Introduction

With the development of science and technology, wearable devices based on wireless body area networks (WBANs) are widely used in health monitoring, motion detection, military activities, and biomedicine [1], [2], [3], [4]. Globally, the degree of aging is becoming serious, and the multifaceted health monitoring of the elderly has become an urgent problem to be solved. As shown in Fig. 1, healthcare applications based on WBAN include detecting postural activities such as sitting, standing, and falling, or monitoring physiological signals such as blood pressure, heart rate, and blood oxygen [5], [6]. In order to realize the real-time external posture monitoring of the elderly, the monitoring equipment is required to have comfortable wearability, sensitivity, and multifaceted monitoring [7]. As an important part of wearable devices, the antenna can not only play the role of signal transmission but also act as an auxiliary detection sensor device [8]. Therefore, high requirements are put forward for wearable antenna sensors, such as compact structure, low loss, sensitivity, circular polarization (CP), and broadband.

Due to the increasingly complex wireless communication environment, more and more interference signals affect the communication quality of the wearable antenna. Since the constant motion of the human body, polarization mismatch occurs. Linearly polarized (LP) antennas are difficult to meet the high requirements of the current communication environment, and CP wearable antennas have received much attention [9], [10]. Yang et al. proposed a dual-band dual CP textile antenna for off-body communications based on the artificial magnetic conductor (AMC) [11]. Although the backward radiation is greatly reduced, its operating bandwidth and axial ratio bandwidth (ARBW) are narrow. Metasurface has attracted much attention as a new type of structure [12], [13], and loading it on patch antennas can broaden the bandwidth and increase the gain of the antenna [14], [15], [16]. Zhang et al. proposed a new type of wearable antenna based on metasurfaces, which not only achieves high gain and broadband but also achieves the characteristics of compact structure and low specific absorption rate [17]. To meet the comfort and bendability requirements of wearable antennas, the materials used for wearable antennas mainly include felt [18], [19], denim [20], polydimethylsiloxane (PDMS) [21], and polyimide (PI) [22]. Zheng et al. proposed a flexible dual-band wearable antenna for WBAN based on the flexible material PI [23]. Although it achieves a low profile and has good bendability, its bandwidth is narrow, and its radiation intensity to the human body is high, which may cause potential harm to the human body. Since wearable antennas need to work near the human body, the high permittivity of human tissue will have a certain impact on the resonant frequency and operating bandwidth of the antenna [24], [25].

In WBAN, the antenna can serve a dual purpose, operating not only as a signal transceiver but also as a sensor, exhibiting versatile functionality. Nie et al. proposed a wearable LP antenna sensor for cervical curvature detection based on metamaterials [26]. The deviation of the resonant frequency of the antenna when the cervical spine is bent can be used to judge the degree of bending. The antenna sensor achieves a sensitivity of 7.5 MHz/1°, but the maximum gain only reaches 6.75 dBi. El Gharbi M et al. proposed an embroidered LP antenna-based sensor integrated into a T-shirt for real-time breathing monitoring [27]. Using resonant frequency shifts, the sensor detects chest movement during various breathing patterns and positions. But the antenna sensor has a low gain of only 1.86 dBi, which results in a short communication distance. Zhang et al. proposed a wearable button CP antenna sensor to realize simultaneous wireless information and power transmission through dual working modes [28]. The design enables omnidirectional communication in the 3.5 GHz WiMAX band and power harvesting in the 5 GHz WLAN band. However, it has a narrow ARBW and no analysis of the sensing performance.

Based on the preceding analysis, it is evident that wearable antennas and antenna sensors commonly exhibit inherent limitations, including restricted bandwidth, low gain, limited flexibility, and elevated electromagnetic emissions toward the human physique. And most of the antennas currently used in sensors are LP, and there are few reports of antenna sensors with CP. These challenges have evolved into significant impediments in the advancement of antenna technology in WBAN, demanding prompt and effective resolution.

In this article, we propose a wearable antenna sensor based on metasurface for assisting fall detection of the elderly. The wearable metasurface antenna sensor (WMAS) proposed in this article exhibits clear advantages in WBAN, and the main contributions are as follows:

• (1).
• The wearable antenna proposed herein, subsequent to the incorporation of a metasurface, has expanded the bandwidth by 189 %. Simultaneously, it has accomplished CP and elevated the antenna's gain.
• (2).
• By introducing a sponge layer as a pressure sensor between the slot and metasurface layers, variations in pressure lead to changes in coupling efficiency, affecting bandwidth and resonance points. This interaction realizes the sensing properties and high sensitivity is obtained.
• (3).
• The proposed antenna sensor is assessed for sensing and bending performance on simulated human tissue and actual subjects. Results affirm the high sensitivity and broadband characteristics. Analysis of radiative behavior on human tissue reveals wide beamwidth and minimal backward radiation. Furthermore, specific absorption rate (SAR) analysis confirms the sensor's compliance with international standards, with radiation well below permissible limits for human exposure.

This article is organized as follows. In Section 2, the design of the metasurface unit is described and the resonance properties are explained. The metasurface structure is analyzed by using the characteristic mode theory (CMT), and its physical characteristics are deeply analyzed. Then the sensor performance based on bandwidth and resonance point variation is studied and analyzed. In Section 3, the performance of the WMAS is measured, and the radiation intensity of WMAS to the human body is discussed by constructing a human tissue model. In Section 4, the conclusion of the full text is presented.

Section snippets

Metasurface unit design

Fig. 2 shows the structure of the proposed metasurface unit. According to the requirements of the wearable antenna, a highly flexible felt is used as the dielectric substrate material with a relative permittivity of 1.2 and a loss tangent of 0.02. The metasurface unit is composed of a truncated ring and a 45° inclined slot in two symmetrical trapezoids. The specific structural parameters are shown in Fig. 2(a) and Table 1. To meet the requirement of CP performance in wearable antennas, the

Wearable metasurface antenna performance analysis

For the wearing comfort of the elderly and the high sensitivity of the sensor, the proposed WMAS uses the flexible material felt as the dielectric substrate. The sponge with a relative permittivity of 1 is used as the pressure sensor since the properties of the sponge will not affect the performance of the WMAS. The fabricated wearable metasurface antenna is shown in Fig. 16, and the performance test is carried out through the vector network analyzer (VNA) and microwave anechoic chamber.

The

Conclusion

In this article, a metasurface-based wearable antenna sensor for elderly fall assistance detection is proposed. Through the design and analysis of the metasurface unit, the magnitude and phase characteristics of the reflection coefficient are studied. The resonance characteristics of the modes and the theoretical principles for achieving CP are further investigated using the CMT of the metasurface unit array. Using the bone-shaped slot antenna as the base antenna, the operating bandwidth is

CRediT authorship contribution statement

Rigeng Wu: Conceptualization, Methodology, Software, Writing – original draft. Jian Dong: Conceptualization, Methodology, Writing – review & editing, Supervision. Meng Wang: Writing – review & editing. Yadgar I. Abdulkarim: Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported in part by the National Natural Science Foundation of China, under Grant number 61801521 and 61971450, in part by the Natural Science Foundation of Hunan Province, under Grant number 2018JJ2533, 2022JJ30052, and 2023JJ40775, and in part by the Fundamental Research Funds for the Central Universities, under Grant number 2018gczd014 and 20190038020050.