Acoustic trapping of microbubbles in complex environments and controlled payload release

Laser-induced microbubble as an in vivo valve for optofluidic manipulation in living Mice's microvessels

Optofluidic regulation of blood microflow in vivo represents a significant method for investigating illnesses linked to abnormal changes in blood circulation. Currently, non-invasive strategies are limited to regulation within capillaries of approximately 10 μm in diameter because the adaption to blood pressure levels in the order of several hundred pascals poses a significant challenge in larger microvessels. In this study, using laser-induced microbubble formation within microvessels of the mouse auricle, we regulate blood microflow in small vessels with diameters in the tens of micrometers. By controlling the laser power, we can control the growth and stability of microbubbles in vivo. This controlled approach enables the achievement of prolonged ischemia and subsequent reperfusion of blood flow, and it can also regulate the microbubbles to function as micro-pumps for reverse blood pumping. Furthermore, by controlling the microbubble, narrow microflow channels can be formed between the microbubbles and microvessels for assessing the apparent viscosity of leukocytes, which is 76.9 ± 11.8 Pa·s in the in vivo blood environment. The proposed design of in vivo microbubble valves opens new avenues for constructing real-time blood regulation and exploring cellular mechanics within living organisms.

Wave-momentum shaping for moving objects in heterogeneous and dynamic media

Light and sound waves can move objects through the transfer of linear or angular momentum, which has led to the development of optical and acoustic tweezers, with applications ranging from biomedical engineering to quantum optics. Although impressive manipulation results have been achieved, the stringent requirement for a highly controlled, low-reverberant and static environment still hinders the applicability of these techniques in many scenarios. Here we overcome this challenge and demonstrate the manipulation of objects in disordered and dynamic media by optimally tailoring the momentum of sound waves iteratively in the far field. The method does not require information about the object's physical properties or the spatial structure of the surrounding medium but relies only on a real-time scattering matrix measurement and a positional guide-star. Our experiment demonstrates the possibility of optimally moving and rotating objects to extend the reach of wave-based object manipulation to complex and dynamic scattering media. We envision new opportunities for biomedical applications, sensing and manufacturing.

Capillary wave tweezer

Precise control of microparticle movement is crucial in high throughput processing for various applications in scalable manufacturing, such as particle monolayer assembly and 3D bio-printing. Current techniques using acoustic, electrical and optical methods offer precise manipulation advantages, but their scalability is restricted due to issues such as, high input powers and complex fabrication and operation processes. In this work, we introduce the concept of capillary wave tweezers, where mm-scale capillary wave fields are dynamically manipulated to control the position of microparticles in a liquid volume. Capillary waves are generated in an open liquid volume using low frequency vibrations (in the range of 10-100 Hz) to trap particles underneath the nodes of the capillary waves. By shifting the displacement nodes of the waves, the trapped particles are precisely displaced. Using analytical and numerical models, we identify conditions under which a stable control over particle motion is achieved. By showcasing the ability to dynamically control the movement of microparticles, our concept offers a simple and high throughput method to manipulate particles in open systems.

Ultra-stable nano-micro bubbles in a biocompatible medium for safe delivery of anti-cancer drugs

We conducted a series of experimental investigations to generate laser-stimulated millimeter bubbles (MBs) around silver nanoparticles (AgNPs) and thoroughly examined the mechanism of bubble formation within this nanocomposite system. One crucial aspect we explored was the lifetime and kinetics of these bubbles, given that bubbles generated by plasmonic nanoparticles are known to be transient with short durations. Surprisingly, our findings revealed that the achieved lifetime of these MBs extended beyond seven days. This impressive longevity far surpasses what has been reported in the existing literature. Further analysis of the experimental data uncovered a significant correlation between bubble volume and its lifetime. Smaller bubbles demonstrated longer lifetimes compared to larger ones, which provided valuable insights for future applications. The experimental results not only confirmed the validity of our model and simulations but also highlighted essential characteristics, including extended lifetime, matching absorption coefficients, adherence to physical boundary conditions, and agreement with simulated system parameters. Notably, we generated these MBs around functionalized AgNPs in a biocompatible nanocomposite medium by utilizing low-power light excitation. By readily binding potent cancer drugs to AgNPs through simple physical mixing, these medications can be securely encapsulated within bubbles and precisely guided to targeted locations within the human body. This capability to deliver drugs directly to the tumor site, while minimizing contact with healthy tissues, can lead to improved treatment outcomes and reduced side effects, significantly enhancing the quality of life for cancer patients.

