3D numerical simulation of acoustophoretic motion induced by boundary-driven acoustic streaming in standing surface acoustic wave microfluidics

A Review of SAW-Based Micro- and Nanoparticle Manipulation in Microfluidics

Surface acoustic wave (SAW)-based microfluidics has emerged as a promising technology for precisely manipulating particles and cells at the micro- and nanoscales. Acoustofluidic devices offer advantages such as low energy consumption, high throughput, and label-free operation, making them suitable for particle manipulation tasks including pumping, mixing, sorting, and separation. In this review, we provide an overview and discussion of recent advancements in SAW-based microfluidic devices for micro- and nanoparticle manipulation. Through a thorough investigation of the literature, we explore interdigitated transducer designs, materials, fabrication techniques, microfluidic channel properties, and SAW operational modes of acoustofluidic devices. SAW-based actuators are mainly based on lithium niobate piezoelectric transducers, with a plethora of wavelengths, microfluidic dimensions, and transducer configurations, applied for different fluid manipulation methods: mixing, sorting, and separation. We observed the accuracy of particle sorting across different size ranges and discussed different alternative device configurations to enhance sensitivity. Additionally, the collected data show the successful implementation of SAW devices in real-world applications in medical diagnostics and environmental monitoring. By critically analyzing different approaches, we identified common trends, challenges, and potential areas for improvement in SAW-based microfluidics. Furthermore, we discuss the current state-of-the-art and opportunities for further research and development in this field.

Dependence of acoustophoretic aggregation on the impedance of microchannel's walls

BACKGROUND AND OBJECTIVES

Acoustofluidic manipulation of particles and biological cells has been widely applied in various biomedical and engineering applications, including effective separation of cancer cell, point-of-care diagnosis, and cell patterning for tissue engineering. It is often implemented within a polydimethylsiloxane (PDMS) microchannel, where standing surface acoustic waves (SSAW) are generated by sending two counter-propagating ultrasonic waves on a piezoelectric substrate.

METHODS

In this paper, we develop a full cross-sectional model of the acoustofluidic device using finite element method, simulating the wave excitation on the substrate and wave propagation in both the fluid and the microchannel wall. This model allows us to carry out extensive parametric analyses concerning the acoustic properties of the fluid and the microchannel wall, as well as the dimensions of the channel, to explore their influences on the acoustic field, fluid flow and microparticle aggregation.

RESULTS

Our findings demonstrate an order-of-magnitude enhancement in acoustic pressure amplitude and aggregation speed and a reduction in the particle threshold radius to submicron levels, which can be achieved through adjustments to the channel height and the difference in acoustic impedance between the channel wall and the fluid. The optimum channel heights are determined, which depend on the acoustic properties of the channel wall. The particle trajectories, movements along pressure nodal planes, and terminal positions are identified, with relative strength between the radiation force and the streaming force compared in different combinations of parameters.

CONCLUSIONS

This work demonstrates that finetuning the dimensions and acoustic properties of the fluid and microchannel wall in acoustofluidic device can greatly enhance particle aggregation throughput and reduce constraints on particle size. Our findings offer valuable insights into device design and optimization.

Enhanced Particle Trap: Design and Simulation of Pillar-Based Contactless Dielectrophoresis Microfluidic Devices

Contactless and conventional dielectrophoresis (DEP) microfluidic devices are extensively utilized in lab-on-a-chip applications, particularly for cell isolation and analysis. Nonetheless, these devices typically operate at low throughput and require high applied voltages, posing limitations for microfluidic cell isolation and separation. Addressing these challenges, this study explores the utilization of diverse micro-pillar geometries within the microfluidic device to augment THP-1 cell trapping efficiency numerically using FEM modeling. Furthermore, the simulations examine the influence of pillar gap and quantity on cell trapping efficiency in a contactless DEP device. Notably, elliptical pillars demonstrate superior cell trapping efficiency at elevated flow rates compared to alternative configurations, making the microchip more amenable for high-throughput cell separation, trapping, and isolation applications. Remarkably, employing elliptical pillars in a contactless DEP microfluidic chip yields nearly 100% cell trapping efficiency at higher flow rates. Ellipse configuration showed 122% higher cell trap efficiency at the maximum flowrate compare to the previous study with circular configuration. Additionally, it is observed that reducing the gap between pillars correlates with enhanced cell trapping efficiency. Simulation outcomes indicate that employing two rows of elliptical pillars with a 40-µm gap achieves optimal performance. The findings of this investigation underscore the importance of pillars in contactless DEP devices and provide valuable insights for future designs of such microfluidic devices.

A traveling surface acoustic wave-based micropiezoactuator: A tool for additive- and label-free cell lysis

We propose a traveling surface acoustic wave (TSAW)-based microfluidic method for cell lysis that enables lysis of any biological entity, without the need for additional additives. Lysis of cells in the sample solution flowing through a poly (dimethyl siloxane) microchannel is enabled by the interaction of cells with TSAWs propagated from gold interdigitated transducers (IDTs) patterned onto a LiNbO3 piezoelectric substrate, onto which the microchannel was also bonded. Numerical simulations to determine the wave propagation intensities with varying parameters including IDT design, supply voltage, and distance of the channel from the IDT were performed. Experiments were then used to validate the simulations and the best lysis parameters were used to maximize the nucleic acid/protein extraction efficiency (>95%) within few seconds. A comparative analysis of our method with traditional chemical, physical and thermal, as well as the current microfluidic methods for lysis demonstrates the superiority of our method. Our lysis strategy can hence be used independently and/or integrated with other nucleic acid-based technologies or point-of-care devices for the lysis of any pathogen (Gram positives and negatives), eukaryotic cells, and tissues at low voltage (3 V) and frequency (33.17 MHz), without the use of amplifiers.

