A Miniaturized Novel-Shape Dual-Band Antenna for ... · shape radiating patch, and open-ended slots in the ground ... and head phantoms. II. METHODOLOGY The aim of this paper is to

774 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 67, NO. 2, FEBRUARY 2019

A Miniaturized Novel-Shape Dual-BandAntenna for Implantable Applications

Farooq Faisal and Hyoungsuk Yoo , Member, IEEE

Abstract— In this paper, a miniaturized novel-shape dual-bandimplantable antenna operating in the industrial, scientific, andmedical bands (902–928 MHz and 2.4–2.4835 GHz) is devel-oped for battery-powered implants. The Rogers ULTRALAM(εr = 2.9 and tanδ = 0.0025) liquid crystalline polymer mate-rial with 0.1 mm thickness is used as both the substrateand superstrate. By employing the shorting strategy, a flower-shape radiating patch, and open-ended slots in the groundplane, the total volume of the proposed antenna is confined to7 × 7.2 × 0.2 mm3. In a homogeneous skin phantom, the pre-sented antenna has a maximum gain of −28.44 and −25.65 dBiat 928 MHz and 2.45 GHz, respectively. The calculated maximumspecific absorption rate values are in the safe limit and satisfythe IEEE C95.1-1999 and C95.1-2005 safety guidelines. Thedesign, optimization, and analysis of the suggested antenna areperformed using the finite difference time-domain- and finite-element method-based simulators. The results suggest that theflower-shape antenna exhibits fairly omnidirectional radiationpatterns; therefore, it can be used for gastro applications andskin implantations. Through wireless communication link, we val-idated that at both frequencies (928 MHz and 2.45 GHz), 7 Kb/sand 78 Mb/s of data can be transmitted easily over morethan 6 and 1.5 m, respectively. To verify the validity of thedesign and simulation results, measurements are performed bysubstituting the fabricated prototype in the American Society forTesting Materials and head phantoms containing saline solution.The proposed flower-shape antenna shows salient performanceparameters compared with the recently proposed antennas.

Index Terms— Dual-band, flower-shape, implantable antenna,specific absorption rate (SAR).

I. INTRODUCTION

IMPLANTABLE antennas are crucial in establishing thewireless link between the implanted device and the outside

base station. In-body communication offers several advan-tages, including telemetry, access to appropriate medicalrecords, and tracking of patients with implanted devices.Different implantable antennas integrated with differentdevices for various biomedical applications have beendesigned [1]. In biomedical telemetry, data transmission is

Manuscript received December 28, 2017; revised September 11, 2018;accepted October 27, 2018. Date of publication November 9, 2018; dateof current version February 5, 2019. This work was supported by theBasic Science Research Program, through the National Research Foun-dation of Korea, Ministry of Education, Science and Technology, underGrant 2016R1D1A1A099-18140. (Corresponding author: Hyoungsuk Yoo.)

F. Faisal is with the Department of Telecommunication Engineering, Uni-versity of Engineering and Technology, Taxila 47050, Pakistan.

H. Yoo is with the Department of Biomedical Engineering, HanyangUniversity, Seoul 04763, South Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TAP.2018.2880046

allowed between implantable devices and receivers at a certaindistance [2]. The in-body or implanted technology can beutilized in health monitoring to transfer information of thehuman body, such as cardiac rate, blood pressure, and tem-perature. With the development in implanted devices, wirelesshealthcare will be a reality. This will improve the monitoringaccuracy as well as replace the wired discomfort connectivityto the implant in the human body. Thus, beneficial informationcan be achieved for diagnosis and subsequent treatment.

The MedRadio band occupying the frequency spectrumof 401–406 MHz is often used for wireless implantabledevices. However, antennas operating in lower frequencybands cater for narrow bandwidths, thus leading to low datarates and low-resolution images [3]. Owing to the smallbandwidth, bulky size, low data rate, and inadequate reso-lution images in the MedRadio band, the higher industrial,scientific, and medical (ISM) bands (902–928 MHz and2.4–2.4835 GHz) are primarily preferred for certain wirelessmedical implants [4], [5].

