Recent Advances of Wearable Antennas in Materials, Fabrication Methods, Designs, and Their Applications: State-of-the-Art

Stretchable Sensor Materials Applicable to Radiofrequency Coil Design in Magnetic Resonance Imaging: A Review

Wearable sensors are rapidly gaining influence in the diagnostics, monitoring, and treatment of disease, thereby improving patient outcomes. In this review, we aim to explore how these advances can be applied to magnetic resonance imaging (MRI). We begin by (i) introducing limitations in current flexible/stretchable RF coils and then move to the broader field of flexible sensor technology to identify translatable technologies. To this goal, we discuss (ii) emerging materials currently used for sensor substrates, (iii) stretchable conductive materials, (iv) pairing and matching of conductors with substrates, and (v) implementation of lumped elements such as capacitors. Applicable (vi) fabrication methods are presented, and the review concludes with a brief commentary on (vii) the implementation of the discussed sensor technologies in MRI coil applications. The main takeaway of our research is that a large body of work has led to exciting new sensor innovations allowing for stretchable wearables, but further exploration of materials and manufacturing techniques remains necessary, especially when applied to MRI diagnostics.

New Advances in Antenna Design toward Wearable Devices Based on Nanomaterials

Wearable antennas have recently garnered significant attention due to their attractive properties and potential for creating lightweight, compact, low-cost, and multifunctional wireless communication systems. With the breakthrough progress in nanomaterial research, the use of lightweight materials has paved the way for the widespread application of wearable antennas. Compared with traditional metallic materials like copper, aluminum, and nickel, nanoscale entities including zero-dimensional (0-D) nanoparticles, one-dimensional (1-D) nanofibers or nanotubes, and two-dimensional (2-D) nanosheets exhibit superior physical, electrochemical, and performance characteristics. These properties significantly enhance the potential for constructing durable electronic composites. Furthermore, the antenna exhibits compact size and high deformation stability, accompanied by greater portability and wear resistance, owing to the high surface-to-volume ratio and flexibility of nanomaterials. This paper systematically discusses the latest advancements in wearable antennas based on 0-D, 1-D, and 2-D nanomaterials, providing a comprehensive overview of their development and future prospects in the field.

A Review of Clothing Components in the Development of Wearable Textile Antennas: Design and Experimental Procedure

Wearable antenna systems have attracted significant research efforts during the last decade and a rich pool of review papers can be found in the literature. Each scientific work contributes to various fields of wearable technology focusing, mainly, on constructing materials, manufacturing techniques, targeting applications, and miniaturization methods. In this review paper, we examine the use of clothing components in wearable antenna technology. By the term "clothing components" (CC), dressmaking accessories/materials such as buttons, snap-on buttons, Velcro tapes, or zips are considered. In light of their utilization in the development of wearable antennas, the clothing components can play a triple role: (i) that of a clothing item, (ii) that of an antenna part or the main radiator, and (iii) that of an integration means of the antennas into clothes. One of their advantages is that they consist of conductive elements, integrated into the clothes, which can be effectively exploited as operating parts of wearable antennas. This review paper includes classification and description of the clothing components used so far in the development of wearable textile antennas with an emphasis on designs, applications and performance. Furthermore, a step-by-step design procedure for textile antennas that use clothing components as a functional part of their configuration is recorded, reviewed, and described in detail. The design procedure takes into account the detailed geometrical models required for the clothing components and the way they are embedded into the wearable antenna structure. In addition to the design procedure, aspects of experimental procedures (parameters, scenarios, and processes) that should be followed in wearable textile antennas with an emphasis on antennas that use clothing components (e.g., repeatability measurements) are presented. Finally, the potential of textile technology through the application of clothing components into wearable antennas is outlined.

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

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.

Flexible Antennas for a Sub-6 GHz 5G Band: A Comprehensive Review

The ever-increasing demand and need for high-speed communication have generated intensive research in the field of fifth-generation (5G) technology. Sub-6 GHz 5G mid-band spectrum is the focus of the researchers due to its meritorious ease of deployment in the current scenario with the already existing infrastructure of the 4G-LTE system. The 5G technology finds applications in enormous fields that require high data rates, low latency, and stable radiation patterns. One of the major sectors that benefit from the outbreak of 5G is the field of flexible electronics. Devices that are compact need an antenna to be flexible, lightweight, conformal, and still have excellent performance characteristics. Flexible antennas used in wireless body area networks (WBANs) need to be highly conformal to be bent according to the different curvatures of the human body at different body parts. The specific absorption rate (SAR) must be at a permissible level for such an antenna to be suited for WBAN applications. This paper gives a comprehensive review of the current state of the art flexible antennas in a sub-6 GHz 5G band. Furthermore, this paper gives a key insight into the materials for a flexible antenna, the parameters considered for the design of a flexible antenna for 5G, the challenges for the design, and the implementation of a flexible antenna for 5G.

