Traveling Surface Acoustic Wave Research Articles

On Chip Sorting of Stem Cell-Derived β Cell Clusters Using Traveling Surface Acoustic Waves.

There is a critical need for sorting complex materials, such as pancreatic islets of Langerhans, exocrine acinar tissues, and embryoid bodies. These materials are cell clusters, which have highly heterogeneous physical properties (such as size, shape, morphology, and deformability). Selecting such materials on the basis of specific properties can improve clinical outcomes and help advance biomedical research. In this work, we focused on sorting one such complex material, human stem cell-derived β cell clusters (SC-β cell clusters), by size. For this purpose, we developed a microfluidic device in which an image detection system was coupled to an actuation mechanism based on traveling surface acoustic waves (TSAWs). SC-β cell clusters of varying size (∼100-500 μm in diameter) were passed through the sorting device. Inside the device, the size of each cluster was estimated from their bright-field images. After size identification, larger clusters, relative to the cutoff size for separation, were selectively actuated using TSAW pulses. As a result of this selective actuation, smaller and larger clusters exited the device from different outlets. At the current sample dilutions, the experimental sorting efficiency ranged between 78% and 90% for a separation cutoff size of 250 μm, yielding sorting throughputs of up to 0.2 SC-β cell clusters/s using our proof-of-concept design. The biocompatibility of this sorting technique was also established, as no difference in SC-β cell cluster viability due to TSAW pulse usage was found. We conclude the proof-of-concept sorting work by discussing a few ways to optimize sorting of SC-β cell clusters for potentially higher sorting efficiency and throughput. This sorting technique can potentially help in achieving a better distribution of islets for clinical islet transplantation (a potential cure for type 1 diabetes). Additionally, the use of this technique for sorting islets can help in characterizing islet biophysical properties by size and selecting suitable islets for improved islet cryopreservation.

Traveling Surface Acoustic Wave Induced Removal of NSB Proteins from the Acoustic Biosensor

Traveling Surface Acoustic Wave Induced Removal of NSB Proteins from the Acoustic Biosensor

A surface treatment method for improving the attachment of PDMS: acoustofluidics as a case study

A method for a permanent surface modification of polydimethylsiloxane (PDMS) is presented. A case study on the attachment of PDMS and the lithium niobate (LiNbO3) wafer for acoustofluidics applications is presented as well. The method includes a protocol for chemically treating the surface of PDMS to strengthen its bond with the LiNbO3 surface. The PDMS surface is modified using the 3-(trimethoxysilyl) propyl methacrylate (TMSPMA) silane reagent. The effect of silane treatment on the hydrophilicity, morphology, adhesion strength to LiNbO3, and surface energy of PDMS is investigated. The results demonstrated that the silane treatment permanently increases the hydrophilicity of PDMS and significantly alters its morphology. The bonding strength between PDMS and LiNbO3increased with the duration of the silane treatment, reaching a maximum of approximately 500 kPa. To illustrate the effectiveness of this method, an acoustofluidic device was tested, and the device demonstrated very promising enhanced bonding and sealing capabilities with particle manipulation at a flow rate of up to 1 L/h by means of traveling surface acoustic waves (TSAW). The device was reused multiple times with no fluid leakage or detachment issues. The utility of the presented PDMS surface modification method is not limited to acoustofluidics applications; it has the potential to be further investigated for applications in various scientific fields in the future.

Acoustofluidics-Assisted Coating of Microparticles.