The design guidelines for implantable antennas are reportedin [6]. Patch designs are primarily preferred for implantableantennas owing to their high flexibility in design and shape [7].In recent years, a number of implantable antennas havebeen proposed for telemetry applications. A circular polar-ized (CP) implantable antenna for wireless power transferhaving resonance at 915 MHz ISM band was designed onthe Rogers 3010 [8]. The antenna has the smallest volumeof 153.67 mm3, which is obtained by loading the stubs andthe interdigital capacitors among the stubs. This CP antennais set at 4 mm deep in a single-layer skin box of size100 mm × 100 mm × 50 mm. The simulated bandwidth is35 MHz, and the gain is −29 dBi at 910 MHz. A miniaturizedimplantable antenna of total volume 285.7 mm3 was proposedin [9]. A cube phantom of dimensions 100 mm × 100 mm ×60 mm assigned with human skin properties at 915 MHz wasused. The realized peak gain at 915 MHz is −27 dBi, and theachieved simulated bandwidth in the ISM band is 97 MHz(865–962 MHz). In [10], a broadband CP antenna excitedthrough a single fed was proposed for biomedical applications.An improvement of −10 dB bandwidth along the axial ratiowas observed by creating cross-shape slots in the ground plane.

Moreover, a triband implantable antenna with a serpentine-shape radiating patch was suggested in [11] for skin implants.The three paths on the serpentine-shape radiator are respon-sible for the three bands. The bandwidths at 405, 915,and 2450 MHz are 64, 91, and 105 MHz, respectively.In [12], a dual-band (402–405 MHz and 2.4–2.48 GHz)

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FAISAL AND YOO: MINIATURIZED NOVEL-SHAPE DUAL-BAND ANTENNA FOR IMPLANTABLE APPLICATIONS 775

implantable antenna for glucose monitoring was examined.The achieved simulated bandwidths for the MedRadio andISM bands were 20.4% and 4.2%, respectively, but the volumeof the antenna was large (1265.625 mm3). Another dual-bandantenna with serpentine-shape radiator encompassing MedRa-dio and ISM bands was designed in [13]. Although the gain issufficiently good at the ISM band, the profile of this antenna isstill large, and the corresponding bandwidth for the MedRa-dio band is small. Similarly, a single-band wide-beamwidthimplantable antenna operating at 2.45 GHz [14] and a novelimplantable antenna resonating in the MedRadio and ISMbands [15] were also proposed. In [15], the housing caseof an implantable device (pacemaker) served as an antennafor covering the targeted bands. However, the antenna islarge and exhibits a complex geometry; furthermore, the peakgain values at both bands are low (−31 and −30 dBi at403 and 433 MHz, respectively). Liu et al. [16] reported amulitlayer CP helical antenna for gastro applications. Thismultilayer design contains three open-ended loops at severallayers connected through via holes. However, the antennasuffers a low gain value of −32 dBi at 2.4 GHz and acomplex geometry with a large volume of π × (5.5)2 ×3.18 mm3. Similarly, a novel miniaturized implantable antennaencompassing multiple communication channels [17], a capac-itively loaded patch antenna for biomedical applications [18],and a compact dual-band (402–405 MHz and 2.4–2.48 GHz)antenna for medical implants [19] was proposed. Most ofthe aforementioned antennas are either limited by the rigidsubstrates (thicker) or incompatible with most the electronicdevices.

This paper presents a miniaturized novel flower-shape dual-band antenna operating in the ISM bands. This flower-shapedual-band antenna has a miniaturized volume of 10.08 mm3,achieved using a shorting pin, a flower-shape radiator,and open-ended slots in the ground plane. The suggestedantenna was initially designed and analyzed in the cen-ter of a homogeneous skin phantom (HSP) of dimensions100 mm × 100 mm × 100 mm. Subsequently, its perfor-mance was validated in a heterogeneous environment in thehead, stomach, and small intestine of a human body (realisticDUKE model [20]) with the finite difference time-domain-based simulator [6]. The specific absorption rate (SAR) valuesare calculated in the head. The communication link is validatedby calculating the link budget for three different implantlocations in two different scenarios. For further validationof the simulation results, measurements are conducted byimmersing the fabricated prototype in the American Societyfor Testing Materials (ASTM) [21] and head phantoms.

II. METHODOLOGY

The aim of this paper is to design, fabricate, and test a novelflower-shape dual-band miniaturized implantable antenna thatcan cover the ISM bands. The design, fabrication, and testingof implantable antennas present many challenges. An effectiveimplantable antenna demands biocompatibility, compact size,conformity, and good radiation performance.

Fig. 1. Geometry of the suggested antenna. (a) Front, (b) rear, (c) side, and(d) exploded views.

