Meta-Wearable Antennas—A Review of Metamaterial Based Antennas in Wireless Body Area Networks

materials

Review Meta-Wearable Antennas—A Review of Metamaterial Based Antennas in Wireless Body Area Networks

Kai Zhang 1 , Ping Jack Soh 2,3 and Sen Yan 1,*

1 School of Information and Communications Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] 2 Advanced Communication Engineering (ACE) Centre of Excellence, Universiti Malaysia Perlis, Kangar 01000, Malaysia; [email protected] 3 Faculty of Electronic Engineering Technology, Universiti Malaysia Perlis, Arau 02600, Malaysia * Correspondence: [email protected]

Abstract: Wireless Body Area Network (WBAN) has attracted more and more attention in many sectors of society. As a critical component in these systems, wearable antennas suffer from several serious challenges, e.g., electromagnetic coupling between the human body and the antennas, different physical deformations, and widely varying operating environments, and thus, advanced design methods and techniques are urgently needed to alleviate these limitations. Recent developments have focused on the application of metamaterials in wearable antennas, which is a prospective area and has unique advantages. This article will review the key progress in metamaterial-based antennas for WBAN applications, including wearable antennas involved with composite right/left-handed transmission lines (CRLH TLs), wearable antennas based on metasurfaces, and reconﬁgurable wearable antennas based on tunable metamaterials. These structures have resulted in improved performance of wearable antennas with minimal effects on the human body, which consequently will result in more reliable wearable communication. In addition, various design methodologies of meta-wearable antennas are summarized, and the applications of wearable antennas by these methods are discussed. Keywords: metamaterial; wearable antenna; metasurface; wireless body area network Citation: Zhang, K.; Soh, P.J.; Yan, S. Meta-Wearable Antennas—A Review of Metamaterial Based Antennas in Wireless Body Area Networks. 1. Introduction Materials 2021, 14, 149. https://doi.org/10.3390/ma14010149 The emergence of the Internet has brought about tremendous changes in the communication of human. Wireless Body Area Network (WBAN) is one of the emerging

Received: 20 November 2020 technologies that is capable of enabling the communication between people and things. Accepted: 28 December 2020 The development of body area network will result in a more context-aware and personal- Published: 31 December 2020 ized communication in an intelligent wireless environment. WBAN is a network distributed around human, which is mainly used to detect and transmit physiological data of users, Publisher’s Note: MDPI stays neu- and cooperate with other networks to integrate the human into the overall network [1–5]. tral with regard to jurisdictional clai- Through WBAN, communication and data synchronization can take place to complete the ms in published maps and institutio- other communication networks, such as wireless sensor networks and mobile communi- nal afﬁliations. cation networks [1,2,6,7]. An important application of WBAN is in healthcare, where the body area network can transmit physiological information obtained from patients through various physiological sensors, such as blood pressure, blood sugar concentration, temperature, weight, and heartbeat [1,8–10] to the hospital’s medical monitoring equipment or the Copyright: © 2020 by the authors. Li- user’s personal mobile terminal [11]. In entertainment, a personal media device with high censee MDPI, Basel, Switzerland. This article is an open access article speed communication capability will enable augmented/virtual/mixed reality interaction distributed under the terms and con- with users, and wirelessly communicate with a device such as glasses [12] or headset [13]. ditions of the Creative Commons At- In military applications, WBAN can provide personal location and mobile communications tribution (CC BY) license (https:// by a helmet [14,15] or a smart watch [16], etc. Figure1 shows some typical applications of creativecommons.org/licenses/by/ WBANs. 4.0/).

Materials 2021, 14, 149. https://doi.org/10.3390/ma14010149 https://www.mdpi.com/journal/materials Materials 2020, 14, x FOR PEER REVIEW 2 of 20

Materials 2021, 14, 149communications by a helmet [14,15] or a smart watch [16], etc. Figure 1 shows some typi- 2 of 20 cal applications of WBANs.

Medical monitoring Location

Wearable devices

X JT U Game

Virtuality and reality Wireless communication

Figure Figure1. A Wireless 1. A Wireless Body Area Body Network Area Network (WBAN) (WBAN) and its andapplications its applications [1,8,9–16 [1,].8 –16].

Wearable antennas,Wearable as antennas,a vital component as a vital in componentWBAN systems, in WBAN enable systems, wireless enablecommu- wireless com- nication withmunication other devices with on other or off devices human on bodies or off [ human17–43]. Compared bodies [17– to43 ].traditional Compared an- to traditional tennas, the designantennas, of wearable the design antennas of wearable are facing antennas many are development facing many bottleneck developments: The bottlenecks: electromagneticThe coupling electromagnetic between coupling the human between body and the the human antenna, body the and varying the antenna, physical the varying deformationphysicals, the widely deformations, varying operating the widely environments varying operating, and limitations environments, of the fabrica- and limitations of tion process the[27– fabrication33]. Further, process the requirements [27–33]. Further, for these the wearable requirements antennas for these include wearable me- antennas chanical robustness,include mechanicallow-profile, robustness, lightweight, low-profile, user comfort lightweight,, fabrication user simplicity comfort, [17 fabrication–24], simplicity [17–24], wideband [25,26], and multiband [20,27,29]. Thus, advanced design methods wideband [25,26], and multiband [20,27,29]. Thus, advanced design methods and tech- and techniques are urgently needed to address these problems and demands of wearable niques are urgently needed to address these problems and demands of wearable anten- antennas. In recent years, there has been much literature reporting the fabric material nas. In recent years, there has been much literature reporting the fabric material manufac- manufacturing and treatment: Embroidered fabric material, sewn textile materials, woven turing and treatment: Embroidered fabric material, sewn textile materials, woven fabrics, fabrics, materials that are not woven, knitted fabrics, spun fabrics, braiding, coated fabrics materials that are not woven, knitted fabrics, spun fabrics, braiding, coated fabrics through/lamination, printed fabrics, and chemically treated fabrics [18,19]. Furthermore, through/lamination, printed fabrics, and chemically treated fabrics [18,19]. Furthermore, novel forms of flexible devices such as a fully inkjet-printed antenna [30,31], a polydimethyl- novel forms of flexible devices such as a fully inkjet-printed antenna [30,31], a polydime- siloxane (PDMS)-based antenna [21,22], embroidery [32], and a silicone-based antenna [33], thylsiloxane (PDMS)-based antenna [21,22], embroidery [32], and a silicone-based an- and devices combined with new design methods such as substrate-integrated waveguide tenna [33], and devices combined with new design methods such as substrate-integrated (SIW) technology [34], miniature feeding network [23], magneto-electric dipole [35], char- waveguide (SIW) technology [34], miniature feeding network [23], magneto-electric di- acteristic mode theory [27,36,37], textile-type indium gallium zinc oxide (IGZO)-based pole [35], characteristictransistors [ 30mode], and theory thin-film [27, transistor36,37], textile technologies-type indium [24], aregallium presented zinc foroxide special applica- (IGZO)-basedtion transistors scenarios. [30 Furthermore,], and thin-film miniaturization transistor technologies methods, such[24], asare inductor/capacitor-loaded presented for special applicationantennas scenario [38–40s.], Furthermore loop antennas, miniaturization [41], and planar methods, inverted such F antennasas inductor/ca- (PIFA) [42,43] are pacitor-loadedinvolved antennas in WBAN [38–40 devices], loop design,antennas which [41] is, and helpful planar to improve inverted the F design antennas flexibility of the (PIFA) [42,43wearable] are involved antennas. in WBAN devices design, which is helpful to improve the design flexibility of Metamaterialsthe wearable antennas. are widely defined as an artificial periodic structure, in which the Metamaterials are widely defined as an artificial periodic structure, in which the length of the unit cell p is much smaller than the guided wavelength λg, with unusual length of theproperties unit cell notp is availablemuch smaller in nature than in the electromagneticguided wavelength field λ [44g, –with46]. Theunusual use of metamate- properties notrials available has been in hugely nature successful in the electromagnetic in adapting conventional field [44–46 antenna]. The use designs of met- into a wearable amaterials hasform, been including hugely compositesuccessful right/left-handedin adapting conventional transmission antenna lines designs (CRLH into TLs) a based antennas [45–54], zero-order antennas [55,56], metamaterials-inspired antennas [32,57], artificial magnetic conductor (AMC) [58–61], electromagnetic band-Gap (EBG) [62–65], and High- Impedance Surface (HIS) [16]. Wearable antennas have been designed with properties such as multiple band operations [48,55], multiple functionalities [66–69], and gain en- Materials 2020, 14, x FOR PEER REVIEW 3 of 20

wearable form, including composite right/left-handed transmission lines (CRLH TLs) based antennas [45–54], zero-order antennas [55,56], metamaterials-inspired antennas Materials 2021, 14, 149 [32,57], artificial magnetic conductor (AMC) [58–61], electromagnetic band-Gap (EBG)3 of 20 [62–65], and High-Impedance Surface (HIS) [16]. Wearable antennas have been designed with properties such as multiple band operations [48,55], multiple functionalities [66–69], and gain enhancement [70,71] while maintaining a low profile and compact size [56,72,73] duehancement to the development [70,71] while of maintainingthe electromagnetic a low profile metamaterials and compact tech size. Besides [56,72 the,73 ]met duehods to the above,development characteristic of the mode electromagnetic theory is also metamaterials an excellent tech. analysis Besides approach the methods to study above, the charac-met- amaterialsteristic mode-based theory antennas is also [74 an– excellent76], and it analysis improve approachs the efficiency to study of the antennas metamaterials-based design in WBAN.antennas Several [74– 76initial], and attempts it improves have shown the efficiency that it is of an antennas effective designmethod in to WBAN. design wear- Several ableinitial antennas attempts based have on metamaterials. shown that it is an effective method to design wearable antennas basedThis on manuscript metamaterials. reviews the recent progress in the field of wearable antennas with metamaterials.This manuscript Firstly, several reviews designs the recent based progress on CRLH in the TLs, field a typical of wearable 1D metamaterial, antennas with willmetamaterials. be summarized. Firstly, The severaldesigns designs easily realize based ona compact CRLH TLs, size aor typical dual-ba 1Dnd metamaterial, operating bands.will beNext, summarized. the designs Thebased designs on 2D easilymetamaterials, realize a compacti.e., metasurfaces, size or dual-band will be discussed. operating Thebands. main Next,advantages the designs of these based antennas on 2D are metamaterials, the low-profile i.e., and metasurfaces, the wide operating will be discussed. band. The main advantages of these antennas are the low-profile and the wide operating band. Finally, several antennas with reconfigurability will be shown based on reconfigurable Finally, several antennas with reconfigurability will be shown based on reconfigurable metamaterials. The uniqueness of this review is that the design methodologies of the metamaterials. The uniqueness of this review is that the design methodologies of the wearable antennas based metasurface are summarized in recent years, and the merits and wearable antennas based metasurface are summarized in recent years, and the merits drawbacks of the various approaches to design wearable metasurface antennas are com- and drawbacks of the various approaches to design wearable metasurface antennas are pared and discussed. Furthermore, the applications of wearable metasurface antennas de- compared and discussed. Furthermore, the applications of wearable metasurface antennas signed by these methods are described, satisfying the diversified demands in our life. Be- designed by these methods are described, satisfying the diversified demands in our life. sides, the challenges and the directions in metamaterial-based antennas in WBAN in the Besides, the challenges and the directions in metamaterial-based antennas in WBAN in the future are emphasized. future are emphasized.

