, etc. For a reliabletelemetry link, a CP antenna for base station is preferred asit will provide less polarization mismatch loss. For simplicity,an omnidirectional CP antenna (AR = 1 dB) is considered asa receiver with a maximum gain of 2 dBi. Then, based onTable I, the maximum polarization losses (PL) in the capsuleantenna placed in the stomach, small intestine, and colon arefound to be 0.3 dB, 0.65 dB, and 0.4 dB, respectively. Here, weconsidered the maximum polarization loss, as the orientationof the conformal antenna cannot be adjusted when the capsulescans the GI tract. The antenna mismatch loss is negelectedbased on Fig. 12, as the antenna has a good match in differenttissues.

The input power (Pt) to the implanted antenna is limitedto 25 µW (-16 dBm) [12]. The transmitter power level is avital section of the entire capsule endoscopic system. It is dueto the fact that the transmitter power will markedly affect thespeed or data rate of the transmitter. Likewise, high transmitterpower would upgrade the power utilization of the endoscopeand in this way lessen the working time of the endoscopesystem. As indicated in [32], power supply is a major concernin capsule endoscopy system integration. Most industriallyaccessible endoscopic capsules incorporate silver-oxide coinbatteries in the case shell that accommodate around 8–10 h ata voltage of 3 V at 55 mAh, with a normal power conveyanceof 20 mW. By considering this condition, the transmitter power(Pt) was chosen as -4 dBm for the link budget study.

The required antenna power (Rp) is computed as

Rp(dB) =Eb

No+ kTo +Br (1)

where Eb/No represents ideal phase-shift keying in dB, k isthe Boltzmann constant, To is the temperature (in Kelvin), andBr is the bit rate for transmission (in megabits per second).Here, we chose Br = 78 Mbps, as a capsule endoscope requires

high-speed data transmission. The available antenna power(Ap) is given as

Ap(dB) = Pt +Gt +Gr − Lf − PL (2)

where Pt is the transmit power (dBm), Gt is the gain of theproposed antenna (dBi), Gr is the gain of the receiving antenna(dBi), Lf relates the free-space loss (dB), and (PL) representsthe polarization mismatch loss (dB). If d (m) is the separationbetween the transmitter and receiver, then (Lf ) can be writtenas

Lf (dB) = 20 log10(4πd

λ). (3)

Imran et al. demonstrated that, an acceptable margin ofmore than 20 dB is required between Ap and Rp for reliablecommunications [33]. Table III lists the parameters used tocalculate Rp and the Ap. A distance versus margin plot in Fig.17 indicates that, 78 Mbps of data can easily be transmitted adistance of 10 m for different implant locations. The outcomedemonstrates that the communication is conceivable when theantenna is placed in different implant positions. To add more,there is an effective edge for better communication regardlessof the implant locations, unlike the study was done by C. Liuet al. [18]. The calculated distance is significantly higher thanthe previous studies [18]-[19], therefore, the proposed antennais a good fit for the telemetry in the endoscopic capsule devicesat 915 MHz. However, in reality, the antenna in the humanbody ought to have more positions and orientations. The linkbudget analysis is investigated here to initially assess thecommunication capacity of the proposed antenna and externalCP antenna.

IV. CONCLUSION

In this study, a circularly polarized antenna is developedfor a capsule endoscope system in the 915 MHz ISM band.The proposed antenna includes different slots in the groundand patch for miniaturization. These slots can also be usedfor tuning purposes. The circular polarization of the proposedantenna is visualized by using vector current distributions.Circular polarization purity is evaluated in terms of orientationof the capsule. Parametric studies of the antenna are discussedto obtain the optimum design, too. Antenna performance isalso checked by introducing electronic components inside thecapsule, and by placing the antenna in a heterogeneous tissueenvironment under Remcom XFdtd. It is found that, the an-tenna may couple with conductive components for a separationbelow 1.5 mm. From the XFdtd results, we observed that theantenna |S11| is less sensitive towards various tissues, how-ever, polarization purity changes in accordance with differenttissues. The conformal antenna has a maximum simulated gainof -19 dBi (homogeneous muscle) and a minimum gain of -35dBi (small intestine). The maximum simulated AR bandwidthis 29%, and impedance bandwidth is 21.6% in a homogeneousmuscle phantom. Experimental measurements were carried outby using minced pork and by exerting an ASTM phantomcontaining a liquid solution. Special arrangements for theexperiment were considered and discussed by using a printed

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3D capsule. From measurements, the maximum gain wasfound to be -26 dBi, and impedance bandwidth matched thesimulated results. Lastly, a wireless communications link forhigh-speed data transfer was calculated by applying link-budget analysis.

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Rupam Das received a B.Sc. in electrical andelectronics engineering from Chittagong Univer-sity of Engineering and Technology, Chittagong,Bangladesh, in 2011, and an M.Sc in biomedicalengineering from the University of Ulsan, Ulsan,South Korea, in 2013. He is currently workingtoward a Ph.D. in biomedical engineering at theUniversity of Ulsan, Ulsan, South Korea. His currentresearch interests include implantable antennas anddevices, wireless power transfer, metamaterial, andmagnetic resonance imaging safety.

Hyoungsuk Yoo was born in Gyeongsan, Korea, in1977. He received a B.Sc. in electrical engineeringfrom Kyungpook National University, Daegu, Ko-rea, in 2003, and an M.Sc. and Ph.D. in electricalengineering from the University of Minnesota, Min-neapolis, in 2006 and 2009, respectively. In 2009,he was a Postdoctoral Associate with the Center forMagnetic Resonance Research, University of Min-nesota. In 2010, he joined Cardiac Rhythm DiseaseManage ment, Medtronic, MN, as a Senior MRIScientist. He is currently an Associate Professor in

the Department of Biomedical Engineering, School of Electrical Engineering,University of Ulsan, Ulsan, Korea. His research interests include electromag-netic theory, numerical methods in electromagnetics, metamaterials, antennas,implantable devices, and magnetic resonance imaging in high magnetic fieldsystems. Dr. Yoo was awarded Third Prize for the Best Student Paper atthe 2010 IEEE Microwave Theory and Techniques Society InternationalMicrowave Symposium.