Influences of microparticle radius and microchannel height on SSAW-based acoustophoretic aggregation

Design and numerical study of a bidirectional acoustic microfluidic pump enabled by microcantilever arrays

Microfluidics offers a versatile and promising platform for various applications in biomedical and other fields, boasting cost-effectiveness, rapid analysis time, and a compact equipment footprint. However, achieving controlled and versatile microfluidic motion within implantable devices presents a significant challenge. In this study, we propose a novel bidirectional micro-pump design that leverages two sharp-edge microcantilever arrays, driven by ultrasound, to enable selective flow direction by manipulating the ultrasound frequency. Through systematic numerical simulation, we demonstrate the feasibility of this design and further optimize its performance through comprehensive parametric analysis. This work provides valuable guidance for the practical development of sharp-edge-based acoustic micro-pumps, particularly for potential implantable applications such as controlled drug release and in vivo sampling for advanced diagnostics.

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.

Acoustic Streaming Efficiency in a Microfluidic Biosensor with an Integrated CMUT

The effect of microchannel height on acoustic streaming velocity and capacitive micromachined ultrasound transducer (CMUT) cell damping was investigated. Microchannels with heights ranging from 0.15 to 1.75 mm were used in experiments, and computational microchannel models with heights varying from 10 to 1800 micrometers were simulated. Both simulated and measured data show local minima and maxima of acoustic streaming efficiency associated with the wavelength of the `bulk acoustic wave excited at 5 MHz frequency. Local minima occur at microchannel heights that are multiples of half the wavelength (150 μm), which are caused by destructive interference between excited and reflected acoustic waves. Therefore, microchannel heights that are not multiples of 150 μm are more favorable for higher acoustic streaming effectiveness since destructive interference decreases the acoustic streaming effectiveness by more than 4 times. On average, the experimental data show slightly higher velocities for smaller microchannels than the simulated data, but the overall observation of higher streaming velocities in larger microchannels is not altered. In additional simulation, at small microchannel heights (10-350 μm), local minima at microchannel heights that are multiples of 150 μm were observed, indicating the interference between excited and reflected waves and causing acoustic damping of comparatively compliant CMUT membranes. Increasing the microchannel height to over 100 μm tends to eliminate the acoustic damping effect as the local minima of the CMUT membrane swing amplitude approach the maximum value of 42 nm, which is the calculated amplitude of the freely swinging membrane under the described conditions. At optimum conditions, an acoustic streaming velocity of over 2 mm/s in a 1.8 mm-high microchannel was achieved.

An enhanced tilted-angle acoustic tweezer for mechanical phenotyping of cancer cells

Acoustofluidic devices becomes one of the emerging and versatile tools for many biomedical applications. Most of the previous acoustofluidic devices are used for cells manipulation, and the few devices for cell phenotyping with a limitation in throughput. In this study, an enhanced tilted-angle (ETA) acoustofluidic device is developed and applied for mechanophenotyping of live cells. The ETA Device consists of an interdigital transducer which is positioned along a microfluidic channel. An inclination angle of 5° is introduced between the interdigital transducer and the liquid flow direction. The pressure nodes formed inside the acoustofluidic field in the channel deflect the biological cells from their original course in accordance with their mechanical properties, including volume, compressibility, and density. The threshold power for fully converging the cells to the pressure node is used to calculate the acoustic contrast factor. To demonstrate the ETA device in cell mechanophenotyping, and distinguishing between different cell types, further experimentation is carried out by using A549 (lung cancer cells), MDB-MA-231 (breast cancer cells), and leukocytes. The resulting acoustic contrast factors for the lung and breast cancer cells are different from that of the leukocytes by 27.9% and 21.5%, respectively. These results suggest this methodology can successfully distinguish and phenotype different cell types based on the acoustic contrast factor.

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.

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.

Node formation mechanisms in acoustofluidic capillary bridges

Using acoustofluidic channels formed by capillary bridges two models are developed to describe nodes formed by leaky and by evanescent waves. The liquid channel held between a microscope slide (waveguide) and a strip of polystyrene film (fluid guide) avoids solid-sidewall interactions. With this simplification, our experimental and numerical study showed that waves emitted from a single plane surface, interfere and form the nodes without any resonance in the fluid. Both models pay particular attention to tensor elements normal to the solid-liquid interfaces they find that; initially nodes form in the solid and the node pattern is replicated by waves emitted into the fluid from antinodes in the stress. At fluids depths near half an acoustic wavelength, most nodes are formed by leaky waves. In the glass, water-loading reduces node-node separation and forms an overlay type waveguide which aligns the nodes predominantly along the channel. One new practical insight is that node separation can be controlled by water depth. At 0.2 mm water depths (which are smaller than a ¼ wavelength) nodes form from evanescent waves. Here a suspension of yeast cells formed a pattern of small dot-like clumps of cells on the surface of the polystyrene film. We found the same pattern in sound intensity normal, and close, to the water-polystyrene interface. The capillary bridge channel developed for this study is simple, low-cost, and could be developed for filtration, separation, or patterning of biological species in rapid immuno-sensing applications.