Design and Performance Analysis of Compact Wearable Textile Antennas for IoT and Body-Centric Communication Applications

This paper presents two compact textile-based planar dipole and loop antennas for wearable communication applications operating in the 2.4GHz industrial, scientiﬁc, and medical radio (ISM) bands. The antennas were fabricated on a 0.44 mm thin camouﬂaged-military print, cotton jean cloth using conductive copper threads, and sewing embroidery technique to create the radiating structure. Design and performance analyses of the antennas were carried out using simulations; further experiments were performed in anechoic chamber and indoor environment to validate the designs. The experiments were carried out in a free space scenario and on the various locations of the human subject such as the torso and limb joints. The performance of the antennas was investigated based on the reﬂection coeﬃcient in normal and bent conditions corresponding to the diﬀerent radii of the locations of the human limbs. The antennas perform well in free space and on-body scenarios in ﬂat and bend conditions providing return loss below − 10dB in all cases with an acceptable resonant frequency close to 2.4 GHz due to the antenna bending and body eﬀects. The radiation pattern measurements are also reported in this work for free space and on-body scenarios. It is observed that the presence of the human body signiﬁcantly inﬂuences the antenna radiation pattern which leads to an increase in the front-to-back ratio and also makes the antenna more directive. Overall, the performance of the fabricated embroidered textile antennas was found suitable for various wearable body-centric applications in indoor environments.


Introduction
Wearable communication technologies offer promising solutions for applications in the field of biomedical, consumer electronics, sports, military, and smart home applications. Recent developments in miniaturized and flexible electronic devices have made commercialization of such devices possible which has paved the road for vast utility in wearable Internet of ings (IoT) applications [1]. eir lightweight, low-cost manufacturing, ease of fabrication, and the availability of inexpensive flexible substrates (i.e., papers, textiles, and polymers) make flexible electronics an appealing candidate for the next generation of consumer electronics [2,3].
Research is being carried out at various frequency bands available in the open literature for wireless body area frequency and desired radiation characteristics [10]. Several works have been carried out on rigid substrates which lack flexibility and can be fragile due to the dynamic movements and postures made by the user. To address this problem, various flexible substrates are proposed in the open literature. Commonly used flexible materials for antennas are polyimides (PI), polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), liquid crystal polymer (LCP), Kapton, and paper-based substrates [2,3,[6][7][8][9]11].
Apart from polymer-based substrates, textile-based substrates are also gaining popularity due to the ease of integration of the antenna directly with the clothing as well as due to the flexible and conformal nature of the textile fabrics [12]. Textile materials commonly used as substrates are cotton cloth, felt, denim, fleece, nylon, and polyester. Different fabrication methods are chosen depending on the substrate material, application, and suitability; these methods include inkjet-printing, screen-printing, 3D printing, sewing, and embroidered techniques [2,3,13,14]. Textile material selection in terms of electrical properties and thickness, manufacturing process, feeding method, and the overall antenna design are important aspects to be considered while designing such antennas. Antenna designs such as dipole, loop, spiral, and patch are commonly chosen structures for various communication applications [15][16][17][18][19][20]. Advanced antenna structures like (electromagnetic bandgap) EBG, artificial magnetic conductors (ACM), and substrate-integrated waveguide (SIW) antennas to enhance the performance of the antenna have been reported in open literature [21][22][23].
is paper proposes two compact, low-profile textilebased antennas which can be integrated into the clothing suitable for healthcare, indoor, and military IoTapplications. e proposed design exploits the lightweight and flexible properties of the cotton cloth along with the high conductivity copper threads which are embroidered using sewing technology to form the radiating elements over the textile. e dipole antenna operates at 2.19-3.44 GHz and the loop antenna operates at 2.35-2.81 GHz frequency range in free space scenario. Various antenna parameters such as return loss and radiation patterns have been analysed for free space and on-body scenarios. Flexibility tests have also been carried out by bending the antenna at different radii corresponding to that of the radii of the limb joints of the human subject. e novelty of the work described in this paper lies in the fabrication of highly conformal and flexible textile antennas using highly flexible customized copper threads in an automated manner. Moreover, this paper presents an in-depth analysis and comparison for the two textile antennas for onbody measurements in flat as well as bending scenarios (apart from free space scenarios), which is limited in the open literature. Various positions of the limbs have also been analysed to see the effect of orientation and proximity with the torso/thigh region depending on the antenna location. a 0.44 mm thin jeans cloth has been employed as a substrate which makes the antenna design more challenging. e antenna prototypes are presented in Section 2 along with the simulation and measurement results. Section 3 gives details of the on-body measurement procedure and compares free space and on-body measurements related to S 11 and far-field radiation patterns. Section 4 deals with the study on the influence of the antenna performance when bent at different radii for free space and on-body scenarios and key findings are reported. e conclusion is presented in Section 5 with highlights on the future aspects.

