Flexible RFID Tag Research Articles

Off-the-shelf UHF RFID-based sensors for corrosion characterization of coated steel

The application of the UHF RFID technique in non-destructive testing (NDT) and structural health monitoring (SHM) has gained increasing attention due to its wireless, battery-less, and cost-effective attributes. It offers a promising approach for SHM and the Internet of Things (IoTs). This paper reports commercial off-the-shelf (COTS) flexible UHF RFID tag-based sensors for corrosion characterization on coated mild steel. Two types of COTS flexible UHF RFID tags with different bends are fabricated as sensors. The received signal strength indicator (RSSI) measurement is implemented to characterize the corrosion. Three parts (T-match area, dipole arms, and dipole loadings) of the two tags are tested for sensing purposes, and comparison and discussion of sensitivity, read range, and results are provided. This study successfully validates the feasibility of the proposed tag-bent method for corrosion characterization undercoating. It can be concluded that the dipole arm part of the applied COTS tags is the most sensitive area.

Projection Stereolithography 3D Printing High-Conductive Hydrogel for Flexible Passive Wireless Sensing.

Hydrogel-based electronics have inherent similarities to biological tissues and hold potential for wearable applications. However, low conductivity, poor stretchability, nonpersonalizability, and uncontrollable dehydration during use limit their further development. In this study, projection stereolithography 3D printing high-conductive hydrogel for flexible passive wireless sensing is reported. The prepared photocurable silver-based hydrogel is rapidly planarized into antenna shapes on substrates using surface projection stereolithography. After partial dehydration, silver flakes within the circuits form sufficient conductive pathways to achieve high conductivity (387 S cm-1). By sealing the circuits to prevent further dehydration, the resistance remains stable when tensile strain is less than 100% for at least 30 days. Besides, the sealing materials provide versatile functionalities, such as stretchability and shape memory property. Customized flexible radio frequency identification tags are fabricated by integrating with commercial chips to complete the accurate recognition of eye movement, realizing passive wireless sensing.

Digital light 3D printed fast- and controlled degradation of covalent hydrogel networks

Digital light processing (DLP)-based 3D printing of hydrogels with sophisticated structures has been widely used in myriads of fields. However, the inherent covalently crosslinked network of hydrogels limits their further applications, such as working as sacrificial molds. In this work, a photocurable hydrogel system with labile covalent bonds suitable for DLP based printing to fabricate 3D complex hydrogel structures with fast- and controlled degradation ability is reported. The hydrogel precursor is composed of polyethylene glycol 400 (PEG400), a water soluble photoinitiator, and PEGDA575-Do, a degradable photocurable diacrylate monomer that can be degraded in aqueous condition via reversable aza-Michael addition reaction. The mechanical properties of formed hydrogels can be regulated by changing the PEG content and water content, and the degradation rate can be regulated by changing parameters such as PEG400 content, water content and the treating temperature. Moreover, the DLP printed degradable hydrogels networks can be used as sacrificial molds to fabricate structures from materials that cannot be directly used for DLP 3D printing (e.g. thermoplastic polycaprolactone (PCL), epoxy-based resin, and chocolate). As proofs-of-concept, a microfluidic chip is fabricated by using the biocompatible 3D printed hydrogel as a sacrificial template. Similarly, a flexible RFID tag and a flexible self-powered device are also prepared, demonstrating the potential applications in the field of flexible electronics.

A new structure of a miniature and flexible printed UHF RFID TAG

A new Miniaturized RFID tag has been designed and optimized using meander theory and split ring resonator theory. The proposed RFID tag is designed and adapted for an UHF chip with Zchip=30.39-i*211Ohm. The proposed RFID tag is simulated using an EM solver and supported to operate in UHF sub-band [866.9-870.9MHz] with a band width of 4MHz. A prototype has been fabricated using inkjet method on a polyimide substrate and has been tested using a RFID base station which has proved an excellent agreement between theoretical reading range and experiment which is over up 4m.The miniaturized RFID tag is highly suitable for applications require a middle range between 4 and 8m and a flexible compact RFID tag for integration on items such clothing application and access control.

