Design of Metamaterial Based Efficient Wireless Power Transfer

sensors

Article Design of Metamaterial Based Efﬁcient Wireless Power Transfer System Utilizing Antenna Topology for Wearable Devices

Tarakeswar Shaw 1 , Gopinath Samanta 2, Debasis Mitra 1, Bappaditya Mandal 3,* and Robin Augustine 3

1 Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India; [email protected] (T.S.); [email protected] (D.M.) 2 Department of Electronics and Telecommunication Engineering, C. V. Raman Global University, Bhubaneswar 752054, Odisha, India; [email protected] 3 Microwaves in Medical Engineering Group, Electrical Engineering, Division of Solid-State Electronics, Uppsala University, 752 36 Uppsala, Sweden; [email protected] * Correspondence: [email protected]

Abstract: In this article, the design of an efficient wireless power transfer (WPT) system using antenna-based topology for the applications in wearable devices is presented. To implement the wearable WPT system, a simple circular patch antenna is initially designed on a flexible felt substrate by placing over a three-layer human tissue model to utilize as a receiving element. Meanwhile, a high gain circular patch antenna is also designed in the air environment to use as a transmitter for designing the wearable WPT link. The proposed WPT system is built to operate at the industrial, scientific and medical (ISM) band of 2.40–2.48 GHz. In addition, to improve the power transfer efficiency (PTE) of the system, a metamaterial (MTM) slab built with an array combination of 3 × 3 unit cells has been employed. Further, the performance analysis of the MTM integrated system Citation: Shaw, T.; Samanta, G.; is performed on the different portions of the human body like hand, head and torso model to present Mitra, D.; Mandal, B.; Augustine, R. the versatile applicability of the system. Moreover, analysis of the specific absorption rate (SAR) has Design of Metamaterial Based been performed in different wearable scenarios to show the effect on the human body under the Efficient Wireless Power Transfer System Utilizing Antenna Topology standard recommended limits. Regarding the practical application issues, the performance stability for Wearable Devices. Sensors 2021, analysis of the proposed system due to the misalignment and flexibility of the Rx antenna is executed. 21, 3448. https://doi.org/10.3390/ Finally, the prototypes are fabricated and experimental validation is performed on several realistic s21103448 wearable platforms like three-layer pork tissue slab, human hand, head and body. The simulated and measured result confirms that by using the MTM slab, a significant amount of the PTE improvement Academic Editor: Tran Quang Trung is obtained from the proposed system.

Received: 6 April 2021 Keywords: wireless power transfer; wearable antenna; electromagnetic radiation; radiative near-ﬁeld; Accepted: 13 May 2021 wearable devices; metamaterial; power transfer efﬁciency Published: 15 May 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. In the past few decades, a remarkable advancement in the biomedical field brings a radical change in the health monitoring and treatment of patients. Recently, wearable devices such as body sensors and smart electronic devices are widely used to observe the critical health conditions of patients [1–3]. These devices are used to construct a wireless body sensor network (WBSN), which enables constant monitoring and acquisition of phys- Copyright: © 2021 by the authors. iological data like body temperature, blood pressure, glucose level and electrocardiogram Licensee MDPI, Basel, Switzerland. (ECG), etc., of the patients and transfer these vital information’s to the connected healthcare This article is an open access article system [4]. However, attaining the practical and functional connectivity between the differ- distributed under the terms and conditions of the Creative Commons ent sections of the body sensors remains a technical challenge. Usually, wires are broadly Attribution (CC BY) license (https:// used to connect devices in clinical and research settings that disrupt physical activities creativecommons.org/licenses/by/ and not suitable for continuous use. To power the wearable devices, a high demand exists 4.0/). for designing a simple, compact and efficient wireless power transfer (WPT) system that

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reduced the dependency on battery capacity and eliminates the wire connections around the body. Since the experimental demonstration of wireless electricity performed by Nikola Tesla in his pioneer work [5], there has been an increasing interest in both industrial and academic communities to design an efficient WPT system [6]. The WPT system finds its resourceful applicability in the field of charging portable devices [7], vehicles [8] and even for implantable medical devices [9] wirelessly. In recent times, significant research efforts have also been given to construct the non-radiative inductive and magnetic resonant coupling-based WPT system [10–16] for wearable electronic devices. In Refs. [10,11], a magnetic resonance wireless power transfer (MR-WPT) based approach has been presented to power both wearable and implantable devices over a short transfer distance. In these studies [10,11], the transmitting (Tx) and receiving (Rx) elements are designed over the flexible polyester fiber using an adhesive copper sheet. In [12], a non-planar wearable WPT system with improved power transfer through stretchable magnetic composites has been presented. Furthermore, a four coil-based non-planar hand wearable WPT system is designed using magnetic resonance coupling in Ref. [13]. Furthermore, a wearable textile embroidered Rx antenna connected to a rectifier circuit is used to design the MR-WPT system in [14]. Moreover, a comparative study to develop the wearable WPT system on the different dielectric materials is presented using a strongly coupled magnetic resonance approach [15]. However, the use of lumped capacitor and multi-layered (12 layers) Tx structure in [15] increases the design complexity as well as introduces additional losses to the WPT system. Most recently, in [16], a resonant inductive WPT is designed using an embroidered textile coil for a smart cycling glove. The construction of Tx and Rx elements in Refs. [14,16] with perfect accuracy over a textile material using embroidered is quite complicated and cumbersome. In order to obtain better power transfer efficiency (PTE) from the reported MR-WPT systems [10–16], the Tx and Rx elements must be excited at the same resonant frequency. Furthermore, the performance of the non-radiative WPT system highly depends upon the magnetic coupling between the Tx and Rx section, and the PTE reduced rapidly due to an increase in transmission distance [17]. In addition, the efficiency of the non-radiative system is highly affected by the lateral and angular misalignment effect between the Tx and Rx elements. These issues of the non-radiative WPT system have limited the practical applicability to power the wearable devices. Hence, an effective technique is highly preferred to power the wearable devices wirelessly by mitigating the overhead issues. In this regard, the radiating principle of the antenna may also be considered to construct an effective WPT system for wearable devices. This is because; the antenna- based radiative WPT system provides flexibility in transfer distance along with better misalignment tolerance [18]. Though, huge research work has been made by the various groups to construct the radiating principle-based WPT system for air [19] and implantable devices [20–22]. However, to the best of our knowledge, any substantial research effort has not been made in the literature to design a radiating principle-based wearable WPT system. This study aims to address the knowledge gap by designing an antenna topology- based WPT system for wearable devices. To construct the wearable WPT system, two simple circular patch antennas are designed to use as Tx and Rx elements. Further, the PTE of the proposed system has been improved by using a zero-index metamaterial (ZIM) slab consists of both-sided ring unit cells. The performance stability analysis of the proposed system is exhibited on the human hand, head and torso model. The analysis of specific absorption rate (SAR) has been performed to illustrate the effect of radiation on the human body. Furthermore, some metamaterial-based approach has been discussed to control the SAR in the standard limit. Moreover, the performance study of the proposed system due to the misalignment and flexibility of the Rx antenna is accomplished. Finally, to establish the proposed concept, experimental validation is performed with and without MTM slab. The measured and simulated result shows more than 9% efficiency enhancement due to the application of the MTM slab. Sensors 2021, 21, x FOR PEER REVIEW 3 of 19

99 the proposed concept, experimental validation is performed with and without MTM slab. 100 The measured and simulated result shows more than 9% efficiency enhancement due to 101 the application of the MTM slab.

102 2. Design of Wearable Wireless Power Transfer System 103 Sensors 2021, 21, 3448 In this section, to construct the wearable WPT system, the design of the3 of Tx 20 and Rx 104 antenna is presented. Furthermore, the configuration of the simulation setup to construct 105 the WPT link by employing the designed Tx and Rx antenna has been illustrated. 2. Design of Wearable Wireless Power Transfer System 106 2.1. ConstructionIn this section, of the toWearable construct Receiving the wearable (Rx) WPT and system,Transmitting the design (Tx) of Antennas the Tx and Rx antenna is presented. Furthermore, the configuration of the simulation setup to construct 107 theIn WPTthe case link of by wearable employing applications, the designed Tx simplicity and Rx antenna and better has been performance illustrated. are the prime 108 concern for designing the WPT system. Keeping the requirement in mind, initially, we 109 designed2.1. Construction a simple of inset-fed the Wearable circular Receiving patch (Rx) andantenna Transmitting over (Tx)a flexible Antennas substrate with a full 110 ground planeIn the caseto use of wearableas an Rx applications, element. The simplicity configuration and better of performance the Rx patch are antenna the prime is shown concern for designing the WPT system. Keeping the requirement in mind, initially, we 111 in Figure 1a. The antenna is constructed over the felt substrate having the permittivity of designed a simple inset-fed circular patch antenna over a flexible substrate with a full 112 1.63 groundand loss plane tangent to use of as 0.044 an Rx with element. a thickness The configuration of 1 mm. of The the Rx optimization patch antenna of is the shown Rx antenna 113 is performedin Figure1a. by The placing antenna it is over constructed the three-la over theyer felt human substrate tissue having model, the permittivity as shown of in Figure 114 1b. The1.63 andelectrical loss tangent property of 0.044 of withdifferent a thickness huma ofn 1 mm.tissue The is optimizationmentioned of in the Table Rx antenna 1. The dimen- 115 sionis of performed the wearable by placing Rx antenna it over the is three-layeradjusted humanto operate tissue at model, the ISM as shown of 2.45 in GHz. Figure The1b. param- The electrical property of different human tissue is mentioned in Table1. The dimension of 116 eters of the wearable antenna are included in Table 2. the wearable Rx antenna is adjusted to operate at the ISM of 2.45 GHz. The parameters of the wearable antenna are included in Table2.

(a) (b)

117 Figure 1.Figure (a) Schematic 1. (a) Schematic configuration conﬁguration of the of the Rx Rx patch patch antenna. antenna. ((bb)) Three-layer Three-layer phantom phantom tissue tissue (skin, (skin, fat and fat muscle) and muscle) model model 118 used in HFSSused in simulator. HFSS simulator.

Table 1. Electric property of human tissue model at 2.45 GHz [23,24]. 119 Table 1. Electric property of human tissue model at 2.45 GHz [23,24]. Tissue Model Relative Permittivity Conductivity, Tissue(Thickness) Model (εr) σ (S/m)Conductivity, Relative Permittivity (εr) (Thickness)Skin (1.5 mm) 38.0 1.46 σ (S/m) SkinFat (1.5 (10.5 mm) mm) 5.28 38.0 0.104 1.46 FatMuscle (10.5 (40 mm) mm) 52.73 5.28 1.74 0.104 Muscle (40 mm) 52.73 1.74 Table 2. Dimensions of the Rx wearable patch antenna.

