Wearable Textile UHF-RFID Sensors: A Systematic Review

21 Oct.,2022


Since the standard EPC Gen2 was first published in 2004, the physical and logical requirements for UHF RFID tags and readers have been defined, which give researchers a basic framework to design a UHF-RFID system integrating many kinds of sensors for different applications. In the beginning, in the textile industrial field, the UHF-RFID technique was mainly applied for identification purposes for application such as clothing manufacturing, inventory control, warehousing, distribution, logistics and automatic object tracking. Currently, UHF-RFID tags are gradually used for daily living applications with sensing capabilities.

As shown in Figure 6 , a brief history of textile UHF-RFID and sensing technologies preceding current UHF-RFID sensors is provided. From 2007 to 2010, the textile UHF-RFID technique was not yet mature and it was mainly focused on feasibility and simple applications with just tags, such as size-optimizing exploration for the textile UHF-RFID antenna in Figure 6 a, accessories trace and production process monitor in Figure 6 b and exploration for the relation between conductivity and different sewing methods in Figure 6 c,d. Then from 2010 to 2015, the textile UHF-RFID technique was gradually applied for simply sensing uses with just the UHF-RFID antennas and ICs, such as the textile UHF-RFID strain sensors for monitoring human bodily functions and movements, as shown in Figure 6 e,f, textile UHF-RFID tag for performance exploration in Figure 6 g, RSS-based passive UHF-RFID for indoor localization applications in Figure 6 h and textile UHF-RFID with broad impedance bandwidth for tire performance monitoring in Figure 6 i. Up to now, textile UHF-RFID integrated with textile sensors has been getting more attention, and many novel textile materials on different kinds of sensing fields have been tested. There are some typical applications, such as UHF-RFID strain sensors with copper-coated fabric, as shown in Figure 6 j, graphene-based UHF-RFID sensors for moisture monitoring in Figure 6 k and sweat sensing in Figure 6 l, silver-plated thread UHF-RFID sensors for environment humidity monitoring in Figure 6 m,n, accelerometer-based UHF-RFID sensor for patients’ activity recognition in Figure 6 o, and equally important, reliability exploration in Figure 6 p.

5.1. State-of-the-Art of Textile UHF-RFID Sensors Applications

In modern society, electronic devices are always closely relevant to specific application fields. The rule is also suitable for current researches of textile UHF-RFID sensors. The UHF-RFID technique started to be applied in combination with the textile technique and then textile sensors about 15 years ago. However, up to now there have not been enough research achievements applied for current production and living, which also means the huge development prospect is worth paying attention to.

A medical-based UHF-RFID body-worn sensor was fabricated and tested for monitoring fluid accumulation in the lungs, which was integrated as part of the garment on various locations such as front, back and shoulders, as shown in Figure 7 a [ 67 ]. In this work, textiles made by e-fabrics (conductive polymer fibers) were evaluated with a microstrip transmission line structure, which demonstrates the e-fiber transmission line surface had electrical equivalence to metallic but inflexible surfaces of copper transmission lines. Note that some useful fabrication methods were adopted, such as bundled fibers for improving conductivity and assistant yarn for avoiding abrasion damage of the silver coatings on the e-fiber’s polymer core. The important achievement was to use the same e-fibers to fabricate the medical-based UHF-RFID body-worn sensor for lung monitoring. In addition, textile versions were found to be nearly equivalent to the metal one even after being repetitively flexed, washed, and dried. This work gave a pioneering application for textile sensors for lung monitoring in medical-based fields.

For the flexible feature of textiles, a deformation-monitoring-based UHF-RFID strain sensor was proposed for structural health monitoring applications, as shown in Figure 7 b [ 68 ]. In this work, a novel dual-interrogation mode was applied for the design of the textile UHF-RFID strain sensor, which provided a large identification coding capacity for the UHF-RFID sensor. The dual-interrogation mode consisted of reading range extraction mode for the threshold-power-required chip-enabled approach and RCS-based (radar cross section) sensing mode for the chipless approach. In fact, the range extraction mode relied on the read range changing with the applied strain, while the RCS-based sensing mode was directly linked to the frequency shift depending on the strain changing. Note that here the strain was related to the electrical length of embroidered UHF-RFID sensor structures. This work proved the feasibility of double modes for the design of textile UHF-RFID strain sensors and it is worth considering in future research, but certainly, some important validation measurements such as bending, environment impacts and washing for the performance and reliability need to be considered.

In another example of the textile UHF-RFID strain sensor, as shown in Figure 7 c [ 69 ], a notable evaluation for the elongation from an attached object was implemented, compared to the last example in Figure 7 b. This textile UHF-RFID strain sensor was based on silver-plated material fabricated by plain knitting and designed into two separate parts, the feeding loop and the radiating antenna. This design makes the radiating antenna part fully stretchable while the IC attached to the feeding loop could be non-stretchable, which avoids the reliability challenges caused by mechanical stresses from clothing-integrated electronics. In this work, this textile UHF-RFID strain sensor was integrated on the shirt, the performance of which was examined on-body by means of backscattered signal power measurements under strain and in unloaded conditions. The results revealed that the strain sensitivity was great and the achievements had the potential for future smart monitoring applications. However, for real applications such as a controller in an embodied game, as mentioned in the paper, some safety and reliability validation measurements were expected to be considered, such as the performance impact after washing or working in a high electromagnetic interference (EMI) area.

