In recent years, there has been a rapid development in the wearable industry. The growing number of wearables has led to the demand for new lightweight, flexible wearable antennas. In order to be applicable in IoT wearable devices, the antennas must meet certain electrical, mechanical, manufacturing, and safety requirements (e.g., specific absorption rate (SAR) below worldwide limits). However, the assessment of SAR does not provide information on the mechanisms of interaction between low-intensity electromagnetic fields emitted by wearable antennas and the human body. In this paper, we presented a detailed investigation of the SAR induced in erythrocyte suspensions from a fully textile wearable antenna at realistic (net input power 6.3 mW) and conservative (net input power 450 mW) conditions at 2.41 GHz, as well as results from in vitro experiments on the stability of human erythrocyte membranes at both exposure conditions. The detailed investigation showed that the 1 g average SARs were 0.5758 W/kg and 41.13 W/kg, respectively. Results from the in vitro experiments demonstrated that the short-term (20 min) irradiation of erythrocyte membranes in the reactive near-field of the wearable antenna at 6.3 mW input power had a stabilizing effect. Long-term exposure (120 min) had a destabilizing effect on the erythrocyte membrane.

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Wearable Antennas for Sensor Networks and IoT Applications: Evaluation of SAR and Biological Effects

Wireless energy harvesting, a technique to generate direct current (DC) electricity from ambient wireless signals, has recently been featured as a potential solution to reduce the battery size, extend the battery life, or replace batteries altogether for wearable electronics. Unlike other energy harvesting techniques, wireless energy harvesting has a prominent advantage of ceaseless availability of ambient signals, but the common form of technology involves a major challenge of limited output power because of a relatively low ambient energy density. Moreover, the archetypal wireless energy harvesters are made of printed circuit boards (PCBs), which are rigid, bulky, and heavy, and hence they are not eminently suitable for body-worn applications from both aesthetic and comfort points of view. In order to overcome these limitations, textile-based wireless energy harvesting architectures have been proposed in the past decade. Being made of textile materials, this new class of harvesters can be seamlessly integrated into clothing in inherently aesthetic and comfortable forms. In addition, since clothing offers a large surface area, multiple harvesting units can be deployed to enhance the output power. In view of these unique and irreplaceable benefits, this paper reviews key recent progress in textile-based wireless energy harvesting strategies for powering body-worn electronics. Comparisons with other power harvesting technologies, historical development, fundamental principles of operation and techniques for fabricating textile-based wireless power harvesters are first recapitulated, followed by a review on the principal advantages, challenges, and opportunities. It is one of the purposes of this paper to peruse the current state-of-the-art and build a scientific knowledge base to aid further advancement of power solutions for wearable electronics.

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Textile Materials for Wireless Energy Harvesting

Recent advances in the development of innovative textronic products are often related to the implementation of radio-frequency identification (RFID) technology. Such devices contain components of wireless telecommunications systems, in which radiofrequency circuits should be designed taking into account not only the frequency band or destined application, but also the dielectric properties of the materials. As is known from the theory of RFID systems, the dielectric permittivity and loss angle of the substrates significantly affect the performance of RFID transponders. Therefore, the knowledge on the variability of these parameters is highly important in the context of developing new solutions in textronic devices with the RFID interface. According to the plan of studies, at the beginning, the comprehensive characterization and determination of the dielectric parameters of various types of textile substrates were carried out. On this basis, the influence of fabrics on the performance of textronic RFID (RFIDtex) tags was characterized with numerical calculations. As the RFIDtex transponders proposed by the authors in the patent PL231291 have an outstanding design in which the antenna and the chip are located on physically separated substrates and are galvanically isolated, the special means had to be implemented when creating a numerical model. On the other hand, the great advantage of the developed construction was confirmed. Since the impedance at the chip’s terminals is primarily determined by the coupling system, the selected fabrics have relatively low impact on the efficiency of the RFIDtex transponder. Such an effect is impossible to achieve with classical designs of passive or semi-passive transponders. The correctness of the simulations was verified on the exemplary demonstrators, in threshold and rotation measurements performed at the laboratory stand.

