Design of On-Body Epidermal Antenna on AMC Substrate for UHF RFID in Healthcare

This article presents a compact AMC structure used as a shielding element for a generic wearable RFID tag at UHF frequencies for on-body applications, with an overall footprint limited to an area of only <inline-formula> <tex-math notation="LaTeX">$0.03~\lambda _{0}^{2}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$41.4\times82.8$ </tex-math></inline-formula> mm). Thanks to the isolation provided by the AMC planar structure, the tag antenna gain and reading range are increased by about one order of magnitude in comparison with the case of a conventional tag attached to the human body. The designed antenna is platform tolerant, with very good robustness and isolation with respect to the human body, exhibiting a high reliability. The AMC structure is implemented on a thin, flexible, and biocompatible high permittivity silicon-doped dielectric substrate, with apertures both in the substrate and in the ground plane to allow skin transpiration. Therefore, the presented device can be effectively used also as an epidermal antenna, allowing the “on-skin” sampling of the most typical health parameters. The presented configuration has been designed using CST Studio Suite. A prototype has been fabricated and fully characterized, and measured results are in very good agreement with simulations.

Abstract-This article presents a compact AMC structure used as a shielding element for a generic wearable RFID tag at UHF frequencies for on-body applications, with an overall footprint limited to an area of only 0.03 λ 2 0 (41.4 × 82.8 mm).Thanks to the isolation provided by the AMC planar structure, the tag antenna gain and reading range are increased by about one order of magnitude in comparison with the case of a conventional tag attached to the human body.The designed antenna is platform tolerant, with very good robustness and isolation with respect to the human body, exhibiting a high reliability.The AMC structure is implemented on a thin, flexible, and biocompatible high permittivity silicon-doped dielectric substrate, with apertures both in the substrate and in the ground plane to allow skin transpiration.Therefore, the presented device can be effectively used also as an epidermal antenna, allowing the "on-skin" sampling of the most typical health parameters.The presented configuration has been designed using CST Studio Suite.A prototype has been fabricated and fully characterized, and measured results are in very good agreement with simulations.Index Terms-Artificial magnetic conductor (AMC), epidermal antenna, healthcare, RFID antenna.

