Wave Sensor Research Articles

Recent Progress in Surface Acoustic Wave Sensors Based on Low-Dimensional Materials and Their Applications

Benefitting from high sensitivity, rapid response, and cost-effectiveness, surface acoustic wave (SAW) sensors have found extensive applications across various fields, including biomedical diagnostics, environmental monitoring, and industrial automation. Recently, low-dimensional materials have shown great potential in enhancing the performance of SAW sensors due to their exceptional physical, optical, and electronic properties. This review explores recent advancements in the fundamental mechanisms, design, fabrication and applications of SAW sensors based on low-dimensional materials. Specifically, the utilization of low-dimensional materials, including zero-, one- and two-dimensional materials, as sensing materials in SAW sensors are summarized. Their applications in SAW-based gas sensing, ultraviolet light sensing, humidity sensing, as well as biosensing are discussed. Furthermore, major challenges and future perspectives regarding employing low-dimensional materials to enhance SAW sensors are highlighted, providing valuable insights for future research and development in this field.

Functional Materials Based Surface Acoustic Wave Sensors: A Mini Review

Functional Materials Based Surface Acoustic Wave Sensors: A Mini Review

Functional Materials Based Surface Acoustic Wave Sensors: A Mini Review

Functional Materials Based Surface Acoustic Wave Sensors: A Mini Review

Analysis of Multivariable Sensor Responses to Multi-Analyte Gas Samples in the Presence of Interferents and Humidity.

This work presents an adaptive sensor signal-processing approach to enable quantification, using a single gas sensor or a small sensor array, of multianalyte mixtures of aromatic hydrocarbons in the presence of various interferents and humidity for environmental-monitoring applications. Dynamic sensor responses are analyzed by extracting multivariable sensing parameters to provide necessary sensitivity and selectivity. This is achieved by integrating the Levenberg-Marquardt-modified, exponentially weighted, recursive-least-squares-estimation (LM-modified EW-RLSE) algorithm and principal-component analysis (PCA). Achieving measured detection limits as low as 3 μg/L (≤1 ppm by volume) for 6 target analytes, the system exhibits excellent PCA cluster separation for all analytes in the mixtures, with reliable identification and accurate quantification, even in the presence of various interferents. Concentration errors of approximately ±5% are obtained for mixtures containing up to 6 BTEX compounds (including chemical isomers) and up to 4 interferents. Additionally, the study investigates the impact of humidity on the polymer/plasticizer-coated shear-horizontal surface acoustic wave (SH-SAW) sensors, demonstrating accurate concentration estimation in a relative humidity range from dry nitrogen to 65%. This sensing-and-multivariate-signal-processing approach is a promising candidate for reliable environmental monitoring in real-world applications.

Single-Ended Surface Acoustic Wave Pressure Resonators

Surface acoustic wave (SAW) sensors exhibit advantageous attributes for pressure detection, including compact dimensions, cost-effectiveness, facile integration, elevated sensitivity, and a high quality factor (Q value). In this study, the single-ended resonant configuration of SAW pressure sensing element based on ST-X cut quartz was simulated and fabricated. The influence of the interdigitated transducer (IDT) and applied pressure on the resonance frequency of SAW were simulated and analyzed. The designed phase velocity (3159.344 m s−1) without IDT is closest to the theoretical phase velocity (3158 m s−1) of SAW propagation in the substrate, and the relative error is about 0.043%. The designed phase velocity of SAW dropped to 3149.198 m s−1 due to the loading of the IDT mass. With an increase in applied pressure from 0 to 350 kPa, the resonance frequency of the SAW decreases from 157.05 to 154.02 MHz, yielding a maximum linear pressure sensitivity of approximately 8.6 kHz kPa−1. The measured center frequencies of the fabricated single-ended SAW devices are predominantly clustered around 157 MHz, exhibiting a deviation of 0.46 MHz from the simulated results. The present work establishes a foundation for subsequent experimental investigations into pressure sensing.

Machine-Learning-Based Depression Detection Model from Electroencephalograph (EEG) Data Obtained by Consumer-Grade EEG Device.

