Modeling of Microdevices for SAW-Based Acoustophoresis - A Study of Boundary Conditions

A discussion about the velocity distribution commonly used as the boundary condition in surface acoustic wave numerical simulations

Surface acoustic waves in combination with microfluidics has become an attractive research field regarding its various medical and biological applications. It is sometimes preferred to solve just the fluid domain and apply some boundary conditions to represent other components rather than performing a coupled numerical solution. To account for the piezoelectric actuation, a conventional velocity distribution built by superposing the left-going and right-going surface waves is commonly used as the boundary condition, its correctness is assessed here by comparing it to a coupled solution. It was shown that the actual leaky surface acoustic wave in coupled solution has different wavelengths in its real and imaginary parts, sometimes gets out of being sinusoidal, and has a different form compared to the superposed formula. For the phase differences other than 0 and π between the left and right electrodes, the distance between the electrodes affects the streaming and acoustic fields in the microchannel thereby leading to deviations in particle traces. Furthermore, the ratio of the horizontal to vertical components of the surface wave was extracted from the coupled solutions and compared to its previously reported values. The sensitivity analysis showed that for small particles, this ratio does not affect the streaming pattern but changes its velocity magnitude causing a time lag. For larger particles, the ratio altered the movement direction. This study suggests not replacing the piezoelectric actuation with the boundary condition to avoid inaccuracy in resulting fields that are being used in calculations of particle tracing and acoustic radiation forces.

Surface Acoustic Wave-Based Microfluidic Device for Microparticles Manipulation: Effects of Microchannel Elasticity on the Device Performance

Size sorting, line focusing, and isolation of microparticles or cells are fundamental ingredients in the improvement of disease diagnostic tools adopted in biology and biomedicine. Microfluidic devices are exploited as a solution to transport and manipulate (bio)particles via a liquid flow. Use of acoustic waves traveling through the fluid provides non-contact solutions to the handling goal, by exploiting the acoustophoretic phenomenon. In this paper, a finite element model of a microfluidic surface acoustic wave-based device for the manipulation of microparticles is reported. Counter-propagating waves are designed to interfere inside a PDMS microchannel and generate a standing surface acoustic wave which is transmitted to the fluid as a standing pressure field. A model of the cross-section of the device is considered to perform a sensitivity analysis of such a standing pressure field to uncertainties related to the geometry of the microchannel, especially in terms of thickness and width of the fluid domain. To also assess the effects caused by possible secondary waves traveling in the microchannel, the PDMS is modeled as an elastic solid material. Remarkable effects and possible issues in microparticle actuation, as related to the size of the microchannel, are discussed by way of exemplary results.

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.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part I

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. For the first time, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is experimentally revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C). In Part I of this study, we focus on the fluid flow close to the center of the surface acoustic wave field, while in Part II the outer regions with strong acoustic gradients are investigated. By systematic variations of the SAW-wavelength λ_SAW and channel height H, a transition from vortex pairs extending over the entire channel width W to periodic flows resembling the pseudo-standing wave field is revealed. An adaptation of the electrical power, however, only affects the velocity scaling. Based on the experimental data, a validated numerical model was developed in which critical material parameters and boundary conditions were systematically adjusted. Considering a Navier slip length at the substrate-fluid interface, the simulations provide a strong agreement with the measured velocity data over a large frequency range and enable an energetic consideration of the first and second-order fields. Based on the results of this study, critical parameters were identified for the particle size as well as for channel height and width. Progress for the research on SAW-based separation systems is obtained not only by these findings but also by providing all experimental velocity data to allow for further developments on other sites.

Periodic Rayleigh streaming vortices and Eckart flow arising from traveling-wave-based diffractive acoustic fields

Recent studies have demonstrated that periodic time-averaged acoustic fields can be produced from traveling surface acoustic waves (SAWs) in microfluidic devices. This is caused by diffractive effects arising from a spatially limited transducer. This permits the generation of acoustic patterns evocative of those produced from standing waves, but instead with the application of a traveling wave. While acoustic pressure fields in such systems have been investigated, acoustic streaming from diffractive fields has not. In this work we examine this phenomenon and demonstrate the appearance of geometry-dependent acoustic vortices, and demonstrate that periodic, identically rotating Rayleigh streaming vortices result from the imposition of a traveling SAW. This is also characterized by a channel-spanning flow that bridges between adjacent vortices along the channel top and bottom. We find that the channel dimensions determine the types of streaming that develops; while Eckart streaming has been previously presumed to be a distinguishing feature of traveling-wave actuation, we show that Rayleigh streaming vortices also results. This has implications for microfluidic actuation, where traveling acoustic waves have applications in microscale mixing, separation, and patterning.

Quantitative assessment of parallel acoustofluidic device

The advantage of ultrasonic fields in harmless and label-free applications intrigued researchers to develop this technology. The capability of acoustofluidic technology for medical applications has not been thoroughly analyzed and visualized. Toward efficient design, in this research, flowing fluid in a microchannel excited by acoustic waves is fully investigated. To study the behavior of acoustic streaming, the main interfering parameters such as inlet velocity, working frequency, displacement amplitude, fluid buffer material, and hybrid effect in a rectangular water-filled microchannel actuated by standing surface acoustic waves are studied. Governing equations for acoustic field and laminar flow are derived employing perturbation theory. For each set of equations, appropriate boundary conditions are applied. Results demonstrate a parallel device is capable of increasing the inlet flow for rapid operations. Frequency increment raises the acoustic streaming velocity magnitude. Displacement amplitude amplification increases the acoustic streaming velocity and helps the streaming flow dominate over the incoming flow. The qualitative analysis of the hybrid effect shows using hard walls can significantly increase the streaming power without depleting excessive energy. A combination of several effective parameters provides an energy-efficient and fully controllable device for biomedical applications such as fluid mixing and cell lysis.