A Flexible and Low-Cost UHF RFID Tag Antenna for Blood Bag Traceability

A new low-profile flexible RFID tag antenna operating in the ultra-high frequency (UHF) European band (865 MHz–868 MHz) is proposed for blood bag traceability. Its structure combines inductive and capacitive parts with nested slots allowing for the achieving of conjugate impedance matching with the IC-chip. The whole electrical parameters of the environment (substrate, bag, and blood) were considered for the design of the tag antenna. A good agreement was obtained between the measurements and electromagnetic simulations for the input impedance of the tag antenna in the UHF band. A reading range close to 2.5 m was experimentally obtained. Therefore, this tag antenna could be effective and useful in future RFID systems for blood bag monitoring, thus improving patient safety in healthcare infrastructures.

High Capacity and Miniaturized Flexible Chipless RFID Tag Using Modified Complementary Split Ring Resonator

This paper aims to produce a high data capacity and miniaturized flexible chipless RFID tag based on the frequency signature using the Modified Complementary Split Ring Resonator (MCSRR). The proposed 19 bits chipless RFID tag using the frequency shifting technique consists of five slotted overlaying MCSRR with Different Width (MCSRR with DW) structures and the dimension of 48 mm $\times48$ mm. The structure is designed by using a flexible (Polyethylene Terephthalate ) PET substrate with permittivity of 0.2. The operating frequency is between 0.9 GHz and 2.7 GHz. The advancement of slotted overlaying MCSRR with DW structures has successfully miniaturized the chipless RFID tag structure to about $107~{\mathrm {mm}}^{2}$ / bit, $0.02\lambda _{mid}^{2}/\mathrm {bit}$ and 0.09 GHz/bit by maximizing the number of resonators in a limited space and minimizing the frequency separation between the resonators. The omnidirectional tag antenna is incorporated with the proposed MCSRR structure using the retransmission measurement method. The log-periodic antenna with a gain of 5–7 dBi is used for this measurement to improve the range distance between tag and reader system. Based on the retransmission measurement involving the antenna tag, the 19 bits chipless RFID tag which consists of five MCSRR with DW structures can be detected with a maximum range distance of 30 cm and a power transmitted level of 30 dBm.

Scalable Ultrahigh-Speed Fabrication of Uniform Polycrystalline Thin Films for Organic Transistors.

The fabrication of organic semiconductor thin films by printing technologies is expected to enable the low-cost production of devices such as flexible display drivers, RF-ID tags, and various chemical/biological sensors. However, large-scale high-speed fabrication of uniform semiconductor thin films with adequate electrical properties for these devices remains a big challenge. Herein, we demonstrate an ultrafast and scalable fabrication of uniform polycrystalline thin films with 100% surface coverage using liquid crystalline semiconductors such as 2-phenyl-7-decyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) and 2.7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8), at a rate of 3 orders of magnitude higher than before, i.e., 40 mm/s (2.4 m/min) or more by dip-coating in the drainage regime. Organic transistors fabricated with polycrystalline thin films of Ph-BTBT-10 show average mobilities of 4.13 ± 0.75 cm2/(V s) in the bottom-gate-bottom-contact configuration and 10.90 ± 2.40 cm2/(V s) in the bottom-gate-top-contact configuration comparable to those of the devices prepared with single-crystalline thin films. More importantly, these films almost maintain the FET performance when the substrate size is extended up to 4 square inch. The present findings are available for other liquid crystalline semiconductors and bring us one step closer to the realization of printed electronics.

Long read range and flexible UHF RFID tag antenna made of high conductivity graphene‐based film

An antenna made of a graphene-based film with organic polyimide precursor of high conductivity 1.1 × 106 S m−1 and thickness 30 μm, operating in the ultrahigh frequency (UHF) band for radio frequency identification applications is presented in this article. The antenna is optimized to have a conjugate match to the impedance of the chip by tuning the design parameters. Tags are fabricated and tested using the designed antenna, which are shown to have realized gain above −1.5 dBi and radiation efficiency beyond 90% in the whole UHF band from 860 to 960 MHz. The read range of proposed tag is greater than 12.3 m over the entire UHF band with a maximum value of 14 m at 920 MHz. In addition, the flexibility of the tags is demonstrated. After 2000 cycles of bending and stretching, the read range only decreases by 4.5 m comparing to the initial state at 915 MHz.

