Acoustic Microstreaming Research Articles

Acoustic microstreaming near a plane wall due to a pulsating free or coated bubble: velocity, vorticity and closed streamlines

Acoustic microstreaming due to an oscillating microbubble, either coated or free, is analytically investigated. The detailed flow field is obtained and the closed streamlines of the ring vortex generated by microstreaming are plotted in both Eulerian and Lagrangian descriptions. Analytical expressions are found for the ring vortex showing that its length depends only on the separation of the microbubble from the wall and the dependence is linear. The circulation as a scalar measure of the vortex is computed quantitatively identifying its spatial location. The functional dependence of circulation on bubble separation and coating parameters is shown to be similar to that of the shear stress.

Tunable microfluidic standing air bubbles and its application in acoustic microstreaming.

Microbubbles are often used in chemistry, biophysics, and medicine. Properly controlled microbubbles have been proved beneficial for various applications by previous scientific endeavors. However, there is still a plenty of room for further development of efficient microbubble handling methods. Here, this paper introduces a tunable, stable, and robust microbubble interface handling mechanism, named as microfluidic standing air bubbles (μSABs), by studying the multiphysical phenomena behind the gas-liquid interface formation and variation. A basic μSAB system consists specially structured fluidic channels, pneumatic channels, and selectively permeable porous barriers between them. The μSABs originate inside the crevice structures on the fluidic channel walls in a repeatable and robust manner. The volumetric variation of the μSAB is a multiphysical phenomenon that dominated by the air diffusion between the pneumatic channel and the bubble. Theoretical analysis and experimental data illustrate the coupling processes of the repeatable and linear μSAB volumetric variation when operated under common handling conditions (control pneumatic pressure: -90 kPa to 200 kPa). Furthermore, an adjustable acoustic microstreaming is demonstrated as an application using the alterable μSAB gas-liquid interface. Derived equations and microscopic observations elucidate the mechanism of the continuous and linear regulation of the acoustic microstreaming using varying μSAB gas-liquid interfaces. The μSAB system provides a new tool to handle the flexible and controllable gas-liquid interfaces in a repeatable and robust manner, which makes it a promising candidate for innovative biochemical, biophysical, and medical applications.

Acoustic microstreaming due to an oscillating contrast microbubbles near a substrate: Velocity, vorticity and closed streamlines

Intravenously injected microbubbles used as ultrasound contrast enhancing as well as drug delivery agents are encapsulated by a nanometer-thick layer of lipids, proteins, or polymers to stabilize them against premature dissolution. Here, acoustic microstreaming due to an oscillating microbubble, either coated or free, responsible for sonoporation and other bioeffects is analytically investigated. The detailed flow field is obtained, and the closed streamlines due to the ring vortex are plotted in both Eulerian and Lagrangian descriptions. Analytical expressions are found for the ring vortex showing that its length depends only on the separation of the microbubble from the wall and the dependence is linear. The circulation as a scalar measure of the vortex is computed quantitatively identifying its spatial location. The functional dependence of circulation on bubble separation and coating parameters was shown to be similar to that of the shear stress. [Work supported partially by NSF CBET 1602884 and GWU.]

A mathematical model of sonoporation using a liquid-crystalline shelled microbubble

In recent years there has been a great deal of interest in using thin shelled microbubbles as a transportation mechanism for localised drug delivery, particularly for the treatment of various types of cancer. The technique used for such site-specific drug delivery is sonoporation. Despite there being numerous experimental studies on sonoporation, the mathematical modelling of this technique has still not been extensively researched. Presently there exists a very small body of work that models both hemispherical and spherical shelled microbubbles sonoporating due to acoustic microstreaming. Acoustic microstreaming is believed to be the dominant mechanism for sonoporation via shelled microbubbles. Rather than considering the shell of the microbubble to be composed of a thin protein, which is typical in the literature, in this paper we consider the shell to be a liquid-crystalline material. Up until now there have been no studies reported in the literature pertaining to sonoporation of a liquid-crystalline shelled microbubble. A mathematical expression is derived for the maximum wall shear stress, illustrating its dependency on the shell’s various material parameters. A sensitivity analysis is performed for the wall shear stress considering the shell’s thickness; its local density; the elastic constant of the liquid-crystalline material; the interfacial surface tension and; the shell’s viscoelastic properties. In some cases, our results indicate that a liquid-crystalline shelled microbubble may yield a maximum wall shear stress that is two orders of magnitude greater than the stress generated by commercial shelled microbubbles that are currently in use within the scientific community. In conclusion, our preliminary analysis suggests that using liquid-crystalline shelled microbubbles may significantly enhance the efficiency of site-specific drug delivery.