120 Table 2. DimensionsLr = Wr of the RxR rwearable patchW antenna.1 W2 Wfr 70 mm 28.2 mm 13 mm 0.5 mm 3.7 mm Lr = Wr Rr W1 W2 Wfr 70 mm 28.2 mm 13 mm 0.5 mm 3.7 mm In cases of wearable applications, either the Rx element is directly placed over the human body, or an air gap is considered between the body and antenna. Both states are 121 consideredIn cases of and wearable studied during applications, the design either of wearable the Rx Rx element antenna. Theis directly simulated placed return over the 122 humanloss body, characteristics or an air of thegap Rx is antenna considered by placing between over the the body body and and considering antenna. an airBoth gap states are 123 considered(d) are shown and instudied Figure2 during. The measured the design result of by we placingarable the Rx antenna antenna. on the The three-layered simulated return pork slab is also included in Figure2. A good agreement between the simulated and

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124 loss characteristics of the Rx antenna by placing over the body and considering an air gap 125 (d) are shown in Figure 2. The measured result by placing the antenna on the three-layered 126 pork slab is also included in Figure 2. A good agreement between the simulated and meas- 127 ured results has been observed. Furthermore, it can be seen from the return loss charac- Sensors 2021, 21, 3448Sensors 2021, 21, x FOR PEER REVIEW 4 of 20 4 of 19 128 teristics that the resonance frequency is retained at 2.45 GHz from simulations and at 2.46 129 GHz from the measurement. We also investigated the gain of the Rx patch antenna for the 130 124 on-bodymeasured and results airloss gap characteristics has scenario. been observed. of However, the Rx Furthermore, antenna for by th plac ite cansakeing be over of seen clarity,the from body the andon returnly considering the loss simulated an air gap and 131 125 measuredcharacteristics E-plane(d that) are and the shown resonance H-plane in Figure frequency radiation 2. The measured is retained pattern result at of 2.45 by the GHzplacing Rx from patchthe simulations antenna by directly on andthe three-layered placing on 132 126 the humanat 2.46 GHz skin frompork tissue the slab measurement. is model also included (in We simulation) in also Figure investigated 2. A andgood the agreementon gain the of pork the between Rx slab patch the (in antenna simulated measurement) and meas- is 127 for the on-bodyured and results air gap has scenario. been observed. However, Furthermore, for the sake it of can clarity, be seen only from the simulated the return loss charac- 133 plotted in Figure 3. The maximum gain attained from the wearable Rx antenna is 1.86 dBi 128 and measuredteristics E-plane that and the H-plane resonance radiation frequency pattern is retain of theed Rx at patch2.45 GHz by directly from simulations placing and at 2.46 134 129 fromon the the simulation, humanGHz skin from tissue while the model measurement. 0.87 (in dBi simulation) from We also th ande investigat measurement. on the porked the slab gain (in Furthermore, of measurement) the Rx patch isantennathe simulated for the 135 130 radiationplotted efficiency in Figureon-body3. Theof andthe maximum airwearable gap gainscenario. attainedantenna However, from is 18.37 the for wearable th %,e sakeand Rx of the antenna clarity, full isonwidth 1.86ly the dBi at simulated half maxima and 136 131 (FWHM)from the is simulation,foundmeasured to be while E-plane 70.8°/63.4° 0.87 dBiand fromH-plane in the measurement.radiationE/H plane. pattern Moreover, Furthermore, of the Rx patcha the det simulated byailed directly analysis placing of on the 132 radiation efﬁciencythe human of the skin wearable tissue antenna model (in is18.37 simulation) %, and theand full on widththe pork at halfslab maxima (in measurement) is 137 wearable antenna characteristics◦ ◦ for both on-body and an air gap (d) scenario is presented 133 (FWHM) is foundplotted to bein Figure 70.8 /63.4 3. Thein maximum the E/H plane. gain attained Moreover, from a detailed the wearable analysis Rx ofantenna the is 1.86 dBi 138 134 in Tablewearable 3. It antenna canfrom becharacteristics the seen simulation, from forthe while both tabulated on-body0.87 dBi from anddata an th that aire measurement. gap although (d) scenario Furthermore,the is presentedresonance the simulatedfrequency 139 135 of thein Tableantenna3. It can radiationremains be seen efficiency fromthe thesame, tabulatedof the but wearable dataa slight that antenna although change is 18.37 the has resonance%, beenand the seen frequency full width in the of at halfmatching maxima of 140 136 returnthe antennaloss and remains(FWHM) gain theof is same, thefound antenna. but to abe slight 70.8°/63.4° Such change stabilityin has the been E/H seenobtainedplane. in Moreover, the matchingfrom athe det of ailed returnwearable analysis antenna of the 137 loss and gainwearable of the antenna. antenna Such characteristics stability obtained for both from on-body the wearable and an air antenna gap (d) mayscenario be is presented 141 may be due to the presence of the full ground plane and the broadside directive radiation 138 due to the presencein Table of 3. the It fullcan groundbe seenplane from andthe tabulated the broadside data directivethat although radiation the resonance pattern frequency 142 139 patternof the of patch. the of Allpatch. the the antenna simulationsAll the remains simulations are performedthe same, arebut by utilizingaperformed slight change the commercially by has utilizing been seen available thein the commercially matching of 143 140 availablesoftware software ANSYSreturn HFSS. ANSYS loss and HFSS. gain of the antenna. Such stability obtained from the wearable antenna 141 may be due to the presence of the full ground plane and the broadside directive radiation 142 pattern of the patch. All the simulations are performed by utilizing the commercially 143 available software ANSYS HFSS.

144 144 145 145 FigureFigure 2. Return 2. Return lossFigure property loss 2. propertyReturn of the wearable loss ofproperty Rxthe antenna. wearable of the wearable Rx antenna. Rx antenna.

(a) (b)

146 Figure 3. RadiationFigure 3. pattern Radiation of Rx pattern antenna. of Rx (a) antenna. E-plane and (a) E-plane (b) H-plane. and (b) H-plane.

147 (a) (b)

146 Figure 3. Radiation pattern of Rx antenna. (a) E-plane and (b) H-plane.

147

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148 Table 3. Simulated performance of the Rx antenna due to placement on the body and considering Sensors 2021, 21, 3448 5 of 20 149 an air gap (d).

d Operating |S11| Gain Table 3. Simulated(mm) performance of theFrequency Rx antenna due (GHz) to placement on the body(dB) and considering an (dBi) air gap (d). 0 (On Body) 2.45 −33.71 1.86

d 1 Operating 2.45 |S11| −33.16 Gain 1.87 (mm) 2 Frequency (GHz)2.45 (dB) −32.81 (dBi) 1.89 − 0 (On Body)3 2.45 2.45 33.71−31.60 1.86 1.92 1 2.45 −33.16 1.87 − 150 2To construct the 2.45 WPT link, an inset-fed32.81 circular patch 1.89 antenna with a full ground 151 plane3 has also been designed 2.45 to use as −a31.60 Tx element in the air 1.92 environment. The geometry 152 of the Tx antenna is shown in Figure 4a. Herein, a patch antenna is utilized as a Tx element 153 To constructdue to its the simplicity, WPT link, an broadside inset-fed circular radiation patch pattern antenna withand acapability full ground to plane integrate with the sys- has also been designed to use as a Tx element in the air environment. The geometry of the 154 tem easily. To obtain high gain from the Tx antenna, it is designed on a low-loss dielectric Tx antenna is shown in Figure4a. Herein, a patch antenna is utilized as a Tx element due 155 substrate having the dielectric permittivity of 2.2 and loss tangent 0.0009 with a thickness to its simplicity, broadside radiation pattern and capability to integrate with the system 156 easily. Toof obtain 1.58 mm. high gainThe fromdimension the Tx antenna,of the Tx it isantenna designed is on also a low-loss optimized dielectric to operate at the same 157 substratefrequency having the dielectric(2.45 GHz) permittivity of the Rx of element 2.2 and loss to tangentobtain 0.0009better with performance a thickness from the designed 158 of 1.58 mm.system. The dimensionThe optimized of the Txparameter antenna isof also the optimized Tx antenna to operate to operate at the sameat 2.45 GHz is given in 159 frequencyTable (2.45 4. GHz) Furthermore, of the Rx element the simulated to obtain betterand measur performanceed return from loss the designed property of the Tx patch is system. The optimized parameter of the Tx antenna to operate at 2.45 GHz is given in 160 shown in Figure 4b. The resonance frequency obtained from the measured result is 2.44 Table4. Furthermore, the simulated and measured return loss property of the Tx patch 161 is shownGHz in Figure for the4b. antenna. The resonance Further, frequency the simulated obtained an fromd measured the measured radiation result ispattern characteristic 162 2.44 GHzis for illustrated the antenna. in Figure Further, 5. the The simulated maximum and measured gain achieved radiation from pattern the char-Tx antenna is 7.56 dBi 163 acteristicfrom is illustrated the simulation, in Figure 5while. The maximum7.13 dBi is gain achi achievedeved from from the the measurement. Tx antenna is Moreover, the sim- 164 7.56 dBi fromulated the radiation simulation, efficiency while 7.13 dBiof isthe achieved Tx antenna from the is measurement. 92.7%, and Moreover,the FWHM is found to be 165 the simulated74.7°/75.1° radiation in efﬁciencythe E/H plane. of the Tx antenna is 92.7%, and the FWHM is found to be 74.7◦/75.1◦ in the E/H plane.

(a) (b)

166 Figure Figure4. (a) Schematic 4. (a) Schematic of the of Tx the antenna. Tx antenna. (b ()b Simulated) Simulated and measuredmeasured return return loss loss characteristics. characteristics.