Fabricated Textile Antenna Prototype and
Performance Analysis e proposed textile dipole and loop antenna structures comprise the radiating elements (two arms and rectangular loop structure, resp.) fabricated using the copper metal threads over a jeans cotton cloth fabric. ese flexible and conformal copper threads or yarns were developed by twisting multiple thin copper wires, each wire's diameter being 0.071 mm. e antennas are designed to work at 2.4 GHz, ISM band using the jeans cotton material as the substrate with a relative permittivity of 1.67 [12]. e copper threads are embroidered into the substrate material of the antennas to form the radiating elements. e thickness of the copper thread is 0.345 mm throughout and the substrate thickness is around 0.44 mm. e final schematic and dimensions of the antennas are present in Figure 1(a) for the dipole antenna and Figure 1(b) for the loop antenna. e length of the dipole antenna arm is calculated by half-wavelength of the desired frequency, that is, 2.4 GHz. e separation between the two antenna arms and the length of those arms are important factors that are sensitive to the change in the output frequency at which the antenna performs. To verify the proposed design performance before physically fabricating them using the embroidery sewing tool, a CST model was simulated with the same dimensions. e sewing machine used to fabricate the copper-threadbased antennas over the textile material is Bernina B720 [24]. is machine has a Bernina hook system, which gives it a maximum sewing and embroidery speed of 1000 stitches per minute. e maximum stitch width the tool can provide is 5.5 mm with a maximum stitch length of 6 mm. e antenna design models were integrated with the tool using the "Bernina Embroidery Software Designer Plus" software which was purchased along with the sewing tool itself. Figure 1(c) shows the sewing process in which a dipole antenna pattern is being embroidered using copper thread over the denim cloth fabric.
A thin 3 mm foam (which provides no changes in the relative permittivity) was applied on the backside of the antenna fabric to add extra support to the textile during free space and body-centric measurements. e fabricated prototype of the textile dipole antenna and loop antenna is presented in Figures 1(d) and 1(e), respectively. A 50 Ω SMA connector was carefully soldered to the edges of the dipole and rectangular loop structures. e performance of the wearable textile antennas was measured through a programmable 2-port vector network analyser (PNA-E8364C) in terms of reflection coefficient (S 11 ) and compared with the simulated results obtained from CST microwave studio. Radiation pattern measurements were also carried out in the anechoic chamber at the RF and Microwave Lab, CARE, IIT Delhi, and with those obtained from the simulation results. Good agreement is found between the simulation and measurement results for both proposed antennas. Figure 2(a) presents S 11 results for the simulated and measured textile dipole antenna structure. It is observed that both results provide below −10 dB return loss and resonate in the 2.4 GHz, ISM band, and perform well in a free space/indoor environment. e deviations observed in the simulated and measured results can be possibly attributed to the radiating material in these textile antennas being conductive threads in comparison to a uniform sheet of copper in the simulated design, as well as to a difference in the permittivity of the substrate employed in the simulations from that of the jeans cloth substrate in these antennas. e presence of a 50-ohm SMA conductor soldered to the antenna structure can also bring about some variation between the measured and simulated results. Figures 2(b) and 2(c) present the simulated and measured radiation pattern of the dipole antenna for E-plane and H-plane. Similar radiation patterns are observed for both cases, hence validating the performance of the fabricated antenna.