A 3-bit fully inkjet-printed flexible chipless RFID for wireless concentration measurements of liquid solutions

A fully inkjet-printed flexible chipless RFID tag is presented in this paper. It is based on a coplanar waveguide (CPW) coupled to a multiresonator circuit to encode the information in the frequency domain. Two cross-polarized ultra-wideband (UWB) antennas connected to the CPW receive and transmit the signals. As a proof-of-concept, three spiral resonators are used to encode a 3-bit signature, which can be easily expanded to more bits by adding more resonators. The RFID tag is applied for wireless concentration measurements of liquid solutions by characterizing the insertion loss response. Water/sodium chloride (NaCl) solution and water/isopropanol solution are measured wirelessly by the sensor. A capillary tube is placed on one of the resonators to allow the interaction between the sensor and the solutions. By the observations of measurements, different parameters are used to quantify the sensitivity. The change of the insertion loss at the resonant frequency |ΔS21| and the half-power 3-dB bandwidth, ΔBW are used to analyze the water/NaCl solution. |ΔS21| and the shift of resonant frequency, |Δfres| are used to analyze the water/isopropanol sample. The proposed RFID shows the potential for measuring liquid solutions wirelessly.

Beyond Respiration

Sleep sound-activities including snore, cough and somniloquy are closely related to sleep quality, sleep disorder and even illnesses. To obtain the information of these activities, current solutions either require the user to wear various sensors/devices, or use the camera/microphone to record the image/sound data. However, many people are reluctant to wear sensors/devices during sleep. The video-based and audio-based approaches raise privacy concerns. In this work, we propose a novel system TagSleep to address the issues mentioned above. For the first time, we propose the concept of two-layer sensing. We employ the respiration sensing information as the basic first-layer information, which is applied to further obtain rich second-layer sensing information including snore, cough and somniloquy. Specifically, without attaching any device to the human body, by just deploying low-cost and flexible RFID tags near to the user, we can accurately obtain the respiration information. What's more interesting, the user's cough, snore and somniloquy all affect his/her respiration, so the fine-grained respiration changes can be used to infer these sleep sound-activities without recording the sound data. We design and implement our system with just three RFID tags and one RFID reader. We evaluate the performance of TagSleep with 30 users (13 males and 17 females) for a period of 2 months. TagSleep is able to achieve higher than 96.58% sensing accuracy in recognizing snore, cough and somniloquy under various sleep postures. TagSleep also boosts the sleep posture recognition accuracy to 98.94%.

Self-Assembled Functionally Graded Graphene Films with Tunable Compositions and Their Applications in Transient Electronics and Actuation.

The facile fabrication of functionally graded all-graphene films using a single-step casting process is reported. The films consist of a self-assembled graphene oxide (GO) precursor that can be reduced to different levels on an active metal substrate. Control of processing conditions such as the underlying substrate metal and the film-drying environment results in an ability to tailor the internal architecture of the films as well as to functionally grade the reduction of GO. A gradient arrangement within each film, where one side is electrically conductive reduced GO (rGO) and the other side is insulating GO, was confirmed by scanning electron microscopy, Raman, X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy characterization studies. All-graphene-based freestanding films with selectively reduced GO were used in transient electronic applications such as flexible circuitry and RFID tag antennas, where their decommissioning is easily achieved by capitalizing on GO's ability to readily dissociate and create a stable suspension in water. Furthermore, the functionally graded structure was found to exhibit differential swelling behavior, and its potential applications in graphene-based actuators are outlined.