167 Table 4. DimensionsTable 4. Dimensions of the Tx patch of antenna.the Tx patch antenna. You R L L W You t Rt 1 L1 2 Lft2 Wft 90 mm90 mm 24.4 mm 24.4 mm 19.3 mm 19.3 0.3mm mm 0.3 4.88 mm mm 4.88 mm

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(a) (b)

168 Figure 5. The radiation(a) pattern of the Tx antenna. (a) E-plane( band) (b) H-plane.

Figure 5. The radiation pattern of the Tx antenna. (a) E-plane and (b) H-plane. 169 168 Figure2.2. Design 5. The radiationof Wearable pattern WPT of Link the Tx antenna. (a) E-plane and (b) H-plane. 170 2.2. Design of WearableThe schematic WPT Link configuration of the WPT system by integrating the designed Tx and 171 169 The schematicRx antennas2.2. configuration Design from of the Wearable previous of the WPT WPT section Link system is by pres integratingented in theFigure designed 6a. The Tx distance and between the 172 170 Rx antennasTx fromantenna the previousThe and schematic the section surface configuration is presentedof Rx antenna inof Figurethe isWPT marked6a. sy Thestem distance by by d 1integrating. The between transfer thethe designeddistance Tx(d1 and) has 173 171 Tx antennabeen and chosen theRx surfaceantennas for ofthe from Rx proposed antenna the previous is system marked section to by operat isd1 pres. Theentede transferin the in Figureradiative distance 6a. The (near-fieldd1) distance has region between of the the 172 been chosen for the proposed system to operate in the radiative near-field1 region of the 1 174 Tx antenna.Tx antenna In this and region, the surface the radiated of Rx antenna beams is markedare confined by d . Thethat transfer provides distance better ( dsystem) has 173 Tx antenna. In thisbeen region, chosen the for radiated the proposed beams aresystem confined to operat thate providesin the radiative better system near-field region of the 175 performance with enhanced transmission distance and less sensitive to misalignments 174 performance withTx enhanced antenna. transmission In this region, distance the radiated and less beams sensitive are to confined misalignments that provides [19]. better system 176 [19]. The variation of transmission characteristics (|S21|) with the increase of transmission 175 The variation ofperformance transmission with characteristics enhanced (|transmissionS21|) with thedist increaseance and of less transmission sensitive to misalignments 177 distance (d1) in the near-field region is presented in Figure 6b. It can be observed from the 176 distance (d1) in the[19]. near-field The variation region of is presentedtransmission in Figurecharacteristics6b. It can (| beS21 observed|) with the from increase the of transmission 178 21 1 177 figure thatfigure the strength distancethat the of ( dstrength |1S) 21in| the is decreasednear-field of |S | regionis with decreased the is presented increase with of in thed Figure1. Thisincrease 6b. is dueIt canof to d be the. Thisobserved is due from to the the 179 178 spreadingspreading naturefigure of radiating nature that theof waves radiatingstrength and unwantedof waves |S21| andis attenuation decreased unwanted with in theattenuation the free increase space in of of the d1 Tx.free This space is due of to the the Tx 180 179 antenna. Itantenna. can alsospreading It be can seen also nature from be the ofseen radiating figure from that thewaves 60 figure mm and distance unwantedthat 60 provides mm attenuation distance the high inprovides valuethe free spacethe high of the value Tx 181 180 of |S21| (–8.54of |S21 dB),|antenna. (–8.54 which dB), It offerscan which also better be offers seen PTE. betterfrom In the the PTE. next figure section,In thethat next60 a metamaterialmm section, distance a metamaterial provides slab is the high slab value is de- 182 181 designed andsigned utilizedof and |S to21utilized| concentrate(–8.54 dB),to concentrate which or reduce offers the or better spreading reduce PTE. the nature In spreadingthe ofnext the section, radiating nature a metamaterial wavesof the radiating slab iswaves de- from the Tx antenna to improve the PTE of the system. 183 182 from thesigned Tx antenna and utilized to improve to concentrate the PTE or reduceof the system.the spreading nature of the radiating waves 183 from the Tx antenna to improve the PTE of the system.

(a) (b) (a) (b) 184 Figure 6. (a) Schematic of the wearable WPT system. (b) Simulated result for the variation of transmission strength (|S21|) 184 185Figure 6. (witha) Schematic transferFigure distance of 6. the(a )wearable ( Schematicd1). WPT of the system. wearable (b) WPTSimulated system. result (b) Simulatedfor the variation result forof transmission the variation ofstrength (|S21|) 185 with transfer distance (d1). transmission strength (|S21|) with transfer distance (d1). 186 186

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187 3.3. MetamaterialMetamaterial IntegratedIntegrated ProposedProposed WearableWearable WPTWPT System System 187 3. Metamaterial Integrated Proposed Wearable WPT System 188 TheThe proposedproposed wearable wearable WPT WPT systems systems integrated integrated with with an MTM an MTM slab containingslab containing an array an 188 The proposed wearable WPT systems integrated with an MTM slab containing an 189 ofarray 3 × 3of both-sided 3 × 3 both-sided ring unit ring cell unit is shown cell is inshown Figure in7. Figure The MTM 7. The is placed MTM atis aplaced distance at ofa dis-d2 189 array of 3 × 3 both-sided ring unit cell is shown in Figure 7. The MTM is placed at a dis- 190 fromtance the of d wearable2 from the Rx wearable antenna. Rx The antenna. both-sided The both-sided MTM ring MTM unit cell ring configuration unit cell configuration is used to 190 tance of d2 from the wearable Rx antenna. The both-sided MTM ring unit cell configuration 191 improveis used to the improve performance the performance of the proposed of the WPT prop systemosed WPT is shown system in is Figure shown8. in In Figure the figure, 8. In 191 is used to improve the performance of the proposed WPT system is shown in Figure 8. In 192 thethe orientationfigure, the oforientation the exciting of electromagneticthe exciting electromagnetic wave to the MTM wave structure to the MTM is also structure presented. is 192 the figure, the orientation of the exciting electromagnetic wave to the MTM structure is 193 Thealso MTMpresented. structure The MTM is designed structure on is an designed inexpensive on an FR4 inexpensive dielectric FR4 substrate, dielectric having substrate, the 193 also presented. The MTM structure is designed on an inexpensive FR4 dielectric substrate, 194 permittivityhaving the permittivity of 4.4, loss tangent of 4.4, loss of 0.02 tangent with of a thickness0.02 with ofa thickness 1.6 mm. Theof 1.6 dimensions mm. The dimen- of the 194 having the permittivity of 4.4, loss tangent of 0.02 with a thickness of 1.6 mm. The dimen- 195 MTMsions structureof the MTM are structure optimized are to obtainoptimized zero-index to obtain property zero-index at 2.45 property GHz by at following 2.45 GHz the by 195 sions of the MTM structure are optimized to obtain zero-index property at 2.45 GHz by 196 methodologyfollowing the discussed methodology in [25 discussed]. The optimized in [25]. parametersThe optimized of the parameters ring structure of the are ring included struc- 196 following the methodology discussed in [25]. The optimized parameters of the ring struc- 197 inture the are caption included of Figure in the8 .caption The transmission of Figure 8. (|S The21 |)transmission and reflection (|S21 (|S|) 11and|) characteristicreflection (|S11 of|) 197 ture are included in the caption of Figure 8. The transmission (|S21|) and reflection (|S11|) 198 thecharacteristic ring unit cell of the is shown ring unit in Figurecell is 9showna. The in extracted Figure 9a. effective The extracted refractive effective index propertyrefractive 198 characteristic of the ring unit cell is shown in Figure 9a. The extracted effective refractive 199 ofindex the unitproperty cell isof also the shownunit cell in is Figure also shown9b. The inparameter Figure 9b. extractionThe parameter is performed extraction by is 199 index property of the unit cell is also shown in Figure 9b. The parameter extraction is 200 theperformed approach by mentionedthe approach in mentioned [26]. It can in be [26]. observed It can be from observed Figure from9b that Figure the ring9b that unit the 200 cellperformed provides by the the zero-index approach characteristicmentioned in over[26]. theIt can entire be observed ISM (2.40–2.48 from Figure GHz) frequency9b that the 201 ring unit cell provides the zero-index characteristic over the entire ISM (2.40–2.48 GHz) 201 band.ring unit Herein, cell provides the efficiency the zero-index of the proposed characteristic system over is improved the entire by ISM concentrating (2.40–2.48 GHz) the 202 frequency band. Herein, the efficiency of the proposed system is improved by concentrat- 202 radiatingfrequency energy band. ofHerein, the Tx the antenna efficiency over of the the Rx proposed antenna utilizingsystem is the improved zero-index by concentrat- behaviour 203 ing the radiating energy of the Tx antenna over the Rx antenna utilizing the zero-index 203 ofing the the MTM radiating slab. energy As per of Snell’s the Tx law, antenna when ov aer spreading the Rx antenna electromagnetic utilizing the wave zero-index passes 204 behaviour of the MTM slab. As per Snell’s law, when a spreading electromagnetic wave 204 throughbehaviour a zero-index of the MTM slab, slab. the As refracted per Snell’s wave law, becomes when a normal spreading to the electromagnetic slab. Henceforth wave by 205 passes through a zero-index slab, the refracted wave becomes normal to the slab. Hence- 205 utilizingpasses through the MTM a zero-index slab, the gain slab, of the the refracted Tx antenna wave is enhancedbecomes normal by focusing to the the slab. incoming Hence- 206 forth by utilizing the MTM slab, the gain of the Tx antenna is enhanced by focusing the 206 waveforth inby theutilizing direction the ofMTM broadside. slab, the Subsequently, gain of the Tx the antenna efficiency is enhanced of the wearable by focusing WPT the is 207 incoming wave in the direction of broadside. Subsequently, the efficiency of the wearable 207 enhancedincoming significantly.wave in the direction of broadside. Subsequently, the efficiency of the wearable 208 WPT is enhanced significantly. 208 WPT is enhanced significantly.

209 209 210 Figure 7. Configuration of the proposed MTM-based wearable WPT system. 210 FigureFigure 7.7. ConﬁgurationConfiguration ofof thethe proposedproposed MTM-basedMTM-based wearablewearable WPT WPT system. system.

