Textile Antennas for Wearable Radio Frequency Applications

Textiles and Light Industrial Science and Technology (TLIST) Volume 2 Issue 3, July 2013

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Textile Antennas for Wearable Radio Frequency Applications Zheyu Wang*, Lanlin Zhang, John L. Volakis ElectroScience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University 1320 Kinnear Rd., Columbus, OH 43212, USA *

wang.1460@osu.edu; zhang.471@osu.edu; volakis.1@osu.edu

Abstract We present a novel class of embroidered textile radio frequency (RF) circuit and antennas based on electrically conductive metal-polymer fibers (E-fibers) to realize wearable RF electronics woven on daily garments. The Efibers are composed of high strength and flexible polymer core covered with a metallic coating. These E-fibers exhibit very low electrical loss and excellent mechanical conformalty and flexibility. Computerized embroidery machinery and double layer stitching were employed to achieve high conductivity and precise fabrication of the efiber textiles onto regular clothing. Using this process, prototypes of textile antenna prototypes were fabricated and tested. Their RF characteristics were measured in off-body (freestanding) and on-body configurations. The obtained measurement data demonstrated that the textile antennas exhibited excellent RF performances comparable to conventional copper antennas. This is in addition to their excellent mechanical flexibility. Importantly, the textile antennas can be inconspicuously woven onto clothing, without affecting comfort, fashionality, and washability.

Those novel textile antennas provided solutions to future RF functionalized and fashionable garments for wearable high data rate wireless communications and for wireless health monitoring.

Keywords Textile Antenna; Wearable Antenna; Electronic Fiber; RF Electronic

Introduction Personal wireless communication devices require reliable wideband operation to deliver high data rates.

In particular, the wireless signals need to retain quality of service (QoS) regardless of the individuals’ movements or surrounding environment. Wearable radio frequency (RF) electronic devices are capable of offering excellent QoS because they can be placed at multiple locations within the garments. Further, as shown in Fig. 1, due to the large surface areas available on the body, wearable RF electronics can provide excellent functionality, in case of use and

comfort. Similarly, for antennas, large apertures can be realized to overcome size restrictions in handheld devices (e.g. cell phones, PDAs, etc) [Volakis, et al.

2012; Zhang, Wang, and Volakis 2012].

FIG. 1 WEARABLE RF ELECTRONICS FOR HIGH SPEED COMMUNICATIONS, HEALTH MONITORING, AND RFIDS

Wearable electronics have been of interest for more than a decade [Post et al. 2000; Martin et al. 2009; hamedi et al. 2007; SQE 2011; Zhang, Wang, Psychoudakis, and Volakis, 2012; Ouyang and Chappell 2008; Wang, Zhang, Bayram, and Volakis, 2012; Kuhn and Child 1998; Kim et al. 2008; Salonen et al. 2004; Locher and Troster 2008; Schwarz et al. 2011].

A challenge in designing wearable RF electronics is to achieve excellent RF performance while the attire is concurrently comfortable and attractive [Zhang, Wang, and Volakis, 2012]. To meet this requirement, electronic textiles have been reported to realize RF electronic devices [Zhang, Wang, Psychoudakis, and Volakis, 2012; Ouyang and Chappell 2008; Wang, Zhang, Bayram, and Volakis, 2012; Kuhn and Child 1998; Kim et al. 2008; Salonen et al. 2004; Locher and Troster 2008; Schwarz et al. 2011]. These textile-based devices have demonstrated good electrical performance and mechanical flexibility. However, most of the reported electronic textiles so far are thin metallic wires, metallic tapes into clothes or sputtered

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nanoparticles [Kuhn and Child 1998; Kim et al. 2008; Salonen et al. 2004; Locher and Troster 2008; Schwarz et al. 2011]. As such, the corresponding surfaces experience metal fatigue after repetitively flexing, making them unsuitable for wearable electronics. On the contrary, integration of conductive fibers into textiles is more feasible due to the intrinsic flexibility and conformality of the fibers [Wang et al. 2012; Wang, Zhang, Psychoudakis, and Volakis 2012]. Among possible conductive fibers are: (1) metal-coated highstrength polymer fibers (E-fibers) [Wang, Zhang, Bayram, and Volakis 2012],[Kim et al. 2008] and (2) conductive nanoparticle impregnated fibers [Mehdipour et al. 2010]. The metal-coated polymer fibers used in this study are preferred due to their low material losses over others.