Background/Objectives: There have been attempts to detect depression using medical-grade electroencephalograph (EEG) data based on a machine learning approach. EEG has garnered interest as a method for assessing brainwaves by attaching electrodes to the scalp to obtain electrical activity in the brain. Recently, machine learning has been applied to the EEG data to detect depression, with encouraging results. Specifically, studies using medical-grade EEG data have shown that depression can be accurately detected. However, there is a need to expand the range of applications by achieving a score with machine learning using simpler consumer-grade brain wave sensors. At present, a sufficient score has not been achieved.; Methods: To improve the score of depression detection, we quantified various EEG indices to train models such as power spectrum, asymmetry, complexity, and functional connectivity. In addition, feature selection was performed to ensure that the model learns only promising EEG indices for depression detection. The feature selection methods were Light Gradient Boosting Machine (LightGBM) feature importance, mutual information, ReliefF and ElasticNet coefficients. The selected EEG indices were learned by the LightGBM model, which is reported to be as accurate as the latest deep learning models. In cross-validation, the independence of test and training data was ensured to avoid excessively calculated score; Results: The results showed that the Macro F1 score was 91.59%, suggesting that a consumer-grade EEG can detect depression. In addition, analysis of the EEG indices selected by feature selection indicated that the Macro F1 score was about 80% for single EEG indices such as differential entropy in the frequency band β and functional connectivity in the left frontal region in the frequency band 1-128 Hz; Conclusions: Although the data were obtained from a consumer-grade EEG, the results suggest that these EEG indices are promising for detection depression.

A SAW Wireless Passive Sensing System for Rotating Metal Parts

Passive wireless surface acoustic wave (SAW) sensors are very useful for on-site monitoring of the working status of machines in complex environments, such as high-temperature rotating objects. For rotating parts, it is difficult to realize real-time and continuous monitoring because of the unstable sensing signal caused by the continuous change of the relative position of the rotating part to the sensor and shielding of the signal. In our SAW sensing system, we propose a loop antenna integrated with the rotating part to obtain a stable sensing signal owing to its omnidirectional radiation pattern. Methodologies for determining the antenna dimension, system operating frequency, and procedures for designing a SAW sensor tag are discussed in this paper. By fully utilizing the influence of metal rotor on antenna performance, the antenna needs no impedance matching elements while it provides sufficient gain, which equips the antenna with nearly zero temperature drift at a wide temperature-sensing range. Experimental verification results show that this sensing system can greatly improve the stability of the sensing signal significantly and can achieve a temperature sensing accuracy of ~1 °C at different rotational speeds, demonstrated by the feasibility of the loop antenna for monitoring the working status of rotating metal parts.

Physical Sensors Based on Lamb Wave Resonators

A Lamb wave is a guided wave that propagates within plate-like structures, with its vibration mode resulting from the coupling of a longitudinal wave and a shear vertical wave, which can be applied in sensors, filters, and frequency control devices. The working principle of Lamb wave sensors relies on the excitation and propagation of this guided wave within piezoelectric material. Lamb wave sensors exhibit significant advantages in various sensing applications due to their unique wave characteristics and design flexibility. Compared to traditional surface acoustic wave (SAW) and bulk acoustic wave (BAW) sensors, Lamb wave sensors can not only achieve higher frequencies and quality factors in smaller dimensions but also exhibit superior integration and multifunctionality. In this paper, we briefly introduce Lamb wave sensors, summarizing methods for enhancing their sensitivity through optimizing electrode configurations and adjusting piezoelectric thin plate structures. Furthermore, this paper systematically explores the development of Lamb wave sensors in various sensing applications and provides new insights into their future development.

Angle-dependent ion-beam etching of RuAl thin films for structuring GHz-frequency electronics

The ruthenium aluminide (RuAl) alloy is a promising electrode material for wireless surface acoustic wave sensors working under harsh conditions at high temperatures. However, during the structuring of RuAl thin films using ion-beam etching, etched material can redeposit at the edges of the electrodes and form objects, so-called fences, on top of the structured features. These decrease the high-temperature stability and lead to an undesired alteration of the sensor performance. In this work, the angle-dependent ion-beam etching of RuAl thin films was investigated to inhibit the formation of such fence structures. The etch rate was determined as a function of the etching angle between ion-beam and sample surface in a range between 90° and 40°. Furthermore, finger structures with pitches below 500 nm, which are required for devices working in the intended GHz regime, were patterned to study the influence of the etching angle on the profile of the RuAl electrode fingers using transmission electron microscopy and energy-dispersive x-ray spectroscopy. The results show that an etching angle of 50° results in the highest etch rate. For etching angles of 50° and below, the width of the fences is reduced below 10 nm, so they break off during standard resist removal procedures. Such low etching angles lead to shadowed areas on the side of structured features in which unetched material remains. However, this material can be removed by using a two-step etching process combining a 50° step with a 90° step. This process is capable of structuring fence-free trapezoidal-shaped electrode finger profiles. Therefore, the developed process is well suited for the fabrication of high-temperature GHz-frequency RuAl electrodes.