Rapid immunodiagnostics of multiple viral infections in an acoustic microstreaming device with serum and saliva samples.

There is a growing need to screen multiple infections simultaneously rather than diagnosis of one pathogen at a time in order to improve the quality of healthcare while saving initial screening time and reduce costs. This is the first demonstration of a five-step protein array assay for the multiplexed detection of HIV, HPV and HSV antibodies on an integrated microfluidic system. HIV, HPV and HSV reactive antibodies from both serum and saliva were rapidly detected by acoustic streaming-based mixing and pumping to enable an integrated, rapid and simple-to-use multiplexed assay device. We validated this device with 37 serum and saliva samples to verify reactivity of patient antibodies with HIV, HPV and HSV antigens. Our technology can be adapted with different protein microarrays to detect a variety of other infections, thus demonstrating a powerful platform to detect multiple putative protein biomarkers for rapid detection of infectious diseases. This integrated microfluidic protein array platform is the basis of a potent strategy to delay progression of primary infection, reduce the risk of co-infections and prevent onward transmission of infections by point-of-care detection of multiple pathogens in both serum and oral fluids.

Acoustic Streaming and Its Applications

Broadly speaking, acoustic streaming is generated by a nonlinear acoustic wave with a finite amplitude propagating in a viscid fluid. The fluid volume elements of molecules, d V , are forced to oscillate at the same frequency as the incident acoustic wave. Due to the nature of the nonlinearity of the acoustic wave, the second-order effect of the wave propagation produces a time-independent flow velocity (DC flow) in addition to a regular oscillatory motion (AC motion). Consequently, the fluid moves in a certain direction, which depends on the geometry of the system and its boundary conditions, as well as the parameters of the incident acoustic wave. The small scale acoustic streaming in a fluid is called “microstreaming”. When it is associated with acoustic cavitation, which refers to activities of microbubbles in a general sense, it is often called “cavitation microstreaming”. For biomedical applications, microstreaming usually takes place in a boundary layer at proximity of a solid boundary, which could be the membrane of a cell or walls of a container. To satisfy the non-slip boundary condition, the flow motion at a solid boundary should be zero. The magnitude of the DC acoustic streaming velocity, as well as the oscillatory flow velocity near the boundary, drop drastically; consequently, the acoustic streaming velocity generates a DC velocity gradient and the oscillatory flow velocity gradient produces an AC velocity gradient; they both will produce shear stress. The former is a DC shear stress and the latter is AC shear stress. It was observed the DC shear stress plays the dominant role, which may enhance the permeability of molecules passing through the cell membrane. This phenomenon is called “sonoporation”. Sonoporation has shown a great potential for the targeted delivery of DNA, drugs, and macromolecules into a cell. Acoustic streaming has also been used in fluid mixing, boundary cooling, and many other applications. The goal of this work is to give a brief review of the basic mathematical theory for acoustic microstreaming related to the aforementioned applications. The emphasis will be on its applications in biotechnology.

Parallel Label‐Free Isolation of Cancer Cells Using Arrays of Acoustic Microstreaming Traps

AbstractA new parallel label‐free isolation platform for isolating cancer cells (>10 µm) from biological samples using an array of acoustic microstreaming traps is demonstrated. The new microstreaming trapping platform offers high isolation efficiency and treatment in spiked diluted (1:2) blood samples without labeling, extra processing steps or sheath flows (common to other label‐free cell‐separation methods), hence, simplifying the microfluidic set‐up and operation. The versatility of this label‐free method is illustrated by the size‐dependent isolation of synthetic colloids and by three different models of breast cancer cells, showing a remarkably high isolation efficiency (95 ± 5%), being ≈100% for MCF‐7. Extensive experimental data and theoretical simulations confirm that the local acoustic microstreaming produced by the micropillar trap can discriminate and isolate microparticles based solely on their size. Moreover, the parallel isolation generated by the trap array arrangement is capable of effectively improving the capturing efficiency. The tunable and reversible properties of the acoustic microstreaming allow for cell capturing and optical detection within the trapping area, followed by cell release, enrichment, and collection for further studies. This label‐free trapping strategy can open numerous new opportunities for rapid and sensitive detection and capture of cancer cells in biological samples for meaningful clinical applications.