I. INTRODUCTION
T HE recent advance in pervasive remote assistance and healthcare digitalization is favoring the diffusion and development of biomedical services and systems, able to empower patients to have a central role in their personal care and health diagnostic and monitoring [1], [2].The growth of these services and applications calls for the study of new challenges and alternative approaches for the design of wearable, implantable, and biocompatible devices.The latter can rely on electromagnetic fields for gathering and analyzing several categories of data (physiological, emotional, behavioral, environmental), aiming to improve citizens' wellbeing and health [2].
In this framework, wearable electronics represent an emerging research trend that combines multidisciplinary backgrounds such as classic electronics and electromagnetics, material science, communication, and mechanics [3], [4], [5], [6].Wearable electronics and sensors can be easily deployed and/or integrated over users' garments, enabling a hugeness of new applications for human health and wellness.The most effective way to collect the data acquired by wearable sensors is using wireless technology, and the well-assessed passive radio frequency identification (RFID) technology is certainly one of the best candidates [7], also allowing easy interfacing with the increasingly widespread the Internet of Things [8].As a matter of fact, RFID technology is based on communication through electromagnetic backscattering.It requires just a small battery-less IC transponder, which is fully compatible with wearable, flexible, and thin devices ensuring a reading range up to 10 m in the UHF frequency band [7].Furthermore, most recent RFID ICs integrate also some physical sensing capabilities (e.g., temperature and humidity [9]) or, complementary, analog ports, and A/D converters to straightforwardly interface external sensors.As a consequence, RFID platforms can retrieve some important data and information about the state of the human body by exploiting the simple interaction between the tag and the skin [10], with no need for dedicated and complex electronics that should be integrated into the wearable device otherwise [11].
The main drawback of this solution is due to the high losses of the platform on which the wearable device is attached, the human body, which strongly degrades the performance of the RFID tag antenna, decreasing the efficiency and the reading range, detuning the antenna resonant frequency, and deteriorating the radiated electromagnetic far-field.All these effects are strictly related to the antenna size, layout, and operating frequency.Moreover, the geometrical and electromagnetic parameters of the human body, on which the antenna is attached, may also differ based on the antenna location (e.g., head, chest, leg, arm) even on the same person [12], [13], [14], [15], [16].On-body platform tolerant devices are then a must.Such devices should be designed to seamlessly, unobtrusively, and comfortably adhere to the textured and soft surfaces, typical of the human body, and are required to provide a low density of surface conductors.Therefore, preferably, they should be designed using narrow conductor traces and thin dielectric substrates [3], [4], [5], [6].The optimal substrate structure must be flexible, with a relatively high dielectric constant and low losses (for example a suitable doped silicone substrate, which can allow an adequate size reduction, obtaining a compact and comfortable epidermal device) [11], [17].The radiating part of the device, which will ensure the correct transfer of data and information, requires an adequate UHF-RFID tag antenna that should be fabricated through the same technology of the sensor device.
An on-body antenna typically consists of aperture antennas, wherein an arbitrary shaped slot is etched into a metal patch [18], [19], planar inverted-F antennas [13], [15], [20], [21], or patch antennas.The antenna-body coupling effect can be reduced by using appropriate insulating substrates [22], or suitably enlarging the antenna's ground plane [21], [23], allowing effective shielding between the antenna and the human body.This ensures better insulation, but robustness is achieved at the expense of a larger antenna, leading to an obtrusive and uncomfortable device, especially at the UHF band, although a proper shape of the ground plane can be chosen with a reduced impact on the antenna extent [13], [15], [16], [24].Moreover, such wearable antennas (both aperture, patch, and PIFAs) require thick dielectric substrates and/or suitable air gaps to properly work in proximity to the body tissues.
An alternative, and most effective, solution to obtain a more performing device is found in artificial magnetic conductors (AMCs) [25], [26], [27].This solution can be exploited by the designer to preserve the RFID tag antenna radiation characteristic, thanks to the capability of an AMC to isolate an antenna from the surrounding environment.Moreover, the use of the AMC as a tag support will enhance the RFID antenna gain, which is strongly reduced by the coupling with the human body when the tag is directly attached to it [28].
AMC supporting structures play a pivotal role as effective shielding elements, thereby enhancing the performance of onbody antennas.This inherent characteristic of AMCs, which constitutes their primary advantage, can be effectively leveraged for a generic antenna attached to the AMC, provided that the antenna size is smaller than the AMC structure.The generic antenna supported by the AMC can take the form of a wire antenna or rely on a planar design.The optimal performance may require a gap between the antenna and the AMC.This gap can be easily implemented using suitable materials such as foams or supporting dielectric structures.The increase in antenna gain, achieved through the isolation offered by the planar AMC structure, depends on the layout of the supported antenna.For example, in the case of a planar inverted-F antenna (see [42], [46]) with an existing ground plane, the gain improvement due to the AMC is limited to a few decibels.Conversely, for antennas without inherent isolation, such as our generic tag or the structures reported in [45] (a printed monopole) or in [34] (a printed Yagi-Uda antenna), the gain improvement can exceed 10 dB.This highlights the crucial role of supporting AMC in achieving a satisfactory realized gain under typical operating conditions.Additionally, the resulting structure demonstrates platform tolerance, exhibiting good isolation and robustness with respect to the working environment.
In this work, we present the design, characterization, and validation of a relatively small AMC structure used as a shielding element for a wearable RFID tag at UHF frequencies.A generic RFID tag antenna has been attached to the miniaturized AMC, suitably designed to limit the overall footprint of the radiating antenna to an area of only 0.03 λ 2 0 (41.4 × 82.8 mm), a very small size if compared to the state of the art for an AMC-based antenna (see Table I).This compact radiating device is designed to operate within the RFID UHF frequency band, at the central frequency of 900 MHz.The performance of the tag in two specific operating conditions has been investigated and compared, i.e., when directly attached to the human body and when placed upon a tailored AMC structure, which shields it from the wearer.A substantial increase of the tag antenna gain (and, consequently, of the reading range) of about one order of magnitude has been achieved, thanks to the isolation provided by the AMC planar structure, which can be considered a significant improvement in the platform-tolerant device performance.The gain increase (14 dB) is considerable, also if compared to the typical increment achieved by similar proposed devices [27], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46] (see Table I) and allows the wearable antenna to reach a satisfying reading range in typical operating conditions.
The designed antenna is considered platform tolerant, presenting very good isolation and robustness with respect to the human body, a satisfying reading range, and high reliability.
In addition, we adopted very thin dielectric substrates for both the AMC and RFID tag, purposely realizing and shaping the AMC substrate to exhibit an adequately high dielectric constant, low losses, flexibility, biocompatibility, and breathability (thanks to holes in the substrate).For the same reason, also the AMC ground plane is properly shaped, proposing a thin and flexible on-body antenna.As a consequence, our RFID tag can be used also as an epidermal antenna, allowing the "on-skin" sampling of typical health parameters, such as surface/body temperature, pH, humidity, skin impedance, deformations, fluid exchanges at the skin interface and electrophysiological potentials.This tag can operate without interfering with the local metabolism of the skin, since its breathable substrate does not perturb the skin's local temperature and transpiration, resulting in a minimal impact on local sweating and heat exchanges.In Table I, we compared our device also with two epidermal antennas proposed in recent literature [47], [48], which show unsatisfying performance in terms of realized gain, since they work attached to the skin, without any countermeasure with respect to the interaction with the human body.

