Focused Surface Acoustic Waves Research Articles

Capillary-based, multifunctional manipulation of particles and fluids via focused surface acoustic waves

Surface acoustic wave (SAW)-enabled acoustofluidic technologies have recently attracted increasing attention for applications in biology, chemistry, biophysics, and medicine. Most SAW acoustofluidic devices generate acoustic energy which is then transmitted into custom microfabricated polymer-based channels. There are limited studies on delivering this acoustic energy into convenient commercially-available glass tubes for manipulating particles and fluids. Herein, we have constructed a capillary-based SAW acoustofluidic device for multifunctional fluidic and particle manipulation. This device integrates a converging interdigitated transducer to generate focused SAWs on a piezoelectric chip, as well as a glass capillary that transports particles and fluids. To understand the actuation mechanisms underlying this device, we performed finite element simulations by considering piezoelectric, solid mechanic, and pressure acoustic physics. This experimental study shows that the capillary-based SAW acoustofluidic device can perform multiple functions including enriching particles, patterning particles, transporting particles and fluids, as well as generating droplets with controlled sizes. Given the usefulness of these functions, we expect that this acoustofluidic device can be useful in applications such as pharmaceutical manufacturing, biofabrication, and bioanalysis.

Symmetry and Nonlinearity of Spin Wave Resonance Excited by Focused Surface Acoustic Waves

AbstractThe use of a complex ferromagnetic system to manipulate GHz surface acoustic waves (SAWs) is a rich current topic under investigation, but the high‐power nonlinear regime is under‐explored. Focused SAWs are introduced, which provide a way to access this regime with modest equipment. The symmetry of the magneto‐acoustic interaction can be tuned by interdigitated transducer design which can introduce additional strain components. Here, the impact of focused acoustic waves versus standard unidirectional acoustic waves in significantly enhancing the magnon‐phonon coupling behavior is compared. Analytical simulation results based on the modified Landau–Lifshitz–Gilbert theory show good agreement with experimental findings. Nonlinear input power dependence of the transmission through the device is also reported. This experimental observation is supported by the micromagnetic simulation using mumax3 to model the nonlinear dependence. These results pave the way for extending the understanding and design of acoustic wave devices for the exploration of acoustically driven spin wave resonance physics.

Experimental study on acoustic-induced microbubbles fusion

Droplets/microbubbles fusion technology is a key technology for many biochemical medical applications. Here, an acoustic-controlled approach to drive microbubble fusion based on focused surface acoustic wave (FSAW) is presented. When passing through the arc-shaped channel, adjacent microbubbles will decelerate, contact, and fuse induced by the FSAWs. Due to the cooperation of the FSAW and the arc-shaped flow channel structure, the microbubbles are more easily captured and fused with the adjacent microbubbles. The effects of input voltage and pressure input parameters on the microbubble fusion are studied. Relying on electrical input parameters, microbubble fusion can be effectively achieved, providing a new idea for precise gas fuse control. The regulation of the microbubbles fusion by FSAW is revealed, showing potential in the applications of the precise control of gases on a microfluidic chip.

Microfluidics-assisted synthesis of hydrogel microparticles with acoustic-magnetic control

Using one microfluidic device integrating acoustic and magnetic control, we propose a novel preparation method of magnetic fluorescent polyacrylamide hydrogel microspheres and calcium alginate hydrogel microparticles in this paper. The effect of the two interdigital transducers in different position on the droplets generation is studied. By tuning the input parameters of the two focused interdigital transducers (FIDTs), the excited focused surface acoustic wave (FSAW) can induce the generation of magnetic droplets with different sizes flexibly. Then the prepared magnetic droplets are cross-linked after crossing the continuous phase into the laminar flow induced by magnetic force. This method can avoid the direct reaction between the dispersed phase and the continuous phase in the channel, and enable the synthesis of magnetic fluorescent polyacrylamide hydrogel microspheres and calcium alginate hydrogel microparticles more easy and simple.

