Textile Antennas for Biomedical Telemetry: How Safe Are They?

By Ping Jack Soh, Guy A. E. Vandenbosch, Fwen Hoon Wee, Marco Mercuri, Andre van den Bosch, Marta Martínez-Vázquez and Dominique M. M.-P. Schreurs

The emergence of Body Area Networks (BAN) for applications in healthcare and medical monitoring have triggered an extensive research effort into utilizing conformal materials for wireless communications. This option is promising as the applicability of such materials has been successfully demonstrated recently [[1]-[4]]. The user’s mobility and comfortableness can be hindered by the use of wires, besides the increased risk of system failure. Despite the existence of contactless monitoring systems, the use of such conformal antennas to relay measurements collected from clothing-integrated sensors is certainly a more favourable approach, as certain biological signals are more accurately collected with body worn sensors. Due to the need for Extra-BAN (EBAN) communication which requires seamless data transfer across various wireless standards, these antennas are also preferred to be operable across a multitude of wireless applications.

Antenna performance parameters, i.e., reflection and radiation characteristics are expected to be affected by coupling and absorption by the human body. Moreover, placement of such radiating structures in close-proximity to the human user poses operational safety hazard to the user. This article highlights the safety evaluation of these antennas, defined as the Specific Absorption Rate (SAR). This parameter is a measure of power (heat) distribution in a body medium, and characterizes the absorption of the electromagnetic energy by the human tissues operating in the vicinity of an antenna/electronic device. A set of dual band antennas fabricated fully using textiles is evaluated at 2.45 GHz and 5.2 GHz, where they are operational with at least -10 dB of reflection coefficient. They are designed based on the planar inverted-F antenna (PIFA) topology, which consists of a ground plane, shorting wall and radiator, as shown in Fig. 1. The prototyping materials – felt as the substrate, ShieldIt Super and copper foil as the conducting elements – are commercially available, which gives an idea of the resulting SAR from these common materials. Details of the topology, materials, and dimensions are provided in [5], while their fabrication procedure is provided in [4].

Figure 1: The dual-band PIFA (DBPIFA): (a) schematic, (b) SH FPIFA (fabricated) with D = 24 mm, GL = 44 mm, GW = 34 mm, SW = 4mm, h = 6 mm, fL = fW = 9 mm, WF = 1 mm.

SAR simulations are carried out using a realistic Hugo human body model using a commercial electromagnetic solver, CST Microwave Studio. The voxel model is truncated to optimize simulation time and antennas are placed 10 mm from its chest. A commercial, certified system available at IMST GmbH is utilized to perform the measurements. Antennas are similarly placed for experimental evaluation on the Specific Anthropomorphic Mannequin (SAM). Their evaluation setup is shown in Fig. 2.

Figure 2: Measurement and simulation setups: (a) top view of the SAM on the DASY-4 system; (b) AUT placement at the measurement location (bottom), (c) calculated Hugo voxel model, and (d) AUT placement on the truncated model.

Table 1: Summary of simulated and measured SAR.

In summary, our results shown in Table 1 indicate that both simulation and measurements are well below the regulatory limit of 2 W/kg averaged over 10 g of tissue [6]. Moreover, the small differences between them demonstrate a good simulation-measurement agreement. Larger simulation-measurement SAR differences at 5.2 GHz are due to the larger expected measurement uncertainties of 10.5%. All measurements are also well below their respective simulated equivalent, indicating that the simulator successfully estimated the worst-case on-body SAR, which is useful to enable a reasonable safety design margin. Higher SAR is observed for copper foil prototype compared to ShieldIt. Besides their varying conductivities, this is mainly caused by their thicknesses difference. The higher resulting SAR is also increased at the higher frequency due to the tissue properties – conductivity for the same tissue in the antennas’ vicinity increases with frequency. The other reason for the low resulting SAR, besides the materials factor, is the antenna topology. The use of a full ground plane on the antennas’ reverse side enabled effective shielding of the human body against radiation. Thus, we hope that this evaluation has provided valuable pointers to the choice of topology and materials for further textile antenna development for on body communications.

Acknowledgements
The authors would like to acknowledge the financial support from the Malaysian Ministry of Education (MOE), COST Action ic1102-VISTA (Versatile, Integrated and Signal-Aware Technologies for Antennas) and the Hercules Foundation.