In this paper, we present prototypes of E-fiber textile antennas, fabricated using automatic embroidery machines to realize precise stitching [Wang, Zhang, Bayram, and Volakis 2012]. RF characteristics of the textile antennas and circuits have been measured at radio frequencies, and also compared with those of copper antennas. It is demonstrated that textile devices have comparable performance to that of copper, making them promising for wearable applications.

Fabrication of Embroidered Conductive Textiles Conductive Textile Threads Silver coated Amberstrand® fibers, were used to form the conductive textile surfaces for RF applications.

These E-fibers are depicted in Fig. 2. Such fibers have a

(a)

FIG. 2 METALLIZED PBO MULTIPLE-STRAND FIBER.

diameter of ~15 µm, composed of a 10 µm highstrength p-phenylene-2, 6-benzobisoxazole (PBO [Toyobo 2005]) core and a 2-3 µm thick metallic coating (see their Scanning Electron Microscope (SEM) image in [Lobodzinski and Laks 2009]). They exhibit a low D.C. resistivity of 0.8 Ω/m [Carroll, Martin, and Miller], coupled with excellent mechanical strength and flexibility due to the PBO core. Measurement of the fiber conductivity over radio frequencies (0.1 to 20GHz) showed that the effective conductivity of the E-fiber surface is 3.5×106 S/m, comparable to that of copper [Chung et al. 2011]. It is noted that this high conductivity is critical to realize efficient RF applications.

Textile Surfaces for RF Applications The embroidery process was adopted for the E-fibers using a computerized sewing machine. As shown in Fig. 3(a), antenna and circuit designs were translated into embroidery patterns, followed by digitization of the stitching locations. As described in [Wang, Zhang, Bayram, and Volakis 2012], the E-fibers were precisely and firmly couched onto only one side of the textiles.

As such, possible abrasion damage of the metallic fiber

(b)

FIG. 3 TEXTILE-BASED WEARABLE ANTENNAS AND RF CIRCUITS: (a) FABRICATION OF EMBROIDERED TEXTILE SURFACE AND (b) PROTOYPES OF FLEXIBLE TEXTILE CIRCUIT AND ANTENNAS.

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coatings was avoided, as the E-fibers did not go through the textile. Prototype E-fiber antennas were fabricated (see Fig. 3(b)) and mechanically conformable onto different on-body locations. To explore E-fiber viability in constructing multilayer RF circuits, a process for multilayer microstrip circuit structure was also developed using via pins made by E-fibers [Wang, Zhang, Bayram, and Volakis 2012].

Characterization of Textile RF Circuit

Poutput , port 2

S11 (dB ) = 10 × log10

Poutput , port1

Pinput , port1 Pinput , port1

-0.2

21

(dB)

-0.4 -0.6 -0.8 -1

The RF performance of the E-fiber embroidered circuits were tested using a 5 cm-long 50 Ω microstrip transmission line [Pozar 2005]. As depicted in Fig. 4(a), both the microstrip line and ground plane (GND) were embroidered onto the textile surface and backed by a PDMS substrate [Wang, Zhang, Bayram, and Volakis 2012]. The PDMS (polydimethylsiloxane) was used to integrate with textile surfaces in multilayer structures.

Meanwhile, a copper version of the same structure was fabricated and used as reference. The electrical loss of the textile circuit was evaluated by measuring the S-parameters (S11 and S21) using a two-port Agilent N5230A network analyzer. As defined in [Pozar 2005], S21 is the power transmission coefficient and S11 is the power reflection coefficient, both expressed in decibel scale as given below:

S 21 (dB ) = 10 × log10

(a) 0

S

The embroidery process was tailored to achieve high conductivity of the E-fiber textile surfaces. Initially, 332 strands of E-fibers were bundled to form thicker threads. Concurrently, double-layer embroidery was employed to apply a second layer of E-fiber surface atop the first one to form the conductive patterns [Wang, Zhang, Bayram, and Volakis 2012]. The resultant physical discontinuities in the embroidered surface were minimized and were always less than λ/20, with λ being the free space wavelength at the operation frequency.

