Standing surface acoustic wave (SSAW)-based microfluidic cytometer.

Detection of circulating tumour cells (CTCs) plays a crucial role in the diagnosis of tumour metastasis and evaluation of therapeutic effect on prognosis. The low number of the CTCs, however, makes their isolation from the peripheral blood very difficult. In this paper, a microfluidic device has been developed, which consists of a pair of straight interdigital transducers (IDTs) and focused interdigital transducers (FIDTs), to separate tumour cells from red blood cells (RBCs) in the blood stream by surface acoustic waves (SAWs). The standing SAWs (SSAW) generated by the straight IDTs enabled placing the CTCs and RBCs at the pressure nodes while the FIDTs generated the focused travelling pulsed SAWs (TSAWs) to push the CTCs away from the blood cells to achieve the isolation of CTCs. Using this mechanism, this device was capable of concentrating and separating 2 μm and 5 μm polystyrene particles and about 94.2% of particles with the diameter of 5 μm could be collected at the outlet. Moreover, 90% ± 2.4% of U87 glioma cells could be isolated from the RBCs when the cycle number of the TSAWs was 720 k. In the enrichment process, this microfluidic device totally relied on the SSAW without any additional assistance of the flow focusing. For the separation, unlike the bidirectional acoustic radiation force in SSAW, the unidirectional force generated by TSAWs consistently pushed the particles along the wave propagation, improving the sorting performance. This multi-stage device may provide an alternative method for sorting CTCs in a reliable and flexible manner.

Laser-induced fluorescence integrated in a microfluidic immunosensor for quantification of human serum IgG antibodies to Helicobacter pylori

By exciting standing surface acoustic waves (SAWs), a monomer solution can be shaped into a wavy structure. By applying ultraviolet (UV) polymerization, a linear undulate microarray can be fabricated on the polymer material using one-dimensional standing SAWs. When two-dimensional standing SAWs are applied, a latticed microarray, which presents periodically distributed bumps and wells, can be fabricated. The periodicity of the undulate microarray is dependent on the SAW wavelength. Also, the undulating amplitude of the microarray is tunable when applying different input voltages to generate SAWs. The integrated standing SAWs and UV polymerization process provide a rapid method for creating periodic surface patterns.

Deposition of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT-PSS) particles using standing surface acoustic waves and electrostatic deposition method for the rapid fabrication of transparent conductive film

A portable microfluidic flow cytometer based on simultaneous detection of impedance and fluorescence

LIFGO: A modular laser-induced fluorescence detection system based on plug-in blocks

In this paper, the theory for acoustic radiation force exerted by standing surface acoustic waves (SSAWs) is extended to a compressible sphere in inviscid fluids. The conventional theory, developed in plane standing waves, fails to predict the radiation force incident on particles in the SSAW. Our extended formulas reveal that, in the direction normal to the piezoelectric substrate, the acoustic radiation force cannot only push the sphere away, but also pull it back towards the substrate. In the direction parallel to the substrate, both the magnitude of the radiation force and the equilibrium positions for particles can be actively tuned by changing the Rayleigh angle. This is exceedingly meaningful for particle positioning by tilted angle SSAWs and SSAWs. Furthermore, the Rayleigh angle can also be used to actively regulate the resonance point and peak for bubbles acted on by SSAWs. The extended theory can thus be used in the design of SSAWs for manipulating particles and bubbles.

Identification and analysis of phytoplankton is important for understanding the environmental parameters that are influenced by the oceans, including pollution and climate change. Phytoplanktons are studied at the single cell level using conventional light-field and fluorescence microscopy, but the technique is labour intensive. Flow cytometry enables rapid and quantitative measurements of single cells and is now used as an analytical tool in phytoplankton analysis. However, it has a number of drawbacks, including high cost and portability. We describe the fabrication of a microfluidic (lab-on-a-chip) device for high-speed analysis of single phytoplankton. The device measures fluorescence (at three wavelength ranges) and the electrical impedance of single particles. The system was tested using a mixture of three algae (Isochrysis Galbana, Rhodosorus m., Synechococcus sp.) and the results compared with predictions from theory and measurements using a commercial flow cytometer (BD FACSAria). It is shown that the microfluidic flow cytometer is able to distinguish and characterise these different taxa and that impedance spectroscopy enables measurement of phytoplankton biophysical properties.

Particle manipulation and patterning have gained tremendous attention in chemical, biomedical, and manufacturing studies. Hydrogels are usually used for applications in soft robots, biosensing, as well as tissue engineering. In this study, we investigated a nanoparticle manipulation method based on standing surface acoustic waves (SAWs). The SAW device consists of a piezoelectric lithium niobate (LiNbO3) substrate with a pair of interdigital transducers (IDTs). Finite element simulations were performed to understand the mechanisms of the SAW device as well as reveal the acoustic pressure field and electric potential field generated by the device. In addition to numerical studies, proof-of-concept experiments were performed by using a fabricated SAW device for patterning both silicon dioxide (SiO2) nanoparticles and multi-walled carbon nanotubes (MWCNTs) in a hydrogel solution.

