Standing Surface Acoustic Waves Research Articles

On-Chip Label-Free Sorting of Living and Dead Cells.

With the emergence of various cutting-edge micromachining technologies, lab on a chip is growing rapidly, but it is always a challenge to realize the on-chip separation of living cells from cell samples without affecting cell activity and function. Herein, we report a novel on-chip label-free method for sorting living and dead cells by integrating the hypertonic stimulus and tilted-angle standing surface acoustic wave (T-SSAW) technologies. On a self-designed microfluidic chip, the hypertonic stimulus is used to distinguish cells by producing volume differences between living and dead cells, while T-SSAW is used to separate living and dead cells according to the cell volume difference. Under the optimized operation conditions, for the sample containing 50% of living human umbilical vein endothelial cells (HUVECs) and 50% of dead HUVECs treated with paraformaldehyde, the purity of living cells after the first separation can reach approximately 80%, while after the second separation, it can be as high as 93%; furthermore, the purity of living cells after separation increases with the initial proportion of living cells. In addition, the chip we designed is safe for cells and can robustly handle cell samples with different cell types or different causes of cell death. This work provides a new design of a microfluidic chip for label-free sorting of living and dead cells, greatly promoting the multi-functionality of lab on a chip.

Investigation on a cascaded inertial and acoustic microfluidic device for sheathless and label-free separation of circulating tumor cells

High-precision and high-purity acquisition of tumor cells from whole blood is vital for early disease detection and diagnosis. Here, we investigated a cascaded inertial and acoustic microfluidic device for sheathless and label-free separation of circulating tumor cells (CTCs) from the blood through numerical methods. We introduced a spiral microfluidics channel in the first stage (1st) for cell focusing and rough sorting to improve chip integration and reduce the dependence on sheath flow and extra syringe pumps. In the 1st, we simulated the spiral microfluidic with a rectangular cross section to determine the key parameters affecting the migration kinetics of blood cells and tumor cells. Under the influence of Dean drag force and inertial lift force, blood cells migrate toward the inner side of the channel, while CTCs flow out close to the outer side. A flow rate of 400 μl/min was optimized for the operating flow rate. To improve and further enhance the 1st sorting efficiency and purity, we introduced tilted angle standing surface acoustic wave (SSAW) in the second stage (2st). Based on the parametric study, the SSAW with 33.3 MHz, tilted angle with 5°, and acoustic pressure amplitude with 0.7 MPa was selected as the operating parameter. The product of the 1st is used as input for the 2st acoustofluidic unit, enabling a more accurate separation process to obtain CTCs. The simulation results show that the inertial microfluidic units arranged in the first stage help to improve throughput and assist in 2st acoustofluidic separation, and the cascaded chip has accomplished a separation performance of nearly 100% in terms of purity and efficiency.

Velocity and Direction Adjustment of Actuated Droplets Using the Standing Wave Ratio of Surface Acoustic Waves (SAW)

Droplet actuation using surface acoustic wave (SAW) technology has recently been widely employed in “lab-on-a-chip” applications. In this article, SAWs generated by interdigital transducers (IDTs) were used to actuate microdroplets, where their velocity and direction could be adjusted by changing only the excitation phase shift, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> , of the voltage applied to the two IDTs. Specifically, the equation of the vibration of the mixed traveling surface acoustic waves (TSAWs) and standing surface acoustic waves (SSAWs) formed on the substrate surface operating in the exciter–exciter mode has been reported by the authors. An analytical expression for the acoustic standing wave ratio has been derived and given as a function of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> and the spatial phase difference. It can be seen from the expression that the directions of the TSAWs will be opposite to each other, in the range of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> given by (0, π) and (π, 2π), and the component of the TSAWs can be adjusted by changing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> in each direction. Following the theoretical analysis discussed here, two IDTs have been excited to generate a mixture of TSAWs and SSAWs. A series of experiments was carried out to control the velocity and direction of the actuated droplets, by changing only <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> . In addition, an experiment performed to compare the techniques shows that the upper limit of the velocity of the actuated droplets can be significantly increased using the exciter–exciter mode, showing that it has the potential to be an alternative method to the traditional exciter–absorber mode.

