Wireless Strain Research Articles

Biaxially stretchable metamaterial absorber with a four-dimensional printed shape-memory actuator

Among the various methods for strain sensing, the metamaterial absorbers (MMAs) stand out due to their dual capabilities. Specifically, MMAs facilitate the wireless detection of deformations in the target and operate independently of any external power source. However, conventional research has a limitation in that stretchable strain sensors are unable to deform themselves autonomously, which puts constraints on being efficiently utilised in special environments where human intervention is difficult. Herein, we propose a wireless, power-independent, biaxial strain sensor equipped with self-shape and frequency recovery capability that addresses the limitations of existing wireless strain sensors through the unprecedented integration of a 4D-printed shape memory actuator and a biaxially stretchable MMA. The novel integration with the shape memory actuator enables the stretchable MMA to autonomously recover to its original shape and absorption frequency after being heated to 70 °C for a few minutes. This smart functionality enables the resulting wireless strain sensor based on the proposed idea to revert to the original state when sensing a new target without requiring human intervention. The highly sensitive biaxial sensing capability is as follows. When stretched horizontally from 0 % to 30 %, the absorption frequency of the proposed biaxially stretchable MMA demonstrates a linear change from 9.75 GHz to 7.94 GHz, exhibiting a high sensitivity of 4.3 × 10^7 Hz/%. Similarly, when stretched vertically from 0 % to 30 %, the absorption frequency linearly changes from 7.35 GHz to 6.01 GHz, indicating a sensitivity of 5.9 × 10^7 Hz/%. Accordingly, the wireless biaxial sensing capability of the proposed stretchable MMA, as well as its shape-recovery functionality facilitated by the 4D-printed actuator are highly effective for remote strain measurement in environments where direct human involvement is impractical.

Frequency‐Locked Wireless Multifunctional Surface Acoustic Wave Sensors

AbstractSurface acoustic waves (SAWs) have shown great potential for developing sensors for structural health monitoring (SHM) and lab‐on‐a‐chip (LOC) applications. Existing SAW sensors mainly rely on measuring the frequency shifts of high‐frequency (e.g., >0.1 GHz) resonance peaks. This study presents frequency‐locked wireless multifunctional SAW sensors that enable multiple wireless sensing functions, including strain sensing, temperature measurement, water presence detection, and vibration sensing. These sensors leverage SAW resonators on piezoelectric chips, inductive coupling‐based wireless power transmission, and, particularly, a frequency‐locked wireless sensing mechanism that works at low frequencies (e.g., <0.1 GHz). This mechanism locks the input frequency on the slope of a sensor's reflection spectrum and monitors the reflection signal's amplitude change induced by the changes of sensing parameters. The proof‐of‐concept experiments show that these wireless sensors can operate in a low‐power active mode for on‐demand wireless strain measurement, temperature sensing, and water presence detection. Moreover, these sensors can operate in a power‐free passive mode for vibration sensing, with results that agree well with laser vibrometer measurements. It is anticipated that the designs and mechanisms of the frequency‐locked wireless SAW sensors will inspire researchers to develop future wireless multifunctional sensors for SHM and LOC applications.

Highly Robust and Self-Adhesive Soft Strain Gauge via Interface Design Engineering.

Highly robust soft strain gauges are rapidly emerging as a promising candidate in the fields of vital signs and machine conditions monitoring. However, it is still a key challenge to achieve high-performance strain sensing in these sensors with mechanical/electrical robustness for long-term usage. The multilayer structural design of sensors enhances sensing performance while the interfacial connection of heterogeneous materials between different layers is weak. Herein, inspired by the efficient perception mechanism of scorpion slit sensilla with tough interface interconnections, the synergy of ultra-high electrical performance and mechanical robustness is successfully achieved via interface design engineering. The developed multilayer soft strain gauge (MSSG) exhibits a strain sensitivity beyond 105, a lower detection limit of 8.3µm, a frequency resolution within 0.1Hz, and cyclic stability over 63000 strain cycles. Also, the tough interface improves the level of heterogeneous integration in the MSSG which allows to endure different stresses. Furthermore, an MSSG-based wireless strain monitoring system is developed that enables applications on different complex dynamic surfaces, including accurate identification of human throat activity and monitoring of rolling bearing conditions.

Flexible Graphene Film-Based Antenna Sensor for Large Strain Monitoring of Steel Structures.