A. Geometry of the Flower-Shape Antenna

Fig. 1(a)–(d) portrays the front, rear, side, and explodedviews of the flower-shape antenna, respectively. It consists ofa superstrate, radiating element, substrate, and ground plane.The radiating element is backed by a 0.1 mm-thick RogersULTRALAM 3850HT (εr = 2.9 and tanδ = 0.0025) liquidcrystalline polymer (LCP) substrate. The LCP materials areextensively used for semiconductor packaging and the fabrica-tion of printed circuit boards [22], [23]. Effective permittivityof the material changes with variations in its thickness [24].The LCP materials are typically available in 0.025–3 mmthickness. Owing to the flexibility [25], biocompatibility [26],and FDA approval, the Rogers ULTRALAM, which is atype of LCP materials, is suitable for medical applications.A similar biocompatible material with the same thicknessis used for the superstrate. Therefore, the total volume ofthe flower-shape antenna is 7 mm × 7.2 mm × 0.2 mm(10.08 mm3). The radiating element of the flower-shapeantenna shown in Fig. 1 can be divided into three primaryparts. The upper flower-shape part, the U-shape part, andthe middle part comprise the rectangular strip and the twoidentical branches. The upper flower-shape part includes thefeed. A 50 � coaxial feed having a radius of 0.3 mm isused for the excitation of the antenna. The upper flower-shaperadiating part is designed by creating four semicircular smallslots in a circle of radius 1.7 mm. The middle part provides aconnection between the upper flower-shape and U-shape parts.In addition, the two branches having circles with triangularcuts in the middle part aid in impedance matching and tuning.The length (L) and width (W) of the two side strips of theU-shape part shown in Fig. 1(a) are crucial in tuning as well asin the impedance matching of the two resonance frequencies(particularly the 2.4 GHz ISM band). As L increases, bothresonance frequencies shift toward the left side, and vice versa.As W increases from 1.2 mm, both resonance modes move


Fig. 2. Initial simulation setup for the flower-shape antenna (Unit: mm).

toward the upper side of the frequency spectrum. Similarly,both resonance modes can be moved to the left side relativeto the modes corresponding to W = 1.2 mm by decreasing W.Thus, both resonance frequencies of the flower-shape antennacan easily be tuned from these two parameters.

A shorting pin (via) assists in the miniaturization by dou-bling the antenna size. Therefore, a shorting pin with diam-eter 0.6 mm is employed in the proposed design. Similarly,the current paths in the ground plane considerably affectthe performance of the antenna and provide the possibilityof miniaturization. Consequently, a slotted ground plane isattached to the back of the substrate. The slots in the groundplane increase the capacitance, thus causing the resonancemodes to move toward the lower side of the frequencyspectrum.

B. Simulation Environment

The proposed flower-shape antenna is initially designedand analyzed in the center of a HSP of dimensions100 mm × 100 mm × 100 mm, as shown in Fig. 2. Thepermittivity (εr ) and conductivity (σ ) values assigned to theskin phantom are εr = 41.33 and 38 and σ = 0.872 and1.45 S/m at frequencies of 928 MHz and 2.45 GHz, respec-tively [17], [27]. The antenna is set at the center point of theskin phantom, and the separation distance from the air to theantenna in this case is 50 mm. Similarly, the distance betweenthe each edge of antenna and radiation boundaries is greaterthan λo/4 at 928 MHz.

The proposed flower-shape antenna was obtained in foursuccessive steps, as demonstrated by Fig. 3. It is evident thatthe proposed flower-shape antenna is designed by modifyinga basic circular shape patch antenna. The modifications inthe circular-shape patch antenna resulted in different sizes oftriangular and circular cuts and slots. These cuts and slots ofspecific sizes are created to achieve better antenna performanceparameters. Fig. 4 illustrates the S11 (reflection coefficient)comparison for the four involved steps in designing the flower-shape antenna. It is obvious that the step-by-step modifi-cation improves the antenna performance in terms of S11.Two weak resonance modes with S11 < −3 dB appearedat 1.14 and 3.48 GHz in the first step. As mentioned earlier,

Fig. 3. Designing steps of the flower-shape antenna.

Fig. 4. Reflection coefficient (S11) comparison for the design steps.