Asymmetrically aligned focused acoustic waves for enhancing sensing performance of electrochemical microarrays

Microelectrode-based electrochemical detection methods have been extensively applied in microfluidic sensors, but there are significant challenges for achieving fast and efficient contact between analytes and the microarray electrodes and, thus, enhancing the sensing performance. In this paper, we develop a technique using asymmetrically aligned focused surface acoustic waves (FSAWs) to enhance sensitivity of microarray electrodes detection. Effects of various focusing angles of the FSAW devices on the values and distributions of acoustic wave amplitudes were analyzed using finite element simulations, and torques, which determine the acoustic streaming velocity, were calculated as a function of values and distributions of amplitude. Based on simulation results, the FSAW device with a focusing angle of 30° was used to investigate sensitivity of microarray electrochemical sensors. The maximum value of instantaneous current was increased up to 11 times, researching a current value of 4.3 μA with the applied FSAWs. This developed electrochemical sensing platform shows great potentials for highly sensitive food quality control and biochemical detections.

Noncontact Manipulation of Intracellular Structure Based on Focused Surface Acoustic Waves

Cell orientation is essential in many applications in biology, medicine, and chemistry, such as cellular injection, intracellular biopsy, and genetic screening. However, the manual cell orientation technique is a trial-and-error approach, which suffers from low efficiency and low accuracy. Although several techniques have improved these issues to a certain extent, they still face problems deforming or disrupting cell membranes, physical damage to the intracellular structure, and limited particle size. This study proposes a noncontact and noninvasive cell orientation method that rotates a cell using surface acoustic waves (SAWs). To realize the acoustic cell orientation process, we have fabricated a microdevice consisting of two pairs of focused interdigital transducers (FIDTs). Instead of rotating the entire cell, the proposed method rotates the intracellular structure, the cytoplasm, directly through the cell membrane by acoustic force. We have tested the rotational manipulation process on 30 zebrafish embryos. The system was able to orientate a cell to a target orientation with a one-time success rate of 93%. Furthermore, the postoperation survival rate was 100%. Our acoustic rotational manipulation technique is noninvasive and easy to use, which provides a starting point for cell-manipulation-essential tasks, such as single-cell analysis, organism studies, and drug discovery.

Focused surface acoustic wave induced nano-oscillator based reservoir computing

We demonstrate using micromagnetic simulations that a nanomagnet array excited by surface acoustic waves (SAWs) can work as a reservoir. An input nanomagnet is excited with focused SAW and coupled to several nanomagnets, seven of which serve as output nanomagnets. To evaluate memory effect and computing capability, we study the short-term memory (STM) and parity check (PC) capacities, respectively. The SAW (4 GHz carrier frequency) amplitude is modulated to provide a sequence of sine and square waves of 100 MHz frequency. The responses of the selected output nanomagnets are processed by reading the envelope of their magnetization states, which is used to train the output weights using the regression method. For classification, a random sequence of 100 square and sine wave samples is used, of which 80% are used for training, and the rest are used for testing. We achieve 100% training and 100% testing accuracy. The average STM and PC are calculated to be ∼4.69 and ∼5.39 bits, respectively, which is indicative of the proposed acoustically driven nanomagnet oscillator array being well suited for physical reservoir computing applications. The energy dissipation is ∼2.5 times lower than a CMOS-based echo-state network. Furthermore, the reservoir is able to accurately predict Mackey-Glass time series up to several time steps ahead. Finally, the ability to use high frequency SAW makes the nanomagnet reservoir scalable to small dimensions, and the ability to modulate the envelope at a lower frequency (100 MHz) adds flexibility to encode different signals beyond the sine/square waves classification and Mackey-Glass predication tasks demonstrated here.

Microfluidic acoustic valve for capturing locomotive microorganism without anesthesia

The capture of locomotive small organisms is challenging but interesting. In this study, we developed a method to capture Paramecium with acoustic valve. Constructed by the focused surface acoustic waves (FSAW), acoustic valve can be used to block the dispersed phase in liquid flow. Under the joint action of acoustic radiation force and drag force, Paramecium can be successfully confined in a narrow space in the channel without losing its movement ability. By adjusting the applied voltage and the inlet pressure, the Paramecium can be allowed to pass, but the Paramecium cysts get captured. This acoustic device is potential in biochemical experiments, providing a new method for capturing or sorting micro swimmers.

