Secondary Acoustic Radiation Forces Research Articles

3D printed large amplitude torsional microactuators powered by ultrasound

Here, we introduce a design, fabrication, and control methodology for large amplitude torsional microactuators powered by ultrasound. The microactuators are 3D printed from two polymers with drastically different elastic moduli as a monolithic compliant mechanism, and contain precisely engineered capsules with multiple orifices that serve as stators and rotors. Secondary acoustic radiation forces generated among the encapsulated air bubbles controllably rotate the rotor that is supported and stabilized by two torsion bars. The capsules are designed according to the simulations of an analytical model that captures the dynamics of the water-air interface vibrations, which we rigorously validate using a laser Doppler vibrometer and phase-contrast imaging. An in-depth experimental sensitivity analysis is conducted to optimize the arrangement of the rotor and the stators. Integration of experimental results with finite element analysis of the twisting bars and analytical modelling of acoustic phenomena allows us to compute the secondary acoustic radiation forces and the angular displacement of the rotor for a given input pressure. The versatility of the design framework and the robust performance of the printed actuators enable the development of a new class of microscale machines and soft robotic devices that are actuated and controlled by sound waves.

Three-dimensional array of microbubbles sonoporation of cells in microfluidics.

Sonoporation is a popular membrane disruption technique widely applicable in various fields, including cell therapy, drug delivery, and biomanufacturing. In recent years, there has been significant progress in achieving controlled, high-viability, and high-efficiency cell sonoporation in microfluidics. If the microchannels are too small, especially when scaled down to the cellular level, it still remains a challenge to overcome microchannel clogging, and low throughput. Here, we presented a microfluidic device capable of modulating membrane permeability through oscillating three-dimensional array of microbubbles. Simulations were performed to analyze the effective range of action of the oscillating microbubbles to obtain the optimal microchannel size. Utilizing a high-precision light curing 3D printer to fabricate uniformly sized microstructures in a one-step on both the side walls and the top surface for the generation of microbubbles. These microbubbles oscillated with nearly identical amplitudes and frequencies, ensuring efficient and stable sonoporation within the system. Cells were captured and trapped on the bubble surface by the acoustic streaming and secondary acoustic radiation forces induced by the oscillating microbubbles. At a driving voltage of 30 Vpp, the sonoporation efficiency of cells reached 93.9% ± 2.4%.

Preliminary study to scale up microbubble generation with acoustic wave

This research focuses on the utilization of acoustic wave to control the generation of microbubbles from a capillary tube, aiming for mass production with porous media. Initially. ultrasonic standing wave is employed to generate small-size microbubble since high amplitude is required for the pipette with smaller pore size which has high resistance. However, for large bubble generation, secondary acoustic radiation force between bubbles is increased and induces bubble coalescence. Pulsing wave can reduce the force strength by reducing wave cycles (apparent frequency) and increasing the distance between bubbles by controlling bubble generation frequency. Finally, maximum bubble generation frequency is calculated at each bubble size based on calculation of limiting factors such as gas permeability and secondary acoustic radiation force. The research will contribute to generate wide size range of multiple bubbles in aqueous solution and set up its limitation regarding bubble generation frequency through theoretical calculation. The data will be utilized for multiple bubble generation with porous media.

Ultrasound Microrobots with Reinforcement Learning

AbstractUltrasound is an attractive modality for controlling micro/nanorobots due to penetrating deep into tissue, not being affected by the opaque nature of animal bodies, and generating a broad range of forces. However, ultrasound microrobots have poor navigation capabilities and the number of parameters involved in its motion make it extremely challenging for a human controller to accurately predict and manually correct the microrobot's position in real time. Herein, reinforcement learning control strategy is implemented to learn microrobot dynamics by accurately identifying micro/nanorobots (object detection and tracking) and manipulating them with ultrasound. This work demonstrates autonomous navigation of ultrasound microrobots in a fluidic environment. The propulsion strategy relies on the combined action of the primary and secondary radiation forces. Microswarms are formed through the secondary acoustic radiation force, while the primary acoustic radiation force guides the microrobots along a desired trajectory. Microrobots are trained using more than 100 000 images to study their unexpected dynamics. The control of the microrobots is validated, illustrating a good level of robustness and providing the microrobots with computational intelligence that enables them to navigate independently in an unstructured environment without outside assistance.