End-to-end design of wearable sensors

Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors.

Design of Flexible Meander Line Antenna for Healthcare in Wireless Body Area Network Systems

A flexible meander line monopole antenna (MMA) is presented in this paper. The antenna can be worn for on-and off-body applications. The overall dimension of the MMA is 37 mm x 50 mm x 2.37 mms, The MMA was manufactured and measured, and the results matched with simulation results. The MMA design shows a bandwidth of up to 1282.4 (450.5) MHz and provides gains of 3.03 (4.85) dBi in two operating bands, respectively, showing omnidirectional radiation patterns in free space. While worn on the chest or arm, bandwidths as high as 688.9 (500.9) MHz and 1261.7 (524.2) MHz, and the gains of 3.80 (4.67) dBi and 3.00 (4.55) dBi were observed. The experimental measurements of the read range confirmed. Clinical Relevance- Wireless Medical Body Area Network (WMBAN) technology allows for continuous monitoring and analysis of patient health data to improve the quality of healthcare services.

Digitally-embroidered liquid metal electronic textiles for wearable wireless systems

Electronic textiles capable of sensing, powering, and communication can be used to non-intrusively monitor human health during daily life. However, achieving these functionalities with clothing is challenging because of limitations in the electronic performance, flexibility and robustness of the underlying materials, which must endure repeated mechanical, thermal and chemical stresses during daily use. Here, we demonstrate electronic textile systems with functionalities in near-field powering and communication created by digital embroidery of liquid metal fibers. Owing to the unique electrical and mechanical properties of the liquid metal fibers, these electronic textiles can conform to body surfaces and establish robust wireless connectivity with nearby wearable or implantable devices, even during strenuous exercise. By transferring optimized electromagnetic patterns onto clothing in this way, we demonstrate a washable electronic shirt that can be wirelessly powered by a smartphone and continuously monitor axillary temperature without interfering with daily activities.

Fabrication and Performance Analysis of 3D Inkjet Flexible Printed Touch Sensor Based on AgNP Electrode for Infotainment Display

It is possible to employ printed capacitive sensors in car bezel applications because of its lower cost and higher detecting capabilities. In this paper, a flexible sensor for automotive entertainment applications has been developed using an electrode flexible sensor with an interdigitated pattern printed on it using screen printing and 3D printing fabrication processes. Design concerns such as electrode overlap, electrode gap and width on capacitance changes, and production costs were studied. In addition, a new generation of flexible printed sensors has been developed that can outperform conventional human–machine interface (HMI) sensors. The capacitance of the design pattern may be optimized by using a 15mm overlap and 0.5mm electrode line width. Due to the precision of interpolation, overlap has a larger effect on sensor performance than it would have without it.

Design and Evaluation of a Button Sensor Antenna for On-Body Monitoring Activity in Healthcare Applications

A button sensor antenna for on-body monitoring in wireless body area network (WBAN) systems is presented. Due to the close coupling between the sensor antenna and the human body, it is highly challenging to design sensor antenna devices. In this paper, a mechanically robust system is proposed that integrates a dual-band button antenna with a wireless sensor module designed on a printed circuit board (PCB). The system features a small footprint and has good radiation characteristics and efficiency. This was fabricated, and the measured and simulated results are in good agreement. The design offers a wide range of omnidirectional radiation patterns in free space, with a reflection coefficient (S11) of −29.30 (−30.97) dB, a maximum gain of 1.75 (5.65) dBi, and radiation efficiency of 71.91 (92.51)% in the lower and upper bands, respectively. S11 reaches −23.07 (−27.07) dB and −30.76 (−31.12) dB, respectively, with a gain of 2.09 (6.70) dBi and 2.16 (5.67) dBi, and radiation efficiency of 65.12 (81.63)% and 75.00 (85.00)%, when located on the body for the lower and upper bands, respectively. The performance is minimally affected by bending, movement, and fabrication tolerances. The specific absorption rate (SAR) values are below the regulatory limitations for the spatial average over 1 g (1.6 W/Kg) and 10 g of tissues (2.0 W/Kg). For both indoor and outdoor conditions, experimental results of the range tests confirm the coverage of up to 40 m.

