Flexible Surface Acoustic Wave Research Articles

Achieving consistency of flexible surface acoustic wave sensors with artificial intelligence

Flexible surface acoustic wave technology has garnered significant attention for wearable electronics and sensing applications. However, the mechanical strains induced by random deformation of these flexible SAWs during sensing often significantly alter the specific sensing signals, causing critical issues such as inconsistency of the sensing results on a curved/flexible surface. To address this challenge, we first developed high-performance AlScN piezoelectric film-based flexible SAW sensors, investigated their response characteristics both theoretically and experimentally under various bending strains and UV illumination conditions, and achieved a high UV sensitivity of 1.71 KHz/(mW/cm²). To ensure reliable and consistent UV detection and eliminate the interference of bending strain on SAW sensors, we proposed using key features within the response signals of a single flexible SAW device to establish a regression model based on machine learning algorithms for precise UV detection under dynamic strain disturbances, successfully decoupling the interference of bending strain from target UV detection. The results indicate that under strain interferences from 0 to 1160 με the model based on the extreme gradient boosting algorithm exhibits optimal UV prediction performance. As a demonstration for practical applications, flexible SAW sensors were adhered to four different locations on spacecraft model surfaces, including flat and three curved surfaces with radii of curvature of 14.5, 11.5, and 5.8 cm. These flexible SAW sensors demonstrated high reliability and consistency in terms of UV sensing performance under random bending conditions, with results consistent with those on a flat surface.

Acoustic wave devices based on piezoelectric/ferroelectric thin films for high-frequency communication systems and sensing applications

Surface acoustic wave (SAW) and bulk acoustic wave (BAW) based radio-frequency (RF) filters dominate in handsets markets mainly because they have miniature size so multiple filters can be used but take up very small board area. Besides, due to their inherent advantages, such as low insertion loss, relatively high resonant frequency, and miniature size, their application in the physical and biochemical sensing fields has also gradually expanded. The successful implementation requires specific knowledge of acoustic wave properties, the working mechanisms, material properties, and device design. So, in this review, we survey the SAW and BAW filter technology by discussing the working principles and the comparison between the operation frequency band allocations. In addition, the approaches to enhance the performance including the selection of single crystal/ferroelectric materials for BAW and the structure design optimization for SAW are introduced with discussions for further improvement. Then, flexible SAW and BAW devices are also presented and their new application opportunities in the fields of skin-like electronics and wearable health monitoring devices can be foreseen. Finally, the potential challenges of high frequency, wide bandwidth, miniaturization, and compatibility with the integrated circuits (IC) manufacturing process are overviewed as well.

Recent Progress in Flexible Surface Acoustic Wave Sensing Technologies.

In this work, the major methods for implementing flexible sensing technology-flexible surface acoustic wave (SAW) sensors-are summarized; the working principles and device characteristics of the flexible SAW sensors are introduced; and the latest achievements of the flexible SAW sensors in the selection of the substrate materials, the development of the piezoelectric thin films, and the structural design of the interdigital transducers are discussed. This paper focuses on analyzing the research status of physical flexible SAW sensors such as temperature, humidity, and ultraviolet radiation, including the sensing mechanism, bending strain performance, device performance parameters, advantages and disadvantages, etc. It also looks forward to the development of future chemical flexible SAW sensors for gases, the optimization of the direction of the overall device design, and systematic research on acoustic sensing theory under strain. This will enable the manufacturing of multifunctional and diverse sensors that better meet human needs.

Recent Progress in Polymeric Flexible Surface Acoustic Wave Devices: Materials, Processing, and Applications (Adv. Mater. Technol. 21/2023)

Recent Progress in Polymeric Flexible Surface Acoustic Wave Devices: Materials, Processing, and Applications (Adv. Mater. Technol. 21/2023)

Recent Progress in Polymeric Flexible Surface Acoustic Wave Devices: Materials, Processing, and Applications

Recently, there has been a remarkable increase in interest in piezoelectric thin films, particularly zinc oxide (ZnO) and aluminum nitride (AlN), deposited on flexible polymeric substrates. This is due to the rapid expansion of fields such as robotics, wearable devices, and the Internet of Things (IoT). These thin‐film layered structures have a wide range of applications, such as energy harvesting, sensing and biosensing, microfluidic sorting, pumping and mixing, and telecommunications. One of the most promising platforms is piezoelectric acoustic‐based thin‐film devices on inexpensive, recyclable, or disposable polymeric substrates. Their reliability, reproducibility, conformability, small size, and wireless control capabilities make them a leading candidate for the development of new wireless, fully automated, and digitized microsystems for IoT and wearable devices. This review examines recent advancements in the engineering of ZnO and AlN thin films on polymeric, flexible, and conformable surface acoustic wave (SAW) devices. Topics covered include the materials used, piezoelectric film deposition, device fabrication, and the latest applications of this technology as sensors and actuators.