211 211 212 Figure 8. Schematic view of the both-sided ring structure. Where LM = WM = 24 mm, R1 = 11.25 mm 212 Figure 8. Schematic view of the both-sided ring structure. Where LM = WM = 24 mm, R1 = 11.25 mm 213 Figureand R2 8.= 6Schematic mm. view of the both-sided ring structure. Where LM = WM = 24 mm, R1 = 11.25 mm 213 and R2 = 6 mm. and R2 = 6 mm. 214 214

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(a) (b)

215 FigureFigure 9. (a) 9. Property(a) Property of of the the transmission transmission and and reflection reflecti coefficienton coefficient of the ring of unit the cell. ring (b) unit Extracted cell. effective (b) Extracted refractive effective index. refractive 216 index. The performance enhancement of the WPT system highly depends on the combination of the unit cell array and its placement position (d2) from the receiving antenna [21,22]. 217 Henceforth,The performance a parametric enhancement study has been performed of the WPT to find system out the highly optimal depends combination on of the combina- 2 218 tionunit of cell the array unit as cell well array as its position,and its placementd2, that provides position the better (d ) performancefrom the receiving enhancement antenna [21,22]. 219 Henceforth,from the system. a parametric The simulated study result has for been the various performed combinations to find of theout unit the cell optimal array is combination × 220 ofpresented unit cell in array Table5 as. From well the as table, its position, it can be seen d2, thatthat the provides combination the of better 3 3 arraysperformance offers enhance- a better increment (2.88 dB) in coupling strength (|∆S |) that enhances the PTE of the 221 ment from the system. The simulated result for 21the various combinations of the unit cell system noticeably compared to other combinations. The optimal performance enhancement 222 arrayis obtained is presented for the array in Table combination 5. From of 3 the× 3 table, due to theit can uniform be seen illumination that the effect combination of the of 3 × 3 223 arraysMTM offers slab discussed a better in increment [27]. It can also (2.88 be realizeddB) in coupling from Table 5strength that due to(|∆ incrementS21|) that in enhances the 224 PTEarray of combination, the system thenoticeably performance compared of the system to other is degraded. combinations. This is because The aoptimal single Tx performance 225 enhancementpatch antenna is cannot obtained effectively for the illuminate array combinat the large panelion sizeof 3 of× the3 due MTM to slab,the uniform which illumina- generates a loss in the system [21,27]. Furthermore, the effect of the MTM-slab (with 226 tion effect of the MTM slab discussed in [27]. It can also be realized from Table 5 that due 3 × 3 arrays) for the different placement positions (d2) in the proposed WPT system is 227 to increment in array combination, the performance of the system is degraded. This is presented in Table6. The optimization of d2 is performed to diminish the near field coupling 228 becauseeffect between a single the Tx MTM patch slab antenna and Tx patch. cannot It can effectively be perceived illuminate from the table the that large at 8 panel mm size of the 229 MTMdistance, slab, better which enhancement generates in couplinga loss in strength the system (|∆S21|) [21,27]. is attained. Furthermore, The improvement the effect of the in coupling is attributed to better impedance matching among the MTM slab and the Tx 230 MTM-slab (with 3 × 3 arrays) for the different placement positions (d2) in the proposed patch at the specified distance [21,22,27]. 231 WPT system is presented in Table 6. The optimization of d2 is performed to diminish the 232 nearTable field 5. Simulatedcoupling results effect for between the different the MTM combination slab ofand unit Tx cell patch. arrays It on can MTM-slab be perceived from 233 the(|S table21|= − 8.54that dB at without 8 mm MTM distance, at d1 = 60 better mm). enhancement in coupling strength (|∆S21|) is at- 234 tained. The improvement in coupling is attributed to better impedance matching among Array Frequency |S11| |S21| Increment 235 the MTMCombination slab and the(GHz) Tx patch at the(dB) specified distance(dB) [21,22,27].|∆S 21| (dB) 3 × 3 2.46 −20.86 −6.26 2.88 236 Table 5.4 Simulated× 4 results 2.45 for the different−16.03 combination of− unit8.16 cell arrays on 0.38 MTM-slab (|S21|= 237 −8.54 dB without MTM at d1 = 60 mm). 6 × 6 2.44 −14.47 −8.35 0.19

Array Combina- Frequency |S11| |S21| Increment tion (GHz) (dB) (dB) |∆S21| (dB) 3 × 3 2.46 −20.86 −6.26 2.88 4 × 4 2.45 −16.03 −8.16 0.38 6 × 6 2.44 −14.47 −8.35 0.19

238 Table 6. Simulated results for the different placement position (d2) of the MTM-slab (|S21|= −8.54 239 dB without MTM at d1 = 60 mm).

d2 Frequency |S11| |S21| Increment (mm) (GHz) (dB) (dB) |∆S21| (dB) 6 2.43 −17.01 −6.36 2.18 8 2.46 −20.86 −6.26 2.88 10 2.42 −16.47 −6.98 1.56

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Table 6. Simulated results for the different placement position (d ) of the MTM-slab (|S |= −8.54 dB Sensors 2021, 21, x FOR PEER REVIEW 2 21 9 of 19 without MTM at d1 = 60 mm).

d2 Frequency |S11| |S21| Increment (mm) (GHz) (dB) (dB) |∆S21| (dB) 240 Furthermore,6 2.43to demonstrate−17.01 the effect −of6.36 the zero-index 2.18 MTM slab on the transmis- 8 2.46 −20.86 −6.26 2.88 241 sion strength (|S21|) of the WPT system, a comparative study is performed with and with- 10 2.42 −16.47 −6.98 1.56 242 out the integration of the MTM slab. The comparison of |S21|-characteristic is illustrated 243 in Figure 10, with the variation of transfer distance (d1). It can be clearly perceived from Furthermore, to demonstrate the effect of the zero-index MTM slab on the transmission 244 strengththe figure (|S21 |)that of the the WPT transmission system, a comparative strength study is isenhanced performed withsignificantly and without along with the change 245 theof integrationtransmission of the MTMdistance slab. Thedue comparison to the use of |Sof21 |-characteristicMTM slab. Consequently, is illustrated in the efficiency of the Figure 10, with the variation of transfer distance (d1). It can be clearly perceived from the 246 figureproposed that the wearable transmission system strength is enhancedenhanced significantly consider alongably. with Furthermore, the change of to clarify the reason 247 transmissionbehind the distance performance due to the improvement use of MTM slab. of Consequently, the proposed the efficiency MTM integrated of the WPT system, the 248 proposedPoynting wearable vector system distribution is enhanced for considerably. without Furthermore,and with MTM to clarify integrated the reason system is depicted in behind the performance improvement of the proposed MTM integrated WPT system, the 249 PoyntingFigure vector11a,b, distribution respectively. for without From and the with figure, MTM integrated it can systembe seen is depicted that due in to the integration of 250 FigureMTM 11 slab,a,b, respectively. the radiated From electromagnetic the figure, it can be seen wave that from due to the integration Tx antenna of is concentrated over MTM slab, the radiated electromagnetic wave from the Tx antenna is concentrated over 251 the wearable Rx antenna with higher intensity, which improves the performance of the the wearable Rx antenna with higher intensity, which improves the performance of the 252 systemsystem considerably. considerably.

253 254 FigureFigure 10. 10.Variation Variation in the transmission in the transmission strength with andstre withoutngth with MTM slab.and without MTM slab. Moreover, a comparative performance analysis of the MTM integrated wearable WPT system is done by positioning the Rx antenna over the voxel models like human hand, head and torso. The simulation setup by placing the Rx antenna on the different portions of the human body is shown in Figure 12. This study is accomplished with the optimized 3 × 3 array combination of the unit cell on the MTM slab at d1 = 60 mm and d2 = 8 mm. Figure 13a,b depicts the S-parameter properties of the proposed wearable WPT system applied to different models. From Figure 13b, it can be observed that the strength of the transmission coefficient (|S21|) is enhanced significantly due to the use of MTM slab, which improves the efficiency of the system. For the sake of clarity, the S-parameter

(a) (b)

255 Figure 11. Distribution of poynting vector. (a) Without and (b) with MTM slab.

256 Moreover, a comparative performance analysis of the MTM integrated wearable 257 WPT system is done by positioning the Rx antenna over the voxel models like human 258 hand, head and torso. The simulation setup by placing the Rx antenna on the different 259 portions of the human body is shown in Figure 12. This study is accomplished with the 260 optimized 3 × 3 array combination of the unit cell on the MTM slab at d1 = 60 mm and d2 = 261 8 mm. Figure 13a,b depicts the S-parameter properties of the proposed wearable WPT 262 system applied to different models. From Figure13b, it can be observed that the strength 263 of the transmission coefficient (|S21|) is enhanced significantly due to the use of MTM slab, 264 which improves the efficiency of the system. For the sake of clarity, the S-parameter plots

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240 Furthermore, to demonstrate the effect of the zero-index MTM slab on the transmis- 241 sionplots strength for the (| withoutS21|) of MTM the WPT slab insystem, case of thea comparative human hand, study head and is performed torso model with are not and with- 242 out incorporatedthe integration in Figure of the13. However,MTM slab. a detailed The comparison comparative studyof |S for21|-characteristic the different wearable is illustrated 243 in Figurescenarios 10, with with and the without variation the integration of transfer of thedistance MTM slab (d1). is It furnished can be inclearly Table7 .perceived Herein, from the efficiency of the WPT is calculated from the transmission characteristics (|S |) as, 244 the figure that the transmission strength is enhanced significantly along with21 the change η = |S |2 [20–22]. A change in efficiency improvement, along with a shift in operating 245 of transmission21 distance due to the use of MTM slab. Consequently, the efficiency of the frequency, is observed from Table7. Such variation occurs as the human body is a most 246 proposedcomplex wearable environment system with theis enhanced discrepant consider dielectric propertiesably. Furthermore, of various parts to clarify of human the reason 247 behindtissues the [ 28performance], which usually improvement affects the system’s of the performanceproposed MTM differently. integrated In the WPT proposed system, the 248 Poyntingwearable vector WPT distribution system, the performance for without is analyzedand with for MTM a homogeneous integrated three-layer system is tissue depicted in 249 Figuremodel 11a,b, as well respectively. as for inhomogeneous From the andfigure, irregular it can structures be seen suchthat asdue the to human the integration hand, of head and torso model. Even though the excitation power is kept the same for the Tx 250 MTM slab, the radiated electromagnetic wave from the Tx antenna is concentrated over antenna, but the dielectric property, size and geometry of the different wearable scenarios 251 the varywearable simultaneously. Rx antenna Henceforth, with higher a variation intens inity, the transmissionwhich improves efficiency the improvements performance of the 252 systemand shiftconsiderably. in operating frequency is observed in Table7. From the comparative study for the different wearable scenarios, it is clear that the proposed WPT system may be considered for application in the various wearable applications.

Table 7. Comparative study of the performance for the different wearable scenario with and without (W/O) the integration of the MTM slab.