Fig. 5. Effects of the ground slots on the reflection coefficient (S11).

the two branches and the U-shape part aid in impedancematching; therefore, the two resonance modes become dom-inant in steps 2 and 3. Apart from the increase in theresonance depth, the modifications in steps 2 and 3 shift bothresonance modes toward the left and right sides, respectively.By inserting the ground slots and a shorting pin, the tworesonance modes achieved in step 3 were successfully movedto the desired ISM bands in step 4 without increasing the sizeof the flower-shape antenna.

Fig. 5 shows the behavior of the flower-shape antenna interms of S11 as a function of the ground slots (S1 and S2).


Fig. 6. Effects of the branches angle on the reflection coefficient (S11).

It is noteworthy that by removing the ground slots hav-ing width S1, the proposed flower-shape dual-band antennabecomes a single band operating at 1.63 GHz. The −10 dBbandwidth at 1.63 GHz is 119 MHz with an S11 valueof −15 dB. Fig. 5 also reveals that by eliminating the groundslots having width S2, the resonance of the lower ISM bandshifts to 1.05 GHz, while that of the 2.4 GHz shifts to2.6 GHz. The bandwidths (−10 dB) at 1.05 and 2.6 GHz are188.6 and 236.5 MHz, respectively. Thus, Fig. 5 reveals theimportance of the ground slots for the suggested flower-shapeantenna.

C. Parametric Analysis

This section presents the parametric analysis of the proposedflower-shape implantable antenna. The parametric analysis ofan antenna facilitates in optimization as well as in adaptingthe best dimensional parameters for the antenna depending onthe different scenarios. For this parametric analysis, the sameHSP with the same dielectric properties and dimensions,as shown in Fig. 2, is utilized. Again, the antenna is setat the center point of the phantom to observe the impacton the reflection coefficient owing to the variations in theantenna designing parameters. For our proposed flower-shapeantenna, the important parameters to be considered for theparametric analysis are the angle (Ang) of the branches withthe rectangular strip, the length (L) and width (W) of the sidestrips of the U-shape part, and the width (S1 and S2) andlength (Ls ) of the ground slots, as shown in Fig. 1(a) and (b).A detailed study based on each parameter of the flower-shapeantenna is beyond the scope of this paper. However, basedon the aforementioned parameters, we have observed thatall the parameters have been selected satisfactorily and thesuggested flower-shape antenna can be easily tuned from theseparameters, as will be discussed in the following.

1) Variations in the Branch Angle (Ang): The parametricanalysis in terms of Ang with the middle strip is illustratedin Fig. 6. The Ang with the middle strip is varied from60◦ to 80◦. The reflection coefficient (S11) curves in Fig. 6reveal that both resonance frequencies shift linearly toward

Fig. 7. Effects of the length of the side strips of the U-shape part on thereflection coefficient (S11).

Fig. 8. Effects of the width of the side strips of the U-shape part on thereflection coefficient (S11).

the left side, as Ang decreases from 80◦ to 60◦. Moreover,variations in the return depth are observed as Ang variesfrom 60◦ to 80◦. Thus, we concluded that Ang is crucial inimpedance matching as well as in the tuning to the desiredfrequency bands.

2) Variations in the Side Strips Length (L) of the U-ShapePart: The influence of L of the side strips of the U-shape partof the radiating element is portrayed in Fig. 7. By varying L,both resonance frequencies can be shifted; however, we foundthat the 2.4 GHz ISM band is more sensitive to the variationsin L. Moreover, a third resonance appears at approximately2 GHz by decreasing L, and it becomes increasingly dominantwith a further decrement in L. This paper suggests thatL = 4 mm is an optimal choice for the flower-shape antennato encompass the two ISM bands.

3) Variations in the Side Strips Width (W) of the U-ShapePart: The W of the side strips of the U-shape part of theradiator is varied from 0.8–1.6 mm. Fig. 8 demonstrates thebehavior of the flower-shape antenna in terms of S11 as afunction of W. As shown, both resonance frequencies move


Fig. 9. Effects of the ground slot (S1) on the reflection coefficient (S11).

Fig. 10. Effects of the ground slot (S2) on the reflection coefficient (S11).

to the upper side relative to the return curve of W = 1.2 mmwhen W increases from 1.2 mm. We further found that S11decreases significantly in the 2.4 GHz band owing to variationsin W. Similarly, by decreasing W (W < 1.2 mm), the bandscan be moved toward the left side relative to the return curvefor W = 1.2 mm. It is noteworthy that variations in W exhibitmaximal effects on the 2.4 GHz band, as compared to thelower ISM band.