For Further Reading

1.C. Hertleer, H. Rogier, L. Vallozzi, and L. Van Langenhove, “A Textile Antenna for Off-Body Communication Integrated Into Protective Clothing for Firefighters,” IEEE Trans. on Antennas and Propagation, vol.57, no.4, pp.919-925, 2009.

2. I. Locher, M. Klemm, T. Kirstein, and G. Troster, “Design and Characterization of Purely Textile Patch Antennas,” IEEE Trans. on Advanced Packaging, vol.29, no.4, pp.777-788, 2006.

3. P. J. Soh, G. A. E. Vandenbosch, S. L. Ooi, and N. H. M. Rais, “Design of a Broadband, All-Textile Slotted PIFA,” IEEE Trans. Antennas and Propagation, vol.60, no.1, pp.379-384, Jan. 2012.

4. P. J. Soh, G. A. E. Vandenbosch, S. L. Ooi, and M. R. N. Husna, “Wearable Dual-Band Sierpinski Fractal PIFA using Conductive Fabric,” Electronics Letters, vol.47, no.6, pp.365-367, March 2011.

5. P. J. Soh, S. J. Boyes, G. A. E. Vandenbosch, Y. Huang, and S. L. Ooi, “On-body Characterization of a Dual-Band, All-Textile PIFA,” Progress in Electromagnetic Research, vol.129, pp.517-539, 2012.

6. P. J. Soh, G. A. E. Vandenbosch, F. H. Wee, A. van den Bosch, M. Martinez-Vazquez, D. M. M.-P. Schreurs, “Specific Absorption Rate (SAR) Evaluation of Biomedical Telemetry Textile Antennas”, IEEE MTT-S International Microwave Symposium (IMS), pp. 1-3, June 2-7, 2013.

ABOUT THE AUTHORS

Ping Jack Soh is currently a Senior Lecturer in the School of Computer and Communication Engineering (SCCE), Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia. He received the Bachelor and Master degrees in Electrical Engineering from Universiti Teknologi Malaysia (UTM), Johor, Malaysia, in 2002 and 2005, respectively. From 2002 to 2004, he was a Test Engineer in Venture Corp. before joining Motorola Technology Malaysia as a R&D Engineer in 2005. There, he worked on the characterization and testing of new two-way radio antennas and RF front-ends. In 2006, he joined SCCE-UniMAP as a Lecturer. He is currently on study leave and working towards his Ph.D in the ESAT-TELEMIC Research Division, K.U. Leuven, Belgium. His research interests include planar antennas, flexible/textile antennas, on-body communication, metamaterials, passive microwave components and microwave measurements.

Mr. Soh was the recipient of the CST University Publication Award in 2011 and 2012, the IEEE Antennas and Propagation Society (AP-S) Doctoral Research Award in 2012, and the IEEE Microwave Theory and Techniques Society (MTT-S) Graduate Fellowship for Medical Applications in 2013.

Guy A. E. Vandenbosh received the M.S. and Ph.D. degrees in Electrical Engineering from the Katholieke Universiteit Leuven, Belgium, in 1985 and 1991, respectively. Since 1993, he has been a Lecturer, and since 2005, a Full Professor at the same university. His research interests are in the area of electromagnetic theory, computational electromagnetics, planar antennas and circuits, nano-electromagnetics, EM radiation, EMC, and bio-electromagnetics. His work has been published in ca. 150 papers in international journals and has led to ca. 250 presentations at international conferences. Currently, he leads the Working Group on Software within the European Association on Antennas and Propagation (EuRAAP), he holds the position of chairman of the IEEE Benelux Chapter on Antennas and Propagation, and is secretary of the Belgian National Committee for Radio-electricity (URSI), where he is also in charge of commission E.

Fwen Hoon Wee received the B.Eng. degree in Communication Engineering from Universiti Malaysia Perlis, Perlis, Malaysia in 2009. Currently, she is working towards her Ph.D. degree in the School of Computer and Communication Engineering, Universiti Malaysia Perlis. Her current research interests include dielectric resonator antennas, ferroelectric ceramics, microstrip antennas and array antenna.