2. 2.Wearable Wearable Antennas Antennas Based Based on on Composite Composite Right/ Right/Left-HandedLeft-Handed Transmission Transmission Lines Lines (CRLH(CRLH TLs) TLs) 2.1.2.1. Wearable Wearable Electrical Electrical Small Small Antennas Antennas (ESAs) (ESAs) Based Based on onZeroth Zeroth-Order-Order Resonance Resonance (ZOR) (ZOR) CRLHCRLH TLs TLs is one is one form form of ofelectroma electromagneticgnetic metamaterials, metamaterials, which which usually usually is ispresented presented as asa uniform a uniform period periodicic structure. structure. A Aunit unit cell cell of ofa typical a typical CRLH CRLH TLs TLs is shown is shown in inFigure Figure 2a2,a, includingincluding a parallel a parallel resonant resonant topology topology and and a series a series resonant resonant topology topology.. According According to tothis, this, thethe dispersio dispersionn relation relation is drawn is drawn in inFigure Figure 2b2, b,where where the the Δz∆ isz isthe the length length of ofthe the unit unit cell. cell. TheThe intersection intersectionss between between the the dispersion dispersion curve curve and and ω-ωaxis-axis mean mean that that the the resonator resonator can can generategenerate the the zero zero-order-order resonances. resonances.

ω Z ' ' ∆ ' ∆ LzR CzL / −βc βc

' ωΓ1 ' Y Cz∆ ' ∆ R LzL / ω0

ω Γ2 β∆z ∆z (a) (b)

FigureFigure 2. ( 2.a)( Thea) The circuit circuit model model of ofa unit a unit cell cell of ofwith with composite composite right/left right/left-handed-handed transmission transmission lines lines (CRLH(CRLH TLs) TLs) consists consists of oftwo two structures: structures: A Aparallel parallel resonant resonant topology topology and and a series a series resonant resonant topol- topology, ogy,and and (b )(b the) the dispersion dispersion curve curve is is obtained obtained when when the the electromagnetic electromagnetic wave wave through through the the circuit circuit of the of the unit cell with a length of Δz, where the ωΓ1 and ωΓ2 are the two frequencies of zero-order unit cell with a length of ∆z, where the ωΓ1 and ωΓ2 are the two frequencies of zero-order resonance. resonance. Generally,there are three kinds of forms in CRLH zero-order resonance antennas: Epsilon- negativeGenerally, resonance there (ENR)are three[56 ,kinds68,69, 72of, 73forms], Mu-negative in CRLH zero resonance-order (MNR)resonance [14 ,antennas:48–52], and Epsilondual-negative-negative resonance resonance (DNR) (ENR) [ 53[56,54,68,77,69–,8072].,73 ENR], Mu is- enablednegative by resonance loading shunt(MNR) inductance [14,48– 52in], and the equivalentdual-negative circuit resonance of a conventional (DNR) [53 transmission,54,77–80]. ENR line, is and enabled the effective by loading permittivity shunt of the unit cell is negative. MNR indicates that the series capacitance is loaded in the equivalent circuit of the conventional transmission line, and the effective permeability of the unit cell is negative. DNR represents that both the effective permittivity and effective permeability are negative. Cheng et al. designed a patch loaded with a complementary split-ring resonator (CSRR), which is fabricated on a ﬂexible substrate and folded in a cylindrical shape, Materials 2020, 14, x FOR PEER REVIEW 4 of 20

inductance in the equivalent circuit of a conventional transmission line, and the effective permittivity of the unit cell is negative. MNR indicates that the series capacitance is loaded in the equivalent circuit of the conventional transmission line, and the effective permea-

Materials 2021, 14, 149 bility of the unit cell is negative. DNR represents that both the effective permittivity4 ofand 20 effective permeability are negative. Cheng et al. designed a patch loaded with a complementary split-ring resonator (CSRR), which is fabricated on a flexible substrate and folded in a cylindrical shape, form- ingforming a self a-packaged self-packaged folded folded patch patch antenna antenna as shown as shown in Figure in Figure 3a 3[a77 []77. The]. The advantage advantage of thisof this antenna antenna is isthat that the the cavity cavity of of the the package package can can shield shield the the electromagnetic electromagnetic wave and alleviate electromagnetic interference interference (EMI). (EMI). The The loaded CSRR patch is modeled as anan RLC tank, and the microstrip components c consistingonsisting of the patch and the feeding line are representedrepresented by the capacitive and the inductive parameters, respectively, respectively, which can be modeled as as a ann LC tank, whose equivalent circuit is shown in Figure 33b.b. Essentially,Essentially, thisthis antenna employed thethe principleprinciple ofof DNR.DNR.There There exists exists both both negative negative equivalent equivalent permittivity permittiv- ityand and equivalent equivalent permeability, permeability and, and the the performances performances detail detail are are listed listed in Table in Table1. 1.

Lc1 Lc2 Lf Port 1

Cc1 Cc2

Lr Cr Rr Lp Cp Rp

(a) (b)

Figure 3. AA compact compact omnidirectional omnidirectional self self-packaged-packaged patch patch antenna antenna with with complementary complementary split split-ring-ring resonatorresonator loading for wireless endoscopeendoscope applicationsapplications [[7777].]. ((aa)) TheThe modelmodel ofof thethe packagepackage state state of of the the antenna. (b) Equivalent circuit of the antenna. antenna. (b) Equivalent circuit of the antenna.

Another DNR flexible flexible antenna without via via-holes-holes is designed by Kim et al.al. [78]. The printed circuit board can be represented as the unit cell of CRLH TLs circuit model in Figure 22. This viavia freefree antenna antenna can can significantly significantly alleviate alleviate the the complexity complexity and and fabrication fabrication cost. cost.The proposedThe proposed structure structure is involved is involved with with a vertically a vertically polarized polarized antenna antenna with awith monopole- a mon- opolelike radiation-like radiation pattern, pattern, which which is suitable is suitable for many for many commercial commercial and personal and personal electronics. electronics.An air bridgeAn air isbridge introduced is introduced for reducing for reducing loss, and loss, the and measured the measured radiation radiation efficiency effi- of ciencythe planar of the ZOR planar antenna ZOR confirmsantenna confirms a maximum a maximum improvement improvement of nearly of 10% nearly at the10% central at the centraloperating operating frequency frequency of 2.37 GHz.of 2.37 GHz. Wearable antennasantennas dodo notnot only only require require a a good good radiation radiation performance performance and and a miniature a minia- turedimension dimension but alsobut needalso need to have to have a low a Specificlow Specific Absorption Absorption Rate (SAR)Rate (SAR) for human for hum bodyan bodytissue. tissue. In 2010, In Jung2010, et Jung al. presented et al. presented a flexible a DNR flexible zeroth-order DNR zeroth antenna,-order which antenna, introduced which introducedfour CRLH four TL unit CRLH cells TL and unit possessed cells and aposses zero-phasesed a zero constant,-phase representing constant, representing a minimiza- a minimizationtion [79]. This [ antenna79]. This is antenna fabricated is fabricated on Rodgers on RO3003 Rodgers with RO3003 a thickness with a of thickness 0.5 mm, whose of 0.5 mm,electoral whose length electoral of the length unit cell of isthe designed unit cell to is be designed 0.04 λg. to Its be ZOR 0.04 frequency λg. Its ZOR is 2.45 frequency GHz and is 2.45the10dB GHz bandwidthand the 10dB is 6.5%.bandwidth However, is 6.5%. this However, antenna is this not antenna a good candidate is not a good for a candidate wearable fordevice a wearable because device the most because energy the radiated most energy from it radiated would be from absorbed it would by users’be absorbed bodies. by It usersmay’ lead bodies. to a It high may SAR leadvalue to a high when SAR mounted value when on human mounted bodies. on human In order bodies. to overcome In order tothis overcome defect, Lee this et defect, al. proposed Lee et al a. wristproposed antenna a wrist as the antenna same principle,as the same which principle, is designed which isusing designed a Coplanar using Waveguide a Coplanar Ground Waveguide (CPWG) Ground technique (CPWG) in 2011 technique [80]. The in performance2011 [80]. The of performancethe proposed of antenna the proposed is insensitive antenna to theis insensitive on-body condition to the on- becausebody cond theition ground because plate the on groundthe bottom plate of on the the substrate bottom can of the actually substrate play can an isolator. actually The play finite an isolator. size ground The finite plane size can provide high isolation between the antenna and human tissue and promote the antenna property of radiation. In the wearable antenna field, the research on ENR and MNR zero-order mode antenna is rare. However, the proper utilization of them on other kinds of antennas has been published [48–52,56,68,73]. Considering the difficult implementation of both the capacitors and the inductors, new types of metamaterials that support only ENR or MNR mode is a new direction in wearable ZOR antennas. Compared with DNR antennas, ENR antennas can be utilized as a simple structure antenna [56]. For the foreseeable future, both the ENR Materials 2020, 14, x FOR PEER REVIEW 5 of 20

ground plane can provide high isolation between the antenna and human tissue and promote the antenna property of radiation. In the wearable antenna field, the research on ENR and MNR zero-order mode antenna is rare. However, the proper utilization of them on other kinds of antennas has been published [48–52,56,68,73]. Considering the difficult implementation of both the capacitors and the inductors, new types of metamaterials that support only ENR or MNR mode is a new direction in wearable ZOR antennas. Compared with DNR antennas, ENR anten- Materials 2021, 14, 149 nas can be utilized as a simple structure antenna [56]. For the foreseeable5 future, of 20 both the ENR and the MNR antennas will have a widely use in wearable designs, in addition to DNR structures. and the MNR antennas will have a widely use in wearable designs, in addition to DNR structures.2.2. Wearable Dual-Band Patch Antennas Based on CRLH TLs

2.2. WearableTwo Dual-Band methods Patch are Antennasgenerally Based used on CRLHto design TLs a wearable dual-band patch antenna based on CRLH TL: By either using two different resonant orders or using dual-zero res- Two methods are generally used to design a wearable dual-band patch antenna based on CRLHonances. TL: ByThe either method using of two using different two different resonant ordersresonances or using refers dual-zero to the resonances. two bands either be- Theing method represented of using by two negative different modes, resonances zeroth refers-order to the modes, two bands positive either modes, being or rep- the combina- resentedtions byof negativethem. Yan modes, et al. zeroth-orderpresented two modes, wearable positive antennas modes, orloaded the combinations with CRLH TL topolo- of them.gies using Yan et the al. presentedprinciple twoof two wearable symmetric antennas resonant loaded orders with CRLH [57,58 TL]. More topologies specifically, the usingtwo the resonances principle of are two with symmetric the same resonant mode orders number [57,58 but]. Morewith specifically,different sign thes two (±n). The sub- resonancesstrates areof the with proposed the same modewearable number antennas but with are different a 3 mm signs and (a± 6n). mm The thick substrates felt with relative of thepermittivity proposed wearable of 1.3 and antennas loss tangent are a 3of mm 0.044. and The a 6 mmtwo thickresonances felt with are relative observed per- at 2.45 and mittivity of 1.3 and loss tangent of 0.044. The two resonances are observed at 2.45 and 5.15 GHz, corresponding to the first order of the negative mode and the positive mode. 5.15 GHz, corresponding to the first order of the negative mode and the positive mode. ThisThis configuration configuration results results in a similar in a similar field distribution field distribution and thus similarand thus radiation similar patterns radiation patterns in thein twothe bands.two bands. The antenna The antenna model andmodel its performancesand its performances in S11 and in radiation S11 and patterns radiation patterns are shownare shown in Figures in Figures4 and5 .4 and 5.