Operating Efﬁciency Wearable MTM Slab |S | |S | Efﬁciency Frequency 11 21 Improvement Environment Loading (dB) (dB) (%) (GHz) (%) Three-layer W/O MTM 2.44 −31.12 −8.54 13.94 - Tissue Model With MTM 2.46 −20.86 −6.26 23.66 9.72 W/O MTM 2.41 −17.16 −9.10 12.30 - Human Hand With MTM 2.42 −31.54 −6.51 22.34 11.04 W/O MTM 2.32 −18.24 −12.64 5.45 - Human Head With MTM 2.36 −19.17 −7.44 18.03 12.58 W/O MTM 2.30 −22.19 −14.36 3.67 - 253 Human Torso With MTM 2.33 −19.36 −8.74 13.37 9.7 254 Figure 10. Variation in the transmission strength with and without MTM slab.

(a) (b)

255 FigureFigure 11. Distribution 11. Distribution of ofpoynting poynting vector. vector. (a(a)) Without Without and and (b) with(b) with MTM MTM slab. slab.

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265 for the without MTM slab in case of the human hand, head and torso model are not incor- 266 porated in Figure 13. However, a detailed comparative study for the different wearable 267 scenarios with and without the integration of the MTM slab is furnished in Table 7. 268 Herein, the efficiency of the WPT is calculated from the transmission characteristics (|S21|) 269 as, η = |S21|2 [20–22]. A change in efficiency improvement, along with a shift in operating 270 frequency, is observed from Table 7. Such variation occurs as the human body is a most 271 complex environment with the discrepant dielectric properties of various parts of human 272 tissues [28], which usually affects the system’s performance differently. In the proposed 273 wearable WPT system, the performance is analyzed for a homogeneous three-layer tissue 274 model as well as for inhomogeneous and irregular structures such as the human hand, 275 head and torso model. Even though the excitation power is kept the same for the Tx an- 276 tenna, but the dielectric property, size and geometry of the different wearable scenarios 277 vary simultaneously. Henceforth, a variation in the transmission efficiency improvements 278 Sensors 2021, 21, 3448 and shift in operating frequency is observed in Table 7. From the comparative study11 of 20 for 279 the different wearable scenarios, it is clear that the proposed WPT system may be consid- 280 ered for application in the various wearable applications.

(a)

(b) (c) Sensors 2021, 21, x FOR PEER REVIEW 11 of 19 281 FigureFigure 12. 12.The The scenario scenario of theof the proposed proposed MTM MTM integrated integrat wearableed wearable WPT WPT system system over human,over human, (a) hand, (a) 282 (b)hand, head ( andb) head (c) torso and ( model.c) torso model.

283

(a) (b)

284 Figure 13. ComparisonFigure 13. Comparison in S-parameters in S-parameters for vari for variousous wearable wearable scenariosscenarios such such as three-layer as three-layer tissue, human tissue, hand, human head andhand, head and 285 torso model.torso (a) model.Reflection (a) Reﬂection and (b and) transmission (b) transmission characteristics. characteristics.

286 Table 7. Comparative study of the performance for the different wearable scenario with and without (W/O) the integration 287 of the MTM slab.

Operating Efficiency Wearable MTM Slab |S11| |S21| Frequency Efficiency (%) Improvement Environment Loading (dB) (dB) (GHz) (%) Three-layer Tissue W/O MTM 2.44 −31.12 −8.54 13.94 - Model With MTM 2.46 −20.86 −6.26 23.66 9.72 W/O MTM 2.41 −17.16 −9.10 12.30 - Human Hand With MTM 2.42 −31.54 −6.51 22.34 11.04 W/O MTM 2.32 −18.24 −12.64 5.45 - Human Head With MTM 2.36 −19.17 −7.44 18.03 12.58 W/O MTM 2.30 −22.19 −14.36 3.67 - Human Torso With MTM 2.33 −19.36 −8.74 13.37 9.7

288 4. Misalignment and Bending Analysis of the Proposed MTM Integrated System 289 In the application scenario, it is necessary to estimate the performance stability of the 290 proposed WPT system due to the movement of the human body. Hence, the stability of 291 the system performance is investigated in light of the misalignment between the Tx and 292 Rx antenna. Furthermore, the flexibility effect of the wearable Rx antenna on the system 293 performance is studied.

294 4.1. Misalignment Analysis 295 Perfect alignment among the Tx and Rx antenna is difficult due to the movement of 296 the human posture. Henceforth, the lateral and angular misalignment analysis of the Tx 297 antenna is performed to show the effect on the performance of the proposed WPT system. 298 The analysis of the lateral misalignment is performed by moving the Tx antenna parallelly 299 by keeping the MTM-slab and Rx antenna fixed at their positions, as shown in Figure 14a. 300 In the figure, the lateral offset distance of the Tx antenna is specified by ‘La’. A comparative 301 study about the coupling strengths (|S21|) between the Tx and Rx antenna with the varia- 302 tion of La is presented in Figure 14b. Furthermore, in Table 8, the effect of lateral alignment

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4. Misalignment and Bending Analysis of the Proposed MTM Integrated System In the application scenario, it is necessary to estimate the performance stability of the proposed WPT system due to the movement of the human body. Hence, the stability of the system performance is investigated in light of the misalignment between the Tx and Rx antenna. Furthermore, the ﬂexibility effect of the wearable Rx antenna on the system performance is studied.

4.1. Misalignment Analysis Perfect alignment among the Tx and Rx antenna is difficult due to the movement of the human posture. Henceforth, the lateral and angular misalignment analysis of the Tx antenna is performed to show the effect on the performance of the proposed WPT Sensors 2021, 21, x FOR PEER REVIEW system. The analysis of the lateral misalignment is performed by moving the Tx antenna 12 of 19 parallelly by keeping the MTM-slab and Rx antenna fixed at their positions, as shown in Figure 14a. In the figure, the lateral offset distance of the Tx antenna is specified by ‘La’. A comparative study about the coupling strengths (|S21|) between the Tx and Rx antenna with the variation of L is presented in Figure 14b. Furthermore, in Table8, the effect of 303 on the operating frequencya and transmission strength is clearly presented. It can be per- lateral alignment on the operating frequency and transmission strength is clearly presented. 304 ceivedIt from can be the perceived table fromthat the with table the that enhancement with the enhancement of the of ‘L thea’, ‘L thea’, the transmission transmission strength of 305 the systemstrength is ofdegraded the system along is degraded with along a frequency with a frequency shift. shift.

(a) (b)

306 Figure 14. (Figurea) Configuration 14. (a) Conﬁguration of lateral of lateral misalignment misalignment of of thethe Tx Tx antenna. antenna. (b) Effect (b) Effect on transmission on transmission strength (|S strength21|). (|S21|).

307 Table 8.Table Effect 8. Effect of the of the lateral lateral misalignment misalignment (La (L) ona) theon performance the performance of the proposed of the WPTproposed system. WPT system.

La (mm) 0 10 20 30 La (mm) 0 10 20 30 Frequency (GHz) 2.46 2.41 2.44 2.43 Frequency (GHz) 2.46 2.41 2.44 2.43 |S21| (dB) −6.26 −6.85 −6.89 −7.38 |S21| (dB) −6.26 −6.85 −6.89 −7.38 Furthermore, the study of angular misalignment is also executed by rotating the Tx 308 Furthermore,antenna in the yz-plane the study by keeping of angular the MTM-slab misalignme and Rxnt antenna is also ﬁxed executed at their positions, by rotating the Tx 309 antennaas representedin the yz-plane in Figure by 15 keepinga. The change the inMTM-sl transmissionab and strength Rx antenna (|S21|) is fixed also studied at their positions, for the different values of rotation angle (θr) and presented in Figure 15b. Moreover, for 310 as representeda better understanding in Figure about15a. theThe effect change of angular in transmission misalignment, astrength comparative (|S study21|) is is also studied 311 for thegiven different in Table values9. A slight of rotation change in theangle coupling (θr) and strengths presented (|S 21|) canin Figure be observed 15b. from Moreover, for a 312 better theunderstanding table. about the effect of angular misalignment, a comparative study is 313 given in Table 9. A slight change in the coupling strengths (|S21|) can be observed from 314 the table.

(a) (b)

315 Figure 15. (a) Schematic of the angular misalignment of the Tx antenna. (b) Effect on transmission strength (|S21|).

316 Table 9. Effect of the angular misalignment (θr) on the performance of the proposed WPT system.

θr (degree) 0 10 20 30 Frequency (GHz) 2.46 2.43 2.42 2.45 |S21| (dB) −6.26 −6.69 −7.79 −7.89 317

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303 on the operating frequency and transmission strength is clearly presented. It can be per- 304 ceived from the table that with the enhancement of the ‘La’, the transmission strength of 305 the system is degraded along with a frequency shift.

(a) (b)

306 Figure 14. (a) Configuration of lateral misalignment of the Tx antenna. (b) Effect on transmission strength (|S21|).

307 Table 8. Effect of the lateral misalignment (La) on the performance of the proposed WPT system.

La (mm) 0 10 20 30 Frequency (GHz) 2.46 2.41 2.44 2.43 |S21| (dB) −6.26 −6.85 −6.89 −7.38

308 Furthermore, the study of angular misalignment is also executed by rotating the Tx 309 antenna in the yz-plane by keeping the MTM-slab and Rx antenna fixed at their positions, 310 as represented in Figure 15a. The change in transmission strength (|S21|) is also studied 311 for the different values of rotation angle (θr) and presented in Figure 15b. Moreover, for a 312 better understanding about the effect of angular misalignment, a comparative study is Sensors 2021, 21, 3448 13 of 20 313 given in Table 9. A slight change in the coupling strengths (|S21|) can be observed from 314 the table.

(a) (b)

315 Figure 15.Figure (a) Schematic 15. (a) Schematic of the ofangular the angular misa misalignmentlignment of of thethe Tx Tx antenna. antenna. (b) Effect(b) Effect on transmission on transmission strength (|Sstrength21|). (|S21|).