II. AMC DESIGN
An AMC is a particular electromagnetic bandgap (EBG) structure [49], [50] designed to emulate the properties of an AMC within the designated frequency range.EBG structures, generally characterized by their periodic or sometimes non-periodic nature, exert control over the propagation of electromagnetic waves across a defined frequency band, accommodating all incident angles and polarization states.These periodic structures exhibit resonance at specific frequencies, manifesting filtering characteristics that find applications at microwave frequencies, offering advantages such as compactness and high gain [49], [50].
The AMC operation is based on the resonance of the cavity between the ground plane and the periodic elements.Therefore, AMCs are narrowband devices.The shape of the periodic elements ranges from simple (patches, dipoles) to complex geometries [51] and, due to the resonant behavior of the AMC device, the size of the unit cell must be close to half a wavelength.
By employing an AMC, it is possible to place a planar electric source close (or attached) to the meta-surface without degrading its performance, since the AMC has only a negligible effect on its radiating properties.This could be very important in wearable applications, wherein the AMC features can be exploited to obtain high robustness and isolation between the antenna and its platform (the human body).Moreover, this solution allows a significant increase in the gain of a planar electric source when compared with its gain when it radiates in free space or even when attached to the human body (which represents a significantly worse situation).
AMC structures have been widely used in recent years for several applications: as ground planes for antennas [52], as resonant structures [45], for improving the transmission of waves from nearby wearable antennas [53], for antenna impedance matching [54], for antenna detection [55], for radiating element shielding and SAR reduction [27], and as tunable devices [56].
Unfortunately, AMC unit cells can be cumbersome, mainly in the lower part of the UHF band, wherein even half a wavelength can be an unpractical size in several applications.Therefore, a challenging aspect of AMC design is to minimize the size of the periodic unit cell.In [57] and [58] the AMC device has been miniaturized using convoluted elements and space-filling curves as unit cells, but these solutions reduce the operating bandwidth.
A better solution to obtain the AMC reduction has been achieved in this work by modifying a conventional symmetric square unit cell inspired by the classical bidimensional cell, namely the "Jerusalem cross" [59], [60], whose circuital model can be considered equivalent to a parallel LC circuit.In such EBG structures, the equivalent capacitance C is mainly related to the gap between two adjacent elementary EBG cells, whereas the equivalent inductance L is due to the thin microstrip lines connecting the cells.
We started our design with a conventional symmetric square unit cell [depicted in Fig. 1(a)], which we modified through the incorporation of enhanced features in the form of comb-like arms on each side of the metallic squares [illustrated in Fig. 1(b) and (c)].These arms play a crucial role in extending the path of the current flowing through the structure.Each of these comb-like arms introduces an additional inductance to the cell equivalent parallel LC circuit, determined by the length of the arms.Furthermore, the presence of these arms introduces an additional capacitance, which is influenced by the spacing between these arms.The cumulative effect of these modifications results in the new AMC cell resonating at a lower frequency compared to its conventional counterpart.The overall capacitance of the cell is subject to further fine-tuning through adjustments in the spacing between neighboring cells [intercell distance DL cell , as depicted in Fig. 1(a)].The cell size and its design parameters (intercell spacing DL cell , lengths of comb-like arms L 1 , L 2 , L 3 and their spacing DL a , DL b ) have been carefully selected to ensure resonance at 900 MHz, within the RFID UHF frequency band.

