Microwave Resonators for Wearable Sensors Design: A Systematic Review

Abstract

The field of flexible electronics is undergoing an exponential evolution due to the demand of the industry for wearable devices, wireless communication devices and networks, healthcare sensing devices and the technology around the Internet of Things (IoT) framework. E-tex tiles are attracting attention from within the healthcare areas, amongst others, for providing the possibility of developing continuous patient monitoring solutions and customized devices to accommodate each patient's specific needs. This review paper summarizes multiple approaches investigated in the literature for wearable/flexible resonators working as antenna-based systems, sensors and filters with special attention paid to the integration to flexible materials, especially textiles. This review manuscript provides a general overview of the flexible resonators' advantages and drawbacks, materials, fabrication techniques and processes and applications. Finally, the main challenges and future prospects of wearable resonators are discussed.

Keywords: bending analysis; embroidered resonator; flexible resonator; textile materials; textile resonator; washing analysis; wearable device.

Figure 11

Some examples of printing technique for the design of resonators: (a) inkjet-printed array of SRR on a commercial polyamide film [132]; (b1,b2) inkjet-printed silver rectangle-based SRR resonator array on flexible metamaterial PI film [133]: layout (b1) and prototypes (b2); (c1,c2) inkjet-printed MSRR unit cell array on a very low-cost photo paper [135]: combined SRR and CLS unit cell (c1) and bended prototype (c2).

Figure 1

Graph of a bandpass filter Q-factor.

Figure 2

Frequency shift from (a) without MUT to (b) with MUT in the sensing area.

Figure 3

Some microstrip resonator structures: (a) half-wavelength (λ/2) resonator; (b) quarter-wavelength (λ/4) open-end stub resonator; (c) ring resonator; (d) circular patch resonator; (e) stepped impedance resonator; (f) hairpin resonator.

Figure 4

Topologies of the (a) ELC; (b) single SRR; (c) double SRR; (d) double CSRR; (e) OSRR; (f) OCSRR; (g) SR; (h) CSR; and their equivalent circuit models. The ohmic losses would be considered by adding a series resistance in the model. Models reproduced from references [3,21].

Figure 5

Moisture regains (%) of natural fibers [31,32,33].

Figure 6

Some examples of wearable electronics manufacturing techniques: (a) Wet Etching: flexible split-ring resonator metamaterial structure fabricated on a polypropylene film by chemical etching [94]; (b) Welding and Soldering: components surface-mounted soldered to conductive fabric. Reproduced with permission. Copyright Liza Stark (http://thesoftcircuiteer.net/); (c1,c2) Adhesive Conductive Foil: implementation of (c1) a ring resonator and (c2) a stub resonator on textiles using adhesive copper tape [29]; (d1,d2) Screen Printing: (d1) an illustration of a conventional screen-printing process; (d2) prototype of 60 GHz flexible meta surface made of a unit cell of circular metallic rings embedded with a square-shaped ring and printed on Melinex [95]; (e) Inkjet Printing: split-ring resonator fabricated using an ink-jet printer using ink with silver nanoparticles [77]; (f) 3D Printing: chipless RFID tag made of rectangular slot ring tags, different IDs (left to right): ‘1111111111110’ and ‘1010101010100’ [96]; (g1,g2) Embroidery: (g1) embroidery methods (from left to right): standard embroidery and TFP [92]; (g2) embroidered transmission line loaded with a split-ring resonator on felt substrate with a satin pattern (60% density) [97]; (h1,h2) Weaving and Knitting: (h1) different knitted plain structure-based sensors and (h2) illustrated simulation of plain knit structure composed of conductive yarn and non-conductive yarn made from 100% polyester [93].

Figure 7

Flow chart for the deviation of material’s dielectric characterization.

Figure 8

Conventional resonator coupling methods: (a) loose coupling; (b) enhanced coupling; (c) line-to-ring coupling; (d) matched-stubs coupling.

Figure 9

Microwave liquid sensor based on a metamaterial CSRR: (left) design of the sensor using CST software and (right) S₂₁ response of water–ethanol mixtures for different concentrations and bending radii [108].

Figure 10

Summary of applications of e-textiles wearable technology.

Figure 12

Some examples of fabricated resonator-based sensors for textile applications: (a) dielectric resonator antenna button textile antenna for off-body applications [136]; (b) stub resonator in woolen felt substrate [137]; (c) embroidered SRR for antenna design [138].

Figure 13

Some examples of embroidery technique used in the design of resonator-based sensors and antennas: (a) embroidery of an ORR-based antenna sensor [12]; (b1,b2) fully embroidered transmission line loaded in a squared SRR on a felt substrate [17]: (b1) bended prototype and (b2) study of the bending effect on the frequency and S₂₁ parameter; (c) embroidered double-circular SRR prototype [14].