In the field of wireless strain monitoring, it is difficult for the traditional metal-made antenna sensor to conform well with steel structures and monitor large strain deformation. To solve this problem, this study proposes a flexible antenna strain sensor based on a ductile graphene film, which features a 6.7% elongation at break and flexibility due to the microscopic wrinkle structure and layered stacking structure of the graphene film. Because of the use of eccentric embedding in the feeding form, the sensor can be miniaturized and can simultaneously monitor strain in two directions. The sensing mechanism of the antenna is analyzed using a void model, and an antenna is designed based on operating frequencies of 3 GHz and 3.5 GHz. The embedding size is optimized using a Smith chart and impedance matching principle. Both the simulation and experimental results verify that the resonant frequency and strain magnitude are linearly inversely proportional. The experimental results show that the strain sensitivity is 1.752 kHz/με along the geometric length and 1.780 kHz/με along the width, with correlation coefficients of 0.99173 and 0.99295, respectively.

Advancing Wireless Strain Sensing with Low-cost Printable Supersensitive Radiofrequency Patches

Engineering structures experience changes in material and functional properties due to structural damage, induced by sudden events or gradual degradation over time. Structural Health Monitoring (SHM) is crucial, utilizing real-time monitoring systems with various sensors. Suitable sensors for monitoring structures are vital for future economies. They should feature sensitivity, cost-effectiveness, easy integration, and reduced complexity. Current strain sensors, like strain gauges and Fiber-optic Brag Gratings (FBG), are sensitive but have drawbacks, including wiring complexities, installation time, and limited lifetimes. Introducing wireless sensors as a solution, enables autonomous strain monitoring, overcoming the drawbacks of traditional wired sensors. Chipless Radio Frequency (RF) technology, specifically passive RF sensors, stands out for smart structure development. These sensors, relying on RF readers for power, offer compactness, non-intrusiveness, longer lifetimes, and cost-effectiveness, making them promising for monitoring large structures. This research aims to develop an innovative chipless RF strain sensor for wireless communication, emphasizing enhanced sensitivity and seamless integration into real structures for integrity monitoring. First, the structure and processing technique of such sensors will be presented. Second, an external readout coil connected to a vector network analyzer was used to make electromagnetic coupling with the receiver coil in the sensor and resonate the sensor on its resonant frequency. When experiencing strain, the signal activates a transmission line mechanism, increasing the attenuation along the capacitive element. The consequence is a strong variation in the effective sensor length making a remarkable variation in the resonant frequency. We demonstrate the exceptional gauge factor in the resonant frequency at different strain levels, which gives our sensor a big advantage, compared to pure resistive sensors and geometrically related capacitive sensors. We also demonstrate that such sensors can be fabricated in large quantities at very low cost as they are by design compatible with classical printing technologies.

Biocomposite silk fibroin hydrogel with stretchability, conductivity and biocompatibility for wireless strain sensor

As a natural biopolymer material, silk fibroin with unique mechanical properties can be used in the preparation of biocomposite hydrogels for strain sensors. But, the electromechanical properties of biocomposite hydrogel strain sensors are still insufficient, such as the deterioration of electrical signals and low sensitivity, which need to develop a hydrogel with a stable transmission network for electric conduction. Herein, a silk fibroin biocomposite hydrogel is prepared by incorporating tannic acid and MXene nanosheets into a polyacrylamide and silk fibroin double network. The electromechanical properties of hydrogels are improved by optimizing the proportion of material components. As a result, the double network structure and supramolecular interaction enhance the stretchability of hydrogels (692% fracture strain). The hydrogel also exhibits good biocompatibility and conductivity (0.85 S/m), which shows the application prospect in wearable sensors. The wireless strain sensor assembled by this biocomposite hydrogel presents good portability and sensing performance, such as high sensitivity (gauge factor = 6.04), wide working range (500% strain), and outstanding stability (1000 cycles at 100% strain). The results indicate that the hydrogel strain sensor can be used to monitor human body movement. The biocomposite hydrogel is expected to be applied in the field of wearable strain sensors, and this study can provide a new way for the design of flexible electronic materials.

Wireless wearable system based on multifunctional conductive PEG-HEMA hydrogel with anti-freeze, anti-UV, self-healing, and self-adhesive performance for health monitoring

Polymer gels, such as hydrogels, have been widely applied in biomedicine, flexible electronics, and soft robotics. However, due to the complexity of application environments, preparing hydrogels that can maintain comprehensive performance under harsh conditions remains a challenge. Herein, based on polyethylene glycol (PEG) and water as a mixed solvent and using hydroxyethyl methacrylate (HEMA) as a monomer, a conductive dual-network PEG-HEMA hydrogel with anti-freezing, anti-ultraviolet, self-healing, and self-adhesive functions was prepared. The results show that PEG-HEMA hydrogel remained unfrozen even at ‐20°C, with tensile strength and elongation at break of 49.946 KPa and 529.7%, high resolution (able to measure strains as low as 0.2%), good sensitivity and linearity (GF=1.076, R2=0.999), fatigue resistance (over 1000 cycles), and long-term storage capability (over one week). Furthermore, a small wireless wearable strain sensor system was developed by integrating the hydrogel with a microcontroller, which is capable of monitoring various human motion and physiological signals in real time and exhibiting excellent sensing performance. This work provides a straightforward approach for multifunctional hydrogels, and it is expected to attract widespread attention and applications in fields such as electronic skin, human-machine interaction, and human health monitoring.