Table 9. Effect of the angular misalignment (θr) on the performance of the proposed WPT system. 316 Table 9. Effect of the angular misalignment (θr) on the performance of the proposed WPT system. θ (degree) 0 10 20 30 Sensors 2021, 21, x FOR PEER REVIEW r 13 of 19 θr (degree)Frequency (GHz)0 2.46 2.4310 2.42 20 2.45 30

Frequency|S 21(GHz)| (dB) 2.46−6.26 −2.436.69 −7.79 2.42 −7.89 2.45 |S21| (dB) −6.26 −6.69 −7.79 −7.89 318 4.2. Bending Analysis 4.2. Bending Analysis 317 319 TheThe wearable wearable Rx Rx antenna antenna has beenbeen designeddesigned over over a flexiblea flexible felt felt substrate substrate to eliminate to eliminate 320 thethe difficulty difficulty faced faced with with the the rigid rigid substrate substrate during during the the integration onon thethe humanhuman body. body. To 321 analyzeTo analyze the flexibility the flexibility effect, effect, the the Rx Rx antenna antenna hashas beenbeen wrapped wrapped around around the three-layerthe three-layer 322 cylindricalcylindrical tissue tissue model model of of radius radius R,R, illustratedillustrated in in Figure Figure 16 a.16a. The The variation variation in flexibility in flexibility 323 of theof the Rx Rx antenna antenna is is performed performed by changingchanging the the radius radius R. R. The The effect effect of flexibility of flexibility on the on the 324 couplingcoupling strength strength (|S (|S2121|)|) is is shown shown in in Figure 16 16b,b, with with the the variation variation of R.of FromR. From the figure,the figure, 325 a minora minor change change can can be be perceived perceived duedue toto the the flexibility flexibility of of the the Rx Rx antenna. antenna.

(a) (b)

326 FigureFigure 16. ( 16.a) Geometry(a) Geometry of ofthe the bending bending study study of of the RxRx antenna.antenna. (b ()b Effect) Effect on on transmission transmission (|S21 (|S|)21 strength.|) strength.

327 5. Analysis of Specific Absorption Rate (SAR) 328 The safety of the human body and health is the prime concern against the harmful 329 effects of the electromagnetic (EM) field for human exposure. Therefore, the major chal- 330 lenge regarding the design of an efficient WPT system, particularly for the wearable WPT 331 system, is to maintain the safety standards. Regarding safety, the value of SAR would be 332 taken into consideration for the transmission of high power at enhanced transmission dis- 333 tances. Generally, the SAR is defined by the rate of absorbed energy within the unit mass 334 of the human body when explored in the EM field. The SAR value is also expressed math- 335 ematically by using the following equation [29],

SAR 𝑊/𝐾𝑔 (1)

336 where 𝜎 is the tissue conductivity (S/m), E is the electric field density (V/m), and 𝜌 is the 337 intensity of tissue (Kg/m3). It can be realized from the above expression that the SAR value 338 depends upon the property of human tissue and its density along with the induced E-field 339 from the Tx element. Furthermore, the level of EM radiation absorbed by the human body 340 is directly related to the conductivity of the human organs. Usually, the tissue of human 341 body consists with a lot of water that increases the conductivity. Henceforth, the SAR var- 342 ies on the different body parts of the human [30]. 343 In the present study, the 1-g average SAR distribution for the proposed MTM inte- 344 grated WPT system for the different wearable scenarios is illustrated in Figure 17. From 345 the figure, the obtained peak SAR value in the case of the three-layer tissue, hand, head 346 and torso model are 14.26, 12.41, 8.50 and 2.08 W/kg, respectively. However, regarding 347 the safety of the human body, the maximum average SAR value over the 1-g of cubic 348 tissue model allowed by the IEEE C95.1−1999 standard should be less than or equal to 1.6 349 W/kg [31]. As per the recommendation of the Federal Communications Commission’s 350 (FCC) standards [32], the maximum allowed transmitter output energy fed to the Rx an- 351 tenna at the ISM-bands is at 1 W (30 dBm). Herein, the distribution of the SAR is per-

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5. Analysis of Specific Absorption Rate (SAR) The safety of the human body and health is the prime concern against the harmful effects of the electromagnetic (EM) field for human exposure. Therefore, the major challenge regarding the design of an efficient WPT system, particularly for the wearable WPT system, is to maintain the safety standards. Regarding safety, the value of SAR would be taken into consideration for the transmission of high power at enhanced transmission distances. Generally, the SAR is defined by the rate of absorbed energy within the unit mass of the human body when explored in the EM field. The SAR value is also expressed mathematically by using the following equation [29],

σ|E|2 SAR = [W/kg] (1) ρ

where σ is the tissue conductivity (S/m), E is the electric field density (V/m), and ρ is the intensity of tissue (kg/m3). It can be realized from the above expression that the SAR value depends upon the property of human tissue and its density along with the induced E-field from the Tx element. Furthermore, the level of EM radiation absorbed by the human body is directly related to the conductivity of the human organs. Usually, the tissue of human body consists with a lot of water that increases the conductivity. Henceforth, the SAR varies on the different body parts of the human [30]. In the present study, the 1-g average SAR distribution for the proposed MTM integrated WPT system for the different wearable scenarios is illustrated in Figure 17. From the figure, the obtained peak SAR value in the case of the three-layer tissue, hand, head and torso model are 14.26, 12.41, 8.50 and 2.08 W/kg, respectively. However, regarding the safety of the human body, the maximum average SAR value over the 1-g of cubic tissue model allowed by the IEEE C95.1−1999 standard should be less than or equal to 1.6 W/kg [31]. As per the recommendation of the Federal Communications Commission’s (FCC) standards [32], the maximum allowed transmitter output energy fed to the Rx an- Sensors 2021, 21, x FOR PEER REVIEW 14 of 19 tenna at the ISM-bands is at 1 W (30 dBm). Herein, the distribution of the SAR is performed for the maximum 1 W excitation power set at the Tx patch. Moreover, by decreasing the

352 excitationformed for powerthe maximum at the 1 W transmitter excitation power end, set the at the peak Tx patch. 1-g averageMoreover, by SAR decreas- value should be kept 353 withining thethe excitation standard power limit at the [transmitter20–22]. The end, excitation the peak 1-g poweraverage SAR required value should at the be Tx end to keep the 354 SARkept value within inthe safetystandard limits limit [20–22]. in the The case exci oftation the power three-layer required tissue, at the Tx hand, end to headkeep and torso model 355 the SAR value in safety limits in the case of the three-layer tissue, hand, head and torso 356 aremodel 0.11, are 0.13, 0.11, 0.190.13, 0.19 and and 0.78 0.78 W, W, respectively. respectively.

357

358 FigureFigure 17. 17.Distribution Distribution of SAR of SAR for different for different wearable wearable applications. applications. (a) Three-layer( tissue,a) Three-layer (b) hu- tissue, (b) human 359 man hand, (c) head and (d) torso model. hand, (c) head and (d) torso model. 360 Furthermore, the novel property of metamaterial has also been utilized to reduce the 361 SAR [33–36] without reducing the excitation power at the Tx end. In [33], an MTM struc- 362 ture consists of split-ring resonators (SRRs) has been used to reduce the electromagnetic 363 interface between the antenna and the human head. Furthermore, an MTM-based perfect 364 artificial magnetic conductor (AMC) has been used to minimize the SAR of a planar in- 365 verted-F antenna [34]. Further, the performance enhancement and SAR reduction of a 366 dual-band flexible antenna have been achieved in [35] by using an MTM-slab at the back- 367 side of antenna in WBAN application. In [36], an MTM reflector was recently designed on 368 a felt textile substrate to reduce the SAR effect and increase the performance of an ultra- 369 wideband antenna for WBAN applications. Moreover, the SAR value of the proposed 370 MTM based wearable WPT system might also be reduced by the approach presented in 371 Refs. [33–36].

372 6. Measurement and Discussion 373 To validate the proposed design concept of the wearable WPT system, the different 374 parts such as Tx, Rx antenna, and the array of 3 × 3 MTM slab have been fabricated. The 375 standard printed circuit board (PCB) technology has been used to design the Tx antenna 376 and MTM slab. The wearable Rx antenna is designed over a felt substrate using an adhe- 377 sive copper sheet with a thickness of 0.035 mm. To sustain the conductivity of uniform 378 surface, a single copper sheet has been utilized to construct both the full ground and ra- 379 diating patch along with microstrip feed without any discontinuity or interconnects. The 380 photographs of the fabricated prototype to build the wearable WPT link are shown in 381 Figure 18. The three-layer pork tissue has been used to consider the realistic multi-layered

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Furthermore, the novel property of metamaterial has also been utilized to reduce the SAR [33–36] without reducing the excitation power at the Tx end. In [33], an MTM structure consists of split-ring resonators (SRRs) has been used to reduce the electromagnetic interface between the antenna and the human head. Furthermore, an MTM-based perfect artificial magnetic conductor (AMC) has been used to minimize the SAR of a planar inverted-F antenna [34]. Further, the performance enhancement and SAR reduction of a dual-band flexible antenna have been achieved in [35] by using an MTM-slab at the backside of antenna in WBAN application. In [36], an MTM reflector was recently designed on a felt textile substrate to reduce the SAR effect and increase the performance of an ultra- wideband antenna for WBAN applications. Moreover, the SAR value of the proposed MTM based wearable WPT system might also be reduced by the approach presented in Refs. [33–36].

6. Measurement and Discussion To validate the proposed design concept of the wearable WPT system, the different parts such as Tx, Rx antenna, and the array of 3 × 3 MTM slab have been fabricated. The Sensors 2021, 21, x FOR PEER REVIEWstandard printed circuit board (PCB) technology has been used to design the Tx antenna 15 of 19 and MTM slab. The wearable Rx antenna is designed over a felt substrate using an adhesive copper sheet with a thickness of 0.035 mm. To sustain the conductivity of uniform surface, a single copper sheet has been utilized to construct both the full ground and radiating patch along with microstrip feed without any discontinuity or interconnects. The photographs 382 ofenvironment the fabricated prototype for measurement to build the wearable purpose WPT link[23]. are Furthermore, shown in Figure 18 the. The measurements are con- 383 three-layerducted by pork placing tissue has the been wearable used to consider antenna the realistic over the multi-layered human environmenthand, head and body (chest). The for measurement purpose [23]. Furthermore, the measurements are conducted by placing 384 thepictures wearable of antenna measurements over the human rega hand,rding head the and realistic body (chest). scenario The pictures of the of wearable platforms like 385 measurementsthe three-layered regarding pork the realistic slab and scenario also of at the the wearable different platforms portions like the of three- the human body integrated 386 layeredwith the pork MTM slab and slab also atare the presented different portions in Figure of the human 19. The body integratedMTM slab with has the been attached over the MTM slab are presented in Figure 19. The MTM slab has been attached over the wearable 387 Rxwearable patch antenna Rx bypatch using antenna Styrofoam. by The using measurements Styrofoam. are performed The withmeasurements the help of are performed with 388 thethe Anritsu help S820Eof the vector Anritsu network S820E analyzer. vector network analyzer.