Elliptical slot based polarization insensitive compact and flexible chipless RFID tag

A miniaturized, polarization insensitive, and fully passive chipless radio frequency identification tag is proposed in this research article. The realized tag is based on slotted elliptical structures in a nested loop fashion with identical lengths and widths of slot resonators. Alteration of data sequence is accomplished by addition and elimination of nested resonators in the geometric structure. The tag is capable to encode 10 bits of data and covers spectral range from 3.6 to 15.6 GHz. The formulated structure demonstrates polarization insensitive characteristic. The data encoding structure is analyzed and optimized for different substrates that are, Rogers RT/duroid/5880, Rogers RT/duroid/5870, and Taconic TLX‐0 over the miniaturized footprint of 22.8 × 16 mm2. The presented tag is robust, novel, compact, and flexible exhibiting a stable response to impinging electromagnetic waves at various angles of incidence.

Oxide TFT Rectifiers on Flexible Substrates Operating at NFC Frequency Range

This paper presents the experimental characterization of different rectifier circuits using indium–gallium–zinc-oxide thin-film transistor technologies either at NFC or a high frequency range (13.56 MHz) of RFID. These circuits include a single ended rectifier, its differential counterpart, a bridge rectifier, and a cross-coupled full wave rectifier. Diodes were implemented with transistors using conventional processing steps, without requiring short channel devices ( $L =15\,\,\mu \text{m}$ ). Hence, there is no need for either extra masks or processing steps unlike the Schottky diode-based implementation. These circuits were fabricated on a PEN substrate with an annealing temperature not exceeding 180 °C. This paper finds a direct application in flexible low-cost RFID tags since they enable integration of the required electronics to implement tags with the same fabrication steps.

Flexible ultra-wideband antenna incorporated with metamaterial structures: multiple notches for chipless RFID application

A coplanar waveguide (CPW) ultra-wideband (UWB) antenna incorporated with metamaterial—split ring resonator structure—that operates from 3.0 to 12.0 GHz is proposed for chipless RFID tag. The 30 mm × 40 mm flexible chipless RFID tag is designed on the fleece substrate (er = 1.35, thickness = 1 mm and tan \( \delta \) = 0.025). A six-slotted modified complementary split ring resonator (MCSRR) is introduced into the ultra-wideband antenna to produce multiple band notches at 3.0, 4.0, 5.0, 6.0 and 7.0 GHz. The frequency shifting technique is introduced for designing a high-capacity chipless RFID tag with compact size. Each MCSRR is able to code in four different allocations (00, 01, 10 and 11). To achieve encoding of 10-bits data (10,234 number), six MCSRRs are proposed with three-slotted MCSRR in the radiator and three-slotted MCSRR in the ground plane.

Design and testing of RFID sensor tag fabricated using inkjet-printing and electrodeposition

The passive RFID tag with an added sensing function is of interest to many applications. In particular, applications where RFID tagging is already considered to be the next step, such as food items, are a specific target. This paper demonstrates a flexible RFID tag sensor fabricated using a low cost technique with an added zero-cost sensing function. It is more specifically applied to the sensing of degradable food, in particular beef meat in our demonstrated example. To reach this, the antenna is designed in such a way to be sensitive to the variation of the dielectric permittivity of the meat over time. The design of the sensing tag as well as its fabrication process are described. The fabrication involves inkjet printing of a silver nanoparticle based ink on a commercial low cost PET film to create a seed layer. It is followed by a copper electrodeposition step on top of the silver pattern to complete the tag to obtain the desired thickness and conductivity of the tag antenna. The results of the electrical tests showed that with the inkjet printing–electrodeposition combination it is possible to produce flexible electrically conductive patterns for practical RFID applications. The tag was then tested in close-to-real-world conditions and it is demonstrated that it can provide a sensing function to detect the consumption limit of the packaged beef.

Study of Voids in the Flexible RFID Tag Inlays Packaged by Anisotropic Conductive Adhesive

RFID is recognized one of the most potential information technologies. The RFID tag is a small tag containing an integrated circuit chip and an antenna. Voids can very often be detected in the non-metalized area of the RFID tags, they generated with unpredictable size and located randomly in the tag. The formation of the voids is the combined action of material properties and bonding parameters. In this work, the formation of the material related voids in the tags was investigated by thermogravimetric analysis. All specimens showed continuous loss of mass to varying degrees during the heating process. The air and moisture entrapped in the polymer matrix in the fabrication process and the process of use are the mainly reason. The loss mass of the etched antenna substrate primarily came from the lamination adhesive has not cured completely. With the simultaneous reaction of thermal stresses and internal vapor/volatile gas pressure drives both pre-existing and newly nucleated voids to grow. In addition, the voids growth under the high temperature (85°C) and high humidity (85%RH) conditions was investigated. The characteristic size of voids increases gradually with the increasing aging time because of the combined effects of the residual stress and the different coefficients of thermal expansion and moisture expansion.