(a)

(b) (c)

FigureFigure 4. A4. compactA compact all-textile all-textile dual-band dual-band antenna antenna loaded loaded with metamaterial-inspired with metamaterial- structureinspired [57structure]. (a) The [57 simulation]. (a) The simulation modelmodel and and the the prototype prototype (in mm). (in mm). (b) The (b) E-ﬁeld The E distribution-field distribution at 2.45 GHz at 2.45 (−1 GH mode)z (− and1 m 5.2ode) GHz and (+1 5.2 mode) GHz in(+1 z-direction. mode) in z-direction. (c)(c S11) S11 curves curves of the of the wearable wearable antenna antenna in simulation in simulation and measurement. and measurement.

Materials 2021, 14, 149 6 of 20 Materials 2020, 14, x FORMaterials PEER 2020 REVIEW, 14, x FOR PEER REVIEW 6 of 20 6 of 20

Figure 5. SimulatedFigure and measured 5. Simulated radiation and measured patterns radiationradiationin the (a) patternslowerpatterns band inin the thein- (plane,(aa)) lowerlower (b) band bandupper in-plane, in -plane, ( b(b)) upper upper band band in-plane, (c) lowerin-plane,band inband-plane, (c) in lower- plane,(c) lower band and in-plane,band (d) upper in-plane, and band (d and) in upper- (planed) upper band [57]. in-planeband in-plane [57]. [57].

In 2018, Lee et al.In proposed 2018,2018, LeeLee a etet compact al.al. proposed dual - aband compact patch dual-banddual antenna-band using patch MNR antenna for usinga MNR forfor aa smart helmet operatingsmart helmet in the 2.4 operating GHz and inin thethe5 GHz 2.42.4 GHzbandsGHz andand [14 5].5 GHzGHzThis antenna bandsbands [[1414 consists].]. This antennaof two consistsconsists ofof twotwo MNR loops, whichMNR are loops, coupled which to each are coupledother to toexcite each a othertwo-order toto exciteexcite resonance aa two-ordertwo- orderusing resonanceresonancethe usingusing thethe outer MNR loop.outer At 2.4 MNR GHz, loop. a ZOR At mode 2.4 GHz, is introduced a ZOR mode by isthe introduced current on by the the outer current loops on the outer loops of the two MNRs,of which the two flows MNRs, in the which same flows flows direction in the with same no direction significant with phase no significant significant difference. phase difference. On the other hand,On at the 5 GHz, other the hand, current at 5 GHz, on the the outer current loop on flows the outer in the loop opposite flows flows direction, in the opposite direction, indicating the firindicatingst mode. The thethe two firstfirst modes mode. will The generate two modes different will generate radiation different patterns, radiation which patterns, whichwhich are suitable for onare- and suitable off-body for on-on communications- and off-bodyoff-body communicationscommunications using a single radiator. usingusing aa singlesingle radiator.radiator. There is very limitedThere literature isis veryvery limitedlimited on structures literature literature providing on on structures structures dual providing-zero providing resonances. dual-zero dual- zeroNone- resonances. resonances. Nonethe- None- theless, a compactless,theless, dual a- compactba a compactnd antenna dual-band dual using-band antennadual antenna zeroth using using-order dual dual resonance zeroth-order zeroth- fororder smart resonance resonance mobile for for smartsmart mobilemobile phone applicationphone was applicationproposedapplication in was was[53] proposed .proposed The two inCRLH in [ 53[53].] . TheTL The structures two two CRLH CRLH, as TL TLshown structures, structures in Figure as, as shown shown in inFigure Figure6 , 6, are excited usingare6, are exciteda foldedexcited using feedline, using a folded a andfolded feedline, each feedline, contributed and and each each contributeda ZOR contributed mode a,ZOR in awhich ZOR mode, modeeach in which , in which each bandeach band being able beingbandto be beingtuned able to ableflexibly. be tunedto be Here, tuned flexibly. the flexibly. performa Here, theHere,nces performances the of performaa part of of ncesthe a part CLRHof a of part the TLs CLRHof the CLRH TLs based TLs antennas are listed in Table1 for comparison. based antennas arebased listed antennas in Table are 1 forlisted comparison. in Table 1 for comparison.

CL1 LR1 LR2 CL1 CL2 LR1 LR2 CL2

CR1 LL1 PortCR 11 LL1 LLPort2 1 CR2 LL2 CR2

Port 1 Port 1 Port 1 Port 1

(a) (a) (b) (b)

Figure 6. FigureA compact 6. A multibandcompactFigure multiband antenna 6. A compact employing antenna multiband employing dual-band antenna dual CRLH-band employing TL CRLH for smart TLdual for mobile-band smart phoneCRLH mobile applicationTL phone for smart [53 mobile].(a) The phone application [53]. (a) The schematic diagram of the antenna with the circuit parameters. (b) The schematicapplication diagram of [ the53]. antenna(a) The schematic with the circuit diagram parameters. of the antenna (b) The with equivalent the circuit circuit parameters. of this antenna. (b) The equivalent circuit ofequivalent this antenna. circuit of this antenna.

Materials 2021, 14, 149 7 of 20

Table 1. Performances among the CRLH TL-based wearable antennas.

Ref. Frequency (GHz) Bandwidth Gain (dBi) Size (λ2) SAR (W/kg) Substrate (εr) [53] 0.88/1.9 16%/29% 0.06/2.2 * 0.08(@0.88) - Substrate (2.41) [57] 2.45/5.2 5.5%/11.2% −3.5/6.6 0.16(@2.45) 0.012/0.25 Felt substrate (1.3) [58] 2.45/5.4 3.8%/7.6% −1.37/4.68 0.39(@2.45) Felt substrate (1.3) [77] 2.4 - −5.2 0.01 - RT/Duroid 5880 (2.2) [78] 2.4 3% - 0.04 - Substrate (2.2) [79] 2.4 6.5% 1.39 0.1 - Rodgers RO 3003 (3.0) [80] 2.4 2.5% −7 - - RT/Duroid 5880 (2.2) * (@0.88) means that the λ is the wavelength in free space at 0.88 GHz.

3. Wearable Antennas Based on Metasurfaces The in-phase reflection characteristic of Artificial Magnetic Conductor (AMC) structures can effectively reduce backward radiation of the wearable antenna while maintaining a low antenna profile, improve gain, and reduce the coupling between the antenna and the human body. Thus, recently, metasurface-based wearable antennas have become a popular method to improve the radiation behavior for wearable antennas.

3.1. Single-Band Wearable Antennas with Metasurfaces It is commonly known that the radiated energy from wearable antennas may poten- tially be harmful to human health [81,82]. Due to its proximity to the human body, the biological tissues in the human body may be affected by the radiated energy on the one hand, whereas on the other hand, the human body may absorb and scatter the radiated energy intended to be directed into free space. The latter deteriorates the performance of the antenna. To alleviate these effects, researchers have adopted AMC structures in low-profile wearable antennas, which will act as a reflector for the radiated energy in specified frequencies. Through analyzing the metasurface antenna design have proposed in literature. The conventional method is that the near-zero reflection phase of the unit cell is set at around the resonance frequency by controlling the topologies of the AMC unit cells [20,61,64,65,83,84], which is also the main way to design a metasurface wearable antenn. For the decrease of the SAR and improvement of the gain, another work in [63] proposed an AMC-backed printed Yagi–Uda antenna for on-body communication. The proposed antenna is made flexible using the combination of copper foil tape and polyester sheet. The antenna operates in the 2.4 GHz industrial, scientific, and medical (ISM) band. The printed Yagi-Uda is used to achieve a high gain, endfire radiation pattern, and decreased path loss. An EBG surface is employed to minimize the frequency detuning of the antenna and to reduce the specific absorption rate (SAR), while increasing the antenna gain when placed on the human body. The antenna gain increased by 60%, whereas SAR is decreased from 8.55 to 0.07 W/kg when the antenna is backed using the EBG. Despite being excellent in terms of gain and SAR, the size of the proposed antenna is still large and may not be suitable for some wearable applications. Alemaryeen et al. studied the effect of crumpling on textile coplanar waveguide (CPW) fed by a monopole antenna integrated with a flexible AMC structure [85]. Kwak et al. proposed a planar inverted-F antenna (PIFA) involved with an AMC structure for body SAR reduction, besides highlighting that an AMC structure without via can control the radiation pattern of the antenna and reduce the electromagnetic wave radiated towards the human body [86]. Agarwal et al. presented a technique of combining the endfire radiated wearable planar antenna with an AMC surface [87]. In this design, a single-layered AMC and a double-layered AMC are combined with a conventional planar Yagi antenna to change the direction of the radiation. Results showed that the double-layered AMC can improve the antenna performance in terms of front-to-back ratio, frequency detuning, radiation efficiency, and SAR. However, the bandwidth of 45 MHz of this antenna is not wide enough in practice. Metasurface structures are introduced in wearable antenna designs to provide a high degree of isolation Materials 2021, 14, 149 8 of 20