Microparticles have been applied in many areas, ranging from drug delivery, diagnostics, cosmetics, personal care, and the food industry to chemical and catalytic reactions, sensing, and environmental remediation. Coating further provides additional functionality to the microparticles, such as controlled release, surface modification, bio-fouling resistance, stability, protection, etc. In this study, the conformal coating of microparticles with a positively charged polyelectrolyte (polyallylamine hydrochloride, PAH) by utilizing an acoustofluidic microchip was proposed and demonstrated. The multiple laminar streams, including the PAH solution, were formed inside the microchannel, and, under the traveling surface acoustic wave, the microparticles traversed through the streams, where they were coated with PAH. The results showed that the coating of microparticles can be achieved in a rapid fashion via a microfluidic approach compared to that obtained by the batch method. Moreover, the zeta potentials of the microparticles coated via the microfluidic approach were more uniform. For the unfunctionalized microparticles, the charge reversal occurred after coating, and the zeta potential increased as the width of the microchannel or the concentration of the PAH solution increased. As for the carboxylate-conjugated microparticles, the charge reversal again occurred after coating; however, the magnitudes of the zeta potentials were similar when using the microchannels with different widths or different concentrations of PAH solution.

Computational optimization of acoustofluidic devices for advanced particle micromanipulation

Acoustofluidic devices, which combine principles of acoustics and microfluidics, have emerged as a promising platform for biological micro-object micromanipulation due to their non-invasive, accurate, rapid, and label-free qualities. Acoustofluidic devices have found utility in various biomedical applications including single-cell studies, point-of-care testing, lab-on-a-chip studies, and tissue engineering. In this work, we present novel device configurations which enable complex and high-resolution control of suspended micro-objects. The studies presented here utilize computational analysis to optimize (1) traveling surface acoustic wave device dimensions, (2) the configuration of a planar acoustic resonator that integrates a structured surface, (3) the thickness of the coupling layer and superstrate materials for bulk-wave transmission, and (4) the shape of acoustically actuated 3-D microstructures. These devices were verified experimentally to generate highly localized acoustic fields and acoustic streaming effects enabling rapid and spatially controllable particle capture, transport, and patterning. In doing so, this work demonstrates that computational analysis is an integral part of the development of acoustofluidic devices for advanced micromanipulation, which have extensive potential in biomedical applications.

Acoustofluidic patterning in glass capillaries using travelling acoustic waves based on thin film flexible platform

Surface acoustic wave (SAW) technology has been widely used to manipulate microparticles and biological species, based on acoustic radiation force (ARF) and drag force induced by acoustic streaming, either by standing SAWs (SSAWs) or travelling SAWs (TSAWs). These acoustofluidic patterning functions can be achieved within a polymer chamber or a glass capillary with various cross-sections positioned along the wave propagating paths. In this paper, we demonstrated that microparticles can be aligned, patterned, and concentrated within both circular and rectangular glass capillaries using TSAWs based on a piezoelectric thin film acoustic wave platform. The glass capillary was placed at different angles along with the interdigital transducer directions. We systematically investigated effects of tilting angles and wave characteristics using numerical simulations in both circular and square shaped capillaries, and the patterning mechanisms were discussed and compared with those agitated under the SSAWs. We then experimentally verified the particle patterns within different glass capillaries using thin film ZnO SAW devices on aluminum (Al) sheets. Results show that the propagating SAWs can generate acoustic pressures and patterns in the fluid due to the diffractive effects, drag forces and ARF, as functions of the SAW device’s resonant frequency and tilting angle. We demonstrated potential applications using this multiplexing, integrated, and flexible thin film-based platform, including patterning particles (1) inside multiple and successively positioned circular tubes; (2) inside a solidified hydrogel in the glass capillary; and (3) by wrapping a flexible ZnO/Al SAW device around the glass capillary.

Numerical study of microdevice with surface acoustic waves for separation of gas mixtures

Recently, it was shown that traveling surface acoustic waves (SAWs) can affect gas flow in microchannels. The effect of SAWs was studied in free-molecular flow regime, and it was shown that SAWs can induce separation of gas mixtures. In the present work, this effect is studied for denser flow regimes, which are more interesting from a practical point of view. The problem is studied numerically using own modification of the direct simulation Monte Carlo method on the example of a neon–argon mixture. The main finding is that SAWs still enhance separation of gas mixtures outflowing into vacuum through a microchannel under all studied rates of gas rarefaction up to Kn≈0.1. Another important practical result is that effect is present for wave speeds typical for existing SAWs (≈1000 m/s) and in a wide range of SAW amplitude to channel height ratios. Influence of other practical aspects, such as channel length, masses of species, and available magnitudes of material surface speed, are also briefly discussed.