,

,

For example, S21=0dB (Poutput,port2/Poutput,port1=1) indicated that all power was delivered with no loss, and for S21 = -3dB (Poutput,port2/Poutput,port1 = 0.5) half of the power was deliveried to the other port. Similarly, S11=0dB (Poutput,port1/Poutput,port1=1) indicated that the power was totally reflected and S11=-10dB (Poutput,port1/Poutput,port1=0.1) implies good impedance matching where only 10% of input power was reflected back). This was critical for

E-fiber: Density 1.6 lines/mm E-fiber: Density 2.0 lines/mm E-fiber: Density 2.7 lines/mm Copper: reference

-1.2 -1.4

0

0.5

1 1.5 2 Frequency (GHz)

2.5

3

(b) FIG. 4 (a) FABRICATED TEXTILE TRANSMISSION LINE (TL) ON A TEXTILE GROUND PLANE (DENSITY: 2.0LINES/MM), (b) S21 OF THE TL FOR DIFFERENT EMBROIDERY DENSITIES.

proper functions of RF antennas.

As shown in [Zhang, Wang, and Volakis 2012], the measured S11 for both textile and copper circuits were shown to be less than -10 dB, indicating good impedance match over the entire measured frequency range. The relation between S21 and the embroidery density was also investigated. It was seen that the insertion loss (-S21) of the textile circuit decreased as the stitching density increased. However, a higher stitching density was difficult to achieve and caused needle breakage. Therefore a stitching density of 2.0 lines/mm was selected as a compromise between low electrical loss and fabrication ease.

From the transmission line measurements, the E-fiber textile demonstrated a very low loss of 0.2 dB/cm at radio frequencies up to 3 GHz. It was found comparable to the RF performances of corresponding copper lines, viz. loss only 0.08 dB/cm higher than copper. As expected, loss increased with frequency.

But of more importance is that the overall loss of Efibers was very low. Importantly, the E-fiber textiles

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sustained their RF performance and structural integrity after repetitively flexing [Wang, Zhang, Bayram, and Volakis 2012]. These characteristics were highly attractive for wearable RF applications.

Textile Antennas for Radio Frequency Applications Textile Dipole Antenna for UHF Communications 1) RF Performance of Textile Dipole Antenna A meandered flare dipole antenna was designed and fabricated using the aforementioned embroidery process [Wang, Zhang, Psychoudakis, and Volakis 2012],[Psychoudakis and Volakis 2009]. It was operated at the 500-600 MHz in the Ultra High Frequency (UHF) band. Such a textile antenna was flexibile on both concave and convex surfaces (see Fig. 5), making it promising for wearable communications.

To operate the dipole antenna, the two antenna branches were activated by a coaxial cable (see Fig. 5(a)) [Balanis 2005]. Both the textile antenna and its copper counterparts were then measured in an outdoor antenna measurement facility.

For reference, the UHF textile dipole was firstly tested in a planar (freestanding) configuration as shown in Fig. 6(a). The measured antenna parameters were the resonance frequency, radiation gain (gain vs. frequency), and radiation pattern (gain vs. Angle of radiation) [Balanis 2005]. As seen in Fig. 6(b), the textile antenna resonated (S11) at 610 MHz, in agreement with its copper counterpart and simulated data (simulation assumed a lossless perfect electrical conductor (PEC) and was done by using Ansys® High Frequency Structural Simulator (HFSS). It is noted that the textile dipole’s realized gain was 1 dB, viz. 0.8 dB lower than that of copper. This lower gain was due to the power loss on the E-fiber surface. Nevertheless, the E-fiber textile antenna provided satisfactory RF performances, along with mechanical benefits, making it feasible for UHF body-worn communications. We further note that the RF performance of the textile antenna did not degrade after repetitive flexing (more than 20 times).