4) Variations in the Width of the Ground Slot (S1): Theeffects on the antenna performance in terms of the S11 owingto variations in S1 are shown in Fig. 9. Again, it is obvious thatthe upper ISM band is more sensitive to S1 compared to thelower one. It is evident from the graphs that when 0.2 mm ≤S1 ≤ 0.4 mm, the alteration in the antenna behavior is minimal.However, when S1 = 1 mm, the upper and lower resonancemodes shift to 2.17 GHz and 877 MHz, respectively. Bothresonance modes are matched adequately if S1 = 0.2 mm;therefore, we chose this setting.

5) Variations in the Width of the Ground Slot (S2):By increasing S2 in the range of 0.4 mm ≤ S2 ≤ 0.8 mm,the two resonances of the ISM bands shift toward the left

Fig. 11. Effects of the ground slot length (Ls ) on the reflectioncoefficient (S11).

side, while for S2 > 0.8 mm, a shift toward the right-handside relative to the curve corresponding to S2 = 0.8 mmis observed. Likewise, a significant reduction occurs whenS2 > 0.8 mm, as shown in Fig. 10. However, we found thatthe minimum S11 value for S2 > 0.8 mm also fluctuates about−10 dB and can provide sufficient bandwidth for operation.It is obvious that S2 = 0.8 mm is the best choice for tuningthe two ISM bands with optimum bandwidths.

6) Variations in the Length of the Ground Slot (Ls): Theinfluences of the ground slot with length Ls on the suggestedantenna behavior are also studied, as demonstrated in Fig. 11.The ground slot length (Ls) is varied from 1.8 to 2.4 mm.These S11 curves clearly show that both ISM bands can easilybe shifted by changing Ls . A shift toward the left-hand sidewas observed in the resonance frequencies with an incrementin Ls . Moreover, the reflection coefficient value at the secondresonance frequency decreases with increasing Ls . It is furthershown from Fig. 11 that both resonance frequencies arematched perfectly with Ls = 2.2 mm; therefore, we selectedthe setting thereof.

It can be summarized that both resonance bands of theflower-shape antenna can be controlled from the parameters,such as Ang, L, W, S1, S2, and Ls . We observed that W,S1, and S2 exhibit minimal effects on the lower ISM bands;however, they can operate as accessorial variables in tuningand matching for them. Moreover, we observed that both res-onance modes decrease and shift linearly owing to variationsin Ls , S1, and Ang, separately. The crucial parameters thatcontrol both operation frequencies are Ang and Ls , which areoptimized specifically for the desired frequencies.

III. RESULTS

As mentioned earlier, the proposed antenna was initiallydesigned and optimized in the center of the HSP in HFSS,as shown in Fig. 2. The dielectric properties of the HSP onboth ISM bands are frequency dependent. To examine the per-formance in a more practical and real scenarios, the proposedantenna is implanted in the head, small intestine, and stomach


Fig. 12. 3-D human voxel model employed for the performance validationof the flower-shape antenna.

Fig. 13. (a) Fabricated flower-shape antennas. (b) Reflection coefficient (S11)measurement in saline solution. (c) Radiation patterns measurement in salinesolution.

of a realistic model (adult male, 97 Kg) in a Remcom XFdtd-based simulator, as shown in Fig. 12. The human model avail-able in Remcom consists of 39 different types of tissues [28].The rectangular box in Fig. 12 shows the portion of the bodyof this model used for the simulations. The selected volume forthe simulation is larger, compared with the volume of the HSP(see Fig. 2). The SAR values are calculated in the head of therealistic model. A prototype of the flower-shape antenna wasfabricated, as shown in Fig. 13(a). Fig. 13(b) and (c) shows themeasured setups for the fabricated flower-shape antenna. TheASTM and head phantoms contain saline solution [29] with anapproximated permittivity = 41.2 and 37.8 and conductivity =1.22 and 1.44 S/m at 928 MHz and 2.45 GHz, respectively.

Fig. 14 compares the measured reflection coefficient withthe simulated one. These curves reveal that the measuredreflection coefficient in the ASTM phantom is almost indis-tinguishable from the simulated one, except for a slightdeviation in the resonance frequencies. This deviation inthe resonance frequencies may be attributed to the fact thatthe superstrate and substrate layers were obtained separately

Fig. 14. Simulated and measured reflection coefficient (S11).