Body-Centric Measurement Scenarios
Body-centric measurements were performed on a human subject in an indoor laboratory environment in the 2.4 GHz ISM frequency band. e male human subject has a height of 167 cm and a weight of 74 kg with an average build.
e wearable antenna was placed on the torso region of the human subject to compare with the free space S 11 results (Figure 3(a)). e wearable textile antennas are able to achieve below −10 dB return loss for onbody scenarios indicating good performance of the antenna when placed in the proximity of the body. An example of the measured and simulated S 11 results of the textile loop antenna is presented in Figure 3(b), for both the free space and the on-body scenarios. For the on-body simulation results, a phantom representing the torso region of the human body with dimensions 200 × 200 × 70 mm 3 is designed in CST microwave studio. e human phantom consists of skin, fat, and muscle as the three layers with electrical properties such as permittivity and conductivity as per data provided in [25] for 2.4 GHz frequency.
e thickness of the skin, fat, and muscle layer is 3 mm, 7 mm, and 60 mm, respectively [26]. A shift of 100 MHz in the resonant frequency is observed when placed over the body (2.4 GHz) in comparison to the free space (2.5 GHz) results which is due to the body effects that influence the antenna performance.
Apart from S 11 measurements, far-field radiation pattern measurements are also carried out for both antennas studied in an anechoic chamber for free space and on-body scenarios. e antenna pattern measurements are performed for E-plane and H-plane to understand the influence of the human subject in both planes.
e results for the textile dipole antenna and loop antenna are presented in

Impact of Antenna Bending
One of the main challenges of the textile wearable antenna design is the uncertain form of the garment surface due to frequent movements performed by the human subject and also the nature of the fabric [29]. Hence, the wearable antennas integrated with the garment also encounter scenarios such as bending, stretching, flexing, and crumpling apart from the normal flat condition [29,30]. is section evaluates the antenna performance under bending conditions for free space and on-body scenarios.    results leading to a shift in the resonant frequency and also the magnitude of the S 11 results. Significant deviation from the flat case results is observed for the lowest bending radii � 25.7 mm as the antenna structure is quite affected by the degree of bending taking place especially for the textile dipole antenna. e bending scenario leads to a change in the resonant length of the antenna leading to variation in the S 11 characteristics. Minimum variation is observed for the highest bending radii � 57.2 mm as this scenario is approaching towards the flat orientation of the antenna. As observed in Figure 6(