Marco Mercuri was born in Calabria, Italy in 1985. He received the Bachelor and Master degrees in electronic engineering from the Università della Calabria (UNICAL), Arcavacata di Rende, Italy, in 2006 and 2009, respectively. He is currently working towards the Ph.D degree within the TELEMIC research group of the Department of Electrical Engineering (ESAT), KU Leuven, Belgium.

His research interests include biomedical applications of microwave/RF, wireless sensors, and microwave/millimeter-wave measurements. Mr. Mercuri is a student member of the IEEE Microwave Theory and Techniques (IEEE MTT-S) and the IEEE Engineering in Medicine and Biology Society. He was the recipient of the 2013 IEEE MTT-S Graduate Fellowship Award. He was also the best interactive forum paper finalist at Automatic RF Techniques Group (ARFTG) Conference on June 2013.

Andre van den Bosch received his certified engineer degree in 2000. Since then he has been with IMST GmbH, Germany as head of dosimetric laboratory. His main focus is the dosimetric assessment of mobile telecommunication devices.

Marta Martínez-Vázquez was born in Santiago de Compostela, Spain, in 1973. She received the M.Sc. and Ph.D. degrees in telecommunication engineering from the Universidad Politécnica de Valencia, Valencia, Spain, in 1997 and 2003, respectively. In 1999 she was granted a fellowship from the Pedro Barrié de la Maza Foundation for postgraduate research at IMST GmbH, in Germany. Since 2000, she has been a full-time Staff Member with the Antennas and EM Modelling Department, IMST GmbH, Kamp-Lintfort, Germany. She has authored and coauthored over 50 publications, including books, book chapters, journal and conference papers, and patents. Her research interests include the design and applications of antennas for mobile communications, planar arrays, sensors, and RF systems.

Dr. Martínez-Vázquez was awarded the 2004 Best Ph.D. award of the Universidad Politécnica de Valencia for her dissertation on small multiband antennas for handheld terminals. She is the chair of the COST IC1102 Action “Versatile, Integrated and Signal-aware Technologies for Antennas (VISTA)”. Previously, she was a member of the Executive Board of the ACE (Antennas Centre of Excellence) Network of Excellence (2004 – 2007) and the leader of its small antennas activity, and the vice-chair of the COST IC0603 Action “Antenna Sensors and Systems for Information Society Technologies” (2007 – 2011). She is a member of the Administrative Committee of the IEEE Antennas and Propagation Society, of the Board of Directors of the European Association of Antennas and Propagation (EurAAP), and of the Technical Advisory Panel for the Antennas and Propagation Professional Network of IET. She is currently a Distinguished Lecturer for the IEEE Antennas and Propagation Society, an editor of the IEEE Antennas and Propagation Magazine, an associate editor of the IEEE Antennas and Wireless Propagation Letters, and a member of the editorial board of the Radioengineering Journal.

Dominique Schreurs received the M.Sc. degree and Ph.D. degree in electronic engineering from University of Leuven (KU Leuven), Belgium. As post-doc fellow, she was visiting scientist with Agilent Technologies (USA), E.T.H. Z¨rich (Switzerland), and NIST (USA). She is now a Full Professor at KU Leuven. Her main research interests concern the (non)linear characterization and modelling of microwave devices and circuits, as well as (non)linear hybrid and integrated circuit design for telecommunications and biomedical applications. She is co-editor of two books, contributor to seven books, and (co-)author of about 100 journal papers and 300 contributions at international conferences.

Prof. D. Schreurs serves on the IEEE MTT-S AdCom as chair of the IEEE MTT-S Education Committee. D. Schreurs is Distinguished Microwave Lecturer for the period 2012-2014. She is also Associate Editor of the IEEE Microwave and Wireless Components Letters, Associate Editor of the International Journal of Microwave and Wireless Technologies, reviewer for various IEEE journals as well as TPRC member of IMS, BioWireless, and EuMW. She is also co-TPC chair of IMWS-Bio in 2013. Beyond IEEE, Prof. D. Schreurs also serves as Technical Chair on the Executive Committee of the ARFTG organization, and initiated the NVNA Users’ Forum as well as the IEEE Women in Microwaves event at the European Microwave Week.