Fig. 15. Reflection coefficient (S11) comparison in different simulation andreal environments.

during fabrication. The superstrate layer is combined withthe substrate by utilizing an adhesive material; therefore,a small air gap may be present between these two layers.Moreover, the properties of the adjoining material may alsobe accountable for this deviance. Nevertheless, the measuredresult is still in an acceptable limit, and its good agreementwith the simulated result can be observed from Fig. 14.

Fig. 15 illustrates the performance comparison based onS11 and bandwidth of the flower-shape antenna in dif-ferent environments. The measured −10 dB bandwidth is180 and 365.4 MHz in the lower and higher ISM bands,respectively. In the HSP case, the bandwidth at the lowerresonance is 197.6 MHz (822.8–1020.4 MHz), while thebandwidths are 231.12, 249.53, and 190.05 MHz in theheterogeneous tissue environment for the head, small intestine,and stomach, respectively. Likewise, the bandwidth for theupper resonance band is 245.3 MHz (2.2738–2.5191 GHz) forthe homogeneous case, and in the heterogeneous environment,the bandwidths are 600, 502.5, and 204 MHz for the head,small intestine, and stomach, respectively. The third resonance


Fig. 16. Far-field gain radiation patterns. (a) 928 MHz. (b) 2.45 GHz.

mode appearing at approximately 2 GHz within the HSPcombines with the fundamental 2.45 GHz resonance mode,thereby forming a wider −10 dB bandwidth in the case ofhead and small intestine. Similarly, for the simulations inthe stomach, the bandwidth at the third appearing resonancemode (2 GHz) is 280 MHz (1.88–2.16 GHz). The significantchange in the reflection coefficients at 2.45 GHz in differenthuman tissues may be attributed to the asymmetrical loadeffect [30] and variations in the electrical properties [31]of the surrounding tissues. Variations in the tissue electricalproperties may have altered the impedance matching of theantenna at the higher resonance frequency (2.45 GHz) [32].Moreover, a small shift was observed in the center frequenciesbecause of the heterogeneous (more realistic) tissue envi-ronment. However, it is noteworthy that the heterogeneousenvironment has minimal effects on the lower as well as onthe upper frequency bands, and the heterogeneous environmentaffects S11 and bandwidth of the flower-shape antenna overallin a positive manner. It is obvious from Fig. 15 that thebandwidths as well as the depth of the resonance modes at928 MHz increase, as the antenna is introduced into the head,small intestine, and stomach.

Fig. 16 compares the measured and simulated far-field gainpatterns. Irrespective of the environment, the gain patterns arealmost identical; nevertheless, the peak values are environmentdependent. The maximum gain values achieved in differentenvironments fit well with the conventional implantable anten-nas gain in the ISM-band telemetry [33], [34]. Moreover,the radiation patterns at both resonance frequencies are nearlyomnidirectional in both the E-plane and H-plane. The antennaradiation pattern depends on the asymmetry and symmetryof the tissue model in which the antenna is supposed to

TABLE I

PERFORMANCE SUMMARY OF THE PROPOSED FLOWER-SHAPEANTENNA IN DIFFERENT ENVIRONMENTS

TABLE II

MAXIMUM SAR VALUES AND ALLOWABLE INPUT POWER

be implanted [33]. An omnidirectional-like radiation can beobserved in the symmetrical tissue model, whereas the radi-ation pattern becomes asymmetrical within the anatomical(heterogeneous) tissue models owing to the inhomogeneity andirregularity [17], [35]. The proposed flower-shape antenna iselectrically small; therefore, when it is surrounded by the HSP,it radiates nearly omnidirectional patterns. The small deterio-ration in the omnidirectional radiation in the deep implantationcondition is due to the asymmetry of the heterogeneous tissueenvironment [6], [36]. Table I summarizes the performance ofthe flower-shape antenna in terms of the bandwidth and gainin different simulations and real environments.