Enhanced standing-wave acoustic levitation using high-order transverse modes in phased array ultrasonic cavities

Airborne acoustic trapping by ultrasonic phased arrays has seen great advances in recent years, and yet the manipulation of objects with different shapes and sizes or heavy particles remains challenging. Here, we demonstrate that the manipulation capabilities of a standing-wave acoustic levitator can be extended by introducing intracavity high-order transverse (HOT) modes in the azimuthal direction, enabling the simultaneous trapping of several objects within a wide range of shapes and sizes with positional and rotational stability, including objects with sizes larger than one wavelength and weights in the scale of millinewtons. The conditions to generate different HOT modes are theoretically analyzed and experimentally implemented. We numerically calculate the pressure distributions, exhibiting good qualitative agreement with the experimental pressure distributions obtained with schlieren images. In addition, we calculate the acoustic force field for several examples of HOT modes and different particle sizes, which leads to a qualitative understanding of the experimental observations.

Mechanistic study of ultrasound and microbubble enhanced cancer therapy in a 3D vascularized microfluidic cancer model

Accumulating evidence has shown that ultrasound exposure combined with microbubbles can enhance cancer therapy. However, the underlying mechanisms at the tissue level have not been fully understood yet. The conventional cell culture in vitro lacks complex structure and interaction, while animal studies cannot provide micron-scale dynamic information. To bridge the gap, we designed and assembled a 3D vascularized microfluidic cancer model, particularly suitable for ultrasound and microbubble involved mechanistic studies. Using this model, we first studied SonoVue microbubble traveling dynamics in 3D tissue structure, then resolved SonoVue microbubble cavitation dynamics in tissue mimicking agarose gels at a frame rate of 0.675 M fps, and finally explored the impacts of ultrasound and microbubbles on cancer cell spheroids. Our results demonstrate that microbubble penetration in agarose gel was enhanced by increasing microbubble concentration, flow rate and decreasing viscosity of the gel, and little affected by mild acoustic radiation force. SonoVue microbubble exhibited larger expansion amplitudes in 2 %(w/v) agarose gels than in water, which can be explained theoretically by the relaxation of the cavitation medium. The immediate impacts of ultrasound and SonoVue microbubbles to cancer cell spheroids in the 3D tissue model included improved cancer cell spheroid penetration in micron-scale and sparse direct permanent cancer cell damage. Our study provides new insights of the mechanisms for ultrasound and microbubble enhanced cancer therapy at the tissue level.

Filtered Lebedev quadrature method for robust and efficient beam shape coefficient estimation in acoustic tweezers calibration

Acoustic tweezers offer a contactless, three-dimensional, and selective approach to trapping objects by harnessing the acoustic radiation force. Precise control of this technique requires accurate calibration of the force, which depends on the object's properties and the spherical harmonics expansion of the incident field through the beam shape coefficients. Previous studies showed that these coefficients can be determined using either the Lebedev quadrature or the angular spectrum methods. However, the former is highly susceptible to noise, while the latter demands extensive implementation time due to the number of required measurement points. A filtered method with a reduced number of points is introduced to address these limitations. Initially, we emphasize the implicit filtering in the angular spectrum method, allowing relative noise insensitivity. Subsequently, we present its unfiltered version, enabling force estimation of a standing field. Finally, we develop a filtered method based on the Lebedev quadrature, requiring fewer points, and apply it to focused vortex beams. Numerical evaluation of the radiation force demonstrates the method's resilience to noise and a reduced need for points compared to previous methods. The filtered Lebedev method paves the way for characterizing high-frequency acoustic tweezers, where measurement constraints necessitate rapid and robust beam shape coefficient estimation techniques.

Calibration of the axial stiffness of a single-beam acoustic tweezers

Single-beam acoustic tweezers have recently been demonstrated to be capable of selective three-dimensional trapping. This new contactless manipulation modality has great potential for many scientific applications. Its development as a scientific tool requires precise calibration of its radiation force, specifically its axial component. The lack of calibration for this force is mainly due to its weak magnitude compared to competing effects such as weight. We investigate an experimental method for the calibration of the axial stiffness of the radiation force by observing the axial oscillations of a trapped bead in a microgravity environment. The stiffness exhibits a linear relationship with the acoustic intensity and is of the mN/m order. Then, a predictive model, loaded with the experimental acoustic field, is compared to the measured stiffness with very good agreement, within a single amplitude coefficient. This study paves the way for the development of calibrated acoustic tweezers.