A. Permittivity-Tailored AMC Supporting Substrate
As previously mentioned, a significant challenge in designing and implementing an AMC structure for UHF frequencies lies in achieving a compact size.Properly designing the periodic structure is a crucial factor in obtaining this outcome.Equally important is the precise design and fabrication of a high-permittivity substrate.Moreover, the substrate dielectric constant should be carefully tailored to match the very strict design constraints: indeed, even slight variations in the substrate properties could lead to a not-effectively-working device because of the narrowband behavior of the AMC structure.
To properly realize a substrate with the above features, we have mixed a silicone rubber matrix with a highly ferroelectric ceramic filler to produce a nanocomposite material having the desired properties [61].Silicone rubber is chosen because it is a biocompatible, nontoxic, and non-irritant material.It typically consists of two components mixed in a 1:1 ratio to initiate the polymerization process, causing the silicone to solidify.During this initial phase, the silicone remains processable, allowing for the addition of filler into the silicone matrix and the pouring of the composite material into the mold before polymerization starts.The mixing process can be performed manually or with specialized mixers to minimize the introduction of air into the composite.The presence of microbubbles of air is the main factor leading to degradation in dielectric properties and can occur during both mixing and casting procedure.The ceramic filler utilized was Barium Titanate (BaTiO 3 ), known for its distinctive electrical properties, including a high dielectric constant and piezoelectric behavior.However, its dielectric constant significantly relies on its lattice structure, which can be hexagonal, tetragonal, or cubic, determined by the production process and sintering temperature [62].Additionally, BaTiO 3 exhibits a Curie Temperature of approximately 120 • C. Beyond this point, it undergoes a lattice transformation to the cubic form, resulting in a reduction of the dielectric constant.Consequently, this value can vary widely, ranging from tens to thousands.For the above reason, the adopted Barium Titanate has been previously characterized in [63] and resulted in a dielectric constant of 250, which remains valid as long as the mixing process is performed at room temperature and, in general, the threshold temperature of 120 • C is never exceeded.To estimate the effective dielectric constant of the final substrate, the Lichtenecker's model [64] was employed, which is defined by the following formula: where ε e f f is the dielectric constant of the final composite material, ε 1 and ε 2 are the dielectric constant of both matrix and filler (3.25 and 250, respectively), and f 1 , f 2 are the volume percentage of the materials.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Our goal was to realize a substrate of the AMC having a value of ε e f f between 6.2 and 6.3 and this result was achieved by utilizing, according to (1), a filler percentage of around 16%.To allow for a correct measurement of dielectric constant and loss tangent, in the same realization process, a bulk dielectric of 10 × 5 × 0.15 cm 3 along with the desired customized dielectric substrate has been created.The electromagnetic characterization of the bulk substrate at 866 MHz, performed using the well-known T-Resonator method [65], resulted in a measured dielectric constant of 6.24 and a loss tangent of 0.0135.Specifically, as for substrate shaping, an additive manufacturing technology called fused filament fabrication (FFF) has been used for 3-D-printing the container (mold) in which the silicone-based composite was cast.Molds and substrates are shown in Fig. 2.
The fabricated doped silicone substrate is flexible and has a relatively high dielectric constant with relatively low losses, hence it allows an adequate size reduction, obtaining a compact and comfortable wearable antenna.