Monitoring Gene Sequences of Staphylococcus aureus Using a Love-Mode Surface Acoustic Wave Biosensor Coated with Cellulose Acetate/Polyethylenimine Nanofibers and Au Nanoparticles.

Love-mode surface acoustic wave (SAW) sensors show great promise for biodetection applications owing to their low cost, digital output, and wireless passive capability, but their performance is often restricted by the availability of suitable sensitive membrane layers. Herein, a composite layer of electrospun fibers made from cellulose acetate and polyethylenimine, coated with gold nanoparticles, is proposed as a porous and sensitive membrane coated onto a love-mode SAW biosensor for monitoring gene sequences of Staphylococcus aureus. The results showed that the developed sensor exhibited an impressive sensitivity of 122.56 Hz/(nmol/L) for detecting gene sequences of S. aureus, surpassing the sensitivity of conventional SAW sensors employing a bare Au film as the sensitive layer by 5-fold. The analysis revealed a remarkably linear detection (R2 of 0.97827) of S. aureus gene sequences within the range of 0 to 100 nmol/L. The limit of detection was impressively low at 0.9116 nmol/L. The good stability and specificity of the biosensor in liquid environments were demonstrated for clinical diagnostics.

Rapid detection of depression by volatile organic compounds from exhalation

Depression is a pervasive and often undetected mental health condition, which poses significant challenges for early diagnosis due to its silent and subtle nature. To evaluate exhaled volatile organic compounds (VOCs) as non-invasive biomarkers for the detection of depression using a virtual surface acoustic wave sensors array (VSAW-SA). A total of 245 participants were recruited from the Hangzhou Community Health Service Center, including 38 individuals diagnosed with depression and 207 control subjects. Breath samples were collected from all participants and subjected to analysis using VSAW-SA. Univariate and multivariate analyses were employed to assess the relationship between VOCs and depression. The findings revealed that the responses of virtual sensor ID 14, 44, 59, and 176, which corresponded respectively to ethanol, trichloroethylene or isoleucine, octanoic acid or lysine, and an unidentified compound, were sensitive to depression. Taking into account potential confounders, these sensor responses were utilized to calculate a depression detection indicator. It has a sensitivity of 81.6% and a specificity of 81.6%, with an area under the curve of 0.870 (95% CI = 0.816-0.923). Conclusions: exhaled VOCs as non-invasive biomarkers of depression could be detected by a VSAW-SA. Large-scale cohort studies should be conducted to confirm the potential ability of the VSAW-SA to diagnose depression.

Systematic approach for high piezoelectric AlN deposition

Aluminum nitride (AlN) is a wide band gap semiconductor with interesting piezoelectric properties for many applications, including micromechanical systems (MEMS), acoustic wave sensors or energy harvesting devices. The influence of DC reactive magnetron sputtering (DCRMS) deposition parameters on the structural, crystalline, and piezoelectric properties of AlN thin films are presented. The systematic approach of Design of Experiments (DoE) has been used to evaluate the role of magnetron power, nitrogen and argon flows on the deposition process and the film properties. Magnetron power and argon flow have resulted to be the parameters causing the most significant effects on the deposition rate. AlN films have been deposited with a high crystal quality, showing low values of FWHM (0.28) and c-axis orientation parallel oriented respect to the growth direction, as evidenced by X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). These samples also showed a high piezoelectric coefficient (d33) of 5 pC/N for AlN thin films. This work highlights the importance of deposition parameters on the properties of the film and the important role that DoE play for its optimization with a minimum number of depositions.

Innovative surface acoustic wave devices: A solution for real time monitoring and self-cleaning cascade impactor

This paper presents a comparative study of three types of surface acoustic waves sensors used for particulate matter detection along with a theoretical study tackling the possibility to switch to innovative materials called Piezoelectric-on-Insulator. These sensors are placed in a cascade impactor as impaction plates. Monitoring their phase variation allows us to measure the quantity of fine particles present in the air with high accuracy. Until now, the sensors used in our prototype are built on quartz substrate and present a sufficient sensitivity to fine particles. One major concern in our application is the fouling of the sensor’s surface with particles upon long periods of exposure. This shaped our drive to develop a self-cleaning sensor relying on other substrates with stronger electromechanical coupling coefficient (k2) [1]. Hence, the aim of this study is to demonstrate the possibility of using strongly coupled modes on different piezoelectric substrates for an accurate PM detection. Among these, different POI substrates are considered to assess their physical and acoustic properties, especially in terms of k2 and thermal stability, for further optimization enabling their exploitation in the self-cleaning system that is being developed.