The effect of microchannel height on the acoustophoretic motion of sub-micron particles

Acoustophoresis is an effective technique for particle manipulation. Acoustic radiation force scales with particle volume, enabling size separation. Yet, isolating sub-micron particles remains a challenge due to the acoustic streaming effect (ASE). While some studies confirmed the focusing ability of ASE, others reported continuous stirring effects. To investigate the parameters that influence ASE-induced particle motion in a microchannel, this study examined the effect of microchannel height and particle size. We employed standing surface acoustic wave (SSAW) to manipulate polystyrene particles suspended in the water-filled microchannel. The results show that ASE can direct particles as small as 0.31 µm in diameter to the centre of the streaming vortices, and increasing the channel height enhances the focusing effect. Smaller particles circulate in the streaming vortices continuously, with no movement towards the centres. We also discovered that when the channel height is at least 0.75 the fluid wavelength, particles transitioning from acoustic radiation-dominated to ASE-dominated share the same equilibrium position, which differs from the pressure nodes and the vortices’ centres. The spatial distance between particles in different categories can lead to particle separation. Therefore, ASE is a potential alternative mechanism for sub-micron particle sorting when the channel height is accurately adjusted.

Suppression of skyrmion Hall effect via standing surface acoustic waves in hybrid ferroelectric/ferromagnetic heterostructures

Magnetic skyrmion is a promising information carrier for its low critical driven current density, topological stability, and small size, which has been proposed for various devices such as racetrack memory and logic gates. However, the skyrmion Hall effect originating from Magnus force leads to transverse motion, which hinders the development of skyrmionic device applications. Here, we propose artificial tracks built by standing surface acoustic waves (SSAWs) to suppress the skyrmion Hall effect through micromagnetic simulations. We systematically study the dynamics of an isolated skyrmion under SSAWs and driven currents in a prototype of the ferromagnetic skyrmion system. The skyrmion Hall angle changes from 80° to 0°, where the skyrmion motion is along the driven current. An analytical model considering magnetoelastic energy induced by SSAWs is developed, and a linear relation between the current density and the critical SSAW amplitude to eliminate the skyrmion Hall effect is achieved. Furthermore, a reconfigurable multichannel skyrmion racetrack is constructed through the change of SSAW wavelengths. Our work opens a feasible route for the suppression of skyrmion Hall effect via SSAWs.

Acoustic prison for single live cell 3D multi-imaging enabled by light-sheet microscopy

Having a system capable of obtaining high resolution, quantitative, subcellular information of individual live cells through imaging, while minimizing cell damage, is of great importance in many fields of research. In recent years, optofluidic light-sheet fluorescent microscopy (LSFM) has emerged as a powerful tool providing a low photo-toxic imaging method utilising the fluidic environment offered by microfluidics. Here, the benefits of LSFM were integrated with an acoustic single cell prison for precise single cell handling and 3D multi-imaging in a semi-automated manner. The compact, monolithic, acousto-optofluidic platform employed standing surface acoustic waves (SSAWs) to sequentially trap an individual cell on either side of an imaging region, which gathered planar, cross-sectional images of the cell. A cytoplasmic stain was utilised to not only visualize the cell trajectory throughout the imaging process, but also to verify the cell viability post-acoustic exposure. A mitochondrial stain was also used to better demonstrate the resolution capabilities of the device. Through post-image processing, 3D volumetric images of the cell were reconstructed and the results between the first and second rounds of imaging were directly comparable. Furthermore, the acoustic prison advantageously positions the cells in the upper region of the channel, enabling the ability to accurately compare temporal changes in cell morphology; a capability that can lead to advancements in therapeutics and drug delivery to access responses of cells to stimuli over time.