B. AMC Unit Cell Numerical Results
The final AMC design has a unit cell with periodicity D = 41.4 mm (0.12 λ 0 at 900 MHz) and a total thickness of 1.57 mm (0.0047 λ 0 at 900 MHz).The conventional AMC cell with the same geometry will resonate at 1.15 GHz, therefore, the comb-like arms allow a size reduction of 28%.
Both the amplitude and phase of the reflection coefficient of the AMC structure for an incident plane wave are reported in Fig. 3 for different values of the substrate height h s .This is a key parameter for the AMC, which will influence both the resonant frequency and the operating frequency band (defined as the frequency range where the phase varies from −90 • to +90 • [49], [50], [66]).
As reported in the frequency response of the designed unit cell, depicted in Fig. 3, increasing the AMC substrate thickness h s leads to an increase in the operating bandwidth and a reduction of the resonant frequency.Moreover, when the thickness h s is lower than 3 mm, the AMC exhibits only a partial inversion of the phase.However, since epidermal antennas should guarantee unobtrusive and comfortable operation, a thickness above 3 mm is not acceptable, as the substrate height needs to be as small as possible.Therefore, it is crucial to reduce the thickness as much as possible.Thus, to address this constraint, we opted to fix the value of h s at 1.5 mm.Despite the fact that such reduced thickness allows only a partial inversion of the AMC phase, as apparent also from Figs. 3(b) and 4, this is adequate to ensure satisfactory performance for the designed on-body epidermal antenna (AMC + RFID tag), when the antenna is attached to the human body.This is evidenced by the results presented in the next section, which showcase the performance of the designed structure.
In order to ensure adequate perspiration of the skin, both the AMC substrate and ground plane should be drilled, making a sufficient number of holes.The cell of Fig. 1(b) is therefore modified by inserting five small rectangular openings in each of the four quadrants, as shown in Fig. 1(d), giving the structure a band-aid-like layout.
The frequency response of this unit cell is shown in Fig. 4, where it is compared with the cell without holes, showing only

III. ANTENNA NUMERICAL RESULTS AND ON-BODY PERFORMANCE
The AMC designed in the previous section is used as a shielding element for an on-body RFID tag operating within the RFID UHF frequency band, at the central frequency of 900 MHz.A loop antenna attached to the miniaturized AMC structure (see Fig. 5) has been selected.It consists of a standard rectangular metallic loop on a Rogers RT/Duroid 5880 substrate [67].The Duroid has a thickness of h sub = 0.8 mm and dimensions L O_SU B = 32 mm, L V _SU B = 30 mm, with dielectric permittivity ε r = 2.2 and tanδ = 0.0009.The tag metallic traces are made of copper with a thickness of 0.035 mm and a width of w tag = 1 mm.The tag is connected to the Magnus S3 integrated circuit (IC) by Axzon with impedance Z chi p = 4-j93 (corresponding to the RC model indicated in the datasheet [68], with R p = 2073 and C p = 1.9 pF, computed at 900 MHz) and chip sensitivity (i.e., the minimum power required to turn-on the IC) P I C = −13.3dBm p IC = −13.3dBm, by means of a T-match for impedance matching purposes [69].The finite AMC structure placed underneath the tag antenna is composed of an array of 2×1-unit cells (see Fig. 5), which represents the smallest possible size for a finite AMC, hence the proposed on-body/epidermal antenna has a very compact size of 41.4 × 82.8 × 2.405 mm 3 (2.2 × 10 −4 λ 3 0 at 900 MHz).The rectangular tag optimal geometrical parameters L O_T AG = 30 mm and L V _T AG = 28 mm L O_T AG = 30 mm were chosen based on the result of a parametric analysis oriented to the optimization of the far-field gain G.In a similar fashion, the T-match dimensions a = 10 mm and b = 9 mm were chosen in order to match the tag impedance with that of the microchip, so that also the tag realized gain G τ (i.e., the antenna gain G scaled by the impedance mismatch with the microchip [69]) was optimized.Both gain and realized gain are shown in Fig. 6.
In order to highlight the increase in the performance of the tag due to the insertion of the AMC shielding structure, two operating conditions have been investigated and compared: 1) the tag directly attached to the human body and 2) the tag placed upon the tailored AMC structure.
We used a single-layer equivalent model as a numerical phantom to analyze the body-antenna coupling effect on the antenna performance.It consists of a 100 mm-thick material with an area of 200 × 200 mm 2 (see Fig. 6) and with electrical parameters ε r = 55 and σ = 0.94 S/m at 900 MHz [70].
In Figs.7-12, the simulated magnetic field, electric field, and surface current distributions are shown for the operating conditions 1) and 2), respectively.
The presented results clearly highlight that the AMC structure acts as a shielding layer between the antenna and the human body, and the stacked structure (tag on AMC) shows significantly smaller peak values for both electric fields (an order of magnitude lower), magnetic field, and surface current (four times lower), ensuring very good isolation and robustness with respect to the human body.This behavior is confirmed      broadside direction), with a front-to-back ratio greater than 18 dB.
Finally, Fig. 14 shows the increase of the tag antenna gain when it is attached to the AMC structure and placed on the phantom, with respect to the gain of the tag directly attached to the phantom without the support of the AMC.A considerable increment of 14 dB is achieved, which corresponds to a significant improvement in the performance of the proposed on-body/epidermal device, allowing it to reach a very good reading range.The improvement in antenna performance due to the insertion of the AMC is closely linked to the specific design of the supported antenna.The inherent shielding feature of AMCs can be efficiently employed for any generic antenna attached to the AMC, as long as the antenna is smaller than the AMC structure.Looking at the state-of-the-art structures reported in Table I, it is evident that the AMC functions effectively and independently of the supported antenna.The key difference lies in the increase in gain and enhancement in isolation, both of which depend on the particular layout of the antenna under consideration.