Acoustic microbubble propulsion, train-like assembly and cargo transport

Achieving controlled mobility of microparticles in viscous fluids can become pivotal in biologics, biotechniques, and biomedical applications. The self-assembly, trapping, and transport of microparticles are being explored in active matter, micro and nanorobotics, and microfluidics; however, little work has been done in acoustics, particularly in active matter and robotics. This study reports the discovery and characterization of microbubble behaviors in a viscous gel that is confined to a slight opening between glass boundaries in an acoustic field. Where incident waves encounter a narrow slit, acoustic pressure is amplified, causing the microbubbles to nucleate and cavitate within it. Intermittent activation transforms microbubbles from spherical to ellipsoidal, allowing them to be trapped within the interstice. Continuous activation propels ellipsoidal microbubbles through shape and volume modes that is developed at their surfaces. Ensembles of microbubbles self-assemble into a train-like arrangement, which in turn capture, transport, and release microparticles.

Review of Ultrasonic Particle Manipulation Techniques: Applications and Research Advances

Ultrasonic particle manipulation technique is a non-contact label-free method for manipulating micro- and nano-scale particles using ultrasound, which has obvious advantages over traditional optical, magnetic, and electrical micro-manipulation techniques; it has gained extensive attention in micro-nano manipulation in recent years. This paper introduces the basic principles and manipulation methods of ultrasonic particle manipulation techniques, provides a detailed overview of the current mainstream acoustic field generation methods, and also highlights, in particular, the applicable scenarios for different numbers and arrangements of ultrasonic transducer devices. Ultrasonic transducer arrays have been used extensively in various particle manipulation applications, and many sound field reconstruction algorithms based on ultrasonic transducer arrays have been proposed one after another. In this paper, unlike most other previous reviews on ultrasonic particle manipulation, we analyze and summarize the current reconstruction algorithms for generating sound fields based on ultrasonic transducer arrays and compare these algorithms. Finally, we explore the applications of ultrasonic particle manipulation technology in engineering and biological fields and summarize and forecast the research progress of ultrasonic particle manipulation technology. We believe that this review will provide superior guidance for ultrasonic particle manipulation methods based on the study of micro and nano operations.

Phase holograms for the three-dimensional patterning of unconstrained microparticles

Acoustic radiation forces can remotely manipulate particles. Forces from a standing wave field align microscale particles along the nodal or anti-nodal locations of the field to form three-dimensional (3D) patterns. These patterns can be used to form 3D microstructures for tissue engineering applications. However, standing wave generation requires more than one transducer or a reflector, which is challenging to implement in vivo. Here, a method is developed and validated to manipulate microspheres using a travelling wave from a single transducer. Diffraction theory and an iterative angular spectrum approach are employed to design phase holograms to shape the acoustic field. The field replicates a standing wave and aligns polyethylene microspheres in water, which are analogous to cells in vivo, at pressure nodes. Using Gor'kov potential to calculate the radiation forces on the microspheres, axial forces are minimized, and transverse forces are maximized to create stable particle patterns. Pressure fields from the phase holograms and resulting particle aggregation patterns match predictions with a feature similarity index > 0.92, where 1 is a perfect match. The resulting radiation forces are comparable to those produced from a standing wave, which suggests opportunities for in vivo implementation of cell patterning toward tissue engineering applications.

Oxygen-Generating Biomaterials for Translational Bone Regenerative Engineering

Successful regeneration of critical-size defects remains one of the significant challenges in regenerative engineering. These large-scale bone defects are difficult to regenerate and are often reconstructed with matrices that do not provide adequate oxygen levels to stem cells involved in the regeneration process. Hypoxia-induced necrosis predominantly occurs in the center of large matrices since the host tissue's local vasculature fails to provide sufficient nutrients and oxygen. Indeed, utilizing oxygen-generating materials can overcome the central hypoxic region, induce tissue in-growth, and increase the quality of life for patients with extensive tissue damage. This article reviews recent advances in oxygen-generating biomaterials for translational bone regenerative engineering. We discussed different oxygen-releasing and delivery methods, fabrication methods for oxygen-releasing matrices, biology, oxygen's role in bone regeneration, and emerging new oxygen delivery methods that could potentially be used for bone regenerative engineering.