Whole-blood sorting, enrichment and in situ immunolabeling of cellular subsets using acoustic microstreaming

Analyzing undiluted whole human blood is a challenge due to its complex composition of hematopoietic cellular populations, nucleic acids, metabolites, and proteins. We present a novel multi-functional microfluidic acoustic streaming platform that enables sorting, enrichment and in situ identification of cellular subsets from whole blood. This single device platform, based on lateral cavity acoustic transducers (LCAT), enables (1) the sorting of undiluted donor whole blood into its cellular subsets (platelets, RBCs, and WBCs), (2) the enrichment and retrieval of breast cancer cells (MCF-7) spiked in donor whole blood at rare cell relevant concentrations (10 mL−1), and (3) on-chip immunofluorescent labeling for the detection of specific target cellular populations by their known marker expression patterns. Our approach thus demonstrates a compact system that integrates upstream sample processing with downstream separation/enrichment, to carry out multi-parametric cell analysis for blood-based diagnosis and liquid biopsy blood sampling.

Topographical Manipulation of Microparticles and Cells with Acoustic Microstreaming.

Precise and reproducible manipulation of synthetic and biological microscale objects in complex environments is essential for many practical biochip and microfluidic applications. Here, we present an attractive acoustic topographical manipulation (ATM) method to achieve efficient and reproducible manipulation of diverse microscale objects. This new guidance method relies on the acoustically induced localized microstreaming forces generated around microstructures, which are capable of trapping nearby microobjects and manipulating them along a determined trajectory based on local topographic features. This unique phenomenon is investigated by numerical simulations examining the local microstreaming in the presence of microscale boundaries under the standing acoustic wave. This method can be used to manipulate a single microobject around a complex structure as well as collectively manipulate multiple objects moving synchronously along complicated shapes. Furthermore, the ATM can serve for automated maze solving by autonomously manipulating microparticles with diverse geometries and densities, including live cells, through complex maze-like topographical features without external feedback, particle modification, or adjustment of operational parameters.

Making, mapping, and using acoustic nanobubbles for therapy

Acoustically driven bubbles continue to find new therapeutic uses, including drug delivery to tumors, opening the blood-brain barrier, and direct fractionation of tissues for surgical applications. Creating acoustic cavitation at length scales and pressure amplitudes compatible with biology remains a major challenge and could be greatly facilitated by a new generation of nano-scale cavitation nuclei that stretch our current understanding of bubble stability, acoustic microstreaming, and the interaction between cavitating bubbles and biological media. Furthermore, in order to ensure patient safety and treatment efficacy, monitoring the location and activity of the nanobubbles during ultrasonic excitation is essential. It will be shown that Passive Acoustic Mapping (PAM) of sources of nonlinear acoustic emissions enables real-time imaging and control of cavitation activity at depth within the body, thereby making it possible to monitor therapy using solely acoustical means. Combined, these techniques for making, mapping, and using acoustic nanobubbles can enable improved delivery of modern immuno-oncology agents to cells and tumors, needle-free transdermal drug delivery and vaccination, and new spinal therapies to treat the spinal cord or repair the intervertebral disc.

Reproducible bubble-induced acoustic microstreaming for bead disaggregation and immunoassay in microfluidics

The bead-based immunoassay requires not only efficient mixing but also good control of bead-surface-area-to-sample-volume ratio to realise accurate and reproducible detection of low concentration samples. This paper reports the development of a microfluidic platform with the reproducible and efficient bubble-induced micromixing for bead disaggregation and immunoassay of prostate-specific antigen (PSA). The platform consists of a microfluidic chip with a microchamber and rectangular traps for capturing air bubbles and a home-made controller to generate sound wave using an external piezo transducer. Methods for reproducible bubble formation and bubble size control during mixing are explored. The influence of driving voltage, PDMS thickness and the substrate material on the mixing efficiency is characterised by mixing a fluorescence dye and a buffer solution. The optimised acoustic microstreaming is able to break clusters with hundreds of beads and homogenise individual beads over the microchamber. Immunoassay with efficient micromixing has been applied to PSA immunoassay with greatly reduced detection time. This study provides a practical guide for the design and development of the bubble-induced acoustic micromixers for bead disaggregation and on-chip immunoassays.

Ultrasound-enhanced penetration through sclera depends on frequency of sonication and size of macromolecules