IV. SENSITIVITY ANALYSIS AND BENDING ASSESSMENT
To assess the sensitivity and robustness of the proposed on-body epidermal antenna, a comprehensive analysis was conducted to evaluate its performance under various conditions, encompassing different phantom models and bending scenarios.The literature provides varying dielectric constant values for muscle tissue, with discrepancies attributed to factors such as measurement inaccuracies, subject age, or the specific muscle type under consideration.Leveraging literature-derived data for muscle tissues at 900 MHz, we applied parameters sourced from [70], [71], and [72] to simulate the muscle-like phantom.In Fig. 15, we compared the simulated realized gains for the proposed tag attached to the flat muscle-like phantom depicted in Fig. 6 with the electric parameters given by [70] (ε r = 55 and σ = 0.94 S/m), [71] (ε r = 50 and σ = 1.47 S/m for a temperature of T = 15 • C, ε r = 53 and σ = 1.1 S/m for T = 30 • C), and [72] (ε r = 56.6 and σ = 1.33 S/m).Despite the differences in dielectric constant and conductivity among the considered phantoms, the presented results demonstrate minimal impact on the antenna performance.At the design frequency of 900 MHz, the maximum difference among the four curves is only 0.27 dB, showing the low sensitivity of the proposed tag.
Additionally, we also assessed the structure's robustness under bending conditions using a multilayered cylinder simulating a human leg.This model incorporates skin, fat, muscle,    was set to 50 mm, and the skin layer thickness (DR S ) to 3 mm.By varying the thicknesses of the muscle and fat layers, and consequently adjusting the overall leg diameter, we explored different antenna bending diameters (DR T ) ranging from 73 to 98 mm.This parametric exploration allowed us to evaluate the antenna sensitivity also in the context of a multilayer cylindrical phantom model.Despite the substantial variations in layer thickness, the antenna exhibited remarkable stability in performance.At the design frequency of 900 MHz, the maximum difference among the six curves was only 0.6 dB, with a difference lower than 0.4 dB compared to the flat muscle-like phantom in Fig. 6.This resilience across diverse scenarios can be attributed to the high isolation provided by the supporting AMC structure.It highlights the platform tolerance of our structure, showing its low sensitivity to changes in phantom properties and structure, and its robustness when subjected to bending.