SEX DIFFERENCE IN ABSOLUTE AND NORMALIZED FORCE AT FOUR DIFFERENT ISOMETRIC CONTRACTION INTENSITIES

Purpose: When measuring isometric contractions, providing real-time visual feedback differs from the practices in general clinical environment. In addition, even though men and women have clear physical and physiological differences, most of the existing studies analyzed absolute muscle contractions with no distinction between men and women. The aim of this study was to investigate whether there are differences in absolute and normalized hip extension forces measured without visual feedback between men and women. Methods: Twenty-eight healthy adults participated (13 men and 15 women; age=- 22.00±11.44 years; height=165.86±18.30 cm; and weight=61.91±12.34 kg) in the study. Maximum (MVC) and submaximal voluntary contraction forces (75%, 50%, and 25% of MVC, in a random order) of hip extension were measured using a wireless strain gauge and with no visual feedback. Results: Absolute contraction forces measured at four target intensities were significantly greater in men (p

A wireless, implantable bioelectronic system for monitoring urinary bladder function following surgical recovery

Partial cystectomy procedures for urinary bladder-related dysfunction involve long recovery periods, during which urodynamic studies (UDS) intermittently assess lower urinary tract function. However, UDS are not patient-friendly, they exhibit user-to-user variability, and they amount to snapshots in time, limiting the ability to collect continuous, longitudinal data. These procedures also pose the risk of catheter-associated urinary tract infections, which can progress to ascending pyelonephritis due to prolonged lower tract manipulation in high-risk patients. Here, we introduce a fully bladder-implantable platform that allows for continuous, real-time measurements of changes in mechanical strain associated with bladder filling and emptying via wireless telemetry, including a wireless bioresorbable strain gauge validated in a benchtop partial cystectomy model. We demonstrate that this system can reproducibly measure real-time changes in a rodent model up to 30 d postimplantation with minimal foreign body response. Studies in a nonhuman primate partial cystectomy model demonstrate concordance of pressure measurements up to 8 wk compared with traditional UDS. These results suggest that our system can be used as a suitable alternative to UDS for long-term postoperative bladder recovery monitoring.

Wireless Strain Gauge for Monitoring Bituminous Pavements

This paper introduces the implementation of a new device for measuring deformations at the surface layers of bituminous pavement. Using wireless technology, rechargeable remotely, low cost, and easily positioned in a layer by coring after pavement construction, this sensor makes it possible to obtain measurements of the deformation when a vehicle passes by. The development of the wireless sensor is presented as well as its advantages and limitations. It was then tested in the laboratory under a hydraulic press and in situ using a full-scale test of the mobile load simulator (MLS10 type). This system allows simple measurement, gives reliable results, and could be a useful device for the structural monitoring of pavement structures.

Inkjet-Printed Multiwalled Carbon Nanotube Dispersion as Wireless Passive Strain Sensor.

In this study, a multiwalled carbon nanotube (MWCNT) dispersion is used as an ink for a single-nozzle inkjet printing system to produce a planar coil that can be used to determine strain wirelessly. The MWCNT dispersion is non-covalently functionalized by dispersing the CNTs in an anionic surfactant, namely sodium dodecyl sulfate (SDS). The fabrication parameters, such as sonication energy and centrifugation time, are optimized to obtain an aqueous suspension suitable for an inkjet printer. Planar coils with different design parameters are printed on a flexible polyethylene terephthalate (PET) polymer substrate. The design parameters include a different number of windings, inner diameter, outer diameter, and deposited layers. The electrical impedance spectroscopy (EIS) analysis is employed to characterize the printed planar coils, and an equivalent electrical circuit model is derived based on the results. Additionally, the radio frequency identification technique is utilized to wirelessly investigate the read-out mechanism of the printed planar MWCNT coils. The complex impedance of the inductively coupled sensor undergoes a shift under strain, allowing for the monitoring of changes in resonance frequency and bandwidth (i.e., amplitude). The proposed wireless strain sensor exhibits a remarkable gauge factor of 22.5, which is nearly 15 times higher than that of the wireless strain sensors based on conventional metallic strain gauges. The high gauge factor of the proposed sensor suggests its high potential in a wide range of applications, such as structural health monitoring, wearable devices, and soft robotics.