Gap Distance Between Pearl Chains in Acoustic Manipulation

We present a theory to compute the stable gap (interparticle) distance between particle chains collected in the pressure node of an acoustic standing wave. The primary and secondary acoustic radiation forces are the two competing forces that act on the particles during the particle chain formation. The stable equilibrium distance between two chains is reached when both forces are in balance. Most interestingly, the density scattering coefficient appears to the second power in the theoretical prediction of the gap distance, indicating that the particle-chain formation occurs for both particles heavier than the surrounding medium and, notably, also for buoyant particles. Experimentally, the gap distance is evaluated for several different media and particle material combinations and the particle-chain formation is observed for both buoyant particles and particles heavier than the surrounding medium. The theory agrees well with experiments in the cases where the material properties of the medium and the particles are well known.

3D‐Printed Acoustofluidic Devices for Raman Spectroscopy of Cells

Acoustofluidics technology can be used to trap live cells (and also micro/nanoparticles) in microenvironments suitable for cell assays. Herein, a cheap and easy‐to‐fabricate device is proposed that works with Raman spectroscopy for biosensing applications. The device comprises a 3D‐printed microchamber working as a half‐wavelength acoustic resonator. By tuning the resonance frequency with a low voltage (≈4 V), cells or particles are aggregated and levitated in seconds by the action of the acoustic radiation force. Based on finite element simulations, the radiation force field produced inside the device is described. In the cellular enrichment (aggregation) process, a metastable honeycomb lattice is formed mostly due to the cell‐to‐cell attraction caused by the secondary acoustic radiation force. Orderly and metastable levitating aggregates provide an excellent arrangement for Raman spectroscopy to investigate cells individually. Polystyrene particles are used for the device characterization and Raman acquisition process. Biosensing applications are showcased with live murine macrophages J774.A1, which are used in infection assay of leishmaniasis disease. The unique features of the device, e.g., simple fabrication process with cheap materials, simple operation, fast time response, and formation of metastable cellular aggregates; hold a noteworthy potential for applications in life sciences and biotechnology involving cell assays.

"Computational Investigation of Secondary Acoustic Radiation Force Between Cells and Particles Out of Nodal Pressure Line"

The wide applications of acoustofluidic devices which have been studied in recent years, play a key role in cell manipulation and separation [1,2]. In such cases, the first part of acoustic force, which is called primary acoustic radiation force, has been widely investigated in last few decades

Numerical study of the effect of pressure amplitude and frequency on secondary acoustic radiation force between two polystyrene particles in water

Poly(N-isopropylacrylamide) microgels emulate the behavior of natural platelets, promoting wound healing. Our previous studies suggested that ultrasound stimulation of these synthetic platelets further enhances wound healing, by increasing particle displacement and deformation. It is necessary to understand and model the interaction between ultrasound and these particles. We hypothesize that the acoustic radiation force may play a role. Two types of radiation force are considered: a primary radiation force caused by the external acoustic field, and a secondary radiation force resulting from scattering between particles. We present a parametric numerical study of the interaction between polystyrene beads (similar to the synthetic platelets) and an acoustic field, to investigate the effect of magnitude, frequency and bandwidth of excitation field on particle response. This is done via the numerical modeling of the primary and secondary radiation forces using 2-D and 3-D FEM. The 3-D simulations capture the re-scattering effects between the particles. One particle is placed at the origin of the coordinate system (scatterer) and a second particle (probe) is placed in the nodal and anti-nodal pressure planes. The numerical results show a good agreement with the governing theoretical equations proposed by Silva and Bruus, suggesting the relevance of the radiation force hypothesis.

A Quantitative Study of the Secondary Acoustic Radiation Force on Biological Cells during Acoustophoresis

We investigate cell-particle secondary acoustic radiation forces in a plain ultrasonic standing wave field inside a microfluidic channel. The effect of secondary acoustic radiation forces on biological cells is measured in a location between a pressure node and a pressure anti-node and the result is compared with theory by considering both compressibility and density dependent effects. The secondary acoustic force between motile red blood cells (RBCs) and MCF-7 cells and fixed 20 µm silica beads is investigated in a half-wavelength wide microchannel actuated at 2 MHz ultrasonic frequency. Our study shows that the secondary acoustic force between cells in acoustofluidic devices could play an important role for cell separation, sorting, and trapping purposes. Our results also demonstrate the possibility to isolate individual cells at trapping positions provided by silica beads immobilized and adhered to the microchannel bottom. We conclude that during certain experimental conditions, the secondary acoustic force acting on biological cells can dominate over the primary acoustic radiation force, which could open up for new microscale acoustofluidic methods.