from the human body and reduce the SAR values in the tissues. However, most periodic designs are electrically large and feature poor front to back ratio (FBR). An example is a low-profile wearable antenna integrated with a compact EBG structure proposed in [62], with a footprint of only 0.135 λ2. In addition, the significant size reduction compared to literature did not compromise its performance. It operates from 2.17 to 2.83 GHz with a bandwidth of 660 MHz (27%) and a gain of 7.8 dBi. More details are listed in Table2. For broadband, Lin et al. proposed a millimeter-wave band flexible fractal design with a self-similar window-like structure and an AMC as the reflector. This work also demonstrated for the first time that the performance of wearable antennas at mm-wave band, i.e., 20–40 GHz, can be improved using flexible AMC structures [88]. Besides that, Liu et al. studied a flexible windmill-shaped crossed-dipole antenna [89]. It featured a more compact structure with additional bandwidth over traditional dipoles, besides a more aesthetic outlook. Two narrowband resonances that are located close to each other have been combined to achieve a wider bandwidth in this metasurface-based antenna. In addition, a wideband wearable antenna, as shown in Figure7c, which covers a bandwidth of 1.1 GHz (52%) was presented in [90], in which the four bending states are studied and the S11 curves are shown in Figure7a,b. From the examples above, the AMC plane of this antenna plays a significant role in its wideband and backward suppression. Apart from the common requirements of metasurfaces antenna, some special problems have addressed are reported in the literature. Chen et al. in 2016 proposed a lightweight, low-profile, and highly directive antenna for smartwatch applications [16]. It is reported that the properties of the AMC unit cell designed using the conventional method, in which the near-zero reflection phase of the unit cell is set at around the resonance frequency, are significantly different compared to the 2 × 2 array in practice. This is due to its properties being obtained from infinite-size structure simulations. To overcome this, the fractional factorial designs (FFDs) method is applied to analyze such a problem by considering the entire integrated structure. In addition, Kamardin et al. proposed a method to improve the transmission between on-body antennas using textile AMC waveguide sheets [91]. It was found that the AMC sheet actually acts as a subsidiary waveguide, which offers a new independent transmission path to minimize the transmission loss. Meanwhile, the enhancements of antenna structures via the application of metasurface have also be researched. Due to the bandgap characteristics of metasurfaces, such as in EBG structures, they can be effective in improving the isolation of a MIMO antenna system. Kim et al. used this characteristic for this purpose and demonstrated that the correlation coefficient can be reduced in a MIMO monopole antenna [92]. Besides textiles, inkjet printing has been applied to fabricate wearable antennas with metasurfaces [31,93]. These antennas are usually extremely lightweight and highly flexible while maintaining fabrication simplicity and low cost. However, such flexible structures are limited for practical wearable applications for humans, as they require stable performance in a severe environment—the additional flexibility results in performance deterioration and a decrease in mechanical robustness. To better complete complex, multi-functional metamaterial antennas, the general method hardly meets the need for efficient design. Thus, CMA theory is also introduced in the metasurface antenna study. Wen et al. proposed an improved alternative approach to designing AMC-based MIMO antennas in [74]. Instead of employing two antenna elements, a pair of degenerated characteristic modes (CMs) for one of the radiators is proposed in the design of an AMC-based MIMO antenna. A similar approach also is applied in [75,76], and it avoids the complicated design process of antennas and obtains the potential radiation performance of the complex metasurface antennas from the current mods. Materials 2020, 14, x FOR PEER REVIEW 9 of 20 Materials 2021, 14, 149 9 of 20

(a) (b)

(c)

FigureFigure 7. 7.A A wideband wideband textile textile antenna antenna with with a a ring-slotted ring-slotted AMC AMC plane plane [ 90[90].]. ( a()a) The The textile textile antenna antenna bent bent at at two two different different bendingbending radii radiir andr and at at two two axes: axes:r =r = 60 60 mm mm bent bent at atx -axis,x-axis,r =r = 80 80 mm mm bent bent at atx -axis,x-axis,r =r = 60 60 mm mm bent ben att aty -axis,y-axis, and andr =r = 80 80 mm mm bentbent at aty -axis.y-axis. ( b(b)) Reflection Reflection coefficients coefficients of of the the antenna antenna when when bent bent at at the thex -axisx-axis and andy -axisy-axis in in (a ().a). (c ()c The) The photo photo of of the the textile textile antenna: An artificial magnetic conductor (AMC) plane and a feeding structure. antenna: An artificial magnetic conductor (AMC) plane and a feeding structure.

ResearchersResearchers havehave foundfound thatthat finite finite metasurfaces metasurfaces not not only only functioned functioned as as reflecting reflecting groundground planes, planes but, but also also as as the the primary primary radiators radiators or or substratessubstrates withwith effectiveeffective permittivities.permittivities. JiangJiang et et al. al. proposed proposed a a compact compact conformal conformal wearable wearable antenna antenna that that operates operates in in the the 2.36–2.4 2.36–2.4 GHzGHz medical medical body body area area network network band band [94 [].94 The]. The antenna antenna includes includes four four I-shaped I-shaped elements elements as anas AMC an AMC and aand UWB a UWB planar planar monopole monopole antennas. antennas. This metasurface This metas actsurface not acts only not as aonly ground as a planeground for plane improving for improving the isolation the between isolation the between radiator the and radiator the human and the body, human but also body as, the but mainalso as radiator. the main Compared radiator. withCompared conventional with conventional metasurface-based metasurface antennas,-based this antennas, wearable this antennawearable can antenna significantly can significantly increase gain increase and decrease gain and the couplingdecrease betweenthe coupling the antenna between and the theantenna human and body. the human In fact, body. the monopole In fact, the antenna monopole only antenna serves asonly a feed serves to as excite a feed the to AMC excite structure,the AMCand structure, its shape and is its not shape critical is innot impacting critical in the impacting antenna performance.the antenna performance. According toAccording this idea, to the this metasurface idea, the metasurface plane can be plane seen can as a be 2 dimensionseen as a 2 CRLHdimension TLs, CRLH as shown TLs, in as Figureshown8, in and Figure the negative 8, and the order negative resonance order modes resonance working modes in the working left hand in the area left are hand excited area byare the excited feeding by structure,the feeding which structure, reduces which the reduces size of the the antenna size of the drastically antenna [drastically5]. Figure8 [c,5]. dFigure also show 8c, d thealso simulated show the dispersionsimulated dispersion curve and curve S11 in and different S11 in situations, different situations, and Figure and9 shows the simulated SAR of the metasurface antenna over the human body. However, the

Materials 2020, 14, x FOR PEER REVIEW 10 of 20

Materials 2020, 14, x FOR PEER REVIEW 10 of 20 Figure 9 shows the simulated SAR of the metasurface antenna over the human body. Materials 2021, 14, 149 10 of 20 However, the efficiency and the realized gain are also decreased due to its miniaturization. Meanwhile, in [95], a metasurface is considered as a substrate with a high relative Figure 9 shows the simulated SAR of the metasurface antenna over the human body. efﬁciencypermittivityHowe and thever, realized whenthe efficiency gainit is areilluminated and also the decreased realized by a duegain linearly to are its also miniaturization.-polarized decreased wave. due Meanwhile, to its miniaturiza- in [95], a metasurfacetion. Meanwhile, is considered in [95], a as metasurface a substrate withis considered a high relative as a substrate permittivity with when a high it relative is illuminatedpermittivity by a linearly-polarized when it is illuminated wave. by a linearly-polarized wave.

LR CL

L C R L Z LL CR ZY LL CR Reference Plane YX Reference Plane (a) X (b) (a) (b) 180 0 180 Simulated by CST 0 Calculated Simulated by Equivalent by CST Circuit -5 150 150 Calculated by Equivalent Circuit -5 -10 -10 120 120 -15 -15 90 -20 (°) 90 (dB) -20 p (°) (dB)

n=-1 11 p β -25 S

60 n=-1 11 Sim. w/ MS β -25 60 -30S Mea. w/ MS Sim. w/ MS 30 -30 Sim. w/o MS Mea. w/ MS -35 Mea. w/o MS 30 Sim. w/o MS 0 -40 -35 Mea. w/o MS 2.0 2.5 3.0 3.5 4.0 4.5 5.0 2.0 2.2 2.4 2.6 2.8 3.0 0 -40 2.0 2.5 3.0 Frequency3.5 4.0 (GHz) 4.5 5.0 2.0 Frequency2.2 2.4 (GHz) 2.6 2.8 3.0 Frequency(c )(GHz) (dFrequency) (GHz) Figure 8. (a) SimulationFigure model 8. (a) (of Simulationc )dispersion model of unit of cell dispersion of the metasurface of unit cell antenna. of the ( metasurfaceb) The(d equivalent) antenna. circuit (b of) Themetasur- face unit cell. (c) The dispersion curve simulated by CST and calculated by an equivalent circuit. (d) The photo of the Figure 8. (a) Simulationequivalent model of circuit dispersion of metasurface of unit unit cell cell. of the (c) The metasurface dispersion curve antenna. simulated (b) The by CST equivalent and calculated circuit of metasur- compact, low-profileby an metasurface equivalent antenna circuit. ( dand) The its photoS11 curve of the when compact, feeding low-proﬁle structure metasurfacewith/without antenna metasu andrface its plane S11 [5]. face unit cell. (c) The dispersion curve simulated by CST and calculated by an equivalent circuit. (d) The photo of the curve when feeding structure with/without metasurface plane [5]. W/kg compact, low-profile metasurface antenna and its S11W curve/kg when feeding structure with/without metasurface plane [5].

W/kg W/kg

(a) (b)

W/kg W/kg

(a) (b)

W/kg W/kg

(c) (d) Figure 9. Simulated Specific Absorption Rate (SAR) of the metasurface antenna over the human body. (a) The feeding structure without metasurface at 2.65 GHz. (b) The feeding structure with metasurface at 2.65 GHz. (c) The dual band antenna at 2.5 GHz. (d) The dual band antenna at 3.65 GHz [5].

3.2. Dual-Band Wearable Antennas with Metasurfaces (c) (d) A number of researchers have been investigating dual-band wearable antennas Figure 9. Simulated SpecificFigure Absorption 9. Simulatedbased on metasurfaces SpeciﬁcRate (SAR) Absorption [of96 the–104 Rate metasurface]. This (SAR) dual of the-band antenna metasurface characteristic over antenna the for human overwearable the body. human antennas (a) The with feeding structure without metasurfacebody. (a) at The 2.65 feeding GHz. structure (b) The without feeding metasurface structure at 2.65with GHz. metasurface (b) The feeding at 2.65 structure GHz. with (c) The dual band antenna at 2.5 GHz. (d) metasurfaceThe dual band at 2.65 antenna GHz. (c at) The 3.65 dual GHz band [5]. antenna at 2.5 GHz. (d) The dual band antenna at 3.65 GHz [5].

Materials 2021, 14, 149 11 of 20

3.2. Dual-Band Wearable Antennas with Metasurfaces A number of researchers have been investigating dual-band wearable antennas based on metasurfaces [96–104]. This dual-band characteristic for wearable antennas with metasurface is usually realized by exciting the higher modes of the structures or by integrating different resonator shapes. The general design steps of such kind of wearable antennas are as follows [97]: 1. A dual-band antenna is first designed in the two frequency bands of interest. 2. An AMC is designed to operate within the same two bands as the antenna. 3. The AMC is integrated with the antenna and is optimized further in simulations via parametric study. 4. The optimized structure is fabricated and measured. Zhu et al. proposed a dual-band coplanar patch antenna integrated with an electromagnetic bandgap substrate [98,99]. The antenna structure is made using common clothing fabrics and operates in the 2.45 and 5 GHz wireless bands. The EBG consists of 3 × 3 elements and is shown to reduce radiation into the body by over 10 dB and improved gain by 3 dB. The performance of the antenna under different bending conditions and on-body conditions are also investigated. Based on this research, Bai further investigated the performance of a dual-band wearable antenna with an AMC under bending and crumpling conditions. Felt (with a relative permittivity of 1.39) is used as the substrate of the proposed antenna and Zelt conductive textile [100]. The performance of an integrated dual-band wearable antenna with AMC is studied under realistic crumpling conditions using shape deformations commonly found in clothing. Results showed that the influence of the crumpling on the antenna performances is acceptable for normal operation. A dual-band and dual-sense wearable antenna was designed, which operates in GPS and WLAN band with linear and circular polarization, and the AMC plane provides a decrease of backward and improvement of gain and bandwidth. The unite cell and its reflection phase are shown in Figure 10a,b, and the antenna prototype is shown in Figure 10c [101]. Next, Yan et al. proposed an efficiently radiating wearable antenna by replacing the normal metallic ground plane with an AMC plane, as shown in Figure 11. This AMC plane can be regarded as a PMC plane, whereas this antenna utilizes the patch as the radiator in the lower WBAN\WLAN band, and a slot dipole integrated on the patch to enable operation in the upper WLAN band [102], the simulation results are shown in Figure 11b. Such a principle avoids the general drawback commonly found in dual-band antennas with AMC plane such as bandwidth limitation and the increase in fabrication complexity. Similarly, Wang also presented a wearable dual-band antenna. The performance of the proposed antenna is studied via simulations and measurements when the antenna is located on different parts of a real human body and on different human bodies, including both man and woman [103]. Besides that, Velan et al. presented a dual-band wearable fractal-based monopole patch antenna integrated with an EBG structure [104]. The EBG structure reduces the radiation into the human body by more than 15 dB, besides reducing the effects of frequency detuning due to the human body. The performance of the antenna under bending, crumpling, and different on-body conditions have been studied. Finally, its specific absorption rate is also assessed to validate the antenna safety in wearable applications. Materials 2020, 14, x FORMaterials PEER 2021REVIEW, 14, 149 12 of 20 12 of 20

(a) (b)

(c)

Figure 10.FigureA dual-band 10. A dual wearable-band textilewearable antenna textile [101 antenna]. (a) AMC [101 unit]. (a cell) AMC dimensions unit cell of dimensions the AMC unit of cellthe (inAMC mm), (b) reﬂectionunit phase cell of (in the mm), AMC ( unitb) reflection cell, and ( cphase) fabricated of the prototype AMC unit of thecell, antenna. and (c) fabricated prototype of the antenna.