High-Sensitivity Electrochemical Sensors Assisted by Focused Surface Acoustic Wave in Microfluidic Systems

High-Sensitivity Electrochemical Sensors Assisted by Focused Surface Acoustic Wave in Microfluidic Systems

Label-Free Multivariate Biophysical Phenotyping-Activated Acoustic Sorting at the Single-Cell Level.

Biophysical markers of cells such as cellular electrical and mechanical properties have been proven as promising label-free biomarkers for studying, characterizing, and classifying different cell types and even their subpopulations. Further analysis or manipulation of specific cell types or subtypes requires accurate isolation of them from the original heterogeneous samples. However, there is currently a lack of cell sorting ability that could actively separate a large number of individual cells at the single-cell level based on their multivariate biophysical makers or phenotypes. In this work, we, for the first time, demonstrate label-free and high-throughput acoustic single-cell sorting activated by the characterization of multivariate biophysical phenotypes. Electrical phenotyping is implemented by single-cell electrical impedance characterization with two pairs of differential sensing electrodes, while mechanical phenotyping is performed by extracting the transit time for the single cell to pass through microconstriction from the recorded impedance signals. A real-time impedance signal processing and triggering algorithm has been developed to identify the target sample population and activate a pulsed highly focused surface acoustic wave for single-cell level sorting. We have demonstrated acoustic single-particle sorting solely based on electrical or mechanical phenotyping. Furthermore, we have applied the developed microfluidic system to sort live MCF-7 cells from a mixture of fixed and live MCF-7 population activated by a combined electrical and mechanical phenotyping at a high throughput >100 cells/s and purity ∼91.8%. This demonstrated ability to analyze and sort cells based on multivariate biophysical phenotyping provides a solution to the current challenges of cell purification that lack specific molecular biomarkers.

Focused surface acoustic wave locally removes cells from culture surface.

Regenerative medicine and drug development require large numbers of high-quality cells, usually delivered from in vitro culturing. During culturing, the appearance of unwanted cells and an inability to remove them without damaging or losing most if not all the surrounding cells in the culture reduce the overall quality of the cultured cells. This is a key problem in cell culturing, as is the inability to sample cells from a culture as desired to verify the quality of the culture. Here, we report a method to locally remove cells from an adherent cell culture using a 100.4 MHz focused surface acoustic wave (SAW) device. After exposing a plated C2C12 mouse myoblast cell culture to phosphate buffered solution (PBS), ultrasound from the SAW device transmitted into the cell culture via a coupling water droplet serves to detach a small grouping of cells. The cells are removed from an area 6 × 10-3 mm2, equivalent to about 12 cells, using a SAW device-Petri dish water gap of 1.5 mm, a PBS immersion time of 300 s, and an input voltage of 75 V to the SAW device. Cells were released as desired 90% of the time, releasing the cells from the target area nine times out of ten runs. In the one trial in ten that fails, the cells partially release and remain attached due to inter-cellular binding. By making it possible to target and remove small groups of cells as desired, the quality of cell culturing may be significantly improved. The small group of cells may be considered a colony of iPS cells. This targeted cell removal method may facilitate sustainable, contamination-free, and automated refinement of cultured cells.

Compact acousto-optic multimode interference device in (Al,Ga)As.

Multimode interference (MMI) devices are key components in modern integrated photonic circuits. Here, we present acoustically tuned optical switches on an (Al,Ga)As platform that enable robust, compact and fast response systems improving on recently demonstrated technology. The device consists of a 2 × 2 MMI device fine-tuned in its center region by a focused surface acoustic wave (SAW) beam working in the low GHz range. In this way, we can tune the refractive index profile over a narrow modulation region and thus control the optical switching behaviour via the applied SAW intensity. Direct tuning of the MMI device avoids losses and phase errors inherent to arrayed waveguide based switches, while also reducing the dimensions of the photonic circuit.