Flexible Langasite-Based Surface Acoustic Wave Strain Sensor for High-Temperature Operation

Flexible Langasite-Based Surface Acoustic Wave Strain Sensor for High-Temperature Operation

Omnidirectional strain sensing using single flexible acoustic wave device with machine-learning algorithm

Strain sensors are crucial for development of smart systems, providing valuable feedback on the conditions of structures and mechanical components. However, there is a huge challenge for highly accurate detection of both strain intensity and direction (i.e., omnidirectional strain) using one single strain sensor, mainly because only one signal feature is commonly obtained from a single device. To overcome this limitation, we proposed a strategy to achieve omnidirectional strain detection by applying a single flexible surface acoustic wave (SAW) strain sensor, empowered by a machine learning algorithm to analyze multiple signals derived from the same device, simultaneously. Using AlN/flexible glass based SAW devices, we performed omnidirectional strain predictions using eight different machine learning models, and the data were compared with the experimental measurement results. The results showed that the extreme gradient boosting (XGBoost) model showed the highest prediction ability and the best accuracy (i.e., with its coefficient of determination larger than 0.98 and root mean square error less than 0.1) for both strain intensity and direction. This work provides an effective solution for omnidirectional strain sensing using a single device.

Simulation of Flexible Dual-mode Surface Acoustic Wave Resonator for Strain Sensor Application

Based on the requirement of flexible electronic devices (FED)such as surface acoustic wave (SAW) sensors, finite element models for material analysis and SAW resonator design were established. Dual-mode of Rayleigh and Horizontal Shear mode on 0ºY cut LiNbO3 thin film was discovered. The influence of BCB thickness and PI substrate on resonant mode was analyzed. The flexible SAW resonator owned high sensitivity both in Rayleigh mode and Horizontal shear mode under tensile stress and compressive stress by simulation.

Femtosecond Laser Micromachining of the Mask for Acoustofluidic Device Preparation.

Surface acoustic wave (SAW)-based acoustofluidic devices have shown broad applications in microfluidic actuation and particle/cell manipulation. Conventional SAW acoustofluidic device fabrication generally includes photolithography and lift-off processes and thus requires accessing cleanroom facilities and expensive lithography equipment. In this paper, we report a femtosecond laser direct writing mask method for acoustofluidic device preparation. By micromachining of steel foil to form the mask and direct evaporation of metal on the piezoelectric substrate using the mask, the interdigital transducer (IDT) electrodes of the SAW device are generated. The minimum spatial periodicity of the IDT finger is about 200 μm, and the preparation for LiNbO3 and ZnO thin films and flexible PVDF SAW devices is verified. Meanwhile, we have demonstrated various microfluidic functions, including streaming, concentration, pumping, jumping, jetting, nebulization, and particle alignment using the fabricated acoustofluidic (ZnO/Al plate, LiNbO3) devices. Compared to the traditional manufacturing process, the proposed method omits spin coating, drying, lithography, developing, and lift-off processes and thus has advantages of simple, convenient, low cost, and environment friendliness.

Flexible acoustic lens-based surface acoustic wave device for manipulation and directional transport of micro-particles

Microfluidics is an emerging technology that is playing increasingly important roles in biomedical and pharmaceutical research and development. Surface acoustic waves (SAWs) have been combined with microfluidics technology to establish a SAW-based microfluidics technology that uses the unique interaction between the two techniques to manipulate substances effectively in fluids on the surface of a substrate. This paper reports a method to generate SAWs using conventional planar ultrasonic transducers and acoustic lenses. Additionally, this method is introduced to manipulate particles effectively on a substrate surface. It is demonstrated that the particle positions can be manipulated precisely in any direction on the substrate surface, thus enabling high-precision particle manipulation. We also proposed the generation of nonplanar SAWs via appropriate design of the acoustic lens and realized directional particle transport. In addition, structures to enhance forward-propagating acoustic beams are proposed. The proposed method has potential for use in microfluidics and biomedical applications, allowing tasks such as flexible cell manipulation on a chip to be performed without complex design or micromachining.

Flexible Acoustic Lens-Based Surface Acoustic Wave Device for Manipulation and Directional Transport of Micro-Particles

Flexible Acoustic Lens-Based Surface Acoustic Wave Device for Manipulation and Directional Transport of Micro-Particles

Surface Acoustic Wave-Based Flexible Piezocomposite Strain Sensor

A surface acoustic wave (SAW), device composed of polymer and ceramic fillers, exhibiting high piezoelectricity and flexibility, has a wide range of sensing applications in the aerospace field. The demand for flexible SAW sensors has been gradually increasing due to their small size, wireless capability, low fabrication cost, and fast response time. This paper discusses the structural, thermal, and electrical properties of the developed sensor, based on different micro- and nano-fillers, such as lead zirconate titanate (PZT), calcium copper titanate (CCTO), and carbon nanotubes (CNTs), along with polyvinylidene fluoride (PVDF) as a polymer matrix. The piezocomposite substrate of the SAW sensor is fabricated using a hot press, while interdigital transducers (IDTs) are deposited through 3D printing. The piezoelectric properties are also enhanced using a non-contact corona poling technique under a high electric field to align the dipoles. Results show that the developed passive strain sensor can measure mechanical strains by examining the frequency shifts of the detected wave signals.