Dynamically reversible cooperation and interaction of multiple rotating micromotors

Micromotors have been shown to have great potential in various fields (e.g., targeted therapeutics, self-organizing systems), and research on the cooperative and interactive behaviours of multiple micromotors could potentially revolutionize many fields in terms of performing multiple or complex tasks to compensate for the limitations of individual micromotors; however, dynamically reversible transitions among diverse behaviours remain much less explored, and such dynamic transformations are advantageous for achieving complex tasks. Here, we present a microsystem consisting of multiple disk-like micromotors capable of performing reversible transformations between cooperative and interactive behaviours at the liquid surface. The micromotors with aligned magnetic particles in our system have great magnet properties, which provides a strong magnetic interaction with each other and is vital for the whole microsystem. We offer and analyse the physical models among multiple micromotors concerning the cooperative and interactive modes in the lower and higher frequency ranges, respectively, between which the state transformation can reversibly occur. Furthermore, based on the proposed reversible microsystem, the feasibility of the application of self-organization is verified by demonstrating three different dynamic self-organizing behaviours. Our proposed dynamically reversible system has great potential to serve as a paradigm for studying cooperative and interactive behaviours among multiple micromotors in the future.

Optical fiber inclinometer with dynamically controllable excitation length of quantum dots liquid-core waveguide based on a photo-controlled bubble

An ultracompact fiber inclinometer based on a bubble controlled by Marangoni force is proposed in this Letter. By coupling a 980-nm laser, the bubble can suspend in a quantum dots (QDs) liquid-core waveguide (LCW) due to the Marangoni effect. Under the excitation of a 405-nm laser, QDs LCW exhibit green emissions centered at 523 nm. When the tilt angle changes, the position of the bubble changes as well, which causes the variation of the 523-nm fluorescence intensity. The experimental results show that the sensitivity based on the peak intensity ratio (PIR) reaches 0.22/° with a linearity of 0.979 from 0° to 35°. Furthermore, the sensor has excellent stability and repeatability.

Flexible acoustic lens-based surface acoustic wave device for manipulation and directional transport of micro-particles

Microfluidics is an emerging technology that is playing increasingly important roles in biomedical and pharmaceutical research and development. Surface acoustic waves (SAWs) have been combined with microfluidics technology to establish a SAW-based microfluidics technology that uses the unique interaction between the two techniques to manipulate substances effectively in fluids on the surface of a substrate. This paper reports a method to generate SAWs using conventional planar ultrasonic transducers and acoustic lenses. Additionally, this method is introduced to manipulate particles effectively on a substrate surface. It is demonstrated that the particle positions can be manipulated precisely in any direction on the substrate surface, thus enabling high-precision particle manipulation. We also proposed the generation of nonplanar SAWs via appropriate design of the acoustic lens and realized directional particle transport. In addition, structures to enhance forward-propagating acoustic beams are proposed. The proposed method has potential for use in microfluidics and biomedical applications, allowing tasks such as flexible cell manipulation on a chip to be performed without complex design or micromachining.

NIR-II fluorescence visualization of ultrasound-induced blood-brain barrier opening for enhanced photothermal therapy against glioblastoma using indocyanine green microbubbles

Focused ultrasound (FUS)-induced blood-brain barrier (BBB) opening is crucial for enhancing glioblastoma (GBM) therapies. However, an in vivo imaging approach with a high spatial-temporal resolution to monitor the BBB opening process in situ and synchronously is still lacking. Herein, we report the use of indocyanine green (ICG)-dopped microbubbles (MBs-ICG) for visualizing the FUS-induced BBB opening and enhancing the photothermal therapy (PTT) against GBM. The MBs-ICG show bright fluorescence in the second near-infrared window (NIR-II), ultrasound contrast, and ultrasound-induced size transformation properties. By virtue of complementary contrast properties, MBs-ICG can be successfully applied for cerebral vascular imaging with NIR-II fluorescence resolution of ∼168.9 μm and ultrasound penetration depth of ∼7 mm. We further demonstrate that MBs-ICG can be combined with FUS for in situ and synchronous visualization of the BBB opening with a NIR-II fluorescence signal-to-background ratio of 6.2 ± 1.2. Finally, our data show that the MBs-ICG transform into lipid-ICG nanoparticles under FUS irradiation, which then rapidly penetrate the tumor tissues within 10 min and enhance PTT in orthotopic GBM-bearing mice. The multifunctional MBs-ICG approach provides a novel paradigm for monitoring BBB opening and enhancing GBM therapy.

Soundiation: A software in evaluation of acoustophoresis driven by radiation force and torque on axisymmetric objects

Acoustic radiation force and torque arising from wave scattering are commonly used to manipulate micro-objects without contact. We applied the partial wave expansion series and the conformal transformation approach to estimate the acoustic radiation force and torque exerted on the axisymmetric particles. Meanwhile, the translational and rotational transformations are employed to perform the prediction of the acoustophoresis. Although these theoretical derivations are well-developed [Tang and Huang, J. Sound Vibr. 532, 117012 (2022), Tang and Huang, Phys. Rev. E 105, 055110 (2022)], coding the required systems, including generation of the wave function, implementation of the transformations, calculations between modules, etc., is non-trivial and time-consuming. Here, we present a new open-source, matlab-based software, called Soundiation [GitHub Repository: https://github.com/Tountain/SoundiationAcoustophoresis, GPL-3.0 license], to address the acoustic radiation force and torque while supporting the dynamic prediction of non-spherical particles. The implementation is basically generic, and its applicability is demonstrated through the validation of numerical methods. A graphical user interface is provided so that it can be used and extended easily.