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.

Multiple virus sorting based on aptamer-modified microspheres in a TSAW device

Due to the overlapping epidemiology and clinical manifestations of flaviviruses, differential diagnosis of these viral diseases is complicated, and the results are unreliable. There is perpetual demand for a simplified, sensitive, rapid and inexpensive assay with less cross-reactivity. The ability to sort distinct virus particles from a mixture of biological samples is crucial for improving the sensitivity of diagnoses. Therefore, we developed a sorting system for the subsequent differential diagnosis of dengue and tick-borne encephalitis in the early stage. We employed aptamer-modified polystyrene (PS) microspheres with different diameters to specifically capture dengue virus (DENV) and tick-borne encephalitis virus (TBEV), and utilized a traveling surface acoustic wave (TSAW) device to accomplish microsphere sorting according to particle size. The captured viruses were then characterized by laser scanning confocal microscopy (LSCM), field emission scanning electron microscopy (FE-SEM) and reverse transcription-polymerase chain reaction (RT‒PCR). The characterization results indicated that the acoustic sorting process was effective and damage-free for subsequent analysis. Furthermore, the strategy can be utilized for sample pretreatment in the differential diagnosis of viral diseases.

Modeling of red blood cell deformation in a microchannel driven by traveling surface acoustic waves

The static deformation of red blood cells in a microfluidic channel driven by traveling surface acoustic wave (SAW) is theoretically and numerically investigated. The computational framework takes into account the coupling effect of acoustic and mechanic, that is, the propagating acoustic wave generates steady acoustic radiation stress to deform the cell and in turn the deformed cell reshapes the acoustic field. Specifically, the acoustic field is calculated by using a simplified fluid model, which includes the fluid domain with simplified boundary conditions representing the surrounding solids, and the cell is modeled as an elastic biological membrane enclosing with a homogenous fluid. Results show that the SAW generates periodically distributed acoustic pressure nodes in the customized fluid channel, where the cell is predicted to be trapped. The trapped cells are observed to be oriented with their flat nearly perpendicular to the device substrate and stretched along the propagation direction of the SAW, which is consistent with the experimental observations. The parallel cell manipulation is further simulated to illustrate the high-throughput characteristics of the acoustic deformation method. The present computational model is helpful to extract the mechanical properties of cells accurately and design the acoustic microfluidic channel to detect cells reasonably.

An ultra-compact acoustofluidic device based on the narrow-path travelling surface acoustic wave (np-TSAW) for label-free isolation of living circulating tumor cells

Obtaining highly purified intact living cells from complex environments has been a challenge, such as the isolation of circulating tumor cells (CTCs) from blood. In this work, we demonstrated an acoustic-based ultra-compact device for cell sorting, with a chip size of less than 2 × 1.5 cm2. This single actuator device allows non-invasive and label-free isolation of living cells, offering greater flexibility and applicability. The device performance was optimized with different-sized polystyrene (PS) particles and blood cells spiked with cancer cells. Using the narrow-path travelling surface acoustic wave (np-TSAW), precise isolation of 10 μm particles from a complex mixture of particles (5, 10, 20 μm) and separation of 8 μm and 10 μm particles was achieved. The purified collection of 10 μm particles with high separation efficiency (98.75%) and high purity (98.1%) was achieved by optimizing the input voltage. Further, we investigated the isolation and purification of CTCs (MCF-7, human breast cancer cells) from blood cells with isolation efficiency exceeding 98% and purity reaching 93%. Viabilities of the CTCs harvested from target-outlet were all higher than 97% after culturing for 24, 48, and 72 h, showing good proliferation ability. This novel ultra-miniaturized microfluidic chip demonstrates the ability to sorting cells with high-purity and label-free, providing an attractive miniaturized system alternative to traditional sorting methods.

