Recent advances of surface acoustic wave-based sensors for noninvasive cell analysis

Putting piezoelectric sensors into Fano resonances

Piezoelectric resonance sensors are essential to many diverse applications associated with chemical and biological sensing. In general, they rely on continuously detecting the resonant frequency shift of piezoelectric resonators due to analytes accreting on their surfaces in vacuum, gas or fluid. Resolving the small analyte changes requires the resonators with a high quality factor. Here, we propose theoretically and demonstrate experimentally a scheme using a physics concept, i.e., a Fano resonance, to enhance the quality factor rather than optimizing the structure and material of the resonator itself though these are important. The Fano resonance arises due to the interference between a discrete mode and a continuum of modes, leading to the asymmetric and steep dispersion. In our scheme, the as-fabricated piezoelectric sensors are put into the Fano resonance by connecting an external shunt capacitor to them. As a verification case, one-port surface acoustic wave (SAW) resonators on LiNbO3 substrate, incorporating a composite of polymethyl methacrylate (PMMA) and graphene oxide (GO) for humidity sensing, have been fabricated and characterized. We enhance the quality factor by up to a factor of about 8, from 929 for the as-fabricated sensor to 7682 for that with the external shunt capacitor. Our results pave the way for the practical development of piezoelectric resonance sensors with high quality factor.

Rapid tear screening of diabetic retinopathy by a detachable surface acoustic wave enabled immunosensor

BACKGROUND

Diabetic retinopathy (DR), a chronic and progressive microvascular complication of diabetes mellitus, substantially threatens vision and is a leading cause of blindness among working-age individuals worldwide. Traditional diagnostic methods, such as ophthalmoscopy and fluorescein angiography are nonquantitative, invasive, and time consuming. Analysis of protein biomarkers in tear fluid offers noninvasive insights into ocular and systemic health, aiding in early DR detection. This study introduces a surface acoustic wave (SAW) microchip that rapidly enhances fluorescence in bead-based immunoassays for the sensitive and noninvasive DR detection from human tear samples.

RESULTS

The device facilitated particle mixing for immunoassay formation and particle concentration in the droplet, resulting in an enhanced immunofluorescence signal. This detachable SAW microchip allows the disposal of the cover glass after every use, thereby improving the reusability of the interdigital transducer and minimizing potential cross-contamination. A preliminary clinical test was conducted on a cohort of 10 volunteers, including DR patients and healthy individuals. The results demonstrated strong agreement with ELISA studies, validating the high accuracy rate of the SAW microchip.

SIGNIFICANCE

This comprehensive study offers significant insights into the potential application of a novel SAW microchip for the early detection of DR in individuals with diabetes. By utilizing protein biomarkers found in tear fluid, the device facilitates noninvasive, rapid, and sensitive detection, potentially revolutionizing DR diagnostics and improving patient outcomes through timely intervention and management of this vision-threatening condition.

Detachable acoustofluidic droplet-sorter

BACKGROUND

Cell sorting is crucial in isolating specific cell populations. It enables detailed analysis of their functions and characteristics and plays a vital role in disease diagnosis, drug discovery, and regenerative medicine. Fluorescence-activated cell sorting (FACS) is considered the gold standard for high-speed single-cell sorting. However, its high cost, complex instrumentation, and lack of portability are significant limitations. Additionally, the high pressure and electric fields used in FACS can harm cell integrity. In this work, an acoustofluidic device was developed in combination with surface acoustic wave (SAW) and droplet microfluidics to isolate single-cell droplets with high purity while maintaining high cell viability.

RESULT

Human embryonic kidney cells, transfected with fluorescent reporter plasmids, were used to demonstrate the targeted droplet sorting containing single cells. The acoustofluidic sorter achieved a recovery rate of 81 % and an accuracy rate higher than 97 %. The device maintained a cell viability rate of 95 % and demonstrated repeatability over 20 consecutive trials without compromising efficiency, thus underscoring its reliability. Thermal image analysis revealed that the temperature of the interdigital transducer (IDT) during SAW operation remained within the permissible range for maintaining cell viability.

SIGNIFICANCE

The findings highlighted the sensitivity and effectiveness of the developed acoustofluidic device as a tool for single-cell sorting. The detachable microfluidic chip design enables the reusability of the expensive IDT, making it cost-effective and reducing the risk of cross-contamination between different biological samples. The results underscore its capability to accurately isolate individual cells on the basis of specific criteria, showcasing its potential to advance research and clinical applications requiring precise cell sorting methodologies.

A toroidal SAW gyroscope with focused IDTs for sensitivity enhancement

A surface acoustic wave (SAW) gyroscope measures the rate of rotational angular velocity by exploiting a phenomenon known as the SAW gyroscope effect. Such a gyroscope is a great candidate for application in harsh environments because of the simplification of the suspension vibration mechanism necessary for traditional microelectromechanical system (MEMS) gyroscopes. Here, for the first time, we propose a novel toroidal standing-wave-mode SAW gyroscope using focused interdigitated transducers (FIDTs). Unlike traditional SAW gyroscopes that use linear IDTs to generate surface acoustic waves, which cause beam deflection and result in energy dissipation, this study uses FIDTs to concentrate the SAW energy based on structural features, resulting in better focusing performance and increased SAW amplitude. The experimental results reveal that the sensitivity of the structure is 1.51 µV/(°/s), and the bias instability is 0.77°/s, which are improved by an order of magnitude compared to those of a traditional SAW gyroscope. Thus, the FIDT component can enhance the performance of the SAW gyroscope, demonstrating its superiority for angular velocity measurements. This work provides new insights into improving the sensitivity and performance of SAW gyroscopes.

The Recent Development of Acoustic Sensors as Effective Chemical Detecting Tools for Biological Cells and Their Bioactivities

One of the most significant developed technologies is the use of acoustic waves to determine the chemical structures of biological tissues and their bioactivities. In addition, the use of new acoustic techniques for in vivo visualizing and imaging of animal and plant cellular chemical compositions could significantly help pave the way toward advanced analytical technologies. For instance, acoustic wave sensors (AWSs) based on quartz crystal microbalance (QCM) were used to identify the aromas of fermenting tea such as linalool, geraniol, and trans-2-hexenal. Therefore, this review focuses on the use of advanced acoustic technologies for tracking the composition changes in plant and animal tissues. In addition, a few key configurations of the AWS sensors and their different wave pattern applications in biomedical and microfluidic media progress are discussed.