Near-infrared-laser-navigated dancing bubble within water via a thermally conductive interface

Precise manipulation of droplets or bubbles hosts a broad range of applications for microfluidic devices, drug delivery, and soft robotics. Generally the existing approaches via passively designing structured surfaces or actively applying external stimuli, inherently confine their motions within the planar or curved geometry at a slow speed. Consequently the realization of 3D manipulation, such as of the underwater bubbles, remains challenging. Here, during the near-infrared-laser impacting on water, by simply introducing a thermally conductive interface, we unexpectedly observe a spontaneously bouncing bubble with hundreds-of-micrometer diameter at tens-of-Hertz frequency. The unique formation of temperature inversion layer in our system generates the depth-dependent thermal Marangoni force responsible for the bouncing behavior. Both the scaling analysis and numerical simulation agree with observations quantitatively. Furthermore, by controlling the navigation speed of the laser beam, the bubble not only shows excellent steerability with velocity up to 40 mm/s, but also exhibits distinctive behaviors from bouncing to dancing within water. We demonstrate the potential applications by steering the bubble within water to specifically interact with tiny objects, shedding light on the fabrication of bubble-based compositions in materials science and contamination removal in water treatment.

Precise manipulation of droplets or bubbles hosts a broad range of applications for microfluidic devices, drug delivery, and soft robotics. Here, Hu et al. show the manipulation of Marangoni-driven dancing bubbles on water using a near-infrared-laser in a frequency of tens-of-Hertz.

Acoustic Resonance Frequencies of Underwater Toroidal Bubbles

Underwater bubbles display an acoustic resonance frequency close to spherical ones. In order to obtain a resonance significantly deviating from the spherical case, we stabilize bubbles in toroidal frames, resulting in bubbles which can be slender while still compact. For thin tori the resonance frequency increases greatly. Between a pair of bubble rings, we can achieve a flat acoustic pressure field for a critical distance between rings, a condition reminiscent of Helmholtz coils in magnetostatics. This opens the possibility to shape the acoustic field using long tunnels of rings.

Bactericidal Effect of Ultrasound-Responsive Microbubbles and Sub-inhibitory Gentamicin against Pseudomonas aeruginosa Biofilms on Substrates With Differing Acoustic Impedance

The aim of this research was to explore the interaction between ultrasound-activated microbubbles (MBs) and Pseudomonas aeruginosa biofilms, specifically the effects of MB concentration, ultrasound exposure and substrate properties on bactericidal efficacy. Biofilms were grown using a Centre for Disease Control (CDC) bioreactor on polypropylene or stainless-steel coupons as acoustic analogues for soft and hard tissue, respectively. Biofilms were treated with different concentrations of phospholipid-shelled MBs (10⁷-10⁸ MB/mL), a sub-inhibitory concentration of gentamicin (4 µg/mL) and 1-MHz ultrasound with a continuous or pulsed (100-kHz pulse repetition frequency, 25% duty cycle, 0.5-MPa peak-to-peak pressure) wave. The effect of repeated ultrasound exposure with intervals of either 15- or 60-min was also investigated. With polypropylene coupons, the greatest bactericidal effect was achieved with 2 × 5 min of pulsed ultrasound separated by 60 min and a microbubble concentration of 5 × 10⁷ MBs/mL. A 0.76 log (83%) additional reduction in the number of bacteria was achieved compared with the use of an antibiotic alone. With stainless-steel coupons, a 67% (0.46 log) reduction was obtained under the same exposure conditions, possibly due to enhancement of a standing wave field which inhibited MB penetration in the biofilm. These findings demonstrate the importance of treatment parameter selection in antimicrobial applications of MBs and ultrasound in different tissue environments.