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.

High throughput particle sorting based on traveling surface acoustic wave (TSAW) is realized by coupling spiral microchannel and a novel arc electrode

Acoustofluidic technology is an ideal tool for biomedical applications. However, the sorting performance and flux of acoustofluidic chips cannot be achieved simultaneously. In this paper, we propose a method for coupling a novel arc gold interdigital transducer (IDT) with a spiral microchannel. A spiral microchannel is used to focus the target particle (20 μm) inertially to improve the sorting efficiency of the chip. The channel uses a Y-shaped inlet to bind small particles (5 μm) in a fixed stream beam to improve the sorting efficiency of the chip. The fit of arc electrode and spiral flow channel enlarges the acoustic control area, so that the target particles can still be separated effectively at high flow velocity. The results show that when the signal source frequency is 33.7 MHz and the voltage is 5 V, the designed chip can effectively separate the target particles at the flow rate of 25–65 μl min−1, and the sorting purity is 100%. The sorting efficiency decreased with the increase of flow rate, 25 μl min−1: 100 %, 35 μl min−1: 100 %, 45 μl min−1: 94 .8% (within 1.2% error), 55 μl min−1: 92 % (within 1% error), 65 μl min−1: 83 .4% (within 2.4% error). This chip provides an idea for achieving high throughput, high purity and high efficiency cell sorting.

Multifunctional acoustofluidic centrifuge device using tri-symmetrical design for particle enrichment and separation and multiphase microflow mixing

Effective enrichment and separation of particles and mixing of multiphase microflows can be technically challenging in microfluidic sample processing and analysis. Acoustofluidic techniques have advantages such as high biocompatibility, versatility, and simple device design to meet these demands. Here, we present a novel tri-symmetrical acoustofluidic centrifuge based on traveling surface acoustic waves (TSAWs), achieving particle enrichment and bidirectional size-based separation, pushing away the larger particles while enriching the smaller ones, in a rapid time (<6 s). A microfluidic chamber was designed for further extraction of treated particles and cells. Experimental results show that our device can achieve enrichment of particles in a range of sizes and high purity separation (e.g. >90 % in 5 µm and 40 µm sample), demonstrating its sample processing capabilities. The acoustic treatment did not affect cells (consistent with the control group above 80 %). TSAW-induced acoustic streaming enabled efficient mixing (mixing efficiency χ >85 %) of fluorescent multi-phase microflows within a short time (<0.5 s). The underlying physical mechanisms have been verified by numerical simulations.

Acoustofluidic lysis of cancer cells and Raman spectrum profiling.

The lysis of cancer cells inside a sessile droplet was performed using traveling surface acoustic waves (SAWs) without any chemical reagents. Raman spectrum profiling was then carried out to explore detailed cell-derived data. The Rayleigh waves formed by an interdigital transducer were made to propagate along the surface of an LiNbO3 substrate. Polystyrene microparticles (PSMPs) were used to establish mechanical cell lysis effectively, and gold nanoparticles (AuNPs) were added to enhance the Raman signals from the lysed cells by SAWs. The lysis efficiency was evaluated according to the size and concentration of the PSMPs in experiments where the frequency was varied. Lysis occurred mainly by mechanical collision using PSMPs in a high-frequency domain, and the lysis efficiency was improved by increasing the application time and the energy density of the SAWs. Raman signals from the lysed cells were greatly enhanced by nanogaps formed by the AuNPs, which were evenly distributed irrespective of the SAWs through the frequency-independent behavior of the AuNPs. Finally, detailed Raman spectra of MDA-MB-231, malignant breast cancer cells, were acquired, and various organic matter-derived peaks were observed. The 95% confidence region for cells subjected to lysis was more widely distributed than that of cells not subjected to lysis. The proposed SAW platform is expected to facilitate the detection of small quantities and to be applied in biomedical applications.

Image-based cell sorting using focused travelling surface acoustic waves.