389 390 FigureFigure 18. 18.Photographs Photographs of the fabricated of the prototypes. fabricated (a) Receiving prototypes. wearable ( patcha) Receiving antenna, (b) flexibilitywearable patch antenna, (b) c d × 391 offlexibility the wearable of antenna, the wearable ( ) transmitting antenna, patch and (c () )transmitting MTM slab with 3 patch3 array and of unit (d) cells. MTM slab with 3 × 3 array of 392 unit cells.

393 394 Figure 19. Measurement in different environments. (a) Three-layer pork slab, (b) human hand, (c) 395 head and (d) body (chest).

396 During measurement, the distance (d1) between Tx and Rx antenna is considered 60 397 mm, whereas the MTM slab is placed at a distance (d2) of 8 mm from the wearable Rx 398 antenna, as shown in Figure 19. The Tx, Rx antenna and MTM slab are aligned properly 399 on the same horizontal line during the experiment to obtain better performance from the 400 system. A comparative study between the measured efficiency in the different scenarios 401 for without (W/O) and with the integration of MTM slab is shown in Figure 20a. It can be 402 found from the figure that for the use of MTM slab, the efficiency of the proposed system 403 is improved significantly. Herein, the efficiency has been calculated from the measured 404 transmission (|S21|) coefficient characteristics. Furthermore, the detailed analysis of the 405 obtained measured results from the different wearable scenarios with and without the

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382 environment for measurement purpose [23]. Furthermore, the measurements are con- 383 ducted by placing the wearable antenna over the human hand, head and body (chest). The 384 pictures of measurements regarding the realistic scenario of the wearable platforms like 385 the three-layered pork slab and also at the different portions of the human body integrated 386 with the MTM slab are presented in Figure 19. The MTM slab has been attached over the 387 wearable Rx patch antenna by using Styrofoam. The measurements are performed with 388 the help of the Anritsu S820E vector network analyzer.

389 390 Figure 18. Photographs of the fabricated prototypes. (a) Receiving wearable patch antenna, (b) 391 Sensors 2021, 21, 3448 flexibility of the wearable antenna, (c) transmitting patch and (d16) ofMTM 20 slab with 3 × 3 array of 392 unit cells.

393 394 FigureFigure 19. 19.Measurement Measurement in different in environments. different environments. (a) Three-layer pork ( slab,a) Three-layer (b) human hand, pork slab, (b) human hand, (c) (c) head and (d) body (chest). 395 head and (d) body (chest).

During measurement, the distance (d1) between Tx and Rx antenna is considered 60 mm, whereas the MTM slab is placed at a distance (d2) of 8 mm from the wearable Rx 1 396 antenna,During as shown measurement, in Figure 19. The Tx, the Rx antenna distance and MTM (d ) slabbetween are aligned Tx properly and Rx antenna is considered 60 397 onmm, the same whereas horizontal the line MTM during the slab experiment is placed to obtain at bettera distance performance (d from2) of the 8 mm from the wearable Rx system. A comparative study between the measured efficiency in the different scenarios 398 forantenna, without (W/O) as shown and with in the integrationFigure 19. of MTM The slab Tx, is shownRx antenna in Figure 20 anda. It can MTM be slab are aligned properly 399 foundon the from same the figure horizontal that for the useline of MTMduring slab, the efficiency experiment of the proposed to obtain system better performance from the is improved significantly. Herein, the efficiency has been calculated from the measured 400 system. A comparative study between the measured efficiency in the different scenarios transmission (|S21|) coefficient characteristics. Furthermore, the detailed analysis of the 401 obtainedfor without measured (W/O) results and from thewith different the wearableintegration scenarios of withMTM and slab without is theshown in Figure 20a. It can be 402 integrationfound from of the MTMthe figure slab is summarized that for inthe Table use 10. Fromof MT theM table, slab, it can the be observed efficiency of the proposed system that the measured coupling strength |S21| is improved significantly with the presence of 403 MTMis improved slab, which enhances significantly. the efficiency Herein, of the system. the The effici improvementsency has in the been measured calculated from the measured efficiencies are 9.62%, 9.61%, 10.6% and 8.2% in the case of three-layer pork slab, human 404 transmission (|S21|) coefficient characteristics. Furthermore, the detailed analysis of the hand, head and body (chest), respectively. 405 obtainedFurthermore, measured to present theresults MTM effectfrom on thethe PTE different of the system, wearable transfer distance scenarios with and without the (d1) is increased, keeping the value of d2 fixed at 8 mm in the measurement scenario of the three-layer pork slab. The simulated and measured efficiency plot of the WPT system for the three-layer phantom model and pork slab environment is also presented in Figure 20b. From the figure, it can be observed that by using the ZIM slab, the efficiency level of the proposed WPT system is improved significantly, even with the increase in transmission distance. However, some extent of mismatch is seen between the measured and simulated results, which occurred due to the fabrication tolerance as well as misalignment during measurement. Sensors 2021, 21, x FOR PEER REVIEW 16 of 19

406 integration of the MTM slab is summarized in Table 10. From the table, it can be observed 407 that the measured coupling strength |S21| is improved significantly with the presence of 408 MTM slab, which enhances the efficiency of the system. The improvements in the meas- 409 Sensors 2021, 21, 3448 ured efficiencies are 9.62%, 9.61%, 10.6% and 8.2% in the case of three-layer17 of 20 pork slab, 410 human hand, head and body (chest), respectively.

(a) (b)

411 Figure 20. MeasuredFigure 20. andMeasured simulated and simulated PTE characteristics PTE characteristics in in the the differentdifferent wearable wearable scenarios scenarios with and with without and MTM without slab, MTM slab, (a) at d = 60 mm and (b) at different transfer distances. 412 (a) at d1 = 60 mm and1 (b) at different transfer distances. Table 10. Comparative study between the measured results for the different wearable environment with and without 413 Table 10. ComparativeMTM slab. study between the measured results for the different wearable environment with and without 414 MTM slab. Operating Efficiency Measured MTM Slab |S | |S | Efficiency Frequency 11 21 Improvement Environment Loading (dB) (dB) (%) Operating(GHz) (%) Efficiency Measured MTM Slab |S11| |S21| W/O MTMFrequency 2.47 −22.65 −8.78Efficiency 13.24 (%) - Improvement Environment Pork Loading (dB) (dB) Slab With MTM(GHz) 2.42 −17.20 −6.41 22.86 9.62 (%) W/O MTM 2.45 −16.82 −9.87 10.30 - Pork Hand W/O MTM 2.47 −22.65 −8.78 13.24 - − − Slab With MTMWith MTM 2.42 2.41 −17.2025.487 −7.016.41 19.9122.86 9.61 9.62 W/O MTM 2.44 −18.36 −13.20 4.78 - Head W/O MTM 2.45 −16.82 −9.87 10.30 - Hand With MTM 2.40 −23.24 −8.13 15.38 10.60 With MTM 2.41 −25.487 −7.01 19.91 9.61 Body W/O MTM 2.41 −15.28 −15.12 3.07 - (Chest)W/O MTMWith MTM 2.44 2.38 −18.36−18.70 −−13.209.48 11.274.78 8.20 - Head With MTM 2.40 −23.24 −8.13 15.38 10.60 Body W/O MTM Finally, 2.41 a comparative− study15.28 has been furnished−15.12 in Table 11 with3.07 the proposed work - and works reported in the literature. From the table, it can be perceived that the reported (Chest) With MTM work in the 2.38 literature usually−18.70 employs the non-radiative−9.48 inductive11.27 or resonant coupling- 8.20 based approach to design the wearable WPT system, whereas, in the present study, antenna topology has been adopted. The proposed system has been constructed on a compact, 415 Furthermore,flexible substrate to usingpresent a simple the designMTM configuration. effect on the The PTE system of is the also system, applicable transfer for distance 416 (d1) is increased,different wearable keeping scenarios the value such as of the d human2 fixed hand, at 8 head mm and in body.the measurement scenario of the 417 three-layer pork slab. The simulated and measured efficiency plot of the WPT system for 418 the three-layer phantom model and pork slab environment is also presented in Figure 20b. 419 From the figure, it can be observed that by using the ZIM slab, the efficiency level of the 420 proposed WPT system is improved significantly, even with the increase in transmission 421 distance. However, some extent of mismatch is seen between the measured and simulated 422 results, which occurred due to the fabrication tolerance as well as misalignment during 423 measurement. 424 Finally, a comparative study has been furnished in Table 11 with the proposed work 425 and works reported in the literature. From the table, it can be perceived that the reported 426 work in the literature usually employs the non-radiative inductive or resonant coupling- 427 based approach to design the wearable WPT system, whereas, in the present study, an-

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Table 11. Comparative study between the proposed work and works reported in the literature.

Operating Design Transfer Size of the Rx Flexibility of Efﬁciency Application Ref. Frequency Complexity of Distance (mm × mm × mm) Rx (%) Scenario Rx (mm) Muscle [10] 90 × 90 × 1.5 6.78 MHz Yes Low 60 29.4 model [13] π × 142 × 4 14.5 MHz No High 20 45 Arm model Human body [14] 120 × 80 × 0.787 6.78 MHz Yes High 150 46.2 model Both ﬂexible Human hand [15] π × 322 × 0.75 80 MHz Moderate 60 50 and rigid model Human hand [16] 60 × 60 × 0.266 6.78 MHz Yes High 10 64 model 23.66 (Sim.) Human hand, This work 70 × 70 × 1 2.46 GHz Yes Low 60 22.86 (Meas.) head and body Pork slab

7. Conclusions This article introduces an efficient approach to construct a WPT system employing the radiating characteristics of the antenna for the applications of wearable devices. In the proposed system, the simplest configuration of the Tx and Rx antennas are employed to construct the wearable WPT link, which would be suitable for practical applications. The system is designed to operate in the radiative near-field region of the antennas that provide flexibility in the transmission distance. An MTM slab having the zero-index property is successfully utilized to improve the PTE of the system. Herein. for the first time, MTM structure has been employed to improve the efficiency of the antenna topology-based radiative WPT system, particularly to charge the wearable devices. Furthermore, stable performance is reached from the proposed WPT system irrespective to the misalignments and bending effect of the Rx antenna up to a limit. Further, considering the safety of the human body from EM explore, the study of the SAR is performed in detail for the different wearable environments. Moreover, the analysis of simulated and measured results confirms the feasibility of the proposed concept to improve the PTE of the WPT system. It is highly expected that the presented radiation-based WPT approach to charge wearable devices brings exciting possibilities in the near future.