(a) (b)

(c)

FigureFigure 11. A low-profile11. A low-profile dual-band dual textile-band antenna textile antenna with artificial with magneticartificial magnetic conductor conductor plane [102 plane]. (a) The [102 AMC]. (a) plane of the antennaThe and AMC its reflection plane of phase.the antenna (b) S11 and curves its reflection of the dual-band phase. (b antenna) S11 curves in free of spacethe dual and-band on the antenna human in body. (c) The photo offree the space wearable and on antenna. the human body. (c()c )The photo of the wearable antenna. Figure 11. A low-profile dual-band textile antenna with artificial magnetic conductor plane [102]. (a) The AMC plane of the antenna and its reflection phase. (b) S11 curves of the dual-band antenna in free space and on the human body. (c) The photo of the wearable antenna.

(a) (b) Figure 12. Radiation pattern-reconfigurable wearable antenna based on metamaterial structure [105]. (a) The photo of the wearable antenna. (b) Two electrical field distributions at 2.45 GHz in z-direction. The first distribution is Patch mode (n = +1) and second one is monopole mode (n = 0). The arrows in the figures represent the direction of the equivalent magnetic current.

Materials 2021, 14, 149 13 of 20

Table 2. Performances among the metasurface-based wearable antennas.

Ref. Frequency (GHz) Bandwidth Gain (dBi) Size (λ2) SAR (W/kg) Substrate (εr) * 8.6% (@2.3) 1.3 0.048 4.5 2.65 Substrate (2.65) ** 11.5% (@2.65) 2.99 0.1 1.25 [5] 13.6% (@2.2) - 0.036 (@2.45) - 2.45/3.65 Substrate (2.65) 15.7%/2.3% 4.25/7.35 0.2 (@2.45) 0.65/0.37 12% 1.1 0.09 1.88 Kapton polyimide [61] 2.45 18% 4.8 0.28 0.638 (3.5) 15% - 0.04 2.7 [62] 2.4 Substrate (1.7) 27% 7.8 0.14 0.0138 14.5% 3.41 8.85 Polyester [63] 2.4 1.22 1.38% 8.53 0.69 (2.8) - - - RT/duroid 5880 [64] 2.45 0.17 4.88% 6.88 0.244 (2.2) 44.8% 5.15 0.21 1.52 Rogers 3850 [65] 2.45 25% 6.38 0.38 0.0072 (2.9) - - [74] 2.45 0.05 FR-4 (4.3) 3.6% 4.2 0.55 - - - [75] 5.5 Substrate (2.7) 15.8% 7.63 0.25 0.198 24% 4.37 0.11 2.71 [76] 2.45 Substrate (1.6) 16.3% 7.7 0.54 0.04 11% (@6.2) −3.83 0.15 1.22 Leather substrate [83] 5.8 10.7% (@5.8) −0.75 3.84 0.31 (2.3) 153% (2.1–16) 4.3 - [84] 3.5/5.7/10/14 0.29 (@3.5) FR-4 (4.3) 149% (2.3–16) 7.2 0.1 81% 2.45 1.2 9.39 RO3003 [85] 2.45 16.3% 8.41 1.02 0.166 (3.0) 2% 2.8 0.006 1.4 Antenna (3.5) [86] 1.97 2% 4.6 0.05 0.7 AMC (10.2) 0% 4.17 4.2 [87] 2.45 0.16 Styrofoam (1) 5% 2.32 0.714 61% (20–37.52) 7.4 2.5 (@28) - [88] 20–40 Substrate (2.2) 64.4% (20.52–40) 10.3 9.6 (@28) - 78% (5.25–12) 3.5 0.48 (@8) - Panasonic R-F770 [89] 5.7–11 63% (5.7–11) 7.5 1.28 (@8) - (-) 42% 0 - Felt substrate [90] 2.5 1.38 52% 3.38 0.025 (1.44) 8% −8.2 - - Photo paper [93] 2.45 10% 0.86 1.3 - (-) 4.7% 2.0 0.078 11.3 [94] 2.45 Rogers. RO3003 (3) 5.5% 6.2 0.15 0.48 20% (5.5–6.75) 5.2 - [95] 5.5 0.38 Wool felt (1.2) 17% 6.7 0.43 32%/25.8% 1.97/4.2 0.078 (@2.45) 21.41/7.57 Felt substrate [96] 2.45/5.8 8%/27.5% 6.3/6.7 0.25 (@2.45) 0.414/0.9 (1.22) 12%/41% 0.14 (@2.5) Felt substrate [97] 2.5/5.5 - - 56%/32% 0.50 (@2.5) (1.22) 17%/16% 3.9/5.2 0.20 (@2.45) 7.819/6.808 Felt substrate [99] 2.45/5.5 4%/12% 6.4/7.6 0.96 (@2.45) 0.043/0.097 (1.38) - - - [101] 1.575/2.45 0.20 (@1.575) Kevlar (1.66) 7.6%/5.5% 1.98/1.94 0.78/0.71 - - - [102] 2.45/5.5 0.67 (@2.45) Textile (1.3) 12%/16.3% 2.5/4 0.019/0.009 16.3%/3.4% −4.1/2.33 0.05 (@2.45) 8.99/4.08 [103] 2.45/5.8 Polyimide (3.5) 8%/6.8% 5.2/7.7 0.34 (@2.45) 0.7/0.71 70% (1.3–2.7) - 0.4 (@1.8) 5.77/6.62 [104] 1.8/2.45 Jean (1.7) 10.9%/5.08% 1–2 0.81 (@1.8) 0.024/0.016 * the antenna without metasurface; (@2.3) means that the λ is the wavelength in free space at 2.3 GHz; ** the antenna with metasurface.

(a) (b) Materials 2021, 14, 149 14 of 20

4. Reconfigurable Wearable Antennas with Metamaterials The space limitation in wearable antennas and the need for these antennas to operate in multiple wireless standards is spurring the development of reconfigurable antennas. Yan et al. presented a pattern-reconfigurable wearable antenna based on a metamaterial structure, as shown in Figure 12[ 105]. This wearable antenna consists of three CRLH TL unit cells, which are capable(c) of switching between the zero-order resonance and +1 resonance in the patch using switchable stubs connected using vias. The two states of this Figure 11. A low-profile dual-band textile antenna with artificial magnetic conductor plane [102]. (a) antenna operate in the same frequencies but radiate differently, providing a monopole-like The AMC plane of the antenna and its reflection phase. (b) S11 curves of the dual-band antenna in free space and onor the a patch human like body. radiation (c) The photo pattern of the as shownwearable in antenna. Figure 13, and the performances detail are listed in Table3.

(a) (b)

Figure 12. RadiationFigure 12. pattern-reconfigurable Radiation pattern-reconfigurable wearable antenna wearable based antenna on metamaterial based on metamaterial structure [105 structure]. (a) The [105 photo]. of the wearable antenna.(a) The photo (b) Two of the electrical wearable field antenna. distributions (b) Two at electrical 2.45 GHz field in z-direction. distributions The at 2.45 first GHz distribution in z-direction. is Patch mode (n = +1) andThe second first onedistribution is monopole is Patch mode mode (n =(n 0).= +1) The and arrows second in one the is figures monopole represent mode the (n = direction 0). The arrows of the equivalentin the figures represent the direction of the equivalent magnetic current. magnetic current.

Figure 13. The radiation pattern of the antenna in [105]. Measured and simulated radiation patterns. (a) Monopole mode in xz-plane, (b) monopole mode in yz-plane, (c) patch mode in xz-plane, and (d) patch mode in yz-plane.

Materials 2020, 14, x FOR PEER REVIEW 15 of 20

Table 3. Performances among the reconfigurable wearable antennas with metamaterials.

Frequenncy Ref. Bandwidth Gain (dBi) Size (λ2) SAR (W/kg) Substrate (εr) (GHz) 0.086 (State1) 2.9 0.05 Felt substrate [105] 2.4 0.64 0.055 (State2) 4.5 0.01 (1.3) Materials 2021, 14, 149 10% (2.64– Tortuous15 ofCu 20 [106] 2.92 −0.02 0.009 - 2.94) mesh/PDMS (2.8) *- 2.6/0.6 ***0.5 2 RO3003 [107] 2.45/3.3 Table 3. Performances among the reconfigurable**7%/3% wearable 6.2/3.0 antennas with(@2.45) metamaterials. 0.29/- (3) * the antenna without metasurface; ** the antenna with metasurface; *** (@2.45) means that the λ is Ref. Frequenncy (GHz)the wavelength Bandwidth in free space Gain (dBi)at 2.45 GHz. Size (λ2) SAR (W/kg) Substrate (εr) 0.086 (State1) 2.9 0.05 Felt substrate [105] 2.4 0.64 0.055Besides (State2) that, Jang et4.5 al. proposed a method to fabricate0.01 a small semitransparent(1.3) and stretchable antenna using a stretchable micromesh structure, as shown in Figure 14 [106]. Tortuous Cu [106] 2.92This 10% antenna (2.64–2.94) consists of− a0.02 4.7 μm thick Cu 0.009 mesh pattern and - a PDMS layer as the substrate. mesh/PDMS (2.8) The PDMS is flexible and optically transparent, and it can maintain the shape of a mi- *- 2.6/0.6 2 RO3003 [107] 2.45/3.3 cromesh as well as protect the metal*** wire 0.5 (@2.45)from mechanical damage when stretched. The ** 7%/3% 6.2/3.0 0.29/- (3) increase in tensile strain reconfigures the resonant frequency of the antenna almost line- * the antenna without metasurface;arly ** the from antenna 2.46 with to metasurface;2.94 GHz. *** However, (@2.45) means this that antenna the λ is the suffers wavelength from in low free space radiation at 2.45 GHz.efficiency due to the reduced surface currents flowing through the micromesh patch. BesidesNext, a that, wearable Jang etrecon al. proposedfigurable aantenna method with to fabricate AMC structure a small semitransparentconsisting of a folded and stretchableslot and a antennastub was using proposed a stretchable in [107]. micromeshThis antenna structure, operate ass between shown ina singleFigure and14[106 dual] .- Thisband antenna mode, consistswith two of orthogonal a 4.7 µm thick polarizations Cu mesh controlled pattern and by a the PDMS ON/OFF layer states as the of sub- the strate.PIN diodes. The PDMS When is the flexible PIN anddiode optically is in the transparent, ON state, the and stub it can is symmetrical maintain the with shape respect of a micromeshto the CPW as feed well line as protect and does the not metal radiate. wirefrom This mechanicalresults in a damagesingle operating when stretched. frequency The of increasethe antenna. in tensile When strain the reconfiguresPIN switch is the in resonantthe OFF frequencystate, the asymmetrical of the antenna stub almost with linearly respect fromto the 2.46 feed to 2.94line GHz.causes However, the current this on antenna the stub suffers to be fromredirected, low radiation producing efficiency dipole- duelike tora- thediation. reduced Both surface the slot currents and the flowing stub resonate through with the micromeshorthogonal patch.polarizations.

