Holograms for acoustics

An acoustofluidic scanning nanoscope using enhanced image stacking and processing

Nanoscale optical resolution with a large field of view is a critical feature for many research and industry areas, such as semiconductor fabrication, biomedical imaging, and nanoscale material identification. Several scanning microscopes have been developed to resolve the inverse relationship between the resolution and field of view; however, those scanning microscopes still rely upon fluorescence labeling and complex optical systems. To overcome these limitations, we developed a dual-camera acoustofluidic nanoscope with a seamless image merging algorithm (alpha-blending process). This design allows us to precisely image both the sample and the microspheres simultaneously and accurately track the particle path and location. Therefore, the number of images required to capture the entire field of view (200 × 200 μm) by using our acoustofluidic scanning nanoscope is reduced by 55-fold compared with previous designs. Moreover, the image quality is also greatly improved by applying an alpha-blending imaging technique, which is critical for accurately depicting and identifying nanoscale objects or processes. This dual-camera acoustofluidic nanoscope paves the way for enhanced nanoimaging with high resolution and a large field of view.

Non-invasive optogenetics with ultrasound-mediated gene delivery and red-light excitation

Optogenetics has revolutionized the capability of controlling genetically modified neurons in vitro and in vivo and has become an indispensable neuroscience tool. Using light as a probe for selective neuronal activation or inhibition and as a means to read out neural activity has dramatically enhanced our understanding of complex neural circuits. However, a common limitation of optogenetic studies to date is their invasiveness and spatiotemporal range. Direct viral injections into the brain tissue along with implantation of optical fibers and recording electrodes can disrupt the neuronal circuitry and cause significant damage. Conventional approaches are spatially limited around the site of the direct injection and insufficient in examining large networks throughout the brain. Lastly, optogenetics is currently not easily scalable to large animals or humans. Here, we demonstrate that optogenetic excitation can be achieved entirely non-invasively through the intact skull in mice. Using a needle-free combination of focused ultrasound-mediated viral delivery and extracorporeal illumination with red light, we achieved selective neuronal activation at depths up to 4 mm in the murine brain, confirmed through cFos expression and electrophysiology measurements within the treated areas. Ultrasound treatment significantly reduced freezing time during recall in fear conditioning experiments, but remote light exposure had a moderate effect on the freezing behavior of mice treated with viral vectors. The proposed method has the potential to open new avenues of studying, but also stimulating, neuronal networks, in an effort to elucidate normal or dysfunctional brain activity and treat neurological diseases. Finally, the same non-invasive methodology could be combined with gene therapy and applied to other organs, such as the eye and the heart.

Collective guiding of acoustically propelled nano- and microparticles

One of the most important potential applications of motile nano- and microdevices is targeted drug delivery. To realize this, biocompatible particles that can be guided collectively towards a target inside a patient's body are required. Acoustically propelled nano- and microparticles constitute a promising candidate for such biocompatible, artificial motile particles. The main remaining obstacle to targeted drug delivery by motile nano- and microdevices is to also achieve a reliable and biocompatible method for guiding them collectively to their target. Here, we propose such a method. As we confirm by computer simulations, it allows for the remote guiding of large numbers of acoustically propelled particles to a prescribed target by combining a space- and time-dependent acoustic field and a time-dependent magnetic field. The method works without detailed knowledge about the particle positions and for arbitrary initial particle distributions. With these features, it paves the way for the future application of motile particles as vehicles for targeted drug delivery in nanomedicine.

Theoretical Zero-Thickness Broadband Holograms Based on Acoustic Sieve Metasurfaces

Acoustic holography is an essential tool for controlling sound waves, generating highly complex and customizable sound fields, and enabling the visualization of sound fields. Based on acoustic sieve metasurfaces (ASMs), this paper proposes a theoretical design approach for zero-thickness broadband holograms. The ASM is a zero-thickness rigid screen with a large number of small holes that allow sound waves to pass through and produce the desired real image in the target plane. The hole arrangement rules are determined using a genetic algorithm and the Rayleigh–Sommerfeld theory. Because the wave from a hole has no extra phase or amplitude modulation, the intractable modulation dispersion can be physically avoided, allowing the proposed ASM-based hologram to potentially function in any frequency band as long as the condition of paraxial approximation is satisfied. Using a numerical simulation based on the combination of the finite element method (FEM) and the boundary element method (BEM), this research achieves broadband holographic imaging with a good effect. The proposed theoretical zero-thickness broadband hologram may provide new possibilities for acoustic holography applications.

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.

Tunable liquid crystal grating based holographic 3D display system with wide viewing angle and large size

As one of the most ideal display approaches, holographic 3-dimensional (3D) display has always been a research hotspot since the holographic images reproduced in such system are very similar to what humans see the actual environment. However, current holographic 3D displays suffer from critical bottlenecks of narrow viewing angle and small size. Here, we propose a tunable liquid crystal grating-based holographic 3D display system with wide viewing angle and large size. Our tunable liquid crystal grating, providing an adjustable period and the secondary diffraction of the reconstructed image, enables to simultaneously implement two different hologram generation methods in achieving wide viewing angle and enlarged size, respectively. By using the secondary diffraction mechanism of the tunable liquid crystal grating, the proposed system breaks through the limitations of narrow viewing angle and small size of holographic 3D display. The proposed system shows a viewing angle of 57.4°, which is nearly 7 times of the conventional case with a single spatial light modulator, and the size of the reconstructed image is enlarged by about 4.2. The proposed system will have wide applications in medical diagnosis, advertising, education and entertainment and other fields.

High-speed acoustic holography with arbitrary scattering objects

Recent advances in high-speed acoustic holography have enabled levitation-based volumetric displays with tactile and audio sensations. However, current approaches do not compute sound scattering of objects' surfaces; thus, any physical object inside can distort the sound field. Here, we present a fast computational technique that allows high-speed multipoint levitation even with arbitrary sound-scattering surfaces and demonstrate a volumetric display that works in the presence of any physical object. Our technique has a two-step scattering model and a simplified levitation solver, which together can achieve more than 10,000 updates per second to create volumetric images above and below static sound-scattering objects. The model estimates transducer contributions in real time by reformulating the boundary element method for acoustic holography, and the solver creates multiple levitation traps. We explain how our technique achieves its speed with minimum loss in the trap quality and illustrate how it brings digital and physical content together by demonstrating mixed-reality interactive applications.

A Robot Platform for Highly Efficient Pollutant Purification

In this study, we propose a highly efficient robot platform for pollutant adsorption. This robot system consists of a flapping-wing micro aircraft (FWMA) for long-distance transportation and delivery and cost-effective multifunctional Janus microrobots for pollutant purification. The flapping-wing micro air vehicle can hover for 11.3 km with a flapping frequency of approximately 15 Hz, fly forward up to 31.6 km/h, and drop microrobots to a targeted destination. The Janus microrobot, which is composed of a silica microsphere, nickel layer, and hydrophobic layer, is used to absorb the oil and process organic pollutants. These Janus microrobots can be propelled fast up to 9.6 body lengths per second, and on-demand speed regulation and remote navigation are manageable. These Janus microrobots can continuously carry oil droplets in aqueous environments under