Flexible thin-film acoustic wave devices with off-axis bending characteristics for multisensing applications

Flexible surface acoustic wave (SAW) devices have recently attracted tremendous attention for their widespread application in sensing and microfluidics. However, for these applications, SAW devices often need to be bent into off-axis deformations between the acoustic wave propagation direction and bending direction. Currently, there are few studies on this topic, and the bending mechanisms during off-axis bending deformations have remained unexplored for multisensing applications. Herein, we fabricated aluminum nitride (AlN) flexible SAW devices by using high-quality AlN films deposited on flexible glass substrates and systematically investigated their complex deformation behaviors. A theoretical model was first developed using coupling wave equations and the boundary condition method to analyze the characteristics of the device with bending and off-axis deformation under elastic strains. The relationships between the frequency shifts of the SAW device and the bending strain and off-axis angle were obtained, and the results were identical to those from the theoretical calculations. Finally, we performed proof-of-concept demonstrations of its multisensing potential by monitoring human wrist movements at various off-axis angles and detecting UV light intensities on a curved surface, thus paving the way for the application of versatile flexible electronics.

Flexible and bendable acoustofluidics for particle and cell patterning

Surface Acoustic Wave (SAW) based microfluidic devices provide active techniques to manipulate fluid and particles, which can be used for precise and controllable patterning of microparticles and biological cells, with a high efficiency in a non-invasive and contact-free manner. This paper investigates flexible and bendable SAW microfluidic devices and explores the effects of bending and twisting of SAW devices on microparticle and cell patterning, using both experimental and numerical modelling. We showed that bending flexible SAW devices changes the distribution of particle pattern lines significantly. In devices with concave bending the particle pattern lines converge towards the centre of the curvature, whereas for devices with convex bending, they diverge away from it. Comparing the particle patterning using Lamb and Rayleigh wave devices with concave bending, we found that particle alignment is more efficient in the flexural mode Lamb wave device, whereas for the devices with convex bending, the particle patterning is more clear and regular when Rayleigh waves are used. We further investigated the effects of twisting the flexible SAW devices and observed that the particles are patterned into lines parallel to the deformed interdigital transducers (IDTs). We finally patterned yeast cells using our flexible SAW devices, and demonstrated the possibility of using our flexible acoustofluidic device for biomechanical systems such as body conforming technologies, wearable bio-sensors, and flexible point-of-care devices for personalized health monitoring.

Microstructure and bending piezoelectric characteristics of AlN film for high-frequency flexible SAW devices

Flexible surface acoustic wave (SAW) device is one of hot point areas for wearable communication technology because of their ductility and bendability, but the deterioration problem of device performance after bending experiments is limiting its wide application for future wearable communication devices. In this work, the effect of sputtering power on microstructure properties and bending piezoelectric characteristics of aluminum nitride (AlN) film on polyimide (PI) with molybdenum (Mo) buffer layer for high-frequency flexible SAW device was discussed in detail from 200 W to 280 W. Microstructure results show that the high oriented (002) AlN films with narrow full width at half maximum (FWHM) of 0.29° and low roughness of 3.2 nm are obtained under 240 W sputtering power. Piezoelectric and frequency results of SAW devices after cyclic bending indicate that the relative piezoelectric coefficient d*33 is about 8.01 pm/V, and the center frequency of the flexible SAW devices is as high as 4.95 GHz, especially the piezoelectric and frequency characteristics can maintain high satiability after 10,000 bending cycles. This work may be of useful for the flexible high-frequency SAW devices in the wearable communication area.

Enhancing the sensitivity of flexible acoustic wave ultraviolet photodetector with graphene-quantum-dots decorated ZnO nanowires

Graphene quantum dots decorated zinc oxide nanowires (GQDs@ZnO-NWs) were applied to enhance sensing performance of highly flexible and transparent surface acoustic wave (SAW) ultraviolet (UV) photodetectors made on ultra-thin flexible glass. The developed flexible SAW sensors possess better performance than that of the previously developed polymer based flexible SAW devices, due to insignificant acoustic loss of flexible glass substrate. UV sensitivity of the flexible glass based SAW sensors was enhanced by three times, and the response time was shorten by four times after the sensor was coated with the GQDs@ZnO-NWs hybrid nanomaterials. These improvements are mainly attributed to: (1) large specific surface areas of ZnO NWs which can generate a large number of photon-generated carriers; (2) introduction of GQDs which can reduce the carrier recombination rate. The resonant frequency of flexible glass SAW UV photodetectors exhibited a good repeatability and stability in responses to cyclic changes of the UV lights at different wavelengths. They also maintained a good performance under a bending angle of ∼30° for 200 times without apparent degradation, showing the excellent flexibility and stability of the UV photodetector.