Numerical Determination of the Secondary Acoustic Radiation Force on a Small Sphere in a Plane Standing Wave Field.

Two numerical methods based on the Finite Element Method are presented for calculating the secondary acoustic radiation force between interacting spherical particles. The first model only considers the acoustic waves scattering off a single particle, while the second model includes re-scattering effects between the two interacting spheres. The 2D axisymmetric simplified model combines the Gor’kov potential approach with acoustic simulations to find the interacting forces between two small compressible spheres in an inviscid fluid. The second model is based on 3D simulations of the acoustic field and uses the tensor integral method for direct calculation of the force. The results obtained by both models are compared with analytical equations, showing good agreement between them. The 2D and 3D models take, respectively, seconds and tens of seconds to achieve a convergence error of less than 1%. In comparison with previous models, the numerical methods presented herein can be easily implemented in commercial Finite Element software packages, where surface integrals are available, making it a suitable tool for investigating interparticle forces in acoustic manipulation devices.

Acoustic dipole and monopole effects in solid particle interaction dynamics during acoustophoresis.

A method is presented for measurements of secondary acoustic radiation forces acting on solid particles in a plain ultrasonic standing wave. The method allows for measurements of acoustic interaction forces between particles located in arbitrary positions such as in between a pressure node and a pressure antinode. By utilizing a model that considers both density- and compressibility-dependent effects, the observed particle-particle interaction dynamics can be well understood. Two differently sized polystyrene micro-particles (4.8 and 25 μm, respectively) were used in order to achieve pronounced interaction effects. The particulate was subjected to a 2-MHz ultrasonic standing wave in a microfluidic channel, such as commonly used for acoustophoresis. Observation of deflections in the particle pathways shows that the particle interaction force is not negligible under this circumstance and has to be considered in accurate particle manipulation applications. The effect is primarily pronounced when the distance between two particles is small, the sizes of the particles are different, and the acoustic properties of the particles are different relative to the media. As predicted by theory, the authors also observe that the interaction forces are affected by the angle between the inter-particle centerline and the axis of the standing wave propagation direction.

Acoustic fields and forces in drug delivery applications

For ultrasound drug delivery applications, the ability to manipulate and move both the carriers and their payloads provides a way of increasing over efficacy. Both primary and secondary acoustic radiation forces have been used though the optimal acoustic forcing parameters and delivery system are subject to on-going investigations. The primary radiation or Bjerknes force occurs from inhomogeneous propagation of the acoustic field and has been used to direct targeted ultrasound contrast agents or particles in vessels towards endothelial cells or pathological targets at the vessel wall. Secondary Bjerknes forces arise from multiple scattering effects between neighboring particles. For corresponding attractive force regimes, greater particle congregation is possible. Overview of theoretical background and predictions of the acoustic radiation forces is given. The formulation of particle translation by primary radiation in the long wavelength limit is discussed with respect to unsteady drag. Pair wise interaction between particles moving in tandem is compared with formulations for along the lines of center. Nonstationary forcing with respect to amplitude modulation or frequency (chirp) alters the attraction and repulsion regimes compared to continuous forcing. Cellular transport for payloads may be enhanced using more optimally tuned pulse sequences.

The effect of particle density on ultrasound-mediated transport of nanoparticles

A significant barrier to successful drug delivery is the limited penetration of nanoscale therapeutics beyond the vasculature. Building on recent in vivo findings in the context of cancer drug delivery, the current study investigates whether modification of nanoparticle drug-carriers to increase their density can be used to enhance their penetration into viscoelastic materials under ultrasound exposure. A computational model is first presented to predict the transport of identically sized nanoparticles of different densities in an ultrasonic field in the presence of an oscillating microbubble, by a combination of primary and secondary acoustic radiation forces, acoustic streaming and microstreaming. Experiments are then described in which near monodisperse (polydispersity index <0.2) nanoparticles of approximate mean diameter 200 nm and densities ranging from 1.01 g cm−3 to 5.58 g cm−3 were fabricated and delivered to a tissue-mimicking material in the presence or absence of a microbubble ultrasound contrast agent, at ultrasound frequencies of 0.5 MHz and 1.6 MHz and a peak negative pressure of 1 MPa. Both the theoretical and experimental results confirm that denser particles exhibit significantly greater ultrasound-mediated transport than their lower density counterparts, indicating that density is a key consideration in the design of nanoscale therapeutics.

