This review paper summarizes various approaches developed in the literature for antenna sensors with an emphasis on flexible solutions. The survey helps to recognize the limitations and advantages of this technology. Furthermore, it offers an overview of the main points for the development and design of flexible antenna sensors from the selection of the materials to the framing of the antenna including the different scenario applications. With regard to wearable antenna sensors deployment, a review of the textile materials that have been employed is also presented. Several examples related to human body applications of flexible antenna sensors such as the detection of NaCl and sugar solutions, blood and bodily variables such as temperature, strain, and finger postures are also presented. Future investigation directions and research challenges are proposed.

This paper presents a monopole antenna-based sensor for measuring different amounts and concentrations of salt and sugar in water. The proposed antenna sensor consists of a textile monopole antenna with circular-ring and partial ground plane. It is implemented by means of embroidery on a felt textile substrate and it resonates at 2.4 GHz. The textile substrate can absorb liquids through the sensing area incorporated within the monopole antenna structure. Therefore, the substrate dielectric properties are changed according to the liquid properties of the absorbed solution. The proposed antenna sensor uses microwave signals to track different amount and concentrations of salt and sugar in terms of the magnitude of the return loss and resonance frequency shift. The measurements are recorded and compared before and after applying different solutions. The rinsing reliability of the proposed antenna sensor has been also studied. The proposed antenna sensor demonstrates a high sensitivity of 800 MHz/mL with a good correlation with the linear fit ( ${R}^{2}= 0.9737$ ) and 550 MHz/mL with linear response ( ${R}^{2}= 0.9135$ ) for 5% salt and sugar concentration solutions, respectively. To the best of our knowledge, this article demonstrates for the first time the capability of a fully-textile antenna sensor to detect different amounts and concentrations of salt and sugar using microwave signals.

This paper is devoted to a novel textile ultra-wideband (UWB) antenna for wearable applications. The antenna is designed and implemented on a felt textile substrate with attractive features such as a flexible full embroidered topology and a low dielectric loss. In this study, the UWB antenna is evaluated through the reflection coefficient (S11), gain, efficiency and effects of bending. The UWB operation of this compact antenna varies from 3.1 GHz to 11.3 GHz implying a fractional bandwidth of 114%, in which the efficiency is over 60% and the realized gain reaches 4 dB.

In this paper, a feasibility study of a microwave antenna-based sensor is proposed for in vitro experiments for monitoring blood glucose levels. The proposed device consists of a square-ring incorporated within a fully textile monopole antenna to absorb and sense different glucose concentrations, covering patients with different diabetic conditions. The designed antenna-sensor is optimized to operate at 2.4 GHz. The sensing principle is based on the resonance frequency shift of the reflection response of the antenna-based sensor under different glucose levels. The experiments were carried out with blood mimicking by means of aqueous solutions, using D(+)- glucose/water in different concentrations for various diabetic conditions of type-2 diabetes. The performance of the embroidered antenna-based sensor is characterized and validated using a convenient setup for in vitro measurements. The results demonstrated the ability of the proposed antenna-based sensor to cover all the glucose levels of the diabetes range, including hypoglycemia (10–70 mg/dL), normoglycemia (80–110 mg/dL) and hyperglycemia (130–190 mg/dL) with a sensitivity of 350 kHz/(mg/dL). Besides its ability to detect different glucose concentrations of various diabetic conditions, the proposed antenna-sensor presents diverse features such as a simplistic design, compact size, wearability and low cost. The proposed textile device demonstrates a proof of concept for efficient in vitro blood glucose level measurements and diagnostics of diabetes.

Wearable technology has been getting more attention for monitoring vital signs in various medical fields, particularly in breathing monitoring. To monitor respiratory patterns, there is a current set of challenges related to the lack of user comfort, reliability, and rigidity of the systems, as well as challenges related to processing data. Therefore, the need to develop user-friendly and reliable wireless approaches to address these problems is required. In this paper, a novel, full, compact textile breathing sensor is investigated. Specifically, an embroidered meander dipole antenna sensor integrated into an e-textile T-shirt with a Bluetooth transmitter for real-time breathing monitoring was developed and tested. The proposed antenna-based sensor is designed to transmit data over wireless communication networks at 2.4 GHz and is made of a silver-coated nylon thread. The sensing mechanism of the proposed system is based on the detection of a received signal strength indicator (RSSI) transmitted wirelessly by the antenna-based sensor, which is found to be sensitive to stretch. The respiratory system is placed on the middle of the human chest; the area of the proposed system is 4.5 × 0.48 cm², with 2.36 × 3.17 cm² covered by the transmitter module. The respiratory signal is extracted from the variation of the RSSI signal emitted at 2.4 GHz from the detuned embroidered antenna-based sensor embedded into a commercial T-shirt and detected using a laptop. The experimental results demonstrated that breathing signals can be acquired wirelessly by the RSSI via Bluetooth. The RSSI range change was from −80 dBm to −72 dBm, −88 dBm to −79 dBm and −85 dBm to −80 dBm during inspiration and expiration for normal breathing, speaking and movement, respectively. We tested the feasibility assessment for breathing monitoring and we demonstrated experimentally that the standard wireless networks, which measure the RSSI signal via standard Bluetooth protocol, can be used to detect human respiratory status and patterns in real time.

Breathing rate is a crucial vital sign for evaluating well-being and identifying underlying diseases related to the respiration system. This paper presents a fully stretchable triangular loop antenna-based sensor for real-time respiration monitoring. This antenna is integrated into a commercially available T-shirt, which is made of stretchable conductive ink printed on a Thermoplastic polyurethane (TPU) substrate. The sensing mechanism is influenced by shifts in the resonance frequency of a triangular loop antenna sensor, which occur due to thoracic and abdominal deformation during the breathing cycle of inspiration and expiration. The proposed system captures breathing patterns by detecting shifts in the resonance frequency, which are continuously recorded in real-time via Matlab. The Vector Network Analyzer (VNA) was connected to a remote PC via a LAN interface to store the breathing data on a PC host, facilitating data transfer over TCP/IP through the same LAN interface. The proposed antenna-based sensor stands out for its lightweight structure making it convenient to embed in clothing, and its ability to provide continuous monitoring of different breathing patterns including Eupnea, Bradypnea, Tashypnea, Ataxic, Sighing, and Biot’s breathing. The proposed system was tested through experimental measurements, and the obtained results matched well with the standard breathing patterns provided by the World Health Organization.