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.

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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 ( 2 R = 0.9737) and 550 MHz/mL with linear response ( 2 R = 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.

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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.