An Environment-Tolerant Ion-Conducting Double-Network Composite Hydrogel for High-Performance Flexible Electronic Devices

HighlightsNovel double-network (DN) ion-conducting hydrogel (ICH) based on a poly(ionic liquid)/MXene/poly(vinyl alcohol) system (named PMP DN ICH) was synthesized using freeze–thawing and ionizing radiation technology.The PMP DN ICH possesses a multiple cross-linking mechanism and exhibits outstanding ionic conductivity (63.89 mS cm−1), excellent temperature resistance (−60–80 °C) and decent mechanical performance.The well-designed PMP DN ICH shows considerable potential in wearable sensing, energy storage, and energy harvesting.

An anti-swelling chitosan-based hydrogel driven by balance of hydrophilic segment and hydrophobic segment strategy for underwater detection of human motion

Hydrogel-based flexible strain sensors have gained rapid development. However, most of traditional hydrogels were prone to undergo swelling behavior underwater as solvent molecules continuously penetrated, which limited their application scenarios. To make up this deficiency, a chitosan-based anti-swelling hydrogel cross-linked by ionic liquid are presented. The hydrogel is consisted of chitosan (CS) and copolymer of acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA), butyl acrylate (BA) and dimethylaminoethyl methacrylate maleate (DMAEMA-MA). The P(HEMA-BA-AA)/ILs/CS hydrogel exhibited excellent anti-swelling properties driven by balance of hydrophilic segment and hydrophobic segment strategy (equilibrium swelling rate of −3.79 % in water for 100 days). The ionic liquid was introduced into the hydrogel system as cross-linking agent, which could effectively prevent the loss of conductivity of the hydrogel after soaking in water. The synergy effect chain entanglement, H-bonds and electrostatic interactions endowed the hydrogel with excellent mechanical property. Surprisingly, the modulus and toughness of the hydrogel were significantly boosted after a week of submersion. Thus, this anti-swelling hydrogel was employed as wireless strain sensor, which implemented underwater motion monitoring and safety alarms. It is expected that the anti-swelling hydrogel driven by balance of hydrophilic and hydrophobic effects would inspire the development of underwater sensors.

The effect of tensile deformation of knitted fabrics on their electromagnetic shielding ability, electrical resistance, and porosity

Nowadays, electrically conductive textile materials are widely used also for sensing applications in addition to being used as antistatic, electromagnetic shielding, for creating smart textiles, etc. The main aim of this paper is to study the effect of tensile deformation applied on knitted fabrics on their electromagnetic shielding ability, electrical resistance, and porosity to gain knowledge for the construction of textile-based wireless strain sensors. For the experiment, silver-coated yarn was chosen to produce knitted fabrics with two different patterns and three levels of stitch densities. The uniaxial and biaxial deformation was applied to samples and at the same time, the change of electromagnetic shielding ability, electric resistance, and porosity of the sample set was evaluated. It can be summarized, that the vertical stretch has the highest positive effect on the electromagnetic shielding ability and the maximum shielding sensitivity is 12 % compared to other deformation types. In general, the electrical resistance decreases during increased stretch due to the increasing number of contacts between electrically conductive yarns, which causes a decrease in the contact resistance and also a decrease in total electrical resistance. The highest positive effect on the porosity of samples represents biaxial deformation. The finding that the overall shielding efficiency is positively influenced by the electrical conductivity of the sample and at the same time negatively influenced by the increasing porosity during tensile deformation was the motivation to construct a simple regression model for the prediction of the electromagnetic shielding ability of the sample during its extension.

Modeling and Signal Processing of Bulk Acoustic Wave Passive Wireless Strain Sensors

Modeling and Signal Processing of Bulk Acoustic Wave Passive Wireless Strain Sensors

Wireless Strain and Temperature Monitoring in Reinforced Concrete Using Surface Acoustic Wave Sensors

Wireless Strain and Temperature Monitoring in Reinforced Concrete Using Surface Acoustic Wave Sensors

Skin-interfaced self-powered pressure and strain sensors based on fish gelatin-based hydrogel for wireless wound strain and human motion detection