Superharmonic microbubble Doppler effect in ultrasound therapy

The introduction of microbubbles in focused ultrasound therapies has enabled a diverse range of non-invasive technologies: sonoporation to deliver drugs into cells, sonothrombolysis to dissolve blood clots, and blood-brain barrier opening to deliver drugs into the brain. Current methods for passively monitoring the microbubble dynamics responsible for these therapeutic effects can identify the cavitation position by passive acoustic mapping and cavitation mode by spectral analysis. Here, we introduce a new feature that can be monitored: microbubble effective velocity. Previous studies have shown that echoes from short imaging pulses had a Doppler shift that was produced by the movement of microbubbles. Therapeutic pulses are longer (>1 000 cycles) and thus produce a larger alteration of microbubble distribution due to primary and secondary acoustic radiation force effects which cannot be monitored using pulse-echo techniques. In our experiments, we captured and analyzed the Doppler shift during long therapeutic pulses using a passive cavitation detector. A population of microbubbles (5 × 104–5 × 107 microbubbles ml−1) was embedded in a vessel (inner diameter: 4 mm) and sonicated using a 0.5 MHz focused ultrasound transducer (peak-rarefactional pressure: 75–366 kPa, pulse length: 50 000 cycles or 100 ms) within a water tank. Microbubble acoustic emissions were captured with a coaxially aligned 7.5 MHz passive cavitation detector and spectrally analyzed to measure the Doppler shift for multiple harmonics above the 10th harmonic (i.e. superharmonics). A Doppler shift was observed on the order of tens of kHz with respect to the primary superharmonic peak and is due to the axial movement of the microbubbles. The position, amplitude and width of the Doppler peaks depended on the acoustic pressure and the microbubble concentration. Higher pressures increased the effective velocity of the microbubbles up to 3 m s−1, prior to the onset of broadband emissions, which is an indicator for high magnitude inertial cavitation. Although the microbubble redistribution was shown to persist for the entire sonication period in dense populations, it was constrained to the first few milliseconds in lower concentrations. In conclusion, superharmonic microbubble Doppler effects can provide a quantitative measure of effective velocities of a sonicated microbubble population and could be used for monitoring ultrasound therapy in real-time.

Ultrasound-mediated transport of nanoparticles and the influence of particle density

A significant barrier to successful drug delivery in cancer therapy is the limited penetration of nanoscale therapeutics deep into tumors. Ultrasound mediated cavitation has been shown to promote nanoparticle transport, but achieving tumor wide distribution still represents a considerable challenge. The current study investigates the way in which nanoparticle drug-carriers may be designed to enhance penetration under ultrasound exposure. A computational model has been developed to predict the transport of a nanoparticle in an ultrasonic field in the presence of an oscillating microbubble, by a combination of primary and secondary acoustic radiation forces, acoustic streaming, and microstreaming. Experimental investigations were also performed in a tissue-mimicking phantom to study the transport of different types of particle, in the presence or absence of a microbubble ultrasound contrast agent, at ultrasound frequencies of 0.5 MHz and 1.6 MHz with peak pressures in the range of 0–2.0 MPa. Micro- and nanoparticles with contrasting density cores ranging from 1.0 g/cm3 to 19.3 g/cm3 were used for the study. Both the theoretical and experimental results showed that the denser particles exhibit significantly greater ultrasound-mediated transport than their lower density counterparts, indicating that this is a key consideration in the design of nanoscale therapeutics.

Acoustic fields and ultrasound contrast agents

Ultrasound contrast agents are encapsulated gas bubbles which oscillate nonlinearly upon acoustic excitation. Recent developments include targeted agents for molecular imaging applications and ultrasound contrast agents such those with polymer shells specifically designed for high frequency ultrasound applications. Though a variety of models have been proposed since the early development of the agents, many outstanding questions exist on the specifications of the shell and most appropriate model for the different materials. Moreover, only recently have independent mechanical measurements been attempted to advance and facilitate modeling efforts. The various shell models are reviewed and discussed. Validation of models of polymer shell agents are given with respect high frequency acoustic microscopy quantification of the shell elastic properties. Recent work on the modeling and design of agents for high frequency subharmonic excitation based on adaptive signal processing and nonlinear time series methodologies is overviewed. Novel analytical method for treating hydrodynamic interactions with respect to unsteady drag and secondary acoustic radiation forces are outlined with respect to drug delivery and sonoporation applications. The direction and magnitude of coupled oscillations are investigated with respect evolution of the amplitude and phase and the transfer entropy.