Acoustic radiation force of a sphere with a hemispherically split boundary condition in a plane wave

In this paper, an analytical expression of the acoustic radiation force (ARF) for a spherical particle with a hemispherically split impedance boundary in a plane wave is deduced. Numerical calculations are carried out by considering the effect of the magnitude and phase of the acoustic impedance on the ARF. Computation results show that the increase in the magnitude of the acoustic impedance results in an overall decrease in the ARF, whereas the phase of the acoustic impedance results in a decrease in the ARF in the low frequency region. As the frequency increases, the positive phase angle leads to a decrease in the ARF, and the negative phase angle causes the ARF to increase rapidly. For a hemispherically split impedance sphere, the values of the ARF range from those of the rigid sphere and uniform impedance sphere. The finite-element models for the calculation of the ARF of a hemispherically split impedance boundary sphere are established and the correctness of the analytical theory is proved by numerical comparison. This work is expected to contribute theoretical support to the acoustic manipulation of particles with a nonuniform hemispherically split structure.

Amplification of Acoustic Forces Using Microbubble Arrays Enables Manipulation of Centimeter-Scale Objects

Manipulation of macroscale objects by sound is fundamentally limited by the wavelength and object size. Resonant subwavelength scatterers such as bubbles can decouple these requirements, but typically the forces are weak. Here we show that patterning bubbles into arrays leads to geometric amplification of the scattering forces, enabling the precise assembly and manipulation of cm-scale objects. We rotate a 1 cm object continuously or position it with 15  μm accuracy, using sound with a 50 cm wavelength. The results are described well by a theoretical model. Our results lay the foundation for using secondary Bjerknes forces in the controlled organization and manipulation of macroscale structures.

A sound approach to advancing healthcare systems: the future of biomedical acoustics

Newly developed acoustic technologies are playing a transformational role in life science and biomedical applications ranging from the activation and inactivation of mechanosensitive ion channels for fundamental physiological processes to the development of contact-free, precise biofabrication protocols for tissue engineering and large-scale manufacturing of organoids. Here, we provide our perspective on the development of future acoustic technologies and their promise in addressing critical challenges in biomedicine.

When Bubbles Are Not Spherical: Artificial Intelligence Analysis of Ultrasonic Cavitation Bubbles in Solutions of Varying Concentrations

Ultrasonic irradiation of liquids, such as water-alcohol solutions, results in cavitation or the formation of small bubbles. Cavitation bubbles are generated in real solutions without the use of optical traps making our system as close to real conditions as possible. Under the action of the ultrasound, bubbles can grow, oscillate, and eventually collapse or decompose. We apply the mathematical method of separation of motions to interpret the acoustic effect on the bubbles. While in most situations, the spherical shape of a bubble is the most energetically profitable as it minimizes the surface energy, when the acoustic frequency is in resonance with the natural frequency of the bubble, shapes with the dihedral symmetry emerge. Some of these resonance shapes turn unstable, so the bubble decomposes. It turns out that bubbles in the solutions of different concentrations (with different surface energies and densities) attain different evolution paths. While it is difficult to obtain a deterministic description of how the solution concentration affects bubble dynamics, it is possible to separate images with different concentrations by applying the artificial neural network (ANN) algorithm. An ANN was trained to detect the concentration of alcohol in a water solution based on the bubble images. This indicates that artificial intelligence (AI) methods can complement deterministic analysis in nonequilibrium, near-unstable situations.

Ultrasound-Responsive Systems as Components for Smart Materials

Smart materials can respond to stimuli and adapt their responses based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications such as actuation, sensing, or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of time scales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering, and acoustic radiation forces to enable capabilities such as actuation, sensing, payload delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound and poses interesting questions for further investigations.

SonoTweezer: An Acoustically Powered End-Effector for Underwater Micromanipulation

Recent advances in contactless micromanipulation strategies have revolutionized prospects of robotic manipulators as next-generation tools for minimally invasive surgeries. In particular, acoustically powered phased arrays offer dexterous means of manipulation both in air and water. Inspired by these phased arrays, we present SonoTweezer: a compact, low-power, and lightweight array of immersible ultrasonic transducers capable of trapping and manipulation of sub-mm sized agents underwater. Based on a parametric investigation with numerical pressure field simulations, we design and create a six-transducer configuration, which is small compared to other reported multi-transducer arrays (16-256 elements). Despite the small size of array, SonoTweezer can reach pressure magnitudes of 300 kPa at a low supply voltage of 25 V to the transducers, which is in the same order of absolute pressure as multi-transducer arrays. Subsequently, we exploit the compactness of our array as an end-effector tool for a robotic manipulator to demonstrate long-range actuation of sub-millimeter agents over a hundred times the agent's body length. Furthermore, a phase-modulation over its individual transducers allows our array to locally maneuver its target agents at sub-mm steps. The ability to manipulate agents underwater makes SonoTweezer suitable for clinical applications considering water's similarity to biological media, e.g., vitreous humor and blood plasma. Finally, we show trapping and manipulation of micro-agents under medical ultrasound (US) imaging modality. This application of our actuation strategy combines the usage of US waves for both imaging and micromanipulation.