Droplet Ejection at Controlled Angles via Acoustofluidic Jetting.

We study the nozzle-free ejection of liquid droplets at controlled angles from a sessile drop actuated from two, mutually opposed directions by focused surface acoustic waves with dissimilar parameters. Previous researchers assumed that jets formed in this way are limited by the Rayleigh angle. However, when we carefully account for surface tension in addition to the driving force, acoustic streaming, we find a quantitative model that reduces to the Rayleigh angle only when inertia is dominant, and suggests larger ejection angles are possible in many practical situations. We confirm this in demonstrating ejection at more than double the Rayleigh angle. Our model explains the effects of both fluid and input parameters on experiments with a range of liquids. We extract, from this model, a dimensionless number that serves as an analog for the typical Weber number for predicting single droplet events.

Chaotic Mixing of Microdroplets Using Surface Acoustic Waves Device

Small-scale mixing or actively known as micromixing had an utmost importance in the biological and chemical applications using micro total analysis systems (TAS) or lab-onchips. Micromixing is achieved by stirring or agitating liquid or particles. However, in microfluidic technology, it is very difficult to do mixing for a very small amount of liquid including microliter volume. Chemical reactions are often involved in most microfluidic applications, hence making the fluid diffusivity to become very low. Thus, chaotic advection was introduced in this study to shorten the reaction time. Conveniently, a new method was proposed to actively mix micro volume liquid. In this study, we developed focused-surface acoustic wave (F-SAW) fabricated on a piezoelectric substrate to manipulate or mix two different color dyes with low concentration. Operation frequency of 50 MHz was used to generate acoustic waves along the substrate surface and we varied the droplet volume and input power on the F-SAW mixing. Method comparison between passive and active mixing was done by studying the time taken for diffusion mixing. Designing, fabrication and test on Y-junction channel micromixers were done to compare and examine the mixing performance and time. Experimental results showed that mixing by F-SAW device has achieved high efficiency of 91 – 84% in droplet volume of 1 – 5 µL. Therefore, F-SAW mixing has been proven more efficient and fast response compared to diffusion mixing.

Greatly enhanced magneto-optic detection of single nanomagnets using focused magnetoelastic excitation

Magnetization dynamics of nanomagnets directly determine the performance of magnetic storage and memory devices. Here, we report a 10-times enhancement of magnetization dynamics excitation of single nanomagnets using focused surface acoustic waves (SAWs), compared to conventional optical excitation. SAWs are generated via ultrafast optical excitation of an arc-shaped phononic grating and focused onto a single nanomagnet located at the focal spot of the grating. Thanks to the robust resonance excitation, we observe the strain-controlled ultrafast magnetization dynamics in a sub-100 nm single nanomagnet. This improved excitation efficiency was applied to exciting SAWs in four sets of gratings with different pitches using a single laser spot. This enabled selective excitation of any one of four identical nanomagnets at different frequencies simply by tuning an external magnetic field. This all-optical technique provides a method of addressing individual magnetic nano-oscillators and studying their intrinsic magnetization dynamics.

Design of acoustofluidic device for localized trapping.

State of the art acoustofluidics typically treat micro-particles in a multi-wavelength range due to the scale limitations of the established ultrasound field. Here, we report a spatial selective acoustofluidic device that allows trapping micro-particles and cells in a wavelength scale. A pair of interdigital transducers with a concentric-arc shape is used to compress the beam width, while pulsed actuation is adopted to localize the acoustic radiation force in the wave propagating direction. Unlike the traditional usage of geometrical focus, the proposed device is designed by properly superposing the convergent section of two focused surface acoustic waves. We successfully demonstrate a single-column alignment of 15-μm polystyrene particles and double-column alignment of 8-μm T cells in a wavelength scale. Through proof-of-concept experiments, the proposed acoustofluidic device shows potential applications in on-chip biological and chemical analyses, where localized handing is required.