Flexible Dual-Wave Mode AlN-Based Surface Acoustic Wave Device on Polymeric Substrate

In the last decade, the development of new flexible electronics is producing pervasive technologies for the Internet of Things (IoT), robotics and wearables transducers, pushing towards the demonstration of highly compliant low cost components on disposable or recyclable substrates. In recent years, there was a marked improvement in these technologies using organic or inorganic transistors. However, the lack of a high-performance passive-wireless devices in flexible electronics is one of the key bottlenecks for compliant IoT and wearables sensing nodes. One of the main building blocks for sorting this issue out is the development of flexible, bendable and stretchable acoustic-based devices, controllable via wireless antenna. In this work, a flexion characterization of a polymeric-based surface acoustic wave (SAW) device is presented. The flexible SAW is fabricated on polyethylene naphthalate (PEN), employing the high performance of the aluminum nitride (AlN) as a piezoelectric layer. The dual-wave modes, Rayleigh and Lamb, have been characterized. The Lamb waves show better results, in terms of strain sensitivity (1.81E-04 ε) and responsivity (1.156 kHz/με), as compared to the Rayleigh wave (4.29E-04 ε and 0.577 kHz/με, respectively). This polymeric flexiblebased SAW device could pave the way for the development of a passive wireless strain sensor.

Ultrathin Glass-Based Flexible, Transparent, and Ultrasensitive Surface Acoustic Wave Humidity Sensor with ZnO Nanowires and Graphene Quantum Dots.

Flexible electronic devices are normally based on organic polymer substrate. In this work, an ultrathin glass-based flexible, transparent, and ultrasensitive ZnO/glass surface acoustic wave (SAW) humidity sensor is developed using a composite sensing layer of ZnO nanowires (NWs) and graphene quantum dots (GQDs). It shows much larger effective electromechanical coupling coefficients and signal amplitudes, compared to those of flexible polymer-based SAW devices reported in the literature. Attributed to large specific surface areas of ZnO NWs, large numbers of hydrophilic functional groups of GQDs, as well as the formation of p-n heterojunctions between GQDs and ZnO NWs, the developed ZnO/glass flexible SAW sensor shows an ultrahigh humidity sensitivity of 40.16 kHz/% RH, along with its excellent stability and repeatability. This flexible and transparent SAW sensor has demonstrated insignificant deterioration of humidity sensing performance, when it is bent on a curved surface with a bending angle of 30°, revealing its potential applications for sensing on curved and complex surfaces. The humidity sensing and human breathing detection have further been demonstrated for wearable electronic applications using ultrathin glass-based devices with completely inorganic materials.

Conformable surface acoustic wave biosensor for E-coli fabricated on PEN plastic film

Over the last decades, great effort has gone into developing new biosensor technologies for applications in different fields such as disease diagnosis and detection of pollutants in water and food. Global developments in robotic, IoT technologies and in healthcare sensors require new flexible sensor technologies that are low cost and built from sustainable and reusable or recyclable materials. One of the most promising technologies is based on the development of surface acoustic wave (SAW) flexible biosensors, which are highly reproducible, reliable and wirelessly controllable. This work presents for the first time a novel aluminum nitride (AlN)-based conformable SAW immunosensor fabricated on recyclable polyethylene naphthalate. We apply it to the detection of E.Coli using a faster and innovative functionalization method that exploit Protein-A/antibody affinity. A higher sensitivity (Limit of detection-LoD, 6.54*105 CFU/ml) of the Lamb wave traveling on the polymeric device has been obtained in comparison with SAWs traveling on AlN on silicon substrate (LoD, 1.04*106 CFU/ml). Implementation of a finite element method allowed for the estimation of the single E.Coli mass of approximately 9*10−13 g. This work demonstrates the high biosensing potential of flexible polymeric SAW devices for bacteria contamination control in food chain, water and smart packaging.

Three-Dimensional Tetrapodal ZnO Microstructured Network Based Flexible Surface Acoustic Wave Device for Ultraviolet and Respiration Monitoring Applications

Sensitivity of flexible surface acoustic wave (SAW) sensors to ultraviolet (UV) light is enhanced by the application of ZnO micro- and nanostructured networks as sensing layers. In this study, the ...