Author Contributions: Conceptualization, T.S. and D.M.; methodology, T.S.; software, T.S. and G.S.; validation, T.S., G.S., D.M., B.M. and R.A.; formal analysis, T.S.; investigation, T.S., G.S. and D.M.; resources, T.S. and D.M.; data curation, T.S. and G.S; writing—original draft preparation, T.S.; writing—review and editing, G.S., D.M., B.M. and R.A.; visualization, T.S. and D.M.; supervision, G.S., D.M., B.M. and R.A.; project administration, D.M., B.M. and R.A.; funding acquisition, B.M. and R.A. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by the Visvesvaraya Young Faculty Research Fellowship Award, under MeitY, Govt. of India and the Swedish SSF projects, ZeroIOT: Enabling the battery-free Internet of Things (CHI19-0003) and LifeSec: Don’t Hack my Body! (RIT17-0020). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: For research support, D. Mitra acknowledges the Visvesvaraya Young Faculty Research Fellowship Award, under MeitY, Govt. of India. B. Mandal and R. Augustine acknowledge the Swedish SSF projects, ZeroIOT: Enabling the battery-free Internet of Things (CHI19-0003) and LifeSec: Don’t Hack my Body! (RIT17-0020). Conﬂicts of Interest: The authors declare no conﬂict of interest. Sensors 2021, 21, 3448 19 of 20

References 1. Chan, M.; Estève, D.; Fourniols, J.-Y.; Escriba, C.; Campo, E. Smart wearable systems: Current status and future challenges. Artif. Intell. Med. 2012, 56, 137–156. [CrossRef][PubMed] 2. Soh, P.J.; Vandenbosch, G.A.E.; Mercuri, M.; Schreurs, D.M.M.-P. Wearable wireless health monitoring. IEEE Microw. Mag. 2015, 16, 55–70. 3. Seneviratne, S.; Hu, Y.; Nguyen, T.; Lan, G.; Khalifa, S.; Thilakarathna, K.; Hassan, M.; Seneviratne, A. A survey of wearable devices and challenges. IEEE Comm. Sur. Tutor. 2017, 19, 2573–2620. [CrossRef] 4. Chu, B.; Burnett, W.; Chung, J.W.; Bao, Z. Bring on the body NET. Nature 2017, 549, 328–330. [CrossRef] 5. Tesla, N. Apparatus for transmitting electrical energy. U.S. Patent US1119732A, 1 December 1914. 6. Song, M.; Belov, P.; Kapitanova, P. Wireless power transfer inspired by the modern trends in electromagnetics. Appl. Phys. Rev. 2017, 4, 021102. [CrossRef] 7. Hui, S.Y. Planar wireless charging technology for portable electronic products and Qi. Proc. IEEE 2013, 101, 1290–1301. [CrossRef] 8. Shin, J.; Shin, S.; Kim, Y.; Ahn, S.; Lee, S.; Jung, G.; Jeon, S.J.; Cho, D.H. Design and implementation of shaped magnetic- resonance-based wireless power transfer system for roadway-powered moving electric vehicles. IEEE Trans. Ind. Electron. 2014, 61, 1179–1192. [CrossRef] 9. Agarwal, K.; Jegadeesan, R.; Guo, Y.X.; Thakor, N.V. Wireless power transfer strategies for implantable bioelectronics. IEEE Rev. Biomed. Eng. 2017, 10, 136–161. [CrossRef] 10. Kang, S.H.; Choi, J.H.; Harackiewicz, F.J.; Jung, C.W. Magnetic resonant three-coil wpt system between off/in-body for remote energy harvest. IEEE Microw. Wirel. Compon. Lett. 2016, 26, 741–743. [CrossRef] 11. Kang, S.H.; Jung, C.W. Textile resonators with thin copper wire for wearable MR-WPT system. IEEE Microw. Wirel. Compon. Lett. 2017, 27, 91–93. [CrossRef] 12. Lazarus, N.; Bedair, S.S. Improved power transfer to wearable systems through stretchable magnetic composites. Appl. Phys. A 2016, 122, 1–7. [CrossRef] 13. Jeong, M.J.; Yun, T.; Baek, J.J.; Kim, Y.T. Wireless power transmission using a resonant coil consisting of conductive yarn for wearable devices. Text. Res. J. 2016, 86, 1543–1548. [CrossRef] 14. Heo, E.; Choi, K.; Kim, J.; Park, J.; Lee, H. A wearable textile antenna for wireless power transfer by magnetic resonance. Text. Res. J. 2017, 88, 1–9. [CrossRef] 15. Bao, K.; Zekios, C.L.; Georgakopoulos, S.V. A wearable WPT system on flexible substrates. IEEE Antennas Wirel. Propag. Lett. 2019, 18, 931–935. [CrossRef] 16. Wagih, M.; Komolafe, A.; Zaghari, B. Dual-receiver wearable 6.78 MHz resonant inductive wireless power transfer glove using embroidered textile coils. IEEE Access 2020, 8, 24630–24642. [CrossRef] 17. Kurs, A.; Moffatt, R.; Soljacic, M. Simultaneous mid-range power transfer to multiple devices. Appl. Phys. Lett. 2010, 96, 044102. [CrossRef] 18. DeLong, B.J.; Kiourti, A.; Volakis, J.L. A radiating near-field patch rectenna for wireless power transfer to medical implants at 2.4 GHz. IEEE J. Electromagn. RF Microw. Med. Biol. 2018, 2, 64–69. [CrossRef] 19. Gowda, V.R.; Yurduseven, O.; Lipworth, G.; Zupan, T.; Reynolds, M.S.; Smith, D.R. Wireless power transfer in the radiative near field. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1865–1868. [CrossRef] 20. Das, R.; Yoo, H. A multiband antenna associating wireless monitoring and nonleaky wireless power transfer system for biomedical implants. IEEE Trans. Microw. Theory Tech. 2017, 65, 2485–2495. [CrossRef] 21. Shaw, T.; Mitra, D. Metasurface-based radiative near-field wireless power transfer system for implantable medical devices. IET Microw. Antennas Propag. 2019, 13, 1974–1982. [CrossRef] 22. Shaw, T.; Samanta, G.; Mitra, D. Efficient wireless power transfer system for implantable medical devices using circular polarized antennas. IEEE Trans. Antennas Propag. 2020.[CrossRef] 23. Fan, Y.; Huang, J.; Chang, T.; Liu, X. A miniaturized four-element MIMO antenna with EBG for implantable medical devices. IEEE J. Electromagn. RF Microw. Med. Biol. 2018, 2, 226–233. [CrossRef] 24. The “Nello Carrara” Institute of Applied Physics (IFAC). Available online: http://niremf.ifac.cnr.it/tissprop/htmlclie/uniquery. php?func=atsffun&freq=2450000000&tiss=&outform=disphtm&tisname=on&frequen=on&conduct=on&permitt=on&losstan= on&wavelen=on&pendept=on&freq1=2450000000&tissue2=Air&frqbeg=10&frqend=100e9&linstep=100&mode=log&logstep= 5&tissue3=SkinWet&freq3=1000000 (accessed on 12 January 2021). 25. Shaw, T.; Bhattacharjee, D.; Mitra, D. Gain enhancement of slot antenna using zero-index metamaterial superstrate. Int. J. RF Microw. Comput. Aided Eng. 2017, 27, 1–10. [CrossRef] 26. Szabo, Z.; Park, G.-H.; Hedge, R.; Li, E.-P. A unique extraction of metamaterial parameters based on Kramers-Kronig relationship. IEEE Trans. Microw. Theory Tech. 2010, 58, 2646–2653. [CrossRef] 27. Saenz, E.; Gonzalo, R.; Ederra, I.; Vardaxoglou, J.C.; de Maagt, P. Resonant meta-surface superstrate for single and multi-frequency dipole antenna arrays. IEEE Trans. Antennas Propag. 2008, 56, 951–960. [CrossRef] 28. Gabriel, S.; Lau, R.; Gabriel, C. The dielectric properties of biological tissues: III. parametric models for the dielectric spectrum of tissues. Phys. Med. Biol. 1996, 41, 2271–2293. [CrossRef] Sensors 2021, 21, 3448 20 of 20

29. Hosain, M.K.; Kouzani, A.Z.; Tye, S.J.; Abulseoud, O.A.; Amiet, A.; Galehdar, A.; Kaynak, A.; Berk, M. Development of a compact rectenna for wireless powering of a head-mountable deep brain stimulation device. IEEE J. Trans. Eng. Health Med. 2014, 2, 1–13. [CrossRef] 30. Liu, X.; Chen, H.-J.; Alfadhl, Y.; Chen, X.; Parini, C.; Wen, D. Conductivity and frequency dependent specific absorption rate. J. Appl. Phys. 2013, 113, 074902. [CrossRef] 31. IEEE Standards Coordinating Committee. C95.1-1991—IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz. 1999. Available online: https://ieeexplore.ieee.org/document/159488 (accessed on 15 May 2021). 32. Liu, C.; Guo, Y.X.; Sun, H.; Xiao, S. Design and safety considerations of an implantable rectenna for far-field wireless power transfer. IEEE Trans. Antennas Propag. 2014, 62, 5798–5806. [CrossRef] 33. Islam, M.T.; Faruque, M.R.; Misran, N. Reduction of specific absorption rate (SAR) in the human head with ferrite material and metamaterial. Prog. Electromagn. Res. 2009, 9, 47–58. [CrossRef] 34. Kwak, I.S.; Sim, D.U.; Kwon, J.H.; Yoon, Y.J. Design of PIFA with metamaterials for body-SAR reduction in wearable applications. IEEE Trans. Electromagn. Comp. 2016, 59, 297–300. [CrossRef] 35. Wang, M.; Yang, Z.; Wu, J.; Bao, J.; Liu, J.; Cai, L.; Dang, T.; Zheng, H.; Li, E. Investigation of SAR reduction using flexible antenna with metamaterial structure in wireless body area network. IEEE Trans. Antennas Propag. 2018, 66, 3076–3086. [CrossRef] 36. Yalduz, H.; Koç, B.; Kuzu, L.; Turkmen, M. An ultra-wide band low-SAR flexible metasurface-enabled antenna for WBAN applications. Appl. Phys. A 2019, 125, 1–11. [CrossRef]