Microwave Devices for Wearable Sensors and IoT

Abstract

The Internet of Things (IoT) paradigm is currently highly demanded in multiple scenarios and in particular plays an important role in solving medical-related challenges. RF and microwave technologies, coupled with wireless energy transfer, are interesting candidates because of their inherent contactless spectrometric capabilities and for the wireless transmission of sensing data. This article reviews some recent achievements in the field of wearable sensors, highlighting the benefits that these solutions introduce in operative contexts, such as indoor localization and microwave sensing. Wireless power transfer is an essential requirement to be fulfilled to allow these sensors to be not only wearable but also compact and lightweight while avoiding bulky batteries. Flexible materials and 3D printing polymers, as well as daily garments, are widely exploited within the presented solutions, allowing comfort and wearability without renouncing the robustness and reliability of the built-in wearable sensor.

Keywords: 3D printing; IoT; e-Health; electronics; energy harvesting; fall detection; localization; microfluidics; sensors; wearable; wireless power transfer.

Figure 1

Envisioned scenario of an integrated system for wearable sensing [14], indoor localization [15], and (a–h) 3D printing techniques [16] for e-Health applications.

Figure 2

(a) Picture of the prototype, showing the narrowband antenna for power reception, the coupled-line filter loaded with the microfluidic stub resonator, and the rectifier circuit; (b) picture of the system worn as a bracelet [46]. © 2021 IEEE.

Figure 3

DC output voltage level vs. received power for (a) open circuit conditions and (b) adopting an optimized load of 6.5 kΩ [46]. © 2021 IEEE.

Figure 4

(a) Front, (b) back view, and (c) stack-up of the microfluidic biosensor [49].

Figure 5

Simulated and measured reflection coefficient in the case of (a) empty channel and (b) PBS solution and in the presence of fibroblast cells [49].

Figure 6

(a) Schematic view of the operation principles of a module integrating UWB and IMU technologies. #1, #2, #3 represent the three UWB anchors distributed in the environment. (b) Circuitry and UWB antenna worn by the user in the foot. The pink circle highlights the marker point on the user’s shoe adopted during the experiment [54].

Figure 7

(a) Photograph of the RFID reader included in a frame; (b) block diagram of the RF and digital circuitry of the reader for electronic tridimensional beam scanning. The red arrows represent RF signals, whereas the black ones are the digital connections [57]. © 2019 IEEE.

Figure 8

Wearable antenna realized on a denim substrate and connection with the MCU/transceiver circuitry realized on Rogers.

Figure 9

Schematic block diagram representing the principles of operation of a fall detection system based on deep learning algorithms exploiting data coming from wearable sensors [59].

Figure 10

Simulated (a) and fabricated (b) GCPW on Flexible-80A substrate. (c) Comparison between the measured |S₁₂| (dB) curves of the microstrip line and a GCPW realized on Flexible-80A [62]. © 2022 IEEE.

Figure 11

(a) Schematic of the 868 MHz rectenna system layout (where Z_Antenna is the antenna impedance and Z*_Rectifier is the conjugate of the rectifier’s impedance); (b) design of the simulated antenna, and (c) photograph of the realized prototype of the receiving antenna realized on Flexible-80A with adhesive copper [62]. © 2022 IEEE.

Figure 12

(a) Stack-up of the 2.45 GHz rectenna of the receiving antenna with call-out of the transversal cross-section; (b) comparison between the measured rectifier and rectenna efficiencies (%) vs. the RF power received by the coplanar-fed patch antenna.

Figure 13

(a) Schematic of the proposed honeycomb structure with dimensions of the unit cells and thickness of the PLA spacing among them; (b) fabricated prototypes of the microstrip-fed SIW on solid (top) and honeycomb-shaped (bottom) PLA; (c) measured and simulated results of the considered SIWs [75].