2) Body-Worn Textile Antenna The body-worn performance of textile antenna was first simulated by placing the antenna close to a tissue model. As discussed in [Wang, Zhang, Psychoudakis, and Volakis 2012], a truncated tissue model was used, composed of multiple layers of tissue material emulating human tissue properties [Gabriel 1996].

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(a)

(b)

FIG. 5 (a) EMBROIDERED TEXTILE DIPOLE ANTENNA AND ITS COPPER COUTERPARTS. (b) TEXTILE ANTENNA IN CONCAVE AND CONVEX ADAPTATIONS.

(a)

(b)

FIG. 6 MEASUREMENT OF THE TEXTILE DIPOLE ANTENNA: (a) MEASUREMENT SETUP AND (b) S11 AND REALIZED GAIN

FIG. 7 (a) WEARABLE TEXTILE ANTENNA SEWN ON A COTTON SHIRT, (b) MEASUERD RADIATION PATTERNS OF THE TEXTILE AND COPPER ANTENNAS AT 590MHZ FOR DIFFERENT ON-BODY LOCATIONS.

Cloth thickness was also taken into account by including an air gap between the antenna and the tissue. This simulation showed that the body-worn antenna exhibited a small shift in the resultant resonant frequency, compared with the off-body scenario.

Textiles and Light Industrial Science and Technology (TLIST) Volume 2 Issue 3, July 2013

To validate the textile antenna’s body-worn performance, it was was sewn onto a cotton shirt which was placed on a mannequin (see Fig.7(a)). It was noted that the mannequin was filled with tissueemulating liquid composed of water, sugar, and salt, that had an overall relative dielectric constant of εr = 56.7, and conductivity of σ = 0.94 S/m [Wang, Zhang, Psychoudakis, and Volakis 2012]. Fig. 7(b) gave the azimuth radiation patterns of textile antenna when the antenna was placed in front (0°) or back torso (180°) of the mannequin. It was seen that the performance of the body-worn textile antenna matched well with that of copper counterpart. Importantly, the use of multiple body-worn textile antennas can lead to omnidirectional signal coverage, making it very suitable for robust wearable communications [Lee et al. 2011].

3) RF Antenna

Performance

of

Scarf-mounted

(a)

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(b)

FIG. 8 (a) TEXTILE ANTENNA EMBROIDERED ON A SCARF, (b) ON-BODY FREQUENCY RESPONSE.

Textile

The E-fiber antenna’s RF performance was also evaluated when the textile antenna was embroidered on a polyester scarf (see Fig. 8). The measured S11 showed no shift in the antenna resonant frequency fr.

This was critical since frequency detuning implied loss of signal. The textile antenna’s performance was also measured outdoor environment in a windy weather [Zhang, Wang, Psychoudakis, and Volakis, 2012], to ensure repeatable and reliable performance even when the scarf moves in many positions (see Fig. 9). It was observed that when compared to the measurements of antenna fixed on torso, scarf’s location led to higher gain. This was because the scarf was blown far from the body, implying less impact from the lossy body.

Polarization mismatch was, of course, unavoidable when the textile antenna was on the scarf. Because of the scarf’s continuous movement, a statistical analysis of the antenna performance was carried out to investigate its reliability. It was found that 90% probability of the detected gains were over >-23 dB for all five scarf wearing cases [Zhang, Wang, Psychoudakis, and Volakis, 2012]. It was noted that the textile antenna had a gain of -20 dB when placed in front of the torso and -22 dB gain at the back torso.

Analytical results also demonstrated that, regardless of its varying locations, the textile antenna had reliable performance.