Surface acoustic wave induced instability and pattern formation in a liquid polymer film

Recently, standing surface acoustic waves (SAWs) have been demonstrated to stimulate wrinkles on polymer films. Here, we theoretically investigate the underlying mechanism of the acoustic-induced micro-structures on a thin liquid film. The standing SAWs leak into the liquid polymer and generate radiation potential gradients. The strong potential gradients produce acoustic radiation pressure and induce an instability that features the wrinkling structure. Surface stability of the liquid film is firstly investigated by a linear instability analysis and the critical conditions of instability are analytically derived. Theoretical prediction of the film thickness that is favorable for surface instability well agrees with the optimal value found in previous experiments. Balance between the radiation and gravity-capillary pressure at the fluid surface forms the equilibrium equation of the surface profiles. Analytic solution reveals that the component of radiation pressure varying spatially makes the wavelength of the generated surface patterns equal to half of the SAW wavelength, while the constant component of radiation pressure absorbs the film onto the substrate. The interplay between the acoustic waves and the surface morphology is also investigated. Effects of the surface deformation on the radiation pressure is negligible except for that the mean thickness of the liquid approaches the resonance mode where singularity of the radiation pressure occurs.

Numerical Study of Particle Separation through Integrated Multi-Stage Surface Acoustic Waves and Modulated Driving Signals

The manipulation of biomedical particles, such as separating circulating tumor cells from blood, based on standing surface acoustic wave (SSAW) has been widely used due to its advantages of label-free approaches and good biocompatibility. However, most of the existing SSAW-based separation technologies are dedicated to isolate bioparticles in only two different sizes. It is still challenging to fractionate various particles in more than two different sizes with high efficiency and accuracy. In this work, to tackle the problems of low efficiency for multiple cell particle separation, integrated multi-stage SSAW devices with different wavelengths driven by modulated signals were designed and studied. A three-dimensional microfluidic device model was proposed and analyzed using the finite element method (FEM). In addition, the effect of the slanted angle, acoustic pressure, and the resonant frequency of the SAW device on the particle separation were systemically studied. From the theoretical results, the separation efficiency of three different size particles based on the multi-stage SSAW devices reached 99%, which was significantly improved compared with conventional single-stage SSAW devices.

Experimental investigation on microfluidic devices for particle separation using standing surface acoustic waves

In this work, we designed and experimentally tested a microfluidic device for particle separation by using standing surface acoustic waves (SSAW). Emphasis is being placed on an effective microfluidic device being capable of continuous separation of particles in a microchannel design on its size. For this, we developed a miniaturized (e.g., lab-on-a-chip) system for biological and chemical analyses. For demonstration, we experimentally demonstrated that separation of dissimilarly-sized polystyrene particles suspended in water medium was successfully achieved. Numerical simulations were conducted by using COMOSL Multiphysics 5.0 to investigate the effects of sheath layer thickness and particle size on separation efficiency of the microfluidic device. The present numerical and experimental findings were then applied to propose an improved design and fabrication process of Standing surface acoustic waves devices, which has great potential for biological applications.

Methodologies, technologies, and strategies for acoustic streaming-based acoustofluidics

Acoustofluidics offers contact-free manipulation of particles and fluids, enabling their uses in various life sciences, such as for biological and medical applications. Recently, there have been extensive studies on acoustic streaming-based acoustofluidics, which are formed inside a liquid agitated by leaky surface acoustic waves (SAWs) through applying radio frequency signals to interdigital transducers (IDTs) on a piezoelectric substrate. This paper aims to describe acoustic streaming-based acoustofluidics and provide readers with an unbiased perspective to determine which IDT structural designs and techniques are most suitable for their research. This review, first, qualitatively and quantitatively introduces underlying physics of acoustic streaming. Then, it comprehensively discusses the fundamental designs of IDT technology for generating various types of acoustic streaming phenomena. Acoustic streaming-related methodologies and the corresponding biomedical applications are highlighted and discussed, according to either standing surface acoustic waves or traveling surface acoustic waves generated, and also sessile droplets or continuous fluids used. Traveling SAW-based acoustofluidics generate various physical phenomena including mixing, concentration, rotation, pumping, jetting, nebulization/atomization, and droplet generation, as well as mixing and concentration of liquid in a channel/chamber. Standing SAWs induce streaming for digital and continuous acoustofluidics, which can be used for mixing, sorting, and trapping in a channel/chamber. Key challenges, future developments, and directions for acoustic streaming-based acoustofluidics are finally discussed.