3-D Acoustic Tweezers Using a 2-D Matrix Array With Time-Multiplexed Traps

The use of acoustic tweezers for precise manipulation of microparticles in the aqueous environment is essential and challenging for biomechanical applications in vivo. A 3-D acoustic tweezer is developed in this study for 3-D manipulation by using a two-dimensional (2-D) phased array consisting of 256 elements operating at 1.04 MHz. The emission phases of each element are iteratively determined by a backpropagation algorithm to generate multiple acoustic traps. Different traps are multiplexed in time, thus forming synthesized acoustic fields. We demonstrate the 3-D levitation and translation of positive acoustic contrast particles, a major class of bioparticles, in water by different acoustic traps, and compare the positional deviation along the intended path via experimentally measured trajectories. Improved manipulating stability was achieved by multiplexed acoustic traps. The 3-D acoustic tweezers proposed in this study provide a versatile approach of contactless bioparticle trapping and translation, paving the way toward future application of nanodroplet and microbubble manipulations.

Precise micro-particle and bubble manipulation by tunable ultrasonic bottle beams

This paper reports a method to generate tunable bottle beams using an ultrasonic lens, by which the bottle position can be precisely adjusted with the change of the acoustic frequency. Therefore, the position of a single particle or bubble in liquid can be manipulated without using phased array which is costly and huge with complex circuits. Furthermore, we introduced this method to multiple bubble manipulation using acoustic holography. The bottle properties against frequency are theoretically and experimentally analyzed. It is shown that the bottle position depends almost linearly on the operating frequency, which provides a basis for the precise manipulation of bubbles and particles. In addition, the relationship between the acoustic radiation force and the drag force under different incident acoustic pressures is considered, establishing a limit on the moving velocity of the trapped particles. The ultrasonic field observation is further demonstrated by Schlieren imaging system. The proposed method has potential biomedical applications, such as more flexible cell manipulation and targeted drug delivery in vivo, as well as potential applications in the study of chemical reactions between micro objects.

Acoustic hologram optimisation using automatic differentiation

Acoustic holograms are the keystone of modern acoustics. They encode three-dimensional acoustic fields in two dimensions, and their quality determines the performance of acoustic systems. Optimisation methods that control only the phase of an acoustic wave are considered inferior to methods that control both the amplitude and phase of the wave. In this paper, we present Diff-PAT, an acoustic hologram optimisation platform with automatic differentiation. We show that in the most fundamental case of optimizing the output amplitude to match the target amplitude; our method with only phase modulation achieves better performance than conventional algorithm with both amplitude and phase modulation. The performance of Diff-PAT was evaluated by randomly generating 1000 sets of up to 32 control points for single-sided arrays and single-axis arrays. This optimisation platform for acoustic hologram can be used in a wide range of applications of PATs without introducing any changes to existing systems that control the PATs. In addition, we applied Diff-PAT to a phase plate and achieved an increase of > 8 dB in the peak noise-to-signal ratio of the acoustic hologram.

The waves that make the pattern: a review on acoustic manipulation in biomedical research

Novel approaches, combining technology, biomaterial design, and cutting-edge cell culture, have been increasingly considered to advance the field of tissue engineering and regenerative medicine. Within this context, acoustic manipulation to remotely control spatial cellular organization within a carrier matrix has arisen as a particularly promising method during the last decade. Acoustic or sound-induced manipulation takes advantage of hydrodynamic forces exerted on systems of particles within a liquid medium by standing waves. Inorganic or organic particles, cells, or organoids assemble within the nodes of the standing wave, creating distinct patterns in response to the applied frequency and amplitude. Acoustic manipulation has advanced from micro- or nanoparticle arrangement in 2D to the assembly of multiple cell types or organoids into highly complex in vitro tissues. In this review, we discuss the past research achievements in the field of acoustic manipulation with particular emphasis on biomedical application. We survey microfluidic, open chamber, and high throughput devices for their applicability to arrange non-living and living units in buffer or hydrogels. We also investigate the challenges arising from different methods, and their prospects to gain a deeper understanding of in vitro tissue formation and application in the field of biomedical engineering.

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Work on sound waves to spatially control particulate systems is reviewed.

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Classification of surface acoustic waves, bulk acoustic waves, and Faraday waves.

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Sound can be used to arrange, separate, or filter polymer particles.

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Sound can pattern cells in 3D to induce morphogenesis.