We previously employed ultrasound as a needleless approach to deliver macromolecules via the transscleral route to the back of the eye in live animals (Suen et al., 2013). Here, we investigated the nature of the ultrasound-enhanced transport through sclera, the outermost barrier in the transscleral route. Thus, the possible role of cavitation from ultrasound was explored; its effect during and after sonication on scleral penetration was measured; and the dependence on the size of macromolecules was determined. We applied ultrasound frequency from 40kHz to 3MHz at ISATA (spatial-average-temporal-average intensity) of 0.05W/cm2 to fresh rabbit sclera ex vivo. Fluorescent dextran of size 20kDa to 150kDa was used as macromolecular probes. We measured the distance of penetration of the probes through the sclera over 30s during sonication and over 15min after sonication from cryosectioned tissue images. Deeper penetration in the sclera was observed with decreasing frequency. The presence of stable cavitation was further verified by passive acoustic detection. The effect during sonication increased penetration distance up to 20 fold and was limited to macromolecular probes ≤70kDa. The effect post sonication increased penetration distance up to 3 fold and attributed to the improved intrasscleral transport of macromolecules ≥70kDa. Post-sonication enhancement diminished gradually in 3h. As the extent of cavitation increased with decreasing frequency, the trend observed supports the contribution of (stable) cavitation to enhancing transport through sclera. Effect during sonication was attributed to flow associated with acoustic microstreaming. Effect post sonication was attributed to the temporary increase in scleral permeability. Flow-associated effect was more pronounced but only applied to smaller macromolecules.

Enhanced porosity and permeability of three-dimensional alginate scaffolds via acoustic microstreaming induced by low-intensity pulsed ultrasound

The shear stress resulting from the microstreaming induced by low-intensity pulsed ultrasound (LIPUS) has been often used to improve the permeability of cell membrane or porous engineering scaffolds. In the present study, three-dimensional (3-D) scaffold culture systems were constituted to simulate the in vivo microenvironment, providing benefits for cell growth. In order to investigate the mechanism underlying the enhanced porosity and permeability of the 3-D alginate scaffolds by using LIPUS with varied acoustic intensities, two quantitative imaging techniques (i.e. scanning electron microscopy, and laser con-focal imaging) were used to evaluate the porosity and permeability of the 3-D alginate scaffolds. The results suggested that the porosity and permeability of the scaffolds were enhanced by the microbubble-induced microstreaming, and increased with the increasing LIPUS driving intensity. Furthermore, the cell proliferation assessments verified that HeLa cell grew better in the treated 3-D alginate scaffolds, since the LIPUS exposures can improve the scaffold porosity and permeability, leading to better cell growth space and nutrition supply.

Acoustic streaming and the induced forces between two spheres

The ability of acoustic microstreaming to cause a pair of particles to attract or repel is investigated. Expanding the flow around two spheres in terms of a small-amplitude parameter measuring the amplitude of the forcing, the leading order is an oscillating flow field with zero mean representing the effect of the applied acoustic field, while the second-order correction contains a steady streaming component. A modal decomposition in the azimuthal direction reduces the problem to a few linear problems in a two-dimensional domain corresponding to the meridional ($r,z$) plane. The analysis computes both the intricate flow fields and the mean forces felt by both spheres. If the spheres are aligned obliquely with respect to the oscillating flow, they experience a lateral force which realigns them into a transverse configuration. In this transverse configuration, they experience an axial force which can be either attractive or repulsive. At high frequencies the force is always attractive. At low frequencies, it is repulsive. At intermediate frequencies, the force is attractive at large distances and repulsive at small distances, leading to the existence of a stable equilibrium configuration.

Microstreaming generated by two acoustically induced gas bubbles

A theory is developed that describes microstreaming generated by two interacting gas bubbles in an acoustic field. The theory is used in numerical simulations to compare the characteristics of acoustic microstreaming at different frequencies, separation distances between the bubbles and bubble sizes. It is shown that the interaction of the bubbles leads to a considerable increase in the intensity of the velocity and stress fields of acoustic microstreaming if the bubbles are driven near the resonance frequencies that they have in the presence of each other. Patterns of streamlines for different situations are presented.

Selective flow-induced vesicle rupture to sort by membrane mechanical properties.

Vesicle and cell rupture caused by large viscous stresses in ultrasonication is central to biomedical and bioprocessing applications. The flow-induced opening of lipid membranes can be exploited to deliver drugs into cells, or to recover products from cells, provided that it can be obtained in a controlled fashion. Here we demonstrate that differences in lipid membrane and vesicle properties can enable selective flow-induced vesicle break-up. We obtained vesicle populations with different membrane properties by using different lipids (SOPC, DOPC, or POPC) and lipid:cholesterol mixtures (SOPC:chol and DOPC:chol). We subjected vesicles to large deformations in the acoustic microstreaming flow generated by ultrasound-driven microbubbles. By simultaneously deforming vesicles with different properties in the same flow, we determined the conditions in which rupture is selective with respect to the membrane stretching elasticity. We also investigated the effect of vesicle radius and excess area on the threshold for rupture, and identified conditions for robust selectivity based solely on the mechanical properties of the membrane. Our work should enable new sorting mechanisms based on the difference in membrane composition and mechanical properties between different vesicles, capsules, or cells.