Two-Bubble Acoustic Tweezing Cytometry for Biomechanical Probing and Stimulation of Cells

The study of mechanotransduction relies on tools that are capable of applying mechanical forces to elicit and assess cellular responses. Here we report a new (to our knowledge) technique, called two-bubble acoustic tweezing cytometry (TB-ATC), for generating spatiotemporally controlled subcellular mechanical forces on live cells by acoustic actuation of paired microbubbles targeted to the cell adhesion receptor integrin. By measuring the ultrasound-induced activities of cell-bound microbubbles and the actin cytoskeleton contractile force responses, we determine that TB-ATC elicits mechanoresponsive cellular changes via cyclic, paired displacements of integrin-bound microbubbles driven by the attractive secondary acoustic radiation force (sARF) between the bubbles in an ultrasound field. We demonstrate the feasibility of dual-mode TB-ATC for both subcellular probing and mechanical stimulation. By exploiting the robust and unique interaction of ultrasound with microbubbles, TB-ATC provides distinct advantages for experimentation and quantification of applied forces and cellular responses for biomechanical probing and stimulation of cells.

Microbubble array for on-chip worm processing

We present an acoustic non-contact technique for achieving trapping, enrichment, and manipulation of Caenorhabditis elegans using an array of oscillating microbubbles. We characterize the trapping efficiency and enrichment ratio under various flow conditions, and demonstrate a single-worm manipulation mechanism through temporal actuation of bubbles. The reason for oscillating bubbles being versatile in processing worms in a microfluidic environment is due to the complex interactions among acoustic field, microbubbles, fluid flow, and live animals. We explain the operating mechanisms used in our device by the interplay among secondary acoustic radiation force, drag force, and the propulsive force of C. elegans.

Unbinding of targeted ultrasound contrast agent microbubbles by secondary acoustic forces

Targeted molecular imaging with ultrasound contrast agent microbubbles is achieved by incorporating targeting ligands on the bubble coating and allows for specific imaging of tissues affected by diseases. Improved understanding of the interplay between the acoustic forces acting on the bubbles during insonation with ultrasound and other forces (e.g. shear due to blood flow, binding of targeting ligands to receptors on cell membranes) can help improve the efficacy of this technique. This work focuses on the effects of the secondary acoustic radiation force, which causes bubbles to attract each other and may affect the adhesion of targeted bubbles. First, we examine the translational dynamics of ultrasound contrast agent microbubbles in contact with (but not adherent to) a semi-rigid membrane due to the secondary acoustic radiation force. An equation of motion that effectively accounts for the proximity of the membrane is developed, and the predictions of the model are compared with experimental data extracted from optical recordings at 15 million frames per second. A time-averaged model is also proposed and validated. In the second part of the paper, initial results on the translation due to the secondary acoustic radiation force of targeted, adherent bubbles are presented. Adherent bubbles are also found to move due to secondary acoustic radiation force, and a restoring force is observed that brings them back to their initial positions. For increasing magnitude of the secondary acoustic radiation force, a threshold is reached above which the adhesion of targeted microbubbles is disrupted. This points to the fact that secondary acoustic radiation forces can cause adherent bubbles to detach and alter the spatial distribution of targeted contrast agents bound to tissues during activation with ultrasound. While the details of the rupture of intermolecular bonds remain elusive, this work motivates the use of the secondary acoustic radiation force to measure the strength of adhesion of targeted microbubbles.

Novel configurations for acoustophoresis

Separation of particles by their acoustomechanical properties can be accomplished by acoustophoresis, which depends on both primary and secondary acoustic radiation forces. In an acoustic standing wave, particles are moved either toward the pressure nodes or the antinodes depending on the relative contrasts in the particle density and compressibility with the surrounding host liquid. Because particles reradiate the sound waves, there are secondary interparticle forces as well. These secondary forces can either hinder or enhance the separation process. If two species which are to be separated attract each other because of interparticle forces, the separation will be compromised. But if several like particles aggregate, then primary forces are much stronger and the separation occurs more rapidly. When flow is superimposed on the system, as is common in practical applications, then drag forces must also be accounted for. In the present work, novel field configurations have been explored, and the trajectories of particles have been computed for a number of frequencies, flow speeds, field strengths, and property contrasts. Based on these simulations, experiments have been designed to optimize the efficiency of acoustophoresis, especially for biotechnology applications. [Work supported by NASA through Grant NAG8-1351.]