Acoustic-Controlled Bubble Generation and Fabrication of 3D Polymer Porous Materials.

Porous materials have a variety of applications such as catalysis, gas separation, sensing, tissue engineering, sewage treatment, and so on. However, there are still challenges in the synthesis of porous materials with light weight, high porosity, and superhydrophobicity. Herein, we demonstrate one acoustic-controlled microbubble generation method, which is used to synthesize 3D polymer porous materials. The acoustic-controlled microbubble generation based on focused surface acoustic wave (FSAW) is suitable for not only the generation of gas-in-oil microbubbles but also the gas-in-water microbubbles. The size of microbubbles can be real-time controlled by adjusting the frequency or the driving voltage of the FSAW. The as-prepared poly(vinyl alcohol) (PVA) foams composed of microbubbles can be used as a template to fabricate the PVA-based porous gel materials through freezing-thawing cyclic processing, and the various sized bubbles result in different porosity of the PVA-based porous gel materials. Moreover, excellent properties like oleophilicity and superhydrophobicity of the PVA-based porous gel materials can be obtained through a further hydrophobic modification treatment. The oil/water separation experiments have been done to demonstrate the good absorption and reliability of the modified porous gel materials, which are capable of multiple uses.

Focusing surface acoustic waves assisted electrochemical detector in microfluidics.

This article demonstrates a novel electrochemical detection device. The device is composed by two focusing interdigital transducers for exciting focused surface acoustic waves by applying an AC signal, a three-electrode system for electrochemical measurement, and a liquid pool for holding liquid on a LiNbO3 wafer. The amperometry current of ferrocenecarboxylic acid and potassium phosphate buffer solution is used to characterize the detection sensitivity. Two experiments are carried out to optimize the device design. The result shows that the two focusing interdigital transducers with arc degree 30° and distance 5mm can remarkably enhance the liquid mixing rate. Under this condition, the oxidation current is about 27 times larger than that without surface acoustic wave stirring.

Detachable Acoustophoretic System for Fluorescence-Activated Sorting at the Single-Droplet Level.

Droplet-based single-cell sequencing has emerged as a very powerful tool to study the cellular heterogeneity in diseased tissues for a variety of biological problems. However, the current droplet generation with a single particle and cell encapsulation is a random process and suffers from a low yield that is unable to fulfill the high-throughput analysis requirement. In this work, we present a new fluorescence-activated droplet sorting (FADS) system that can isolate single-cell droplets at high accuracy and high yield using a highly focused surface acoustic wave (HFSAW) with a beam width around 50 μm. The acoustic wave is locally coupled into the microfluidic channel for droplet sorting through a micropillar waveguide structure between the channel and the interdigitated transducer (IDT). This detachable acoustic sorting system allows the disposal of the microfluidic channel after a single use to avoid cross-contamination and keeps the expensive IDT device reusable. We have achieved rapid and accurate isolation of single-cell droplets with purity higher than 90% at ∼1 kHz sorting rate with three different encapsulation contents. In addition, with the uniformly produced droplet size at ∼40 μm, the present acoustic FADS system enables effective sorting of small particles down to submicrometer size, which is challenging for existing fluorescence-activated cell sorting systems.

Focused surface acoustic waves induced microdroplets generation and its application for microgels

We demonstrate a novel microdroplets generation method by using focused surface acoustic waves (FSAW). The method differs from previous work in the mechanism and the geometry structure of the chip, which is depend on FSAW rather than flow shearing. The acoustic radiation force arising from FSAW acts on the oil-water interface, breaking up the water into microdroplets whose size can be controlled by tuning the driving voltage and frequency of FSAW. This approach overcomes the limitation of microchannel structure and capillary number of the traditional microfluidic droplets formation methods, which enables a fully electrical control of microdroplets size, with a good uniformity. Further, the FSAW-induced microdroplets are used as templates to prepare microgels with uniform size, offering a new platform for the design and synthesis of monodispersed functional microspheres.