Efecto del ultrasonido endodóntico sobre clorhexidina al 2% en la formación de paracloroanilina. Estudio in vitro

IntroductionChlorhexidine (CHX) in aqueous solution is hydrolysed to p-chloroaniline (PCA), a process accelerated by increasing temperature and pH. Using endodontic ultrasound based on oscillation phenomena, cavitation, and acoustic microstreaming generates heat, affecting the CHX. ObjectiveThe aim of this in vitro study was to identify and quantify the physical-chemical changes, temperature and pH, and the amount of PCA formed by increasing the temperature of 2% CHX by endodontic ultrasound. Materials and methodsSamples of 2% CHX were activated for 30, 60, 90, and 120seconds with endodontic ultrasound 24,500Hz. The pH and temperature were measured before and after activation, as well as the formation and amount of PCA, by reading and recording the result obtained from a standard calibration curve reading at 375nm in a UV-visible light spectrophotometer. ResultsIndependent of time, ultrasound increased the temperature of 2% CHX by 1°C and acidified the solution. No significant changes were recorded in pH and temperature. No staining or precipitates were observed in samples ultrasonically activated at different times. Samples read at 375nm showed no measurable PCA values. Absorption spectra of 2% CHX and 2% CHX activated for more than 60seconds showed different spectral curves, peaks, and absorbance values. ConclusionsUltrasound increased the temperature and acidified the solution of CHX for all application times. No PCA was detected by visible spectrophotometry. Absorption spectra of 2% CHX activated with ultrasound at different times differs from 2% CHX without activation.These differences indicate degradation of CHX and possible presence of PCA.

Reconfigurable microfluidic dilution for high-throughput quantitative assays.

This paper reports a reconfigurable microfluidic dilution device for high-throughput quantitative assays, which can easily produce discrete logarithmic/binary concentration profiles ranging from 1 to 100-fold dilution in parallel from a fixed sample volume (e.g., 10 μL) without any assistance of continuous fluidic pump or robotic automation. The integrated dilution generation chip consists of switchable distribution and collection channels, metering reservoirs, reaction chambers, and pressure-activatable Laplace valves. Following the sequential loading of a sample, a diluent, and a detection reagent into their individual metering chambers, the top microfluidic layer can be reconfigured to collect the metered chemicals into the reaction chambers in parallel, where detection will be conducted. To facilitate mixing and reaction in the microchambers, two acoustic microstreaming actuation mechanisms have been investigated for easy integrability and accessibility. Furthermore, the microfluidic dilution generator has been characterized by both colorimetric and fluorescent means. A further demonstration of the generic usage of the quantitative dilution chip has utilized the commonly available bicinchoninic acid (BCA) assay to analyse the protein concentrations of human tissue extracts. In brief, the microfluidic dilution generator offers a high-throughput high-efficiency quantitative analytical alternative to conventional quantitative assay platforms, by simple manipulation of a minute amount of chemicals in a compact microfluidic device with minimal equipment requirement, which can serve as a facile tool for biochemical and biological analyses in regular laboratories, point-of-care settings and low-resource environments.

Effect of a distant rigid wall on microstreaming generated by an acoustically driven gas bubble

AbstractA theory is developed that describes acoustic microstreaming around a gas bubble undergoing small radial and translational oscillations in the presence of a distant rigid wall. It is shown that the presence of the wall can change the amplitude and the phase of the bubble oscillations in such a way that the intensity of acoustic microstreaming is increased considerably as compared with that generated by the same bubble in an infinite liquid. This occurs if the driving frequency is close to the resonance frequency that the bubble has in the presence of the wall. Equations for acoustic microstreaming in the boundary layer at the wall are also provided.

Theory and experiment on resonant frequencies of liquid-air interfaces trapped in microfluidic devices

Bubble-based microfluidic devices have been proven to be useful for many biological and chemical studies. These bubble-based microdevices are particularly useful when operated at the trapped bubbles' resonance frequencies. In this work, we present an analytical expression that can be used to predict the resonant frequency of a bubble trapped over an arbitrary shape. Also, the effect of viscosity on the dispersion characteristics of trapped bubbles is determined. A good agreement between experimental data and theoretical results is observed for resonant frequency of bubbles trapped over different-sized rectangular-shaped structures, indicating that our expression can be valuable in determining optimized operational parameters for many bubble-based microfluidic devices. Furthermore, we provide a close estimate for the harmonics and a method to determine the dispersion characteristics of a bubble trapped over circular shapes. Finally, we present a new method to predict fluid properties in microfluidic devices and complement the explanation of acoustic microstreaming.