A microfluidic device was developed to transport the aggregated microbubbles to a targeted cell at arbitrary location in liquid solution by introducing the phase-shift to a planar standing surface acoustic wave (SAW). The device consists of two perpendicular pairs of interdigital transducer (IDTs) and a polydimethyl-siloxane microchannel. By adjusting the relative phase between electric voltages applied to the independent IDTs, the microbubbles can be trapped, continuously moved or rotated due to the translation of pressure nodes. We further demonstrate that a SAW is capable of inducing microbubble cluster destruction at a desired location to achieve a single cell’s reparable sonoporation. By controlling the position of the microbubble cluster relative to the targeted cell precisely, the effective size of the collapsing microbubbles is measured to be less than 0.68 times the diameter of microbubble cluster. The sonoporation efficiency and the cell viability are 82.4%± 6.5% and 90%±8.7%, respectively, when the targeted cell is within the effective microbubble destruction region.

Accumulation of particles in a high concentration on a microchannel wall is a common phenomenon in a colloidal fluid. Gradual accumulation/deposition of particles can eventually obstruct the fluid flow and lead to clogging, which seriously affects the accuracy and reliability of nozzle-based printing and causes damage to the nozzle. Particle accumulation in a 100 μm microchannel was investigated by light microscopy, and its area growth in an exponential format was used to quantify this phenomenon. The effects of the constriction angle and alginate concentration on particle accumulation were also studied. In order to reduce the clogging problem, an acoustic method was proposed and evaluated here. Numerical simulation was first conducted to predict the acoustic radiation force on the particles in the fluid with different viscosities. Interdigital transducers (IDTs) were fabricated on the LiNbO3 wafer to produce standing surface acoustic waves (SSAW) in the microchannel. It was found that the actuation of SSAW can reduce the accumulation area in the microchannel by 2 to 3.7-fold. In summary, the particle accumulation becomes significant with the increase of the constriction angle and fluid viscosity. The SSAW can effectively reduce the particle accumulation and postpone clogging.

Particle manipulation via integration of electroosmotic flow of power-law fluids with standing surface acoustic waves (SSAW)

Polymer-based substrate with patterned microstructures has been widely utilized for the development of cell chip in tissue engineering and/or flexible sensors in robotics. The key challenge is to fabricate the polymer-based substrate with the localized patterned microstructures. In this study, we presented a method to fabricate localized microstructures by standing surface acoustic wave (SSAW) and user-defined waveguides. To investigate the working mechanism, we developed a 3D numerical model to analyze the induced acoustic pressure distribution and final generated microstructures. Results demonstrated that the utilized waveguide could localize the acoustic pressure field in a specified region within the prepared photosensitive fluid film based on Rayleigh’s radiation theory and capillary wave motion. By adjusting the SSAW driven modes and using different shaped waveguides, both numerical modeling and experimental tests showed that the localized microstructures can form on the liquid film and then successfully fabricated using ultraviolet (UV) solidification. Therefore, our developed method by using the SSAW and waveguide provides a promising alternative process to fabricate the polymer-based microstructure with localized patterns, and the fabricated polymer-based microstructures could be used for the development of flexible sensors, actuator, and/or soft robotics in future applications.

In modeling the acoustic properties of microfluidics driven by surface acoustic waves (SAW), polydimethylsiloxane (PDMS) is usually used as the microchannel material. In this paper, based on a standing surface acoustic wave (SSAW) driven acoutstophoresis chip, the physical field and particle motion behavior in the microchannel are studied through numerical simulation. The major work includes: (a) the distribution of the acoustic pressure field in the microchannel; (b) the dominant behavior change of the drag force and the acoustic radiation force in the acoustic pressure field, which causes the trajectory change of the polystyrene particles. Based on the lossy-wall boundary conditions, it is shown that the model can simulate the acoustic flow and particle movement caused by leaky SAW in the microchannel. The obtained findings can also provide some suggestions for SAW device design and optimization.

Surface-acoustic-wave (SAW) aerosol generators are very promising systems for applications requiring small aerosol sources and adjustable droplet-size distributions at low aerosol flowrates. However, the droplet-generation mechanism by SAW is yet unclear and dedicated experiments are necessary to gain further insights. In this work, the atomization zone is investigated experimentally to gain more insight into the droplet-generation mechanism by SAW. Three regions are observed in the atomization zone showing different acoustofluidic effects, including the formation of liquid films and patterns in a standing SAW (SSAW) wavefield and droplet generation. In addition, the influence of the humidity of surrounding air on the measured droplet-size distribution of the aerosols generated by SSAW is investigated. Therefore, the air humidity is adjusted at constant air temperature in the sophisticated wind-tunnel Turbulent Leipzig Aerosol Cloud Interaction Simulator combined with an optical particle spectrometer with resolution down to the sub-\textmu{}m scale. Besides the humidity influence on the droplet size in the measurement region, water-saturated air allowed us to estimate the initial droplet-size distribution at the atomization zone on the chip surface. This parameter is crucial for the determination of the underlying liquid-film disintegration mechanism, i.e., the droplets' origin. Hereby, a bimodal log-normal droplet-size distribution is obtained. The mean diameter of the main droplet fraction decreases from 7.2 to 5.3 \textmu{}m with increasing relative humidity from 8.6 to 97%, apparently due to evaporation of smaller droplets in unsaturated air. A second peak corresponds to a mean diameter as small as 600 nm at high humidity conditions, which is likely to correspond to a droplet-generation mechanism not reported so far and extremely difficult to measure or visualize with conventional techniques. The droplet-size measurement results from the aerosol spectrometer are compared and validated in respect to existing droplet-generation models with the results of a phase-Doppler anemometry, a high-resolution, nonintrusive local aerosol-measurement technique.