Scanning X-Ray Diffraction Microscopy of a 6-GHz Surface Acoustic Wave

Surface acoustic waves at frequencies beyond a few GHz are promising components for quantum technology applications. Applying scanning x-ray diffraction microscopy we directly map with a high lateral spatial resolution the complete strain field generated near the $\mathrm{Ga}\mathrm{As}$ surface by a standing surface acoustic wave with wavelength $\ensuremath{\lambda}\ensuremath{\simeq}500$ nm corresponding to frequencies near 6 GHz. We find that the lattice distortions perpendicular to the surface are phase shifted compared to those in propagation direction. Model calculations based on Rayleigh waves confirm our measurements. Our results represent a break through in providing a full characterization of a radio-frequency surface acoustic wave beyond plain imaging.

Numerical Modeling Using Immersed Boundary-Lattice Boltzmann Method and Experiments for Particle Manipulation under Standing Surface Acoustic Waves.

In this work, we employed the Immersed Boundary-Lattice Boltzmann Method (IB-LBM) to simulate the motion of a microparticle in a microchannel under the influence of a standing surface acoustic wave (SSAW). To capture the response of the target microparticle in a straight channel under the effect of the SSAW, in-house code was built in C language. The SSAW creates pressure nodes and anti-nodes inside the microchannel. Here, the target particle was forced to traverse toward the pressure node. A mapping mechanism was developed to accurately apply the physical acoustic force field in the numerical simulation. First, benchmarking studies were conducted to compare the numerical results in the IB-LBM with the available analytical, numerical, and experimental results. Next, several parametric studies were carried out in which the particle types, sizes, compressibility coefficients, and densities were varied. When the SSAW is applied, the microparticles (with a positive acoustic contrast factor) move toward the pressure node locations during their motion in the microchannel. Hence, their steady-state locations are controlled by adjusting the pressure nodes to the desired locations, such as the centerline or near the microchannel sidewalls. Moreover, the geometric parameters, such as radius, density, and compressibility of the particles affect their transient response, and the particles ultimately settle at the pressure nodes. To validate the numerical work, a microfluidic device was fabricated in-house in the cleanroom using lithographic techniques. Experiments were performed, and the target particle was moved either to the centerline or sidewalls of the channel, depending on the location of the pressure node. The steady-state placements obtained in the computational model and experiments exhibit excellent agreement and are reported.

Acoustophoresis separation of particles based on motion modes via tilted-angle standing surface acoustic wave driven by a unidirectional transducer

Acoustophoresis separation technique has attracted great attention due to its superior properties, such as biocompatibility, non-contact, label-free and high-efficiency. In this paper, separation of particles based on motion modes via tilt angle standing surface acoustic wave (TaSSAW) driven by a unidirectional transducer is developed theoretically. It is verified that the designed electrode width controlled unidirectional transducers are effective to improve the intensity of the acoustic field and the acoustic radiation force of the particles in the channel. The results show that when the density and compressibility of the particles are close to those of the fluid and the particle shape is close to spherical, the influences of fluid viscosity and particle shape on the acoustic radiation force are negligible. It is found that in the TaSSAW system the motion modes of the particles are divided into locked mode and drift mode, and they depend on the fluid velocity, acoustic field intensity and tilt angle. polystyrene (PS) particles with radii of 4 and 5 µm are separated based on the differences of motion modes. For further smaller size difference (4.5 and 5 µm) particles, the separation is also realized successfully by making particles move in the same drift mode.

Development of a hybrid acousto-inertial microfluidic platform for the separation of CTCs from neutrophil