Sorting cells is an essential primary step in many biological and clinical applications such as high-throughput drug screening, cancer research and cell transplantation. Cell sorting based on their mechanical properties has long been considered as a promising label-free biomarker that could revolutionize the isolation of cells from heterogeneous populations. Recent advances in microfluidic image-based cell analysis combined with subsequent label-free sorting by on-chip actuators demonstrated the possibility of sorting cells based on their physical properties. However, the high purity of sorting is achieved at the expense of a sorting rate that lags behind the analysis throughput. Furthermore, stable and reliable system operation is an important feature in enabling the sorting of small cell fractions from a concentrated heterogeneous population. Here, we present a label-free cell sorting method, based on the use of focused travelling surface acoustic wave (FTSAW) in combination with real-time deformability cytometry (RT-DC). We demonstrate the flexibility and applicability of the method by sorting distinct blood cell types, cell lines and particles based on different physical parameters. Finally, we present a new strategy to sort cells based on their mechanical properties. Our system enables the sorting of up to 400 particles per s. Sorting is therefore possible at high cell concentrations (up to 36 million per ml) while retaining high purity (>92%) for cells with diverse sizes and mechanical properties moving in a highly viscous buffer. Sorting of small cell fraction from a heterogeneous population prepared by processing of small sample volume (10 μl) is also possible and here demonstrated by the 667-fold enrichment of white blood cells (WBCs) from raw diluted whole blood in a continuous 10-hour sorting experiment. The real-time analysis of multiple parameters together with the high sensitivity and high-throughput of our method thus enables new biological and therapeutic applications in the future.

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.

Unsteady time-averaged streaming in microfluidics using traveling surface acoustic waves

Unsteady time-averaged streaming in microfluidics using traveling surface acoustic waves

A Perturbed Asymmetrical Y-TypeSheathless Chip for Particle Control Based on Adjustable Tilted-Angle Traveling Surface Acoustic Waves (ataTSAWs).

The precise control of target particles (20 µm) at different inclination angles θi is achieved by combining a perturbed asymmetric sheathless Y-type microchannel and a digital transducer (IDT). The offset single-row micropillar array with the buffer area can not only concentrate large and small particles in a fixed region of the flow channel, but also avoid the large deflection of some small particles at the end of the array. The addition of the buffer area can effectively improve the separation purity of the chip. By exploring the manufacturing process of the microchannel substrate, an adjustable tilted-angle scheme is proposed. The use of ataTSAW makes the acoustic field area in the microchannel have no corner effect region. Through experiments, when the signal source frequency was 33.6 MHz, and the flow rate was 20 µL/min, our designed chip could capture 20 µm particles when θi = 5°. The deflection of 20 µm particles can be realized when θi = 15°–45°. The precise dynamic separation of 20 µm particles can be achieved when θi = 25°–45°, and the separation purity and efficiency were 97% and 100%, respectively.

Enhanced Detection in Droplet Microfluidics by Acoustic Vortex Modulation of Particle Rings and Particle Clusters via Asymmetric Propagation of Surface Acoustic Waves.

As a basis for biometric and chemical analysis, issues of how to dilute or concentrate substances such as particles or cells to specific concentrations have long been of interest to researchers. In this study, travelling surface acoustic wave (TSAW)-based devices with three frequencies (99.1, 48.8, 20.4 MHz) have been used to capture the suspended Polystyrene (PS) microspheres of various sizes (5, 20, 40 μm) in sessile droplets, which are controlled by acoustic field-induced fluid vortex (acoustic vortex) and aggregate into clusters or rings with particles. These phenomena can be explained by the interaction of three forces, which are drag force caused by ASF, ARF caused by Leaky-SAW and varying centrifugal force. Eventually, a novel approach of free transition between the particle ring and cluster was approached via modulating the acoustic amplitude of TSAW. By this method, multilayer particles agglomerate with 20 μm wrapped around 40 μm and 20 μm wrapped around 5 μm can be obtained, which provides the possibility to dilute or concentrate the particles to a specific concentration.