The SAR values are calculated by placing the proposedantenna in the head of a realistic model in the RemcomXFdtd simulator. An SAR study is required for indicating themaximum power allowed to the antenna and to meet the safetylimits imposed by the IEEE. For the implantable antennas,1.6 and 2 W/Kg are the SAR limits enforced by the IEEEfor 1 and 10 g tissues, respectively [37]. The SAR valuesat both ISM bands are listed in Table II. These values wereachieved for a transmitter power of 1 W (Watt). To meet thesafety standards, we have found that the maximum allowedtransmitter power (input power) is 3.4 (at 928 MHz) and5.1 mW (at 2.45 GHz) for the 1 g standard, while for the10 g standard, it is 38.07 mW and 49.46 mW at 928 MHz and2.45 GHz, respectively, which are much greater than 25 μW(required transmitter power) [11].

A comparison of the flower-shape implantable antenna withrecently reported studies on implantable antennas is shownin Table III. As shown, the proposed flower-shape antennaexhibits salient properties with a substantially reduced volumecompared to the mentioned implantable antennas. Fig. 17illustrates the distribution of the vector surface currents at928 MHz and 2.45 GHz. At 928 MHz, the current concen-tration is the maximum at the right-hand side of the radiatorsand the edges of the ground slots. Similarly, the contributionof the left-hand side of the radiating element and the edges ofthe left-sided ground slots is dominant in the upper resonance


TABLE III

COMPARISON OF THE FLOWER-SHAPE ANTENNA WITH PRIOR STUDIES

Fig. 17. Surface current distributions. (a) 928 MHz. (b) 2.45 GHz.

mode (2.45 GHz). For both resonance modes, a significantcurrent flow can be detected at the U-shape part of theradiating patch. We observed from the surface current plotsthat a comparatively larger area resonates at the lower ISMband than at the higher band, thus clearly indicating the inverserelation between the resonance frequency and the effectiveresonant area [44]. Moreover, the majority of the currents arenearly in phase with each other, thereby resulting in nearlyomnidirectional radiation patterns (see Fig. 16).

IV. COMMUNICATION LINK ANALYSIS OF

THE FLOWER-SHAPE ANTENNA

For the specification of the data telemetry range, we cal-culated a link budget for an in-to-out body scenario. Suchscenario is important for the medical treatment of patients inhospitals, where data transmit/receive through an implanteddevice to/from an external base station is required. Thiscalculation is colligated with different types of losses, suchas the antenna material and mismatch loss, cable losses, andfree-space losses (L f ) [37]. It is suggested in [11] that for

TABLE IV

PARAMETERS FOR THE LINK BUDGET

a reliable communication, the acceptable margin must begreater than 20 dB; this can be calculated from the differencein the available antenna power (AP ) and the required antennapower (RP ). The important parameters for the link budgetcalculation are shown in Table IV. The required power (RP )can be calculated as

RP (dB) = Eb

No+ K To + Br (1)

where Eb/No = 9.6 dB is the ideal phase shift keying,K represents the Boltzmann’s constant (1.38 × 10−23), To rep-resents the temperature in Kelvin, and Br is the transmissionbit rate in Kilo or Megabits per second (Kb/s or Mb/s).To meet the SAR safety limits enforced by the IEEE, the inputpower (Pt ) of the implanted antennas is limited to −16 dBm(25 μW). Moreover, for avoiding interference with nearbyservices, regulations limiting the effective isotropic radiatedpower (EIRP) of implanted antennas also restrict their max-imum input power. In this case, the EIRP is an importantparameter to compare the transmitting antenna (TX ) radiationsagainst the regulatory restrictions. For instance, the EIRPmust be less than or equal to E I RPm a x , where E I RPm a x =36 and 20 dBm for 915 MHz and 2.45 GHz [45], [46], respec-tively. If frequency hopping is employed in the system, thispower can be increased further. At the lower frequency bands,the propagation of signal will be better in the body comparedwith at higher frequency bands. However, the allowed radiatedpower is higher in the higher frequency bands (ISM bands)than in the lower frequency bands. Furthermore, as indicatedby Ciuti et al. [47], the circuitry and battery power availabilityare primary concerns in implantable devices, which imposefurther power constraints. However, owing to the low gainvalues of the implanted antennas, the maximum input poweris typically limited by the SAR restrictions. As the intendedapplications for the suggested flower-shape antenna are skinimplantation and gastro, two different scenarios were con-sidered based on the input power and bit rate. Most gastroapplicable systems, such as capsule endoscopy, contain silver-oxide batteries that last approximately 9–10 h to provide3 V at 55 mAh with 20 mW power conveyance. Similarly,compared to the gastro applications, in skin implants, forexample, intracranial pressure monitors require low power and


Fig. 18. Link budget for different implant locations. (a) 928 MHz.(b) 2.45 GHz.