Figure 14. The photo of the mechanically reconfigurable electrically small antenna in [106]. The Figure 14. The photo of the mechanically reconﬁgurable electrically small antenna in [106]. The antenna made of flexible wire mesh and a frequency shift is generated in different stretch. antenna made of ﬂexible wire mesh and a frequency shift is generated in different stretch.

5. ConclusionsNext, a wearable reconfigurable antenna with AMC structure consisting of a folded slot andFrom a stub this was review, proposed the design in [107 of ].wearable This antenna antennas operates is rather between different a single from andthe design dual- bandof conventional mode, with antennas. two orthogonal The main polarizations challenge is controlled to ensure that by the the ON/OFF designed stateswearable of the an- PINtenna diodes. still operates When the with PIN minimal diode is coupling in the ON to state, the human the stub body is symmetrical and under withdifferent respect defor- to themations. CPW feed Nonetheless, line and does several not radiate.studies Thishave resultsproven in that a single the integration operating frequency of metasurfaces of the antenna.onto the When antenna the design PIN switch based is incan the significantly OFF state, the improve asymmetrical their performance. stub with respect This review to the feedalso line highlighted causes the the current recent onprogress the stub in tothe be literature redirected, on producingmetamateri dipole-likeal-based wearable radiation. an- Bothtennas the, including slot and the the stub classification resonate with of the orthogonal main approaches polarizations. in their integration. As for the wearable antennas based on CRLH TLs, there are electrically small antennas based on 5. Conclusions ZOR consisting of ENR, MNR, and DNR modes and dual-band patch antennas, and their From this review, the design of wearable antennas is rather different from the design of conventional antennas. The main challenge is to ensure that the designed wearable antenna still operates with minimal coupling to the human body and under different deformations. Nonetheless, several studies have proven that the integration of metasurfaces onto the antenna design based can significantly improve their performance. This review also highlighted the recent progress in the literature on metamaterial-based wearable antennas, including the classification of the main approaches in their integration. As for the wearable antennas based on CRLH TLs, there are electrically small antennas based on ZOR consisting of ENR, MNR, and DNR modes and dual-band patch antennas, and their electromagnetic Materials 2021, 14, 149 16 of 20

property, single-negative, or double-negative material parameter, may exhibit exciting performances, which can be utilized flexibly for different WBAN applications. For the wearable antennas based on metasurface, two methodologies have been presented: One approach is that the zero reflection phase of the unit cell of AMC is design at the resonance frequency so that the feeding antenna can be placed on the AMC reflection plane, decreasing the profile of the wearable antenna Another one is derived from the principle of CRLH TLs that the reflector is as a radiator by exciting the metasurface, and the main merit of it is decrease the size of the wearable antenna dramatically. Finally, three reconfigurable wearable antennas were described briefly. In summary, the radiation properties of these antennas can be improved by using metamaterials as follows: • The radiation properties of wearable antennas can be enhanced by restraining the surface wave and the coupling between antennas and the human body. • A low-profile of a wearable antenna can be realized by using the zero-reflection phase available from metasurfaces such as AMC structures. • The bandwidth of wearable antennas can be broadened by loading reactive metasurfaces. • The direction of radiation and level of gain can be controlled by modification of the field distributions and propagation directions. Nonetheless, with the various requirements of today’s wireless communication systems, it should be emphasized that there is not a single type of wearable antenna that is capable of meeting all of the requirements simultaneously. However, metamaterial-based wearable antennas have been demonstrated to be capable of significantly improve the performance of antennas when applied to the human body compared with traditional antennas. It is foreseeable that wearable antennas would endeavor towards miniaturization, multi- function, multi-band frequency, and broadband in the future, and metamaterials-based antennas, which have unique properties, provide a new approach for these goals. Thus, a deeper understanding of the operation of metamaterials will result in more applications of such structures in future WBAN antennas.

Author Contributions: Conceptualization, K.Z. and S.Y.; writing—original draft preparation, K.Z.; formal analysis, K.Z., P.J.S. and S.Y.; review and editing, K.Z., P.J.S. and S.Y.; supervision and comments, P.J.S. and S.Y.; project administration/funding acquisition S.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 61901351. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References 1. Bariya, M.; Nyein, H.Y.Y.; Javey, A. Wearable sweat sensors. Nat. Electron. 2018, 1, 160–171. [CrossRef] 2. Januszkiewicz, Ł.; Di Barba, P.; Hausman, S. Optimal Design of Switchable Wearable Antenna Array for Wireless Sensor Networks. Sensors 2020, 20, 2795. [CrossRef][PubMed] 3. El Gharbi, M.; Fernández-García, R.; Ahyoud, S.; Gil, I. A Review of Flexible Wearable Antenna Sensors: Design, Fabrication Methods, and Applications. Materials 2020, 13, 3781. [CrossRef][PubMed] 4. Abd Rahman, N.H.; Yamada, Y.; Amin Nordin, M.S. Analysis on the Effects of the Human Body on the Performance of Electro-Textile Antennas for Wearable Monitoring and Tracking Application. Materials 2019, 12, 1636. [CrossRef][PubMed] 5. Zhang, K.; Vandenbosch, G.A.E.; Yan, S. A Novel Design Approach for Compact Wearable Antennas Based on Metasurfaces. IEEE Trans. Biomed. Circuits Syst. 2020, 14, 918–927. [CrossRef][PubMed] 6. Smida, A.; Iqbal, A.; Alazemi, A.J.; Waly, M.I.; Ghayoula, R.; Kim, S. Wideband Wearable Antenna for Biomedical Telemetry Applications. IEEE Access 2020, 8, 15687–15694. [CrossRef] 7. Tian, X.; Lee, P.M.; Tan, Y.J.; Wu, T.L.Y.; Yao, H.C.; Zhang, M.Y.; Li, Z.P.; Ng, K.A.; Tee, B.C.K.; Ho, J.S. Wireless body sensor networks based on metamaterial textiles. Nat. Electron. 2019, 2, 243–251. [CrossRef] 8. Abdi, A.; Ghorbani, F.; Aliakbarian, H.; Geok, T.K.; Rahim, S.K.A.; Soh, P.J. Electrically Small Spiral PIFA for Deep Implantable Devices. IEEE Access 2020, 8, 158459–158474. [CrossRef] Materials 2021, 14, 149 17 of 20

9. Alqadami, A.S.M.; Bialkowski, K.S.; Mobashsher, A.T.; Abbosh, A.M. Wearable Electromagnetic Head Imaging System Using Flexible Wideband Antenna Array Based on Polymer Technology for Brain Stroke Diagnosis. IEEE Trans. Biomed Circuits Syst. 2019, 13, 124–134. [CrossRef] 10. Harun Al Rasyid, M.U.; Lee, B.H.; Sudarsono, A. Wireless body area network for monitoring body temperature, heart beat and oxygen in blood. In Proceedings of the Intelligent Technology and Its Applications (ISITIA), Surabaya, Indonesia, 20–21 May 2015; pp. 95–98. 11. Singh, M.; Jain, N. Performance and Evaluation of Smartphone Based Wireless Blood Pressure Monitoring System Using Bluetooth. IEEE Sens. J. 2016, 16, 8322–8328. [CrossRef] 12. Hong, S.; Kang, S.H.; Kim, Y.; Jung, C.W. Transparent and Flexible Antenna for Wearable Glasses Applications. IEEE Trans. Antennas Propag. 2016, 64, 2797–2804. [CrossRef] 13. Qu, L.; Zhang, R.; Kim, H. High-Sensitivity Ground Radiation Antenna System Using an Adjacent Slot for Bluetooth Headsets. IEEE Trans. Antennas Propag. 2015, 63, 5903–5907. [CrossRef] 14. Lee, H.; Yang, H.; Myeong, S.; Lee, K. Dual-band MNG patch antenna for smart helmet. Electron. Lett. 2018, 54, 1101–1102. [CrossRef] 15. Alqadami, A.S.M.; Trakic, A.; Stancombe, A.E.; Mohammed, B.; Bialkowski, K.; Abbosh, A. Flexible Electromagnetic Cap for Head Imaging. IEEE Trans. Biomed. Circuits Syst. 2020, 14, 1097–1107. [CrossRef][PubMed] 16. Chen, Y.S.; Ku, T.Y. A Low-Profile Wearable Antenna Using a Miniature High Impedance Surface for Smartwatch Applications. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1144–1147. [CrossRef] 17. Gao, G.P.; Hu, B.; Wang, S.F.; Yang, C. Wearable Circular Ring Slot Antenna with EBG Structure for Wireless Body Area Network. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 434–437. [CrossRef] 18. Stoppa, M.; Chiolerio, A. Wearable Electronics and Smart Textiles: A Critical Review. Sensors 2014, 14, 11957–11992. [CrossRef] 19. Almohammed, B.; Alyani, I.; Aduwati, S. Electro-textile wearable antennas in wireless body area networks: Materials, antenna design, manufacturing techniques, and human body consideration—A review. Textile Res. J. 2020.[CrossRef] 20. Paracha, K.N.; Rahim, S.K.A.; Soh, P.J.; Kamarudin, M.R.; Tan, K.G.; Lo, Y.C.; Islam, M.T. A Low Profile, Dual-band, Dual Polarized Antenna for Indoor/Outdoor Wearable Application. IEEE Access 2019, 7, 33277–33288. [CrossRef] 21. Simorangkir, R.B.V.B.; Kiourti, A.; Esselle, K.P. UWB Wearable Antenna with a Full Ground Plane Based on PDMS-Embedded Conductive Fabric. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 493–496. [CrossRef] 22. Sayem, A.M.; Simorangkir, R.B.V.B.; Esselle, K.P.; Hashmi, R.M.; Liu, H.R. A Method to Develop Flexible Robust Optically Transparent Unidirectional Antennas Utilizing Pure Water, PDMS, and Transparent Conductive Mesh. IEEE Trans. Antennas Propag. 2020, 68, 6943–6952. [CrossRef] 23. Zhang, J.H.; Yan, S.; Vandenbosch, G.A.E. A Miniature Feeding Network for Aperture-Coupled Wearable Antennas. IEEE Trans. Antennas Propag. 2017, 65, 2650–2654. [CrossRef] 24. Myny, K. The development of flexible integrated circuits based on thin-film transistors. Nat. Electron. 2018, 1, 30–39. [CrossRef] 25. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Wearable Ultrawideband Technology—A Review of Ultrawideband Antennas, Propagation Channels, and Applications in Wireless Body Area Networks. IEEE Access 2018, 6, 42177–42185. [CrossRef] 26. Poffelie, L.A.Y.; Soh, P.J.; Yan, S.; Vandenbosch, G.A.E. A High-Fidelity All-Textile UWB Antenna with Low Back Radiation for Off-Body WBAN Applications. IEEE Trans. Antennas Propag. 2016, 64, 757–760. [CrossRef] 27. Yoon, J.; Jeong, Y.; Kim, H.; Yoo, S.; Jung, H.S.; Kim, Y.; Hwang, Y.; Hyun, Y.; Hong, W.K.; Lee, B.H.; et al. Robust and stretchable indium gallium zinc oxide-based electronic textiles formed by cilia-assisted transfer printing. Nat. Commun. 2016, 7, 11477. [CrossRef][PubMed] 28. Yan, S.; Volskiy, V.; Vandenbosch, G.A.E. Compact Dual-Band Textile PIFA for 433-MHz/2.4-GHz ISM Bands. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2436–2439. [CrossRef] 29. Hu, X.M.; Sen, Y.; Vandenbosch, G.A.E. Wearable Button Antenna for Dual-Band WLAN Applications with Combined on and off-Body Radiation Patterns. IEEE Trans. Antennas Propag. 2017, 65, 1384–1387. 30. Carey, T.; Cacovich, S.; Divitini, G.; Ren, J.S.; Mansouri, A.; Kim, J.M.; Wang, C.X.; Ducati, C.; Sordan, R.; Torrisi, F. Fully inkjet-printed two-dimensional material field-effect heterojunctions for wearable and textile electronics. Nat. Commun. 2017, 8, 1202. [CrossRef] 31. Genovesi, S.; Costa, F.; Fanciulli, F.; Monorchio, A. Wearable Inkjet-Printed Wideband Antenna by Using Miniaturized AMC for Sub-GHz Applications. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1927–1930. [CrossRef] 32. Hao, J.; Leblanc, A.; Burgnies, L.; Djouadi, A.; Cochrane, C.; Rault, F.; Koncar, V.; Lheurette, E. Textile split ring resonator antenna integrated by embroidery. Electron. Lett. 2019, 55, 508–509. [CrossRef] 33. Quarfoth, R.; Zhou, Y.S.; Sievenpiper, D. Flexible Patch Antennas Using Patterned Metal Sheets on Silicone. IEEE Antennas Wirel. Propag. Lett. 2015, 14, 1354–1357. [CrossRef] 34. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Dual-Band Textile MIMO Antenna Based on Substrate-Integrated Waveguide (SIW) Technology. IEEE Trans. Antennas Propag. 2015, 63, 4640–4647. [CrossRef] 35. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Wearable Dual-Band Magneto-Electric Dipole Antenna for WBAN/WLAN Applications. IEEE Trans. Antennas Propag. 2015, 63, 4165–4169. [CrossRef] Materials 2021, 14, 149 18 of 20