Flexural wave-based soft attractor walls for trapping microparticles and cells

Acoustic manipulation of microparticles and cells, called acoustophoresis, inside microfluidic systems has significant potential in biomedical applications. In particular, using acoustic radiation force to push microscopic objects toward the wall surfaces has an important role in enhancing immunoassays, particle sensors, and recently microrobotics. In this paper, we report a flexural-wave based acoustofluidic system for trapping micron-sized particles and cells at the soft wall boundaries. By exciting a standard microscope glass slide (1 mm thick) at its resonance frequencies <200 kHz, we show the wall-trapping action in sub-millimeter-size rectangular and circular cross-sectional channels. For such low-frequency excitation, the acoustic wavelength can range from 10-150 times the microchannel width, enabling a wide design space for choosing the channel width and position on the substrate. Using the system-level acousto-structural simulations, we confirm the acoustophoretic motion of particles near the walls, which is governed by the competing acoustic radiation and streaming forces. Finally, we investigate the performance of the wall-trapping acoustofluidic setup in attracting the motile cells, such as Chlamydomonas reinhardtii microalgae, toward the soft boundaries. Furthermore, the rotation of microalgae at the sidewalls and trap-escape events under pulsed ultrasound are demonstrated. The flexural-wave driven acoustofluidic system described here provides a biocompatible, versatile, and label-free approach to attract particles and cells toward the soft walls.

Modelling of SAW-PDMS acoustofluidics: physical fields and particle motions influenced by different descriptions of the PDMS domain

In modelling acoustofluidic chips actuated by surface acoustic waves (SAWs) and using polydimethylsilane (PDMS) as a channel material, reduced models are often adopted to describe the acoustic behaviors of PDMS. Here, based on a standing SAW (SSAW) acoustophoresis chip, we compared three different descriptions of a PDMS chamber and looked into in-chamber physical fields and ensuing particle motion processes through finite element (FE) simulations. Specifically, the PDMS domain was considered as an elastic solid material, a non-flow fluid, and a lossy wall, respectively. The major findings include: (a) the shear waves that propagated in a solid PDMS wall did not influence the in-chamber pressure and ARF fields severely, but induced an observable difference in the acoustic streaming (AS) patterns, and distinctly changed the trajectories of polystyrene particles, especially those whose radii were below 1.5 μm; and (b) the equivalent damping coefficients were linearly dependent on the SAW frequency, characterized by a fixed loss per wavelength, indicating the wave leakage at the interface being the main source of the transmission loss of SAWs. Meanwhile, the acoustic radiation force (ARF) can be overestimated when describing PDMS as a lossy wall, especially at the bottom corners of the chamber, which could cause inaccurate predictions of the motion of big particles. Based on the damping mechanism, a rough protocol is provided for scaling of pressure fields between different models. Some suggestions for device designs and operations are also given based on the obtained findings.

Acoustophoretic purification of platelets: feasibility and impact on platelet activation and function

Purity, limited platelet activation, and preservation of platelet function are important stakes of preparation of platelet concentrates (PC) for clinical use. In fact, contaminating red blood cells and leukocytes, as well as activated and/or poorly functional platelets in PC, represents a risk of poor efficiency and adverse side effects during platelet transfusion. Therefore, optimization of preparation and storage of PC is still an active field of research. Shear-induced platelet activation is an unwanted side effect of the hard-spin (up to 5000g) step of centrifugation-based methods currently used in blood banks to prepare PC from whole blood samples. Here, we evaluated the effectiveness of an acoustic-based fractionation device for the isolation of human platelets from whole blood bags. The purity, activation status, and functionality of platelets isolated by acoustopheresis were compared with those of platelets isolated using a reference protocol known to produce limited platelet activation and consisting of two consecutive soft-spin centrifugations (120g and 1200g). Platelet concentration and purity were determined using an automated hematology analyzer. Platelet activation status and platelet reactivity to collagen and thrombin were assessed in flow cytometry by measurement of surface expression of P-selectin and activated integrin αIIbβ3. The ability of isolated platelets to incorporate into a thrombus when transfused to NOD/SCID mice was investigated by intravital microscopy using the ferric chloride-induced thrombosis model. Blood fractionation by acoustophoresis led to the elimination of more than 80% of red blood cells and leukocytes from the platelet fraction, whose mean purity was of 92.8 ± 12.8%. The activation status and reactivity to collagen and thrombin of acoustophoresis-isolated platelets were similar to those of platelets isolated by soft-spin centrifugation. Finally, acoustophoresis-isolated platelets were tethered, adhered to the vessel wall, and incorporated into a growing thrombus following ferric chloride-induced vascular injury. Together, our results indicate that acoustophoresis is a suitable method for the isolation of human platelets with minimal platelet activation and preservation of platelet function.