Biomass-based hydrogels, due to their excellent biocompatibility, can behave as wearable monitoring platforms for healthcare applications and triboelectric sensing devices. However, low tissue adhesiveness, poor stretchability, and bacterial susceptibility cause them to fail to adapt to specific joints with complex movements. Herein, we report a novel biomass-based hydrogel by integrating fish gelatin into polymer networks with in-situ formation of silver nanoparticles (denoted as FG-Ag hydrogel), which achieves great stretchability (2600%), excellent sensitivity (gauge factor = 4), and strong self-adhesion simultaneously. The FG-Ag hydrogel also features a robust antibacterial activity for Escherichia coli and Staphylococcus aureus. Benefitting from these advantages, we have developed a FG-Ag hydrogel-based wearable strain sensor for diverse human-motion detections such as elbow, knee, finger, and wrist bending. Moreover, we have demonstrated an FG-Ag hydrogel-based prototype with an integrated wireless system for real-time strain monitoring on the wound sites. The FG-Ag hydrogel could significantly reduce bacterial infection in vivo and effectively promote wound healing. Additionally, the self-powered pressure sensor and the biomechanical energy harvester also have been demonstrated by the FG-Ag hydrogel based triboelectric nanogenerator (TENG). Accordingly, such FG assistance of hydrogel-based skin-interfaced electronics could provide adequately delicate biomechanical information related to the general health, which furnishes essential technical support for cost-effective, all-green, and highly precise personalized health assessment.

Review of Wireless RFID Strain Sensing Technology in Structural Health Monitoring.

Strain-based condition evaluation has garnered as a crucial method for the structural health monitoring (SHM) of large-scale engineering structures. The use of traditional wired strain sensors becomes tedious and time-consuming due to their complex wiring operation, more workload, and instrumentation cost to collect sufficient data for condition state evaluation, especially for large-scale engineering structures. The advent of wireless and passive RFID technologies with high efficiency and inexpensive hardware equipment has brought a new era of next-generation intelligent strain monitoring systems for engineering structures. Thus, this study systematically summarizes the recent research progress of cutting-edge RFID strain sensing technologies. Firstly, this study introduces the importance of structural health monitoring and strain sensing. Then, RFID technology is demonstrated including RFID technology's basic working principle and system component composition. Further, the design and application of various kinds of RFID strain sensors in SHM are presented including passive RFID strain sensing technology, active RFID strain sensing technology, semi-passive RFID strain sensing technology, Ultra High-frequency RFID strain sensing technology, chipless RFID strain sensing technology, and wireless strain sensing based on multi-sensory RFID system, etc., expounding their advantages, disadvantages, and application status. To the authors' knowledge, the study initially provides a systematic comprehensive review of a suite of RFID strain sensing technology that has been developed in recent years within the context of structural health monitoring.

High-Sensitivity RFID Sensor for Structural Health Monitoring.

Structural health monitoring (SHM) is crucial for ensuring operational safety in applications like pipelines, tanks, aircraft, ships, and vehicles. Traditional embedded sensors have limitations due to expense and potential structural damage. A novel technology using radio frequency identification devices (RFID) offers wireless transmission of highly sensitive strain measurement data. The system features a thin, flexible sensor based on an inductance-capacitance (LC) circuit with a parallel-plate capacitance sensing unit. By incorporating tailored cracks in the capacitor electrodes, the sensor's capacitor electrodes become highly piezoresistive, modifying electromagnetic wave penetration. This unconventional change in capacitance shifts the resonance frequency, resulting in a wireless strain sensor with a gauge factor of 50 for strains under 1%. The frequency shift is passively detected through an external readout system using simple frequency sweeping. This wire-free, power-free design allows easy integration into composites without compromising structural integrity. Experimental results demonstrate the cracked wireless strain sensor's ability to detect small strains within composites. This technology offers a cost-effective, non-destructive solution for accurate structural health monitoring.

Fabrication and verification of vertical graphene-based strain sensor

The purpose of structural health monitoring in civil and mechanical engineering infrastructure is to make a structure safer at a lower cost. Strain, one of the crucial monitoring variables, has an innate relationship with deformation and is sensitive to the degeneration of load-bearing structural elements. In this study, a set of wireless real-time data acquisition and strain monitoring systems based on vertical graphene (VG) were developed. Both the parametric study and the feasibility of the VG-based strain sensor in crack monitoring of concrete structures were investigated. The greatest resistance difference in the cyclic tensile test is 2.99%, and the linearity is 2.4%, proving the newly designed sensor has good repeatability and stability. According to the monotonic tensile test based on a micro-displacement control platform, the relative resistance to change ΔR/R rises as applied strain rises and the repeatability error results in 0.576%. According to the notched concrete beam under the three-point bending test, the VG-based strain sensor's discrepancy from the extensometer's monitoring data is within 11.04%. The findings of this study suggest that the newly developed VG-based strain sensor has the potential for practical application in field strain monitoring, thus reaffirming the feasibility and effectiveness of this novel sensing technology.