V. ON-BODY/EPIDERMAL ANTENNA EXPERIMENTAL VERIFICATION
A prototype of the proposed on-body/epidermal antenna has been realized and fully characterized.The drilled AMC array and ground plane [see Fig. 17(a)] were etched from an adhesive copper sheet and then attached to the substrate.The RFID tag [see Fig. 17(a)] was realized by etching the copper from the Duroid substrate, whereas the microchip was soldered with the help of a bench soldering iron.Moreover, the resulting structure was entirely covered with a thin film of Tegaderm [73] to stabilize the prototype and ensure that all its components were joined together.
Figs. 17(b) and 18 show the measurement setup.The realized on-body antenna was placed above a hydrogel-composed muscle-like phantom (by AET, [74]) with size 200 × 200 × 70 mm, relative permittivity ε r = 54.5 ε r = 54.5 and conductivity σ = 0.6 S/mσ = 0.6 S/m, that was refurbished by water 48 h before use and placed on a rotating platform [see Fig. 15(b)].A circularly polarized antenna (patch with broadside gain G R = 7.5 dBic) was placed 30 cm above the prototype and connected to the TagFormance UHF Pro station for realized gain and radiation pattern measurements purposes (see Fig. 18).Finally, the antenna was also attached to the chest of a volunteer by means of another Tegaderm adhesive substrate in order to perform measurements also in a real case scenario (see Fig. 18).The comparison between simulated and measured realized gains is shown in Fig. 18.The peak values of the measured realized gains are almost equal, whereas a slight difference with the simulated one is observed.Moreover, there is also a small frequency shift between all of them.These differences could be ascribed to uncertainties in prototype realization and dielectric properties of both the muscle-like phantom and volunteer skin.Despite this, measurements and simulations fit very well.Finally, radiation pattern measurements of the tag for θ = 0 • θ = 0 • and θ = 90 • (see Fig. 19) were performed as well.Also, in this case, there is a good agreement between simulations and measurements.
To further verify the proposed AMC decoupling capabilities, a new set of measurements was performed (see Fig. 20).The measurements show the realized gain of the tag + AMC and of the sole tag in three conditions, i.e., air, human body, and human phantom.While the tag + AMC offers the same realized gain regardless of the object to which it is attached (and well in line with simulations), the sole tag is sensibly affected by the operative conditions, suffering detuning, impedance mismatching, and efficiency worsening (the realized gain is sensibly lower and shifted in frequency).
By considering the FRIIS formula, the realized gain can be related to the expected maximum activation distance d With λ the wavelength at 900 MHz, EIRP is the effective isotropic power radiated by the reader, χ the polarization mismatch between reader and tag, and Pic the power sensitivity of the IC.In the case of EIRP = 3.2 W (the maximum power level allowed by ETSI regulations), Pic = −13.3dBm and χ = 1, the expected maximum activation distance d = 3 m.

VI. CONCLUSION
In this work, a novel on-body/epidermal antenna on a miniaturized AMC structure has been designed, exploiting the AMC to improve the performance of healthcare RFID applications.The AMC miniaturization has been achieved by suitably meandering a conventional unit cell and using a house-made high-permittivity and low-losses thin and flexible silicone-doped dielectric substrate.
The antenna works within the RFID UHF frequencies around 900 MHz, and its overall footprint is limited to an area of only 0.03 λ 2 0 (41.4 × 82.8 mm).The isolation provided by the AMC planar structure allows to increase in the tag antenna gain and reading range of about one order of magnitude in comparison with the case of the conventional tag (without AMC) attached to the human body.
The proposed device is platform tolerant, with high reliability, and is implemented on a very thin, flexible, and biocompatible substrate, with an adequate number of holes both in the AMC substrate and ground plane to allow skin transpiration.Such features ensure that the designed structure can be effectively used also as an epidermal antenna, allowing the "on-skin" sampling of the most typical health parameters like body temperature, skin impedance, and electrophysiological potentials.