On-Body Measurements.
On-body antenna bending experiments have been carried out for various locations of the human subject which are the wrist, elbow, shoulder, and knee joints. e antenna bending experiment is performed for two positions of the upper and lower limb while the subject is in a sitting posture. In the first position, the arm is at a 0-degree position with respect to the shoulder joint (as depicted in Figure 7(a)) and the leg is stretched out straight making 0 degrees with respect to the thigh joint (as depicted in Figure 7(b). In the second scenario, the position of the arm is at 90 degrees with respect to the shoulder joint (as depicted in Figure 7(c)) and the leg bent at 90 degrees with respect to the knee joint (as depicted in Figure 7(d)).
As observed in Figures 8(a), 8(b), 9(a), and 9(b) corresponding to the dipole antenna and loop antenna, respectively, the S 11 magnitude and resonant frequency vary for various on-body locations on the antenna. is is attributed to the body effects and also the different curvatures of bending of the textile antenna depending on the radii of      11 parameters with a 90-degree scenario depicting slightly better S 11 resonance. is can be due to the reduction in body effects caused from the torso or thigh region when the limbs are positioned in the 90-degree case.
For the textile loop antenna, higher fluctuation in the magnitude of the S 11 results are observed (Figures 9(a)-9(b)) for the different on-body locations and bending curvatures.    is variation depends on various factors such as the bending radii, surface on which the antenna is placed on the body (directly on the skin/above clothing) and distance from the body surface. e shift in the resonant frequency from the free space results is less for the various bending radii chosen in comparison to that of the textile dipole antenna. e shift ranges from 2.42 GHz to 2.45 GHz of frequency. It can be concluded that the type of antenna, location of the wearable antenna, bending radii of the antenna, and also the orientation of the limbs can cause some variation in the S 11 characteristics such as resonant frequency, impedance bandwidth, and S 11 magnitude. It is also observed that, in all the scenarios considered, the magnitude of the S 11 is below −10 dB and also resonating close to the desired frequency of operation which is the 2.4 GHz ISM band. For the dipole antenna, the shift observed ranges from 10 MHz to 60 MHz and for the loop antenna, the shift observed is in the range of 20 MHz to 50 MHz, for on-body scenarios when compared with the desired 2.4 GHz resonant frequency. Table 2 compares the work described in this paper with the other existing works from the literature [26,[31][32][33][34][35][36][37]. e novelty of the work described in this paper lies in the fabrication of highly conformal, flexible, and compact textile antennas using highly flexible customized copper threads in an automated manner. e fabrication of these low-profile textile antennas was carried out on very thin textile substrates that are suitable for free space and body-centric applications.

Comparison with Related Previous Works
e performance of the two antennas is compared for various bending conditions and on-body scenarios for which a detailed analysis has been provided. It has to be mentioned that such comparison or analysis is very limited considering the research work on textile antennas present in the open literature. e body-centric measurements have been carried out for various locations on the body and also for different orientations of the limbs. e proposed antennas have been designed using a very thin textile jean cloth (of thickness 0.44 mm) and are still able to achieve the desired resonant frequency in normal conditions as well as in bending scenarios. e proposed antennas are fabricated on the jeans cloth using digital embroidery technology with the radiating elements made from highly conformal and flexible customized copper e-threads of diameter 0.345 mm, which gives acceptable results for various on-body locations and limb orientations. e copper yarn is made from twisting multiple copper threads of 0.071 mm in diameter.
e copper e-threads have higher conductivity and provide higher performance in comparison to threads made of other metals (such as silver or steel) that have been employed in previous works [14,35,38]. Most of the works in open literature use conductive fabrics [26,33,36] or copper tapes/foil [31,32,34,37] which are glued over the textile fabric that makes the antenna not durable to withstand repeated use and harsh environments. e use of e-threads (conductive threads) as the radiating elements makes it feasible to integrate the antenna into clothing, thus making the movement of human subject unobtrusive and comfortable.

Conclusion
is work proposes two textile-based antennas for IoT and body-centric communication applications, namely, dipole and loop antenna. e performance of the wearable antenna is investigated in free space and in proximity with the human body. e antennas have been fabricated on cost-effective and commercially available textile cotton cloth and the radiating structure made of conductive copper threads has been formed using the sewing embroidered technique. e proposed antennas are compact and flexible with good agreement between the measured and simulated results operating at 2.4 GHz.
Return loss and radiation pattern characteristics for free space and on-body scenarios have been presented and discussed. e return loss of the antenna shows good performance in free space and on-body scenarios. e antenna radiation pattern becomes more directive for the wearable scenario in comparison to free space which is due to the presence of the human subject. Flexibility tests have also been carried out for different bending radii corresponding to the curvature of the human limbs which are probable locations where the wearable devices can be placed or integrated with the clothing. e performance of the antennas deviates slightly in terms of resonant frequency, the bandwidth of the operation which is attributed to the bending condition and body effects leading to variation in the results. e antennas perform well in free space and on-body scenarios which make them suitable candidates for wearable communication applications.

Data Availability
Data are available upon request to Dr. Richa Bharadwaj (e-mail: richab@care.iitd.ac.in).

Conflicts of Interest
e authors declare that they have no conflicts of interest.