A novel single-feed hybrid reconfigurable microstrip patch antenna for 5G mobile communication and radio frequency energy harvesting applications at 28/38GHz

Reconfigurable antennas have received much attention in RF energy harvesting models owing to their selectivity for operating frequency and polarization. The characteristic of having frequency selectivity and polarization selectivity can be termed as frequency diversity and polarization diversity, respectively. This paper investigates a rectenna device with a new proposed topology in order to eliminate coupling between input and output lines and increase the rectification efficiency with the use of single feed hybrid reconfigurable antenna, switch between 28GHz and 38GHz frequency. Moreover, it is designed to charge a rechargeable battery of 1watt(W). The Reconfiguration mechanism is realized by electronically controlling different states of Switches. PIN Diode (as RLC Equivalent circuit) is used as a switch for ON/OFF states. This antenna mainly comprises rectangular shaped patches (28GHz and 38GHz) with Triango-Truncated edge at the corners. Eighteen PIN Diodes are placed symmetrically throughout the antenna presenting as, S1 & S2 for frequency reconfiguration, S3 to S6 & S7 to S10 connects Triango Truncated edge at the corners for polarization reconfiguration, and for radiation pattern reconfiguration at S11 to S14 & S15 to S18 has been used. The proposed antenna model is capable of simultaneously changing, the radiation patterns as clock and anti-clockwise directions at ±90-degree shift in E and H planes, circularly polarized (CP) states among, Linear Polarization (LP), Right Hand Circularly Polarization (RHCP), and Left Hand Circularly Polarization (LHCP). The current design describes using single antenna for energy harvesting and 5G mobile communication application. This would lead to higher output currents, leading to the ability to efficiently charge a wide variety of batteries. A fully functional prototype has been designed, fabricated and its compound reconfiguration characteristics have been validated for simulated and measured results. For validation of results, the experimental results and the simulation results from the proposed mathematical model were made into comparison, and excellent correlation between the measured and simulated results was obtained.

Exposure Assessment to Radiofrequency Electromagnetic Fields in Occupational Military Scenarios: A Review

(1) Background: Radiofrequency radiations are used in most devices in current use and, consequently, the assessment of the human exposure to the radiofrequency radiations has become an issue of strong interest. Even if in the military field there is wide use of radiofrequency devices, a clear picture on the exposure assessment to the electromagnetic field of the human beings in the military scenario is still missing. (2) Methods: a review of the scientific literature regarding the assessment of the exposure of the military personnel to the RF specific to the military environment, was performed. (3) Results: the review has been performed grouping the scientific literature by the typology of military devices to which the military personnel can be exposed to. The military devices have been classified in four main classes, according to their intended use: communication devices, localization/surveillance devices, jammers and EM directed-energy weapons. (4) Discussion and Conclusions: The review showed that in the exposure conditions here evaluated, there were only occasional situations of overexposure, whereas in the majority of the conditions the exposure was below the worker exposure limits. Nevertheless, the limited number of studies and the lack of exposure assessment studies for some devices prevent us to draw definitive conclusions and encourage further studies on military exposure assessment.

Fabrication and Performance Evolution of AgNP Interdigitated Electrode Touch Sensor for Automotive Infotainment

In the present scenario, a considerable assiduity is provided to develop novel human-machine interface technologies that rapidly outpace the capabilities of display technology in automotive industries. It is necessary to use a new cockpit design in conjunction with a fully automated driving environment in order to enhance the driving experience. It can create a seamless and futuristic dashboard for automotive infotainment application. In the present study, an endeavor was made to equip the In-vehicle bezels with printed capacitive sensors for providing superior sensing capabilities. Silver Nanoparticles based interdigitated pattern electrodes were formed over polycarbonate substrates to make printed capacitive sensors using screen printing process. The developed sensor was investigated to evaluate the qualitative and quantitative measures using direct and in-direct contact of touch. The proposed approach for sensors pattern and fabrication can highly impact on sensor performance in automotive infotainment application due to the excellent spatial interpolation with lower cost, light weight, and mechanical flexibility.

Analysis of Shielding Properties of Head Covers Made of Conductive Materials in Application to 5G Wireless Systems

The introduction of the fifth generation wireless systems caused social emotions regarding the impact of electromagnetic waves on people. Many people who consider themselves to be particularly sensitive to radiation make metal foil head covers (so called “tinfoil hats”) to shield their body from radiation. The aim of this paper is to show how effective the “tinfoil hat” really is when applied to base station radiation in a fifth generation telecommunication system. It presents the results of investigation on effectiveness of these protections in terms of their shielding properties at the frequencies used in fifth generation wireless systems. The research was carried out based on computer simulations. Remcom XFdtd software (software: XFdtd version 7.8.1 manufacturer: Remcom, 315 South Allen Street, Suite 416 State College, PA, USA) utilizing a finite difference time domain method and a numerical model of the head was applied to obtain the data on shielding properties of conductive head covers. It was found that in the case of foil head covers the maximum reduction factor of power density in the head region is approximately 50%. Furthermore, the application of a metal surface shield increases the maximum value of energy absorbed by human tissue in some regions of the head. To overcome this problem, the design of a wire-based shielding structure that does not reduce user comfort is presented as an alternative to the full-metal head cover. For wave propagation in the horizontal plane, its performance is comparable to tinfoil-like structure, but its design makes it much more comfortable for the user.