36. Chen, J.; Berg, M.; Somero, V.; Amin, H.Y.; Prssinen, A. A Multiple Antenna System Design for Wearable Device Using Theory of Characteristic Mode. In Proceedings of the 12th European Conference on Antennas and Propagation, London, UK, 9–13 April 2018; pp. 1–5. 37. Elias, B.B.Q.; Soh, P.J.; Al-Hadi, A.A.; Vandenbosch, G.A.E. Design of a compact, wideband, and flexible rhombic antenna using CMA for WBAN/WLAN and 5G applications. Int. J. Numer. Model. Electron. Netw. Devices Fields 2020, e2841. [CrossRef] 38. Yue, T.W.; Jiang, Z.H.; Werner, D.H. Compact, Wideband Antennas Enabled by Interdigitated Capacitor-Loaded Metasurfaces. IEEE Trans. Antennas Propag. 2016, 64, 1595–1606. [CrossRef] 39. Le, T.T.; Yun, T.Y. Miniaturization of a Dual-Band Wearable Antenna for WBAN Applications. IEEE Antennas Wirel. Propag. Lett. 2020, 19, 1452–1456. [CrossRef] 40. Yue, T.W.; Jiang, Z.H.; Werner, D.H. A Compact Metasurface-Enabled Dual-Band Dual-Circularly Polarized Antenna Loaded With Complementary Split Ring Resonators. IEEE Trans. Antennas Propag. 2019, 67, 794–803. [CrossRef] 41. Maleszka, T.; Pawel, K. Bandwidth properties of embroidered loop antenna for wearable applications. In Proceedings of the 3rd European Wireless Technology Conference, Paris, France, 27–28 September 2010. 42. Casula, G.A.; Montisci, G. A Design Rule to Reduce the Human Body Effect on Wearable PIFA Antennas. Electronics 2019, 8, 244. [CrossRef] 43. Gao, G.P.; Yang, C.; Hu, B.; Zhang, R.F.; Wang, S.F. A Wide-Bandwidth Wearable All-Textile PIFA with Dual Resonance Modes for 5 GHz WLAN Applications. IEEE Trans. Antennas Propag. 2019, 67, 4206–4211. [CrossRef] 44. Caloz, C.; Itoh, T. Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications; Wiley: Hoboken, NJ, USA, 2005. 45. Lai, A.; Caloz, C.; Itoh, T. Composite right/left-handed transmission line metamaterials. IEEE Microw. Mag. 2004, 5, 34–50. [CrossRef] 46. Sanada, A.; Caloz, C.; Itoh, T. Characteristics of the composite right/left-handed transmission lines. IEEE Antennas Wirel. Propag. Lett. 2004, 14, 68–70. [CrossRef] 47. Sun, K.P.; Han, S.; Choi, J.H.; Lee, J.K. Miniaturized Active Metamaterial Resonant Antenna with Improved Radiation Performance Based on Negative-Resistance-Enhanced CRLH Transmission Lines. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 1162–1165. [CrossRef] 48. Ahdi Rezaeieh, S.; Antoniades, M.A.; Abbosh, A.M. Bandwidth and Directivity Enhancement of Loop Antenna by Nonperiodic Distribution of Mu-Negative Metamaterial Unit Cells. IEEE Trans. Antennas Propag. 2016, 64, 3319–3329. [CrossRef] 49. Ahdi Rezaeieh, S.; Antoniades, M.A.; Abbosh, A.M. Miniaturization of Planar Yagi Antennas Using Mu-Negative Metamaterial- Loaded Reflector. IEEE Trans. Antennas Propag. 2017, 65, 6827–6837. [CrossRef] 50. Park, B.; Lee, J. Circularly Polarized Antenna Based on Mu-Negative Transmission Line. In Proceedings of the 8th European Conference on Antennas Propagation, (EuCAP 2014), The Hague, The Netherlands, 6–11 April 2014; pp. 939–941. 51. Ahdi Rezaeieh, S.; Antoniades, M.A.; Abbosh, A.M. Compact Wideband Loop Antenna Partially Loaded With Mu-Negative Metamaterial Unit Cells for Directivity Enhancement. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1893–1896. [CrossRef] 52. Jae-Hyun, P.; Young-Ho, R.; Jeong-Hae, L. Mu-Zero Resonance Antenna. IEEE Trans. Antennas Propag. 2010, 58, 1865–1875. [CrossRef] 53. Li, L.; Jia, Z.; Huo, F.F.; Han, W.Q. A Novel Compact Multiband Antenna Employing Dual-Band CRLH-TL for Smart Mobile Phone Application. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 1688–1691. [CrossRef] 54. Lee, H.M. A Compact Zeroth-Order Resonant Antenna Employing Novel Composite Right/Left-Handed Transmission-Line Unit-Cells Structure. IEEE Antennas Wirel. Propag. Lett. 2011, 10, 1377–1380. 55. Xiong, J.; Lin, X.Q.; Yu, Y.F.; Tang, M.C.; Xiao, S.Q.; Wang, B.Z. Novel Flexible Dual-Frequency Broadside Radiating Rectangular Patch Antennas Based on Complementary Planar ENZ or MNZ Metamaterials. IEEE Trans. Antennas Propag. 2012, 60, 3958–3961. [CrossRef] 56. Park, J.-H.; Ryu, Y.-H.; Lee, J.-G.; Lee, J.-H. Epsilon Negative Zeroth-Order Resonator Antenna. IEEE Trans. Antennas Propag. 2007, 55, 3710–3712. [CrossRef] 57. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Compact All-Textile Dual-Band Antenna Loaded With Metamaterial-Inspired Structure. IEEE Antennas Wirel. Propag. Lett. 2015, 14, 1486–1489. [CrossRef] 58. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Wearable dual-band composite right/left-handed waveguide textile antenna for WLAN applications. Electron. Lett. 2014, 50, 424–426. [CrossRef] 59. Saleem, M.; Li, X.-L. Low Scattering Microstrip Antenna Based on Broadband Artificial Magnetic Conductor Structure. Materials 2020, 13, 750. [CrossRef][PubMed] 60. Cook, B.S.; Shamim, A. Utilizing Wideband AMC Structures for High-Gain Inkjet-Printed Antennas on Lossy Paper Substrate. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 76–79. [CrossRef] 61. Raad, H.R.; Abbosh, A.I.; Al-Rizzo, H.M.; Rucker, D.G. Flexible and Compact AMC Based Antenna for Telemedicine Applications. IEEE Trans. Antennas Propag. 2013, 61, 524–531. [CrossRef] 62. Ashyap, A.Y.I.; Abidin, Z.Z.; Dahlan, S.H.; Majid, H.A.; Shah, S.M.; Kamarudin, M.R.; Alomainy, A. Compact and Low-Profile Textile EBG-Based Antenna for Wearable Medical Applications. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2550–2553. [CrossRef] 63. Abirami, B.S.; Sundarsingh, E.F. EBG-Backed Flexible Printed Yagi–Uda Antenna for On-Body Communication. IEEE Trans. Antennas Propag. 2017, 65, 3762–3765. [CrossRef] Materials 2021, 14, 149 19 of 20

