micromachines Article Design and Evaluation of a Flexible Dual-Band Meander Line Monopole Antenna for On- and Off-Body Healthcare Applications Shahid M Ali 1, Cheab Sovuthy 1,*, Sima Noghanian 2,3,* , Zulfiqur Ali 4 , Qammer H. Abbasi 5 , Muhammad A. Imran 5,6 , Tale Saeidi 1 and Soeung Socheatra 1 1 Department of Electrical and Electronic Engineering, Universiti Teknologi, PETRONAS Bander Seri Iskandar, Tronoh 32610, Perak, Malaysia; [email protected] (S.M.A.); [email protected] (T.S.); [email protected] (S.S.) 2 Wafer LLC, 2 Dunham Rd, Beverly, MA 01915, USA 3 American Public University System, 111 W Congress St, Charles Town, WV 25414, USA 4 Healthcare Innovation Centre, School of Health and Life Sciences, Teesside University, Middlesbrough, Tees Valley TS1 3BX, UK; [email protected] 5 James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK; [email protected] (Q.H.A.); [email protected] (M.A.I.) 6 Artificial Intelligence Research Center (AIRC), College of Engineering and Information Technology, Ajman University, Ajman, United Arab Emirates * Correspondence: [email protected] (C.S.); [email protected] (S.N.) Abstract: The human body is an extremely challenging environment for wearable antennas due Citation: Ali, S.M; Sovuthy, C.; to the complex antenna-body coupling effects. In this article, a compact flexible dual-band planar Noghanian, S.; Ali, Z.; Abbasi, Q.H.; meander line monopole antenna (MMA) with a truncated ground plane made of multiple layers Imran, M.A.; Saeidi, T.; Socheatra, S. of standard off-the-shelf materials is evaluated to validate its performance when worn by different Design and Evaluation of a Flexible subjects to help the designers who are shaping future complex on-/off-body wireless devices. The Dual-Band Meander Line Monopole antenna was fabricated, and the measured results agreed well with those from the simulations. As Antenna for On- and Off-Body a reference, in free-space, the antenna provided omnidirectional radiation patterns (ORP), with a Healthcare Applications. wide impedance bandwidth of 1282.4 (450.5) MHz with a maximum gain of 3.03 dBi (4.85 dBi) in Micromachines 2021, 12, 475. https:// the lower (upper) bands. The impedance bandwidth could reach up to 688.9 MHz (500.9 MHz) and doi.org/10.3390/mi12050475 1261.7 MHz (524.2 MHz) with the gain of 3.80 dBi (4.67 dBi) and 3.00 dBi (4.55 dBi), respectively, Academic Editors: Mark L. Adams on the human chest and arm. The stability in results shows that this flexible antenna is sufficiently and Paulo M. Mendes robust against the variations introduced by the human body. A maximum measured shift of 0.5 and 100 MHz in the wide impedance matching and resonance frequency was observed in both bands, Received: 4 February 2021 respectively, while an optimal gap between the antenna and human body was maintained. This Accepted: 17 March 2021 stability of the working frequency provides robustness against various conditions including bending, Published: 22 April 2021 movement, and relatively large fabrication tolerances. Publisher’s Note: MDPI stays neutral Keywords: WBAN; wearable antenna; on- and off-body communications; SAR with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Currently, advances in the miniaturization of wireless devices and designs of smart wireless networks have allowed a rapid development in wireless body area networks Copyright: © 2021 by the authors. (WBAN) because of their vast demands for numerous applications such as those in health- Licensee MDPI, Basel, Switzerland. care, the military, sports, and electronic gaming [1]. These applications require an easy This article is an open access article integration of flexible and textile-based high-data-rate wireless electronic devices into cloth- distributed under the terms and ing and other wearable gadgets, so that the wearer can easily communicate with various conditions of the Creative Commons devices [2]. One of the key components for body-centric wireless communication (BCWC) Attribution (CC BY) license (https:// is the antenna. BCWC has to work near the human body; therefore, the wearable antenna creativecommons.org/licenses/by/ 4.0/). must meet certain requirements such as being planar, compact, and flexible, and it should Micromachines 2021, 12, 475. https://doi.org/10.3390/mi12050475 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 475 2 of 17 be able to easily be integrated with electronic systems and maintain a reliable link while not causing any discomfort for the user. The human body is a hostile environment for an antenna due to the coupling and absorption of the EM waves [3]. Wearable antennas must be designed and characterized carefully in order to maintain a reliable communication link even under the detuning effects of lossy body tissues, because the human body can deteriorate the performance of the antenna [4]. To add to this complexity, it