Flexible Self-Aligned Amorphous InGaZnO Thin-Film Transistors With Submicrometer Channel Length and a Transit Frequency of 135 MHz

Flexible large area electronics promise to enable new devices such as rollable displays and electronic skins. Radio frequency (RF) applications demand circuits operating in the megahertz regime, which is hard to achieve for electronics fabricated on amorphous and temperature sensitive plastic substrates. Here, we present self-aligned amorphous indium-gallium-zinc oxide-based thin-film transistors (TFTs) fabricated on free-standing plastic foil using fabrication temperatures . Self-alignment by backside illumination between gate and source/drain electrodes was used to realize flexible transistors with a channel length of 0.5 μm and reduced parasitic capacities. The flexible TFTs exhibit a transit frequency of 135 MHz when operated at 2 V. The device performance is maintained when the TFTs are bent to a tensile radius of 3.5 mm, which makes this technology suitable for flexible RFID tags and AM radios.

Small and Flexible Metal Mountable Passive UHF RFID Tag on High-Dielectric Polymer-Ceramic Composite Substrate

A small and flexible metal mountable UHF RFID tag antenna, utilizing a high-permittivity substrate material is presented. The tag is composed of a small single-layer T-matched dipole antenna, on a flexible ceramic (BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) polymer (polydimethylsiloxane) composite substrate, with a thickness of 1.5 mm. The flexibility of the substrate allows it to be mounted on flat and cylindrical metallic surfaces. The performance of the developed tag is evaluated through simulations and measurements on both flat and cylindrical metallic platforms of various sizes. The results show that the tag achieves state-of-the-art size-performance ratio while meeting the key requirements of an affordable RFID tag with a simple and flexible structure.

Design of a Flexible Planar RFID Tag Antenna with Low Performance Degradation from Nearby Target Objects

In this letter, we propose a novel tag antenna that has low performance degradation with nearby dielectric material. We obtained a stable reading performance and a broad matching bandwidth on nearby dielectric materials by employing a T-matching network with thick line width and capacitively slot-loaded arms. We then built the proposed antenna and measured the tag sensitivity to examine the reading characteristics with nearby dielectric materials. The measured results clearly demonstrate stable tag sensitivity with various nearby dielectric materials, such as foam, acrylic-plastic, glass, and ceramic plates. To more closely observe the antenna characteristics with nearby dielectric materials, we also examined the impedance variation and surface current distribution with respect to the dielectric constant of nearby target objects, which ranged from <TEX>$1{\times}{\varepsilon}_0$</TEX> to <TEX>$16{\times}{\varepsilon}_0$</TEX>.

Performance of Printed Polymer-Based RFID Antenna on Curvilinear Surface

The performance of flexible printed RFID tags affixed onto cylindrical containers is dependent on the inductive behavior of the bent antenna on the tag. Conductive polymeric coil antennas were screen printed onto flexible substrates, and the coil resistances, the inductances, and the S-parameters of the antenna coils were measured and analyzed. The RFID dies were mounted onto the antenna coils and the read ranges were characterized as a function of curvature. The results show that the coil inductance decreased slowly with increasing curvature, and the maximum read range of the tags was markedly reduced with the curvature. The decrease in the coil inductance and the maximum read range were hypothesized to vary with the projected bent coil area instead of the geometric coil area. Experimental results confirmed that the maximum read range of an RFID tag affixed on a curvilinear surface can be predicted by the classical inductive coupling model with the bent projected coil area. On the basis of the experimental and analytical results, a reading reliability factor of two is proposed as a design parameter for flexible RFID tags.