64. Abbasi, M.A.B.; Nikolaou, S.; Antoniades, M.A.; Stevanovic, M.N.; Vryonides, P. Compact EBG-Backed Planar Monopole for BAN Wearable Applications. IEEE Trans. Antennas Propag. 2017, 65, 453–463. [CrossRef] 65. El Atrash, M.; Abdalgalil, O.F.; Mahmoud, I.S.; Abdalla, M.A.; Zahran, S.R. Wearable high gain low SAR antenna loaded with backed all-textile EBG for WBAN applications. IET Microw. Antennas Propag. 2020, 14, 791–799. [CrossRef] 66. Ali Esmail, B.; Majid, H.A.; Zainal Abidin, Z.; Haimi Dahlan, S.; Himdi, M.; Dewan, R.; Kamal, A.; Rahim, M.; Al-Fadhali, N. Reconfigurable Radiation Pattern of Planar Antenna Using Metamaterial for 5G Applications. Materials 2020, 13, 582. [CrossRef] 67. Wang, L.B.; See, K.Y.; Zhang, J.W.; Salam, B.; Lu, A.C.W. Ultrathin and Flexible Screen-Printed Metasurfaces for EMI Shielding Applications. IEEE Trans. Electromagn. Compat. 2011, 53, 700–705. [CrossRef] 68. Gajibo, M.M.; Rahim, M.K.A.; Bala, B.D. Reconfigurable epsilon negative metamaterial antenna. In Proceedings of the 2014 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE), Johor Bahru, Malaysia, 8–10 December 2014; pp. 265–267. 69. Senior, D.E.; Yoon, Y. Dual Band Antenna Using the Substrate Integrated Waveguide As An Epsilon Negative Transmission Line. In Proceedings of the 2012 IEEE International Symposium on Antennas Propagation, Chicago, IL, USA, 8–14 July 2012; pp. 1–2. 70. Das, G.K.; Basu, S.; Mandal, B.; Mitra, D.; Augustine, R.; Mitra, M. Gain-enhancement technique for wearable patch antenna using grounded metamaterial. IET Microw. Antennas Propag. 2020, 14, 2045–2052. [CrossRef] 71. Cao, Y.F.; Cai, Y.; Cao, W.Q.; Xi, B.K.; Qian, Z.P.; Wu, T.; Zhu, L. Broadband and High-Gain Microstrip Patch Antenna Loaded With Parasitic Mushroom-Type Structure. IEEE Antennas Wirel. Propag. Lett. 2019, 18, 1405–1409. [CrossRef] 72. Jang, S.; Lee, B. Meta-Structured One-Unit-Cell Epsilon Negative Antenna. Microw. Opt. Technol. Lett. 2009, 51, 2991–2994. [CrossRef] 73. Alu, A.; Engheta, N. Pairing an epsilon-negative slab with a mu-negative slab: Resonance, tunneling and transparency. IEEE Trans. Antennas Propag. 2003, 51, 2558–2571. [CrossRef] 74. Wen, D.L.; Hao, Y.; Munoz, M.O.; Wang, H.Y.; Zhou, H. A Compact and Low-Profile MIMO Antenna Using a Miniature Circular High-Impedance Surface for Wearable Applications. IEEE Trans. Antennas Propag. 2018, 66, 96–104. [CrossRef] 75. Gao, G.P.; Zhang, R.F.; Geng, W.F.; Meng, H.J.; Hu, B. Characteristic Mode Analysis of a Nonuniform Metasurface Antenna for Wearable Applications. IEEE Antennas Wirel. Propag. Lett. 2020, 19, 1355–1359. [CrossRef] 76. Pei, R.; Leach, M.P.; Lim, E.G.; Wang, Z.; Song, C.Y.; Wang, J.C.; Zhang, W.Z.; Jiang, Z.Z.; Huang, Y. Wearable EBG-Backed Belt Antenna for Smart On-Body Applications. IEEE Trans. Ind. Inform. 2020, 16, 7177–7189. [CrossRef] 77. Cheng, X.Y.; Senior, D.E.; Kim, C.; Yoon, Y.K. A Compact Omnidirectional Self-Packaged Patch Antenna with Complementary Split-Ring Resonator Loading for Wireless Endoscope Applications. IEEE Antennas Wirel. Propag. Lett. 2011, 10, 1532–1535. [CrossRef] 78. Yoon Geon, K.; Wonbin, H. Radiation Efficiency-Improvement Using a Via-Less, Planar ZOR Antenna for Wireless ECG Sensors on a Lossy Medium. IEEE Antennas Wirel. Propag. Lett. 2014, 13, 1211–1214. [CrossRef] 79. Jung, T.J.; Kwon, J.H.; Lim, S. Flexible zeroth-order resonant antenna independent of substrate deformation. Electron. Lett. 2010, 46, 740–742. [CrossRef] 80. Lee, J.; Kwak, S.I.; Lim, S. Wrist-wearable zeroth-order resonant antenna for wireless body area network applications. Electron. Lett. 2011, 47, 431–433. [CrossRef] 81. FCC. Available online: https://www.fcc.gov/consumers/guides/wireless-devices-and-health-concerns (accessed on 21 Decem- ber 2020). 82. Review of Published Literature between 2008 and 2018 of Relevance to Radiofrequency Radiation and Cancer. Available online: https://www.fda.gov/media/135043/download (accessed on 21 December 2020). 83. Tak, J.; Hong, Y.; Choi, J. Textile antenna with EBG structure for body surface wave enhancement. Electron. Lett. 2015, 51, 1131–1132. [CrossRef] 84. Negi, D.; Khanna, R.; Kaur, J. Design and performance analysis of a conformal CPW fed wideband antenna with Mu-Negative metamaterial for wearable applications. Int. J. Microw. Wirel. Technol. 2019, 11, 806–820. [CrossRef] 85. Alemaryeen, A.; Noghanian, S. Crumpling effects and specific absorption rates of flexible AMC integrated antennas. IET Microw. Antennas Propag. 2018, 12, 627–635. [CrossRef] 86. Kwak, S.I.; Sim, D.U.; Kwon, J.H.; Yoon, Y.J. Design of PIFA with Metamaterials for Body-SAR Reduction in Wearable Applications. IEEE Trans. Electromagn. Compat. 2017, 59, 297–300. [CrossRef] 87. Agarwal, K.; Guo, Y.X.; Salam, B. Wearable AMC Backed Near-Endfire Antenna for On-Body Communications on Latex Substrate. IEEE Trans. Compon. Packag. Manuf. Technol. 2016, 6, 346–358. [CrossRef] 88. Lin, X.Y.; Seet, B.C.; Joseph, F.; Li, E.F. Flexible Fractal Electromagnetic Bandgap for Millimeter-Wave Wearable Antennas. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 1281–1285. [CrossRef] 89. Liu, X.Y.; Di, Y.H.; Liu, H.; Wu, Z.T.; Tentzeris, M.M. A Planar Windmill-Like Broadband Antenna Equipped With Artificial Magnetic Conductor for Off-Body Communications. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 64–67. [CrossRef] 90. Hussin, E.F.N.M.; Soh, P.J.; Jamlos, M.F.; Lago, H.; Al-Hadi, A.A.; Rahiman, M.H.F. A wideband textile antenna with a ring-slotted AMC plane. Appl. Phys. 2017, 123, 46. [CrossRef] 91. Kamardin, K.; Rahim, M.K.A.; Hall, P.S.; Samsuri, N.A. Vertical and horizontal transmission enhancement between antennas using textile artificial magnetic conductor waveguide sheet. Electron. Lett. 2015, 51, 671–672. [CrossRef] 92. Kim, S.H.; Lee, J.Y.; Nguyen, T.T.; Jang, J.H. High-Performance MIMO Antenna with 1-D EBG Ground Structures for Handset Application. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 1468–1471. [CrossRef] Materials 2021, 14, 149 20 of 20

93. Kim, S.; Ren, Y.J.; Lee, H.; Rida, A.; Nikolaou, S.; Tentzeris, M.M. Monopole Antenna With Inkjet-Printed EBG Array on Paper Substrate for Wearable Applications. IEEE Antennas Wirel. Propag. Lett. 2012, 11, 663–666. [CrossRef] 94. Jiang, Z.H.; Brocker, D.E.; Sieber, P.E.; Werner, D.H. A Compact, Low-Profile Metasurface-Enabled Antenna for Wearable Medical Body-Area Network Devices. IEEE Trans. Antennas Propag. 2014, 62, 4021–4030. [CrossRef] 95. Gao, G.P.; Yang, C.; Hu, B.; Zhang, R.F.; Wang, S.F. A Wearable PIFA with an All-Textile Metasurface for 5 GHz WBAN Applications. IEEE Antennas Wirel. Propag. Lett. 2019, 18, 288–292. [CrossRef] 96. Mersani, A.; Osman, L.; Ribero, J.M. Performance of dual-band AMC antenna for wireless local area network applications. IET Microw. Antennas Propag. 2018, 12, 872–878. [CrossRef] 97. Mantash, M.; Tarot, A.C.; Collardey, S.; Mahdjoubi, K. Design methodology for wearable antenna on artificial magnetic conductor using stretch conductive fabric. Electron. Lett. 2016, 52, 95–96. [CrossRef] 98. Zhu, S.; Langley, R. Dual-band wearable antennas over EBG substrate. Electron. Lett. 2007, 43, 141–143. [CrossRef] 99. Zhu, S.Z.; Langley, R. Dual-Band Wearable Textile Antenna on an EBG Substrate. IEEE Trans. Antennas Propag. 2009, 57, 926–935. [CrossRef] 100. Bai, Q.; Langley, R. Crumpled integrated AMC antenna. Electron. Lett. 2009, 45, 662–663. [CrossRef] 101. Joshi, R.; Hussin, E.F.N.M.; Soh, P.J.; Los, M.F.J.; Lago, H.; Al-Hadi, A.A.; Podilchak, S.K. Dual-Band, Dual-Sense Textile Antenna With AMC Backing for Localization Using GPS and WBAN/WLAN. IEEE Access 2020, 8, 89468–89478. [CrossRef] 102. Yan, S.; Soh, P.J.; Vandenbosch, G.A.E. Low-Profile Dual-Band Textile Antenna with Artificial Magnetic Conductor Plane. IEEE Trans. Antennas Propag. 2014, 62, 6487–6490. [CrossRef] 103. Wang, M.J.; Yang, Z.; Wu, J.F.; Bao, J.H.; Liu, J.Y.; Cai, L.L.; Dang, T.; Zheng, H.X.; Li, E.P. Investigation of SAR Reduction Using Flexible Antenna With Metamaterial Structure in Wireless Body Area Network. IEEE Trans. Antennas Propag. 2018, 66, 3076–3086. [CrossRef] 104. Velan, S.; Sundarsingh, E.F.; Kanagasabai, M.; Sarma, A.K.; Raviteja, C.; Sivasamy, R.; Pakkathillam, J.K. Dual-Band EBG Integrated Monopole Antenna Deploying Fractal Geometry for Wearable Applications. IEEE Antennas Wirel. Propag. Lett. 2015, 14, 249–252. [CrossRef] 105. Yan, S.; Vandenbosch, G.A.E. Radiation Pattern-Reconfigurable Wearable Antenna Based on Metamaterial Structure. IEEE Antennas Wirel. Propag. Lett. 2016, 15, 1715–1718. [CrossRef] 106. Jang, T.; Zhang, C.; Youn, H.; Zhou, J.; Guo, L.J. Semitransparent and Flexible Mechanically Reconfigurable Electrically Small Antennas Based on Tortuous Metallic Micromesh. IEEE Trans. Antennas Propag. 2017, 65, 150–158. [CrossRef] 107. Saeed, S.M.; Balanis, C.A.; Birtcher, C.R.; Durgun, A.C.; Shaman, H.N. Wearable Flexible Reconfigurable Antenna Integrated With Artificial Magnetic Conductor. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2396–2399. [CrossRef]