Fig. 1 .
Fig. 1.(a) Top view of the conventional AMC unit cell.L cell = 41.4 mm; W S1 = 2.55 mm; W S2 = 1.84 mm; DL cell = 0.5 mm; L sq = 17 mm.(b) Top view of the modified AMC unit cell.(c) Zoomed-in view of the region with the comb-like arms.DL a = 1 mm; DL b = 3.85 mm; L 1 = 3 mm; L 2 = 5.2 mm; L 3 = 6.7 mm.The width of each metallic arm is 1 mm.(d) Top view of the modified AMC unit cell with rectangular holes in substrate and ground plane.DL c = 4.15 mm; L H = 2.45 mm.

Fig. 2 .
Fig. 2. (a) Some of the 3-D-printed molds of the realized substrates: the custom one on the left and the bulk one on the right.(b) Silicone-based substrates after the casting process.

Fig. 3 .
Fig. 3. Frequency response of the proposed AMC cell.(a) Reflection coefficient magnitude and (b) phase.

Fig. 4 .
Fig. 4. Frequency response of the meandered AMC screen with and without openings.

Fig. 6 .
Fig. 6.Simulated gain and realized gain of the proposed wearable antenna.

Fig. 7 .
Fig. 7. Magnetic field magnitude on (a) phantom, (b) tag's metallic loop, and (c) tag's dielectric substrate when the tag is directly attached to the phantom.

Fig. 9 .
Fig. 9. Electric field magnitude on (a) phantom, (b) tag's metallic loop, and (c) tag's dielectric substrate when the tag is directly attached to the phantom.

Fig. 10 .
Fig. 10.Electric field magnitude on (a) phantom, (b) tag's metallic loop, (c) tag's dielectric substrate, (d) AMC drilled ground plane, (e) AMC dielectric substrate, and (f) AMC metallic trace when the tag is placed upon the tailored AMC structure.

Fig. 11 .
Fig. 11.Surface current (a) magnitude and (b) vector on the tag's metallic loop when the tag is directly attached to the phantom.

Fig. 12 .
Fig. 12. Surface current (a) magnitude and (b) vector on the AMC drilled ground plane, (c) magnitude and (d) vector on the AMC metallic trace, and (e) magnitude and (f) vector on the tag's metallic loop when the tag is placed upon the tailored AMC structure.

Fig. 13 .
Fig. 13.Far-field pattern (a) when the tag is directly attached to the phantom and (b) when the tag is placed upon the tailored AMC structure.

Fig. 14 .
Fig. 14.Simulated Gain for the tag attached to the AMC structure and placed on the phantom, and for the tag directly attached to the phantom without the support of the AMC structure.

Fig. 15 .
Fig. 15.Comparison between simulated realized gains for the proposed tag attached to the flat muscle-like phantom depicted in Fig. 6 with different electric parameters.

Fig. 16 .
Fig. 16.(a) Three-dimensional view of the multilayer model of the human leg with attached the bent tag (L = 200 mm).(b) Top view of the multilayer model of the human leg.DR S = 3 mm, D b = 50 mm.(c) Comparison between simulated realized gains for the proposed tag attached to the multilayer leg model for different thicknesses of muscle and fat layers, and hence for different bending diameters DR T (as explicitly indicated in the curves labels).
and bone layers (refer to Fig.16for a detailed illustration of this multilayer model and for the electrical parameters assigned to each layer at 900 MHz).The bone diameter (D b )

Fig. 17 .
Fig. 17.(a) Front view and back view of the prototype of the AMC structure of the designed wearable antenna, and front view of the Tag.(b) Antenna attached to the muscle-equivalent gelatinous phantom inserted into the rotating platform.

Fig. 18 .
Fig. 18.Comparison between simulated and measured realized gains.Measures have been performed both with the tag antenna attached to a muscle-like phantom and to a volunteer, according to the setups shown in the figure insets.

Fig. 19 .
Fig. 19.Simulated and measured far-field pattern in the two main cuts (θ = 0 • and θ = 90 • ).The measures have been performed by means of a rotating platform.

Fig. 20 .
Fig. 20.Measured realized gain of the tag + AMC and of the sole tag in three different operative conditions, in air, on the phantom, and on the human body.

TABLE I COMPARISON
WITH THE STATE-OF-THE-ART