Investigation on submicron particle separation and deflection using tilted-angle standing surface acoustic wave microfluidics

With the development of in vitro diagnostics, extracting submicron scale particles from mixed body fluids samples is crucial. In recent years, microfluidic separation has attracted much attention due to its high efficiency, label-free, and inexpensive nature. Among the microfluidic-based separation, the separation based on ultrasonic standing waves has gradually become a powerful tool. A microfluid environment containing a tilted-angle ultrasonic standing surface acoustic wave (taSSAW) field has been widely adapted and designed to separate submicron particles for biochemical applications. This paper investigated submicron particle defection in microfluidics using taSSAWs analytically. Particles with 0.1-1 μm diameters were analyzed under acoustic pressure, flow rate, tilted angle, and SSAW frequency. According to different acoustic radiation forces acting on the particles, the motion of large-diameter particles was more likely to deflect to the direction of the nodal lines. Decreasing the input flow rate or increasing acoustic pressure and acoustic wave frequency can improve particle deflection. The tilted angle can be optimized by analyzing the simulation results. Based on the simulation analysis, we experimentally showed the separation of polystyrene microspheres (100 nm) from the mixed particles and exosomes (30-150 nm) from human plasma. This research results can provide a certain reference for the practical design of bioparticle separation utilizing acoustofluidic devices.

A Simulated Investigation of Lithium Niobate Orientation Effects on Standing Acoustic Waves

The integration of high-frequency acoustic waves with microfluidics has been gaining popularity as a method of separating cells/particles. A standing surface acoustic wave (sSAW) device produces constructive interference of the stationary waves, demonstrating an increase in cell separating efficiency without damaging/altering the cell structure. The performance of an sSAW device depends on the applied input signal, design of the IDT, and piezoelectric properties of the substrate. This work analyzes the characteristics of a validated 3D finite element model (FEM) of LiNbO3 and the effect on the displacement components of the mechanical waves under the influence of sSAWs by considering XY-, YX-, and 1280 YX-cut LiNbO3 with varying electrode length design. We demonstrated that device performance can be enhanced by the interference of multiple waves under a combination of input signals. The results suggest that 1280 YX-cut LiNbO3 is suitable for generating higher-amplitude out-of-plane waves which can improve the effectiveness of acoustofluidics-based cell separation. Additionally, the findings showed that the length of the electrode impacts the formation of the wavefront significantly.

Dual-Wave Acoustofluidic Centrifuge for Ultrafast Concentration of Nanoparticles and Extracellular Vesicles

Extracellular vesicles (EVs) are secreted nanostructures that play various roles in critical cancer processes. They operate as an intercellular communication system, transferring complex sets of biomolecules from cell to cell. The concentration of EVs is difficult to decipher, and there is an unmet technological need for improved (faster, simpler, and gentler) approaches to isolate EVs from complex matrices. Herein, an acoustofluidic concentration of extracellular vesicles (ACEV) is presented, based on a thin-film printed circuit board with interdigital electrodes mounted on a piezoelectric substrate. An angle of 120° is identified between the electrodes and the reference flat of the piezoelectric substrate for simultaneous generation of Rayleigh and shear horizontal waves. The dual waves create a complex acoustic field in a droplet, resulting in effective concentration of nanoparticles and EVs. The ACEV is able to concentrate 20 nm nanospheres within 105 s and four EV dilutions derived from the human prostate cancer (Du145) cell line in approximately 30 s. Cryo-electron microscopy confirmed the preservation of EV integrity. The ACEV device holds great potential to revolutionize investigations of EVs. Its faster, simpler, and gentler approach to EV isolation and concentration can save time and effort in phenotypic and functional studies of EVs.

Three-dimensional modeling and experimentation of microfluidic devices driven by surface acoustic wave

Surface acoustic wave (SAW) technology is proving to be an effective tool for manipulating micro-nano particles. In this paper, we present a fully-coupled 3D model of standing SAW acoustofluidic devices for obtaining particle motion. The "improved limiting velocity method" (ILVM) was used to investigate the distribution of acoustic pressure and acoustic streaming in microchannel. The results show that the distribution of acoustic pressure and acoustic streaming on the piezoelectric substrate surface perpendicular to the acoustic wave propagation direction is inhomogeneous. The motion of micro-particles with diameters of 0.5-, 5-, and 10 μm is then simulated to investigate the interaction of acoustic radiation force and drag force caused by pressure and acoustic streaming. We demonstrate that micro and nanoparticles can move in three dimensions when acoustic radiation force and acoustic streaming interact. This result and method are critical for designing SAW microfluidic chips and controlling particle motion precisely.

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.

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.

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.