Novel Surface Acoustic Wave Strain Sensor with Enhanced High-Temperature Performance and Resistance to Aging

Abstract Surface acoustic wave (SAW) sensors based on Langasite (La3Ga5SiO14, LGS) have emerged as promising candidates for high-temperature strain measurement in passive wireless systems. However, limitations of strain transfer at temperatures above 800°C have hindered the repeatability of sensors due to temperature-induced effects. For instance, the aging of conventional high-temperature adhesives at elevated temperatures restricts the strain transfer between the measured object and the sensor. Moreover, materials like Inconel-600 exhibit a coefficient of thermal expansion significantly larger than the LGS, leading to thermal strains caused by thermal expansion mismatch beyond the maximum range of existing SAW strain sensors. Such factors increase the likelihood of sensor damage during a test. In this paper, we present a novel SAW sensor utilizing an ultra-thin LGS substrate encapsulated within a ceramic coating, which transfers strain by exerting pressure on both sides of the sensor. The strain sensor is mounted on an equal-strain beam inside a high-temperature furnace. The ceramic coating is not tightly bonded to the sensor, thereby mitigating the thermal expansion mismatch between the sensor and the beam at high temperatures. Furthermore, the ceramic coating can withstand large strain caused by thermal expansion while maintaining pressure on the sensor. This innovative SAW sensor enables repeat strain tests up to 800°C, including both calibration and formal tests. The measured strain exhibits an error of less than 5% compared to a standard strain gauge, demonstrating the sensor’s enhanced performance and reliability.

Miniaturized Antenna Design for Wireless and Powerless Surface Acoustic Wave Temperature Sensors.

This paper presents the introduction, design, and experimental validation of two small helical antennae. These antennae are a component of the surface acoustic wave (SAW) sensor interrogation system, which has been miniaturized to operate at 915 MHz and aims to improve the performance of wireless passive SAW temperature-sensing applications. The proposed antenna designs are the normal-mode cylindrical helical antenna (CHA) and the hemispherical helical antenna (HSHA); both designed structures are developed for the ISM band, which ranges from 902 MHz to 928 MHz. The antennae exhibit resonance at 915 MHz with an operational bandwidth of 30 MHz for the CHA and 22 MHz for the HSHA. A notch occurs in the operating band, caused by the characteristics of the SAW sensor. The presence of this notch is crucial for the temperature measurement by aiding in calculating the frequency shifting of that notch. The decrement in the resonance frequency of the SAW sensor is about 66.67 kHz for every 10 °C, which is obtained by conducting the temperature measurement of the system model across temperature environments ranging from 30 °C to 90 °C to validate the variation in system performance.

Design and implementation of shear-horizontal mode surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW) sensors have garnered significant attention due to high performance in gas detection. Love wave as the special mode of SAW has greater attractiveness for molecules detection in gas or liquid. To investigate the detection performance of Love wave SAW (Love-SAW) devices towards toxic gases, the influence of interdigitated electrodes on the propagating performance of Rayleigh and shear horizontal (SH) waves along ST-X and ST-90°X quartz substrates was analyzed by finite element method (FEM). The effect of the guiding layer based on different substrates of SAW devices has been discussed by changing the thickness of the zinc oxide (ZnO) layer. The results indicate that the SH wave excited on ST-90°X quartz substrate couples into the Love wave mode through the ZnO guiding layer, and Rayleigh waves in ST-X quartz do not exhibit mode coupling. The Love-SAW sensor covering the ZnO guiding layer (0.5 µm) has shown high mass sensitivity. The vibration distribution of SH wave excited by different thicknesses of electrodes is different, indicating that the mass-loading is one of the reasons for the transformation of SH wave into Love wave. Furthermore, the 50 ppm, 95 ppm and 110 formaldehyde-load leads the short-circuit resonance frequency of the polyetherimide (PEI) covered Love-SAW devices to drop by 100.1 kHz, 200.2 kHz and 300.3 kHz, respectively. The maximum detection sensitivity reached about 2.73 kHz/ppm. This work can provide valuable insights for designing and investigating the SAW formaldehyde gas sensors.