The ability to fast and precise cell separation is a crucial issue for many biomedical applications and diagnoses. Despite several active and passive cell separation methods, high throughput, biocompatible, and label-free techniques are still demanding. High-accuracy acoustic and high-throughput inertial microfluidics can be integrated by combining acoustic and inertial methods. Accordingly, a new solver and two libraries have been developed in OpenFOAM to simulate particle tracking in a two-stage microchannel for focusing and separating particles simultaneously. In the first stage, the different Aspect Ratios (AR) of a serpentine microchannel with four different configurations were simulated in a range of flow rates to achieve the proper particle focusing. The results showed that the square serpentine configuration with AR=0.2 and flow rate of Q=2 ml/min gives a more favorable particle focusing. This channel was then linked to the second-staged straight channel for acoustic separation of breast cancer cells (MCF-7) and neutrophil white blood cells (WBCs). The separation was also compared for standing and traveling surface acoustic waves over a voltage range, and efficiency and purity were obtained for each outlet. At a flow rate of 2 ml/min, which is about four folds higher than the literature, the separation efficiency for MCF-7 cells was 99.3%, and the purity was 93.5% using standing surface acoustic waves (SSAWs). This high-throughput and high-accuracy proposed microfluidic device promises to be used for Lab-on-Chip (LOC) clinical applications for rapid and early detection before cancer metastasis.

Flexible capacitive pressure sensor array using acoustic-assisted fabrication of microstructures as surface and dielectric layers

Flexible pressure sensors have attracted considerable interests in the fields of intelligent robotics, human-machine interfaces and health monitoring for their portability, flexibility and real-time sensing performance. The microstructures of dielectric layer significantly affect the sensing performance of capacitive pressure sensors, but the common methods to fabricate microstructures always need physical and/or chemical molds, thus may not suit for low-cost fabrication of sensors. The wavy-shaped microstructures on the surface of fluid can be generated with the excitation of standing surface acoustic waves (SSAWs), which is suitable to be used as dielectric layers. Here, we developed a 6 × 6 flexible capacitive pressure sensor array with fingerprint-like surface cover layer and latticed dielectric layer. Both two layers were fabricated through the novel and rapid acoustic-assisted process. The fabricated capacitive sensor array exhibits high sensitivity of 28.8%/N for normal force loading, with spatial resolution of 1.5 mm. It has been successfully used to monitor the vibration of the Adam’s apple during swallowing and detecting the wrist activities in cycles. These studies demonstrate the great potential of the pressure sensor array with acoustically fabricated microstructures in wearable electronics for health monitoring. • A novel flexible pressure sensor array with spatial resolution of 1.5 mm for three-axis distributed force sensing is developed. • A novel and rapid acoustic-assisted process based on SSAWs is used to fabricate the surface layer and dielectric layer with microstructures. • The tactile sensor array features high flexibility and can be easily worn on neck or wrist for health monitoring. • Both Adam’s apple vibration sensing and wrist activity detection can be achieved by the developed tactile sensor array.

Acoustic-assisted 3D printing based on acoustofluidic microparticles patterning for conductive polymer composites fabrication

Multifunctional composites have been widely utilized in soft robots, electronics, batteries, and biomedicine. Meanwhile, several fabrication processes have been proposed to fabricate these functional composites with desired mechanical, electrical, magnetic, and thermal properties. However, patterning functional micro/nanomaterials such as particles, dots, wires, and fibers into complex designs with high spatial resolutions in a matrix remains a technical challenge. Herein, we present an acoustic-assisted 3D printing method enabled by standing surface acoustic waves (SSAWs). The proposed SSAWs-assisted patterning method produces diverse patterning forms of micro/nanomaterials and has almost no special requirements on micro/nanomaterials properties, indicating its great versatility and compatibility. Functional materials are assembled into desired stripe-like, net-like, and dot-like patterns. Multilayer composites with different patterns in each layer are even fabricated through a layer-by-layer printing process. To further demonstrate the potential of this 3D printing method, conductive polymer composite films with programmed electrical anisotropies are printed, yielding a maximum electrical conductivity of 827 S/m. The proposed acoustic-assisted patterning method makes the 3D printing of complex functional composites with customized properties possible, pushing the wide application of functional composites forward.

Bubble-Enhanced Mixing Induced by Standing Surface Acoustic Waves (SSAWs) in Microchannel

BAW-based micromixers usually achieve mixing enhancement with acoustic-induced bubbles, while SAW-based micromixers usually enhance mixing efficiency by varying the configuration of IDTs and microchannels. In this paper, bubble-enhanced acoustic mixing induced by standing surface acoustic waves (SSAWs) in a microchannel is proposed and experimentally demonstrated. Significant enhancement in the mixing efficiency was achieved after the bubbles were stimulated in our acoustofluidic microdevice. With an applied voltage of 5 V, 50 times amplified, the proposed mixing microdevice could achieve 90.8% mixing efficiency within 60 s at a flow rate of 240 μL/h. The bubbles were generated from acoustic cavitation assisted by the temperature increase resulting from the viscous absorption of acoustic energy. Our results also suggest that a temperature increase is harmful to microfluidic devices and temperature monitoring. Regulation is essential, especially in chemical and biological applications.