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Long-term applied sound induces differentiation and tissue formation.

Microbubbles versus Extracellular Vesicles as Therapeutic Cargo for Targeting Drug Delivery

Extracellular vesicles (EVs) and microbubbles are nanoparticles in drug-delivery systems that are both considered important for clinical translation. Current research has found that both microbubbles and EVs have the potential to be utilized as drug-delivery agents for therapeutic targets in various diseases. In combination with EVs, microbubbles are capable of delivering chemotherapeutic drugs to tumor sites and neighboring sites of damaged tissues. However, there are no standards to evaluate or to compare the benefits of EVs (natural carrier) versus microbubbles (synthetic carrier) as drug carriers. Both drug carriers are being investigated for release patterns and for pharmacokinetics; however, few researchers have focused on their targeted delivery or efficacy. In this Perspective, we compare EVs and microbubbles for a better understanding of their utility in terms of delivering drugs to their site of action and future clinical translation.

Bubbles in microfluidics: an all-purpose tool for micromanipulation

In recent decades, the integration of microfluidic devices and multiple actuation technologies at the microscale has greatly contributed to the progress of related fields. In particular, microbubbles are playing an increasingly important role in microfluidics because of their unique characteristics that lead to specific responses to different energy sources and gas-liquid interactions. Many effective and functional bubble-based micromanipulation strategies have been developed and improved, enabling various non-invasive, selective, and precise operations at the microscale. This review begins with a brief introduction of the morphological characteristics and formation of microbubbles. The theoretical foundations and working mechanisms of typical micromanipulations based on acoustic, thermodynamic, and chemical microbubbles in fluids are described. We critically review the extensive applications and the frontline advances of bubbles in microfluidics, including microflow patterns, position and orientation control, biomedical applications, and development of bubble-based microrobots. We lastly present an outlook to provide directions for the design and application of microbubble-based micromanipulation tools and attract the attention of relevant researchers to the enormous potential of microbubbles in microfluidics.

Self-Navigated 3D Acoustic Tweezers in Complex Media Based on Time Reversal

Acoustic tweezers have great application prospects because they allow noncontact and noninvasive manipulation of microparticles in a wide range of media. However, the nontransparency and heterogeneity of media in practical applications complicate particle trapping and manipulation. In this study, we designed a 1.04 MHz 256-element 2D matrix array for 3D acoustic tweezers to guide and monitor the entire process using real-time 3D ultrasonic images, thereby enabling acoustic manipulation in nontransparent media. Furthermore, we successfully performed dynamic 3D manipulations on multiple microparticles using multifoci and vortex traps. We achieved 3D particle manipulation in heterogeneous media (through resin baffle and ex vivo macaque and human skulls) by introducing a method based on the time reversal principle to correct the phase and amplitude distortions of the acoustic waves. Our results suggest cutting-edge applications of acoustic tweezers such as acoustical drug delivery, controlled micromachine transfer, and precise treatment.

Contactless acoustic micro/nano manipulation: a paradigm for next generation applications in life sciences

Acoustic actuation techniques offer a promising tool for contactless manipulation of both synthetic and biological micro/nano agents that encompass different length scales. The traditional usage of sound waves has steadily progressed from mid-air manipulation of salt grains to sophisticated techniques that employ nanoparticle flow in microfluidic networks. State-of-the-art in microfabrication and instrumentation have further expanded the outreach of these actuation techniques to autonomous propulsion of micro-agents. In this review article, we provide a universal perspective of the known acoustic micromanipulation technologies in terms of their applications and governing physics. Hereby, we survey these technologies and classify them with regards to passive and active manipulation of agents. These manipulation methods account for both intelligent devices adept at dexterous non-contact handling of micro-agents, and acoustically induced mechanisms for self-propulsion of micro-robots. Moreover, owing to the clinical compliance of ultrasound, we provide future considerations of acoustic manipulation techniques to be fruitfully employed in biological applications that range from label-free drug testing to minimally invasive clinical interventions.

Phase and amplitude evolution of backscattering by a sphere scanned through an acoustic vortex beam: Measured helicity projections

While acoustic vortex beams have many potential applications, the full implication of the phase information available in scattering experiments has not been developed. The present paper concerns observables in measured near-backward scattering from a sphere in water raster scanned through a first-order acoustic vortex beam. Symmetrically placed transducer elements were operated in a transmit-receive mode. Helicity-dependent projections of the spatial evolution of the scattering were used to display magnitude and phase information. The resulting phase swirl patterns were projection dependent and especially sensitive to the transverse position of the sphere. The magnitude also depended on the sphere's position relative to the beam's axial null.