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The Influence of Textile Substrates on the Performance of Textronic RFID Transponders

One of the major challenges in the development of wearable antennas is to design an antenna that can at the same time satisfy technical requirements and be aesthetically acceptable and suitable for clothing applications. Slotted waveguide antennas made of conductive textiles meet all the demands for integration into clothing. However, the transition from a classic coaxial line into a waveguide remains an open problem and the aim of this paper is to consider and experimentally characterize different possibilities for making this transition.

Textile Slotted Waveguide Antenna: Feeding Considerations

A design of a self-complementary dual-band fifth-generation (5G) antenna for a wireless body area network (WBAN) has been presented in this paper. Like many other advantages, dual-band antennas have the benefit of being able to establish a reliable wireless connection in places that are frequently out of reach. The antenna operates at two popular 5G NR, FR-2 frequency bands: n257, or 28 GHz, and n260, or 39 GHz. FR-2 bands are prominent for high-speed data transactions within a limited range, which is perfect for WBAN applications. The proposed self-complementary single element design has a physical size of 
 
 6
  
 mm
 ×
 8
  
 mm
 ×
 1.59
  
 mm
 
 and is designed with a Rogers RT6002 substrate with a dielectric constant of 2.94. In free space simulations, the antenna performed satisfactorily, achieving around 90% efficiency and having return losses of 16.32 dB and 20.47 dB at the lower and upper frequency bands sequentially. The antenna generated almost omnidirectional patterns at both frequencies with a peak gain of 4.74 dB. By placing the antenna next to a 3D human torso phantom that had been digitally created with accurate human body features, the design’s onbody performance was assessed. Onbody simulations produced favorable performance with a small dispersion from their peak values in some parameters, i.e., efficiency and reflection coefficients but produced more gains: 4.34 dBi at 28 GHz and 6.19 dBi at 39 GHz. For further distance-based investigation, the antenna was placed in five different positions relative to the torso. The test yields 67% efficiency with the minimum gap and 71% at the highest distance from the human body for the lowest band. Though the lower-frequency band produces better results with more gaps, the higher-frequency band performs consistently better in even the closest placement to the human body model by reaching more than 6 dB of gain with above 81% efficiency and wider than 10 GHz of bandwidth. The overall performance indicates this design can be a good solution to complex WBAN scenarios.

Dual-Band Self-Complementary 5G Antenna for Wireless Body Area Network

One of the major challenges in the development of wearable antennas is to design an antenna that can at the same time satisfy technical requirements, be aesthetically acceptable, and be suitable for wearable applications. In this paper, a novel wearable antenna is proposed—textile realization of a slotted waveguide antenna. The antenna is realized using conductive fabric to manufacture the walls of a rectangular waveguide in which the slots were cut out. All connections and cuts are sewn with conductive thread taking over advantages of the traditional process of manufacturing textile objects. The developed slotted waveguide array prototype, containing three slots and designed for operation in the 5.8-GHz ISM band, is experimentally characterized and compared to an equivalent metallic antenna. The achieved operating bandwidth is larger than 300 MHz in both cases. The measured gain of a textile slotted waveguide array is around 9 dBi with a radiation efficiency larger than 50% in the whole operating bandwidth, i.e., the textile array showed a 2 dB lower gain in comparison to the metallic counterpart. The gain is stable in the whole bandwidth and the radiation patterns do not differ. The results demonstrated that such textile antennas are suitable for body-centric communication and sensor systems and can be integrated into clothing, e.g., into a smart safety vest or into a uniform. Further analysis of various realizations of slotted waveguide antennas is presented showing that different versions of the proposed antenna can be used in all three off-body, on-body, and in-body communication scenarios.

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Textile Slotted Waveguide Antennas for Body-Centric Applications

One of the challenges in development of body-centric communication and sensor systems is to design wearable antennas that are technically and aesthetically acceptable to be worn. This paper describes a novel wearable antenna design - textile realization of slotted waveguide antenna. In order to characterize the proposed antenna solution, the measured antenna parameters were compared with the parameters of the equivalent antenna made from aluminium. The realized antenna is suitable for realization of off-body communication mode and can be integrated into clothing, e.g. as a smart belt.

Evita Sopp

Papers

Textile Slotted Waveguide Antennas for Body-Centric Applications

Wearable Slotted Waveguide Textile Antenna

Textile Slotted Waveguide Antenna: Feeding Considerations