Wideband and High-Gain Wearable Antenna Array with Specific Absorption Rate Suppression

In this paper, a wideband and high-gain antenna array with specific absorption rate suppression is presented. By adopting the wideband monopole antenna array and the uniplanar compact electromagnetic band gap (UC-EBG) structure, the proposed wearable antenna array can realize a high gain of 11.8–13.6 dBi within the operating band of 4.5–6.5 GHz. The sidelobe level of the proposed wearable antenna array is less than −12 dB, and the cross polarization in the main radiation direction is less than −35 dB. Benefiting from the UC-EBG design, the specific absorption rate is suppressed effectively, guaranteeing the safety of the proposed antenna array to the human body. The proposed antenna array is processed and tested, and the measurement results show good agreement with the simulation results.

Screen Printing Carbon Nanotubes Textiles Antennas for Smart Wearables

Electronic textiles have become a dynamic research field in recent decades, attracting attention to smart wearables to develop and integrate electronic devices onto clothing. Combining traditional screen-printing techniques with novel nanocarbon-based inks offers seamless integration of flexible and conformal antenna patterns onto fabric substrates with a minimum weight penalty and haptic disruption. In this study, two different fabric-based antenna designs called PICA and LOOP were fabricated through a scalable screen-printing process by tuning the conductive ink formulations accompanied by cellulose nanocrystals. The printing process was controlled and monitored by revealing the relationship between the textiles' nature and conducting nano-ink. The fabric prototypes were tested in dynamic environments mimicking complex real-life situations, such as being in proximity to a human body, and being affected by wrinkling, bending, and fabric care such as washing or ironing. Both computational and experimental on-and-off-body antenna gain results acknowledged the potential of tunable material systems complimenting traditional printing techniques for smart sensing technology as a plausible pathway for future wearables.

Performance of Reconfigurable Antenna Fabricated on Flexible and Nonflexible Materials for Band Switching Applications

In this article, a novel frequency slot-based switchable antenna fabricated on flexible and nonflexible materials is presented for suitable reconfigurable radiations of Bluetooth, WiMAX, and upper WLAN applications. Initially, the performance of this structure was simulated using a CSTTM simulator and evaluated experimentally using a nonflexible FR4 structure. The same antenna was implemented on a flexible (jean) substrate with a relative permittivity of 1.7. The proposed textile antenna prototypes were fabricated by optimal dimensions of an E-shaped slot with a variation on the shape of the ground layer, integrated using a crossed T-shaped strip with ON/OFF switchable state operations. The proposed antenna prototype is compact (20 × 20 mm2), providing switchable radiations with tri bands, has frequencies ranged at 2.36–2.5 GHz for Bluetooth, 3.51–3.79 GHz and 5.47–5.98 GHz for the distinct bands of WiMAX and WLAN, respectively, as well as part of UWB operations.

Design and Evaluation of a Flexible Dual-Band Meander Line Monopole Antenna for On- and Off-Body Healthcare Applications

The human body is an extremely challenging environment for wearable antennas due to the complex antenna-body coupling effects. In this article, a compact flexible dual-band planar meander line monopole antenna (MMA) with a truncated ground plane made of multiple layers of standard off-the-shelf materials is evaluated to validate its performance when worn by different subjects to help the designers who are shaping future complex on-/off-body wireless devices. The antenna was fabricated, and the measured results agreed well with those from the simulations. As a reference, in free-space, the antenna provided omnidirectional radiation patterns (ORP), with a wide impedance bandwidth of 1282.4 (450.5) MHz with a maximum gain of 3.03 dBi (4.85 dBi) in the lower (upper) bands. The impedance bandwidth could reach up to 688.9 MHz (500.9 MHz) and 1261.7 MHz (524.2 MHz) with the gain of 3.80 dBi (4.67 dBi) and 3.00 dBi (4.55 dBi), respectively, on the human chest and arm. The stability in results shows that this flexible antenna is sufficiently 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, respectively, while an optimal gap between the antenna and human body was maintained. This stability of the working frequency provides robustness against various conditions including bending, movement, and relatively large fabrication tolerances.