is worth noting that every individual has a composition of body tissue with different dielectric constants (#) and geometrical parameters [5]. However, the effect of coupling in the human body and wearable antennas can be mitigated by using de-coupling methods such as insolation and proper use of the ground layers. Small antennas with a truncated or no ground plane generally are omnidirectional, and therefore, are more affected by the body’s coupling effect. This effect requires optimizing the antenna-body separation distance effectively [6]. The study and optimization of the antenna with small isolation from the body provide a simple and effective way of combating the detuning effect. Figure1 shows the general architecture in WBAN systems. Figure 1. A general healthcare architecture for integrated meander line antenna. WPPU: wireless patient portable unit; WAPU: wireless-access point unit; LAN: local area network; QoS: quality of service [7]. Numerous flexible textile antennas have been designed and evaluated in WBAN systems. For example, Ala Alemaryeen et al. integrated a planar monopole antenna with a layer of the artificial magnetic conductor (AMC) in [8]. The antenna was completely made of fabric using Pellon for the substrate, and pure copper taffeta for conducting parts. An impedance bandwidth of 4.30–5.90 GHz was observed. Effects of bending and crumpling were studied and the performance of the design on various body models was compared. Stephen J. Boyes et al. in [9] designed an inverted-F antenna on a felt substrate, and its performance was checked on various body locations. The impact of different materials, as well as bending, was studied. The bending effects significantly degraded the antenna’s performance by changes in its distance from the body. Dominique L. Paul et al. in [10] investigated a wideband antenna, working within the band of 0.9–6.0 GHz. The antenna performance was investigated under bending conditions, which showed a considerable change in the antenna’s resonant frequency. In addition, the antenna’s efficiency was improved by adding an electromagnetic bandgap (EBG) surface. In our work, a compact flexible dual-band meander line monopole antenna is pro- posed, and its performance was evaluated on different subjects with various bending and wet conditions, and at two operating bands with different body sizes, in which a wearable antenna is expected to operate for on-/off-body applications. Table1 shows the comparison of this design with similar results from a few papers from the literature and improvements that were achieved. In particular, we have shown that the results are not sensitive to the presence of the human body, and insignificant detuning is observed at both bands when a Micromachines 2021, 12, 475 3 of 17 10 mm distance was maintained. Moreover, these evaluations are influential in determining proper antenna operation on the human body, and also pave the way for unique designs as mentioned by Guy A. E. Vandenbosch et al. in [11,12], and Andrea Ruaro et al. in [13] for future high-performance on-body applications. This paper is arranged as follows: after the introduction, Section2 describes the design steps and the different parameters of the wearable monopole-shaped meander line antenna. The subsections describe the antenna geometry, design strategy, radiation modes, and optimization of the antenna in the on-body model. In Section3, antenna measurement results are presented. The subsection includes the results of reflection coefficients, bandwidth, far-field radiation, and specific absorption rate (SAR) simulation results. Section4 describes the equipment setup and procedure, and free-space and on- and off-body transmission losses and link budget analysis. Finally, Section5 contains the conclusions and future direction. Table 1. Comparison of operating bands, materials, size, bandwidth (MHz), bending effect, and specific absorption rate (SAR) against other flexible antennas published in the literature. Frequency Material Size Bandwidth SAR (W/Kg) Ref. Description (MHz/GHz) Substrate/Patch (W × L) (mm2) (BW) (MHz) (1 g/10 g) 2.4 Fully-textile, large size, single Fully Textile 140.0 [14] Sleeve-Badge 50 × 57 0.34 (10 g) band. Not fully tested on (Multiple Layers) (Free Space) Textile Antenna the body. 2.5/4.2/5.5/6.8 Defected Ground Multiband, small size, not 70/90/350/710 [15] Structure (DGS) Copper/FR4 40 × 40 NA fully textile. Might be difficult (Free Space) with Meander to fabricate. line 2.4/5.2 85/200 (Free 0.2/0.15 Fully textile, dual-band, small Metamaterial- Space) (On-Arm) size, measured on body, use of [16] Felt/ShieldIt 50 × 50 loaded 130/698 0.12/0.25 metamaterial might make it Antenna (On-Chest) (On-Chest) difficult to fabricate. 2.5 ShieldIt/(Jeans 667 Fully textile, small size, single [17] Monopole Patch 50 × 60 NA Cotton) (Free-Space) band, Not tested on the body.