Specific Absorption Rate (SAR) - IEEE Xplore

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Abstract. — This investigation aims at determining the. Specific Absorption Rates (SARs) of textile antennas for on-body biomedical telemetry applications.
Specific Absorption Rate (SAR) Evaluation of Biomedical Telemetry Textile Antennas P. J. Soh1,3, G. A. E. Vandenbosch1, F. H. Wee3, A. van den Bosch2, M. Martinez-Vazquez2, and D. M. M.P. Schreurs1 1

ESAT-TELEMIC Research Div., KU Leuven, Kasteelpark Arenberg 10, 3001 Leuven, Belgium. 2

3

IMST GmbH, Carl-Friedrich-Gauss-Str. 2-4-4, 47475 Kamp-Lintfort, Germany.

School of Computer & Communication Eng., Universiti Malaysia Perlis, 02600 Arau, Perlis, Malaysia.

Abstract — This investigation aims at determining the Specific Absorption Rates (SARs) of textile antennas for on-body biomedical telemetry applications. The dual-band antennas are designed to operate at 2.4 GHz and 5.2 GHz with unidirectional radiations and broad bandwidths. Fractal Planar Inverted-F Antennas (F-PIFAs) are fabricated using different materials to assess factors influencing SAR levels. Simulations and measurements indicated a maximum SAR and uncertainty of 0.6 W/kg and 10%, respectively. Index Terms — Specific Absorption Rate (SAR), multiband antennas, biomedical telemetry, biomedical applications of electromagnetic radiation.

I. INTRODUCTION The emergence of Wireless Body Area Networks (WBAN) for healthcare monitoring, emergency response, search and rescue, military, etc. have triggered extensive research effort into utilizing conformal material alternatives towards realizing a fully-worn system setup. One of such areas is the use of wearable and flexible antennas made from textiles. This option is promising as the applicability of such materials has been successfully demonstrated by previous researchers [1][4]. However, placement of such radiating structures in closeproximity to the human user poses various challenges. Antenna performance parameters such as reflection coefficient, bandwidth, gain, efficiency and radiation characteristics are expected to be affected by coupling and absorption by the human body [5]-[6]. Foreseeing this limitation, wearable antenna researchers have extended their investigation to include the existence of a phantom both early in the design process and in their final evaluations. In addition, effects of bending [7], worn efficiencies [8], and material changes due to environmental conditions [9], besides various methods to limit the influence of body coupling-absorption [10]-[11] are also proposed. However, these investigations typically validate their SARs using commercial numerical software tools. Although important, validation efforts through measurements in a standard, certified facility is often costly due to the scarcity of such equipments. Realizing such potential research vacuum, this work presents a dedicated SAR investigation of textile antennas designed to operate at two center frequencies, i.e., 2.45 GHz and 5.2 GHz. Simulations performed using CST

Microwave Studio on a Hugo Body Model is validated against evaluations in a certified setup compliant to the IEC 62209 standards [12]. First, the antenna design and materials are briefly described before detailing the SAR simulation and measurement procedures. Obtained results will then be presented and discussed prior to a findings summary. II. ANTENNA MATERIALS AND TOPOLOGY The proposed antennas under test (AUTs) are designed and fabricated fully using textiles based on the planar inverted-F antenna (PIFA) structure improved from [3]. This topology features a full ground plane which enables effective shielding against absorption by the human body, besides generating a broad operating bandwidth. Their dual-band characteristic is achieved by employing a fractal radiator on the top layer, which dimensions are estimated according to [4].

Fig. 1. Fabricated prototypes of the proposed fractal PIFAs: (a) PCPTF PIFA, (b) ShieldIt PIFA, (c) copper foil PIFA and (d) schematic with detailed dimensions.

The antenna prototyping materials consist of two textile types, conducting and non-conducting. The former is used to form the radiator, ground plane and shorting wall, whereas a 6 mm thick (t), non-conductive felt with an estimated relative

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dielectric permittivity (İr) of 1.45 and loss tangent (tanį) of 0.044 is used as the substrate. Three antenna types are designed using different conductive materials; (1) ShieldIt Super, (2) Plain Copper Polyester Taffeta Fabric (PCPTF), and (3) conventional copper foil. Their thicknesses and conductivities can be found in [3]-[6] and fabricated prototypes are shown in Fig. 1. The antenna radiator, ground plane and shorting wall are secured onto felt by activating the hot melt on ShieldIt's reverse side at 130oC. Copper foil also features such adhesive reverse side, while manual sewing is used for PCPTF's connection to the felt substrate. RF power is fed to the antennas through 50 ȍ SMA connectors soldered to ShieldIt textile and copper foil at a maximum temperature of 200oC. This step is repeated for PCPTF using a conductive epoxy model 8331 from MG Chemicals.

measurements are İr2.45 = 39.2, ı2.45 = 1.8 S/m and İr5.2 = 36.0, ı5.2 = 4.66 S/m, respectively. Prior to AUT measurements, calibrations are performed near the SAM at both frequencies using standard SAR validation dipoles to ensure that SAR2.45GHz< 6.15 W/kg and SAR5.2GHz