Multiband Textile Antenna for Wearable Communications 1) Multiband Antenna using Textiles For mobile communications, a planar multiband

FIG. 9 MEASURED RADIATION PATTERN OF TEXTILE ANTENNA ON A SCARF IN FIVE DIFFERENT WEARING STYLES.

antenna was designed to operate simultaneously at three bands, namely GSM (850/900MHz), PCS (1800/1900MHz), and WLAN (2450MHz). As shown in Fig. 10(a), the slots and the loaded loop were introduced to realize a good impedance match at all three bands. The antenna was fabricated using E-fibers with a surface accuracy down to 1 mm [Zhang, Wang, and Volakis, 2012].

This precision was critical as it enabled proper functioning of the antenna. The antenna performance was then measured in the anechnoic chamber at The Ohio State University. To characterize the antenna’s operational bandwidth, S11<-10 dB (less than 10% power reflection) was used as the threshold [Balanis 2005]. In Fig. 10(b), measured S11 was plotted and showed that the antenna operated well in the bands from 860-1060 MHz and 1500-2700MHz. As such, it covered all three aforementioned communication bands. The radiation gain of the textile antenna was found to be 2 dB for all three bands, and in good agreement

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with PEC simulations and its copper counterparts [Zhang, Wang, and Volakis, 2012]. That is, the multiband textile antenna was well suited for high quality cellular and WLAN communications.

Antenna Measurement (OSU anechoic chamber)

2) Body-Worn Textile Antenna for Cellular Communications Next, we proceeded to integrate the multiband textile antenna with a mobile device for high quality cellular communications. To do so, the textile antenna was directly embroidered inside a jacket on the shoulder. It was then connected to the receiver of a mobile device to serve as the main antenna of the mobile device. Concurrently, the mobileâ&#x20AC;&#x2122;s original antenna was deactivated. The evaluation of the cellphone signal strength was carried out in an indoor environment with objects and other cellphone users in the room. As depicted in Fig. 11, a full-strength signal (7-bar) was steadily obtained when the textile antenna was mounted on the shoulder of the jacket. By comparison, the original cell antenna provided a full strength signal only when the cellphone was held close to the head (see Fig. 11). In addition, we remark that when the antenna was placed on other body-worn locations, the signal strength was rather low, implying unreliable communication.

(a)

(b) FIG. 10 TRI-BAND TEXTILE ANTENNA: (a) MEASUREMENT SETUP, (b) MEASURED S11

Therefore, the multiband wearable textile antenna demonstrated excellent RF performance to enable high quality and reliable bodyworn communication.

Particularly, its inconspicuous and installation ease in clothing allowed for conformable and fashionable wearable RF electronics.

Conclusions A novel class of embroidered textile antennas and RF circuits was introduced for body-worn communications. Such textile antennas were fabricated using electrically conductive metal-polymer fibers (Efibers). The E-fibers had a low electrical resistivity of 0.8 Ί/m. A unique aspect of the E-fiber over conventional metal was their inherent mechanical strength and flexibility. An automatic embroidery process was developed to realize double layer stitching to achieve a high conductivity for the embroidered RF patterns.

To validate this technology, prototype textile antennas and RF circuits were fabricated and measured for their RF performance. The E-fiber textile transmission line, a fundamental component of RF circuit, was tested carefully. Our measurements demonstrated very low 110

FIG. 11 WEARABLE TEXTILE ANTENNA SYSTEM FOR CELLULAR COMMUNICATIONS

loss of 0.2dB/cm at radio frequencies up to 3 GHz, and only 0.08 dB/cm high than copper.

The fabricated textile antennas also exhibited comparable radiation gains to their copper counterparts in both off-body (freestanding) and onbody configurations. In particular, the textile dipole antenna had only 1 dB lower gain than its copper counterpart, and the multi-band antenna for mobile communications was only 0.5 dB lower. Concurrently, the proposed textile antennas and RF circuits were mechanical flexibile and conformal, allowing for inconspicuous installation onto garments. Such technology provided new avenues of developments on fashionable RF functionalized garments for wearable wireless communications, sensing and health monitoring.

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Zheyu Wang was born in Wuhan, China.

He received his B.S. degree in electrical engineering from University of Electronic Science and Technology of China, Chengdu, China, in 2009. He is currently pursuing the Ph.D. degree at The Ohio State University, Columbus, Ohio.