low data rate for their operation. By considering the above-mentioned situations, Pt is maintained at −16 and −4 dBmfor skin implantation and gastro applications, respectively,which are much lower than the allowable input power shownin Table II. Similarly, Br is fixed at 7 Kb/s and 78 Mb/s forskin implantation (transmission of pressure data) and gastroapplications, respectively [11], [48]. The available antennapower (AP ) can be computed as

AP (dB) = Pt + Gt + Gr − L f (2)

where Pt represents the transmit power in dBm, Gt isthe gain (dBi) of the transmitter antenna (flower-shape) inthe heterogeneous environment, Gr is the receiver antennagain (dBi), and L f represents the free space losses (dB). Forthe transmitter antenna, Gt values used at both frequencies aretissue dependent, whereas Gr is assumed constant at 2 dBi.If the separation between the receiver and transmitter antennasis denoted by d (m), then L f can be computed by

L f (dB) = 20log

(4πd

λ

). (3)

Fig. 18 shows the margin versus distance plot. For a reliablecommunication, the margin in this paper is restricted to 25 dB.It is evident from Fig. 18 that at both frequencies (928 MHzand 2.45 GHz) with an input power of −16 dBm, 7 Kb/s ofdata can be transmitted easily over more than 6 m for differentimplant locations. Similarly, with an input power of −4 dBm,78 Mb/s of data can be transmitted at a distance of more than2.5 and 1.5 m at 928 MHz and 2.45 GHz, respectively, fordifferent implant locations. Moreover, we observed that thetelemetry range for a specific margin decreases with the decre-ment and increment in the gain and data rate, respectively.

V. CONCLUSION

In this paper, a miniaturized flower-shape dual-band antennafor skin implantation and gastro applications was developed.The proposed flower-shape antenna included a shorting pin,open-ended slots in the ground plane, and a specific flower-shape radiator for miniaturization. The open-ended slots aswell as the dimensional parameters of the flower-shape radiatorcould be used for frequency tuning. A parametric analy-sis was performed to obtain the optimal antenna design.The performance of the flower-shape antenna was compared

with the recently reported work. The comparison showed thatthe flower-shape antenna exhibited a substantially compactsize with satisfactory performance. From the SAR calcula-tions, we observed that the maximum allowable power wasmuch higher than that (25 μW) for implantable applications.The omnidirectional radiation patterns of the proposed flower-shape antenna further emphasized its suitable use in skinimplantation and gastro applications. Finally, the wirelesscommunication link was validated by calculating the linkbudget considering two different scenarios.

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Farooq Faisal was born in Buner, Pakistan, in 1993.He received the B.Sc. degree in telecommunicationengineering from the University of Engineering andTechnology, Peshawar, Pakistan, in 2016. He is cur-rently pursuing the M.Sc. degree in telecommunica-tion engineering with the University of Engineeringand Technology, Taxila, Pakistan.

His current research interests include implantableantennas and devices, wireless power transfer,millimeter-wave antennas, frequency selective sur-faces, and EBGs.

Hyoungsuk Yoo (S’07–M’18) received the B.Sc.degree in electrical engineering from KyungpookNational University, Daegu, South Korea, in 2003,and the M.Sc. and Ph.D. degrees in electricalengineering from the University of Minnesota,Minneapolis, MN, USA, in 2006 and 2009,respectively.

In 2009, he joined the Center for MagneticResonance Research, University of Minnesota, as aPost-Doctoral Associate. In 2010, he joined theCardiac Rhythm Disease Management, Medtronic,

Minneapolis, MN, USA, as a Senior MRI Scientist. From 2011 to 2018, hewas an Associate Professor with the Department of Biomedical Engineering,School of Electrical Engineering, University of Ulsan, Ulsan, South Korea.Since 2017, he has been the CEO of E2MR, Seoul, South Korea, a startupcompany. Since 2018, he has been an Associate Professor with theDepartment of Biomedical Engineering, Hanyang University, Seoul. Hiscurrent research interests include electromagnetic theory, numerical methodsin electromagnetics, metamaterials, antennas, implantable devices, andmagnetic resonance imaging in high-magnetic field systems.

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A Miniaturized Novel-Shape Dual-Band Antenna for ... · shape radiating patch, and open-ended slots in the ground ... and head phantoms. II. METHODOLOGY The aim of this paper is to