These above three current kinds of research actually make use of the flexible feature of textile UHF-RFID sensors, which also lead our way to do related researches on this area. In addition to this feature, it is worth knowing that there are many potential features that push the researches on textile UHF-RFID sensors forward.

There is another example for textile UHF-RFID sensors that are sensitive to humidity. As shown in Figure 7 d [ 65 ], it is an environment-based textile UHF-RFID moisture sensor fabricated on a very common substrate thin single-use dishcloth, which consisted of a sensor part and UHF-RFID antenna part. In this work, the performance of the textile UHF-RFID moisture sensor was evaluated by 10 drops of water from wet state to dry state and the evaluation parameter was the read range in office conditions after 5, 10, and 15 min. The result showed the small changes of the read range from 4.7 m in the dry state and 5.2 m in the wet state. From the result, this textile UHF-RFID moisture sensor had certain moisture detection ability, however the changes of the read range were small relative to the humility from 10 drops of water and the impact from impurities in water also needed to be considered. Thus, this kind of application research had the potential to be focused on in the future.

Compared with the textile UHF-RFID moisture sensor mentioned in Figure 7 d, another example of textile UHF-RFID moisture sensors is shown in Figure 7 e [ 70 ], which is only a textile UHF-RFID tag with moisture sensor functionality. In this work, textile UHF-RFID sensors could curve automatically and permanently after being dipped into the water due to the special material, polyvinyl alcohol (PVA). Note that in contrast to the example in Figure 7 d, the test parameter in this work was the change in the backscattered power percentage, which could be measured and compared in order to detect and record the presence of moisture. The comparative measurements in this work proved the results reliable, which showed the potential of the textile UHF-RFID sensors to be applied in environment moisture detection.

In the health-care monitoring field, many kinds of common wearable UHF-RFID sensors on flexible substrates, such as the flexible printed circuit board (FPCB), have been proposed and applied for commercial health-care monitoring applications, whereas textile UHF-RFID sensors for this area are still in the early stages and most investigations for health-care application are at the stage of laboratory research. For example, a health-care-based textile UHF-RFID sweat sensor was proposed for sweat rate measurements, as shown in Figure 7 f [ 6 ]. In this work, the textile UHF-RFID sweat sensor made by screen printing had a noticeable difference in the response of backscattered signal power. The paper explains that the response curves differences are caused by the conductive antenna impedance and material parameter of the textile substrate changing due to the absorbed sweat. This work indicates the high potential of textile UHF-RFID technology in perspiration sensing, but related sweat components were not analyzed, which were attractive and worth considering.

In another example of the health-care-based textile UHF-RFID biosignal pressure sensors, reliable and secure manner for real-time medical data collection was considered by a software framework, which fills the gap between data safety and textile UHF-RFID sensors for health-care monitoring. As shown in Figure 7 g [ 7 8 ], the textile UHF-RFID biosignal pressure sensor named bellyband sensor for infant heart monitoring in the paper was applied on a pregnant mannequin driven by proprietary software to simulate various behaviors. In addition, a modular software framework was developed to both interrogate sensor devices and to store that streaming data for live and post-processing. In this work, considering the missed tag reads, two impact factors were found. One was the delay caused by periodic frequency hopping and another one was the greater distance between the tag and the reader. These research achievements are helpful for future applications, but more other impact factors need to be tested such as the comfort and electromagnetic safety for pregnant women and infants.

In addition to textile UHF-RFID sweat sensors for sweat rate measurements and biosignal pressure sensors for infant heart monitoring, another health-care-based textile UHF-RFID accelerometer sensor, as shown in Figure 7 h, was proposed for alerting on hospitalized patient bed exits, which could work under a super low resolution. In this work, the sensing device could capture ultra low resolution acceleration data from patients and be analyzed by deep convolutional neural network (CNN) architectures automatically to get the discriminate features. The advances of this work were to combine the textile UHF-RFID accelerometer sensor with neural network approaches, which make the low resolution kinematic sensor possible. This is a mature and useful textile UHF-RFID accelerometer sensor, though the reliability of some components such as the mechanical switch in this sensor device needs to be tested.

The last typical example, as shown in Figure 7 i, is a textile UHF-RFID concentration sensor for concentration detection. In this work, the UHF-RFID concentration sensor was printed on a special textile material, which is polyimide flexible substrate, while the sensing antenna was made of copper. The proposed sample is sensitive to the frequency and the concentration of the NaCl solutions and sucrose solutions. Moreover, from the measurement results in the paper, the author proposed that the sensitivity increases with the increase in the percentages of the NaCl and sucrose in water. Although in the work there is no accurate application mentioned, it can give us a research orientation for using total textile UHF-RFID concentration sensors to detect elements in human body fluids.