A novel study on separation of particles driven in two steps based on standing surface acoustic waves

With the development of surface acoustic wave devices, the separation of micron-sized particles based on standing surface acoustic waves (SSAW) has been studied, and the method has the advantages of high efficiency and ease of operation. In this paper, we design a two steps SSAW-based model for separating three different sizes of particles. The particle with the largest diameter is separated in the first sorting step region, and the remaining two particles are separated in the second sorting step region. During the study, we investigate the SSAW device firstly. We chose three different materials for the comparative analysis of the piezoelectric substrates. Next, we investigate the effects of fork-finger pairs and input voltage of the interdigital transducer (IDT) on the stability of the output waveform and sound pressure intensity. Finally, we select the appropriate number of fork-finger pairs and determine the acoustic pressure intensity in the low and high voltage regions respectively, so that three different particles could be successfully sorted in two steps. This research result can provide a certain theoretical basis for practical research fields such as cell sorting and drug detection.

Microfluidic Particle Separation and Detection System Based on Standing Surface Acoustic Wave and Lensless Imaging.

Separation and detection of micro-particles or cells from bio-samples by point-of-care (POC) systems are critical for biomedical and healthcare diagnostic applications. Among various microfluidic separation techniques, acoustophoresis-based technique has the advantages of label-free and good biocompatibility. However, most of the existing separation techniques are bulky and require additional equipment for analysis. We proposed a platform, which integrates an acoustophoresis-based separation device and a lensless imaging sensor into a compact standalone system to tackle this challenge. Standing Surface Acoustic Wave (SSAW) is utilized for label-free particle separation, while lensless imaging is employed for seamless particle detection and counting using self-developed dual-threshold motion detection algorithms. In particular, we specially optimized the design of microfluidic channel and interdigital transducers (IDTs) for higher performance bioparticle separation, designed a heat dissipation system for the suppression of fluid temperature, and proposed a novel frequency-temperature-curve based method to determine the appropriate signal driving frequency for IDTs. At 2 μL/min flow rate, separation efficiency of 93.52% and purity of 94.29% for 15 μm microbead were achieved in mixed 5μm and 15μm microbead solution at a 25 dBm RF driving power, and similar results for mixed 10 μm and 15 μm microbead solution. The results showed that the integrated platform has an excellent capability to seamlessly separate, distinguish, and count microbeads of different sizes. Such a platform and the design methodologies offer a promising POC solution for label-free cell separation and detection in biomedical diagnostics.

A simplified three-dimensional numerical simulation approach for surface acoustic wave tweezers

Standing surface acoustic waves (SSAWs) have been extensively used as acoustic tweezers to manipulate, transport, and separate microparticles and biological cells in a microscale fluidic environment, with great potentials for biomedical sensing, genetic analysis, and therapeutics applications. Currently, there lacks an accurate, reliable, and efficient three-dimensional (3D) modeling platform to simulate behaviors of micron-size particles/cells in acoustofluidics, which is crucial to provide the guidance for the experimental studies. The major challenge for achieving this is the computational complexity of 3D modeling. Herein, a simplified but effective 3D SSAW microfluidic model was developed to investigate the separation and manipulation of particles. This model incorporates propagation attenuation of the surface waves to increase the modeling accuracy, while simplifies the modeling of piezoelectric substrates and the wall of microchannel by determining the effective propagation region of the substrate. We have simulated the SSAWs microfluidics device, and systematically analyzed effects of voltage, tilt angle, and flow rate on the separation of the particles under the SSAWs. The obtained simulation results are compared with those obtained from the experimental studies, showing good agreements. This simplified modeling platform could become a convenient tool for acoustofluidic research.