He has been a Graduate Research Associate at OSU ElectroScience Laboratory since 2009. His research interests involve flexible and wearable antennas and arrays, flexible composite materials, wearable systems and sensors, Body-worn MIMO. His research progress has been reported on BBC News and Discovery News.

Mr. Wang received the Best Paper Award at IEEE iWAT conference, Tucson AZ, in 2012 and the Best Undergraduate Thesis Award from UESTC in 2009.

Lanlin Zhang was born in Nanjing, China. She received her B.S. degree in 2003 from Fudan University, Shanghai, China, her M.S. in 2007 and Ph.D. in 2008, both from Material Science and Engineering, The Ohio State University, Columbus, OH. She is currently working as a Senior Research Associate at the ElectroScience Laboratory, The Ohio State University. Her research areas include design, fabrication and characterization of flexible circuits, dielectric and magnetic materials, nanomaterials and their composites for microwave frequency applications. Her current research focuses on wearable electronics, conformal and load-bearing antennas, low loss metamaterials for non-reciprocal communications, and surface acoustic wave devices for strain sensing. She has published about 40 journal and proceeding papers.

John L. Volakis was born on May 13, 1956 in Chios, Greece and immigrated to the U.S.A. in 1973. He obtained his B.E. Degree, summa cum laude, in 1978 from Youngstown State Univ., Youngstown, Ohio, the M.Sc. in 1979 from the Ohio State Univ., Columbus, Ohio and the Ph.D. degree in 1982, also from the Ohio State Univ.

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He started his career at Rockwell International (1982-84), now Boeing Phantom Works. In 1984, he was appointed Assistant Professor at the University of Michigan, Ann Arbor, MI, becoming a full Professor in 1994. He also served as the Director of the Radiation Laboratory from 1998 to 2000. Since January 2003, he is the Roy and Lois Chope Chair Professor of Engineering at the Ohio State University, Columbus, Ohio and also serves as the Director of the ElectroScience Laboratory. He has carried out research on antennas, computational methods, electromagnetic compatibility and interference, propagation, design optimization, RF materials and metamaterials, multi-physics engineering and bioelectromagnetics. His publications include 8 books (among them: Approximate Boundary Conditions in Electromagnetics, 1995; Finite Element Methods for Electromagnetics, 1998; the classic 4th ed.

Antenna Engineering Handbook, 2007; Small Antennas, 2010; and Integral Equation Methods for Electromagnetics, 2011), over 300 journal papers, over 550 conference papers and 23 book chapters. He has also written several welledited coursepacks on introductory and advanced numerical methods for electromagnetics, and has delivered short courses on antennas, numerical methods, and frequency selective surfaces. In 1998 he received the University of Michigan (UM) College of Engineering Research Excellence award, in 2001 he received the UM, Dept. of Electrical Engineering and Computer Science Service Excellence Award, and in 2010 he received the Ohio State Univ. Clara and Peter Scott award for outstanding academic achievement. Dr. Volakis is listed by ISI among the top 250 most referenced authors; He has graduated/mentored nearly 70 doctoral students/post-docs with 16 of them receiving best paper awards at international conferences.

Dr. Volakis was the 2004 President of the IEEE Antennas and Propagation Society and served on the AdCom of the IEEE Antennas and Propagation Society from 1995 to 1998. He also served as Associate Editor for the IEEE Transactions on Antennas and Propagation from 1988-1992, Radio Science from 1994-97, and for the IEEE Antennas and Propagation Society Magazine (1992-2006), the J. Electromagnetic Waves and Applications and the URSI Bulletin. He chaired the 1993 IEEE Antennas and Propagation Society Symposium and Radio Science Meeting in Ann Arbor, MI., and co-chaired the same Symposium in 2003 at Columbus, Ohio, USA. He was elected Fellow of the IEEE in 1996 and is also a Fellow of the Applied Computational Electromagnetics Society (ACES).

He is a member of the URSI Commissions B and E, and currently served as the Vice Chair of USNC/URSI Commission B.