Portable surface acoustic wave device platform coupled with a paper-based capillary fluidics for real-time biosensing applications

Surface acoustic wave (SAW) sensors have emerged as prominent real-time, label-free biosensors with diverse applications in immunoassays and nucleic acid detection. However, widespread adoption in diagnostics has been impeded by challenges such as miniaturization of instrumentation, microfluidics fabrication and high attenuation in liquid environments. In this study we address these limitations by presenting a portable platform capable of real-time monitoring of a SAW microarray chip comprising up to four sensing channels, combined with a novel paper-based capillary flow channel. For the fabrication of a portable, automated and versatile sensor platform, we integrated a microcontroller, RF components, and a micropump for flow control. Calibration procedures ensured accurate measurements for both amplitude and phase, with a low drift rate (0.72 mdB/h and 1.00 mdeg./h, respectively) and high resolution (2.10 mdB and 6.10 mdeg.), indicating stable operation with minimal interface electronics. The capillary flow channel, implemented by confining the fluid on the sensing surface using a nitrocellulose strip, facilitated laminar and continuous sample flow with minimum damping of the acoustic signal. The sensor system was evaluated with two SAW devices at 100 and 200 MHz using glycerol dilutions within the range of 1 % and 70 %, demonstrating real-time monitoring capabilities and linear correlations between amplitude and phase changes. To prove the biosensing and clinical validity of the SAW newly developed system, DNA adsorption on a poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) functionalized SAW-surface was demonstrated with a detection limit of 0.001 μg/mL. Our study demonstrates the feasibility of a portable SAW sensor system coupled with a low-cost and replaceable capillary flow channel for real-time monitoring and biomolecular detection.

The Effect of Loading W&V:TiO2 Nanoparticles with Noble Metals for CH4 Detection

TiO2 nanoparticles (NPs) doped with W (W:TiO2), double-doped with W and V (W&V:TiO2), and loaded with noble metals (W:TiO2 @Pt/Pd/Ag and W&V:TiO2@Pt/Pd/Ag) were synthesized by laser pyrolysis followed by chemical impregnation and reduction. Due to its exceptional properties, TiO2 is considered a key material being used in a wide range of applications. To improve its detection activity, the increase in the specific surface of the material, and the presence of defects in its structure play a decisive role. Doped and double-doped TiO2 nanoparticles with dimensions in the range of 25–30 nm presented a mixture of phases corresponding to titania, with the anatase phase accounting for the majority (95%). By loading these nanoparticles with small particles of noble metals, a significant increase in the specific surface area by three or even five times the original values was achieved. Sensitive thin films for surface acoustic wave (SAW) sensors were made with the NPs, embedded in polyethyleneimine (PEI) polymer and deposited by spin-coating. Each sensor was tested at CH4 concentrations between 0.4 and 2%, at room temperature, and the best results were obtained by the sensor with NPs doped with V and decorated with Pd, with a limit of detection (LOD) of 17 ppm, due to the strong catalytic effect of Pd.

Detecting low-concentration ammonia with a surface acoustic wave sensor using a silver nanoparticles–graphene–polypyrrole hybrid nanocomposite film

In this study, a surface acoustic wave (SAW) resonator coated with graphene/polypyrrole hybrid nanocomposite films decorated with silver nanoparticles (AgNPs-G/PPy) is proposed for detecting ammonia (NH3) in parts-per-billion concentrations. The AgNPs-G/PPy hybrid nanocomposite film was synthesized via in situ chemical oxidative polymerization. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the AgNPs-G/PPy hybrid nanocomposite film, confirming its successful synthesis and identifying a wrinkled multilayered structure. The AgNPs-G/PPy hybrid nanocomposite film was spin-coated onto the surface of a stress-compensated temperature-cut quartz SAW resonator having an operating frequency of 98.5 MHz to create an NH3 gas sensor. NH3 adsorption by the AgNPs-G/PPy hybrid nanocomposite film modulated the acoustic wave velocity, and the corresponding frequency shift served as a sensing signal. The synergistic interaction between the three constituent materials (AgNPs, graphene, and polypyrrole) enhanced the sensitivity, selectivity, and response speed of the sensor for NH3 detection. At room temperature, the proposed sensor exhibited a positive frequency shift of 568 Hz when exposed to 50 ppb of NH3 gas and a rapid response time of less than 60 s. In addition, the SAW sensor exhibited excellent selectivity.

Ambiently Rapid Response and High Selectivity to NO₂ Enabled by Pd-rGO Composite Film-Coated Surface Acoustic Wave Sensor

Ambiently Rapid Response and High Selectivity to NO₂ Enabled by Pd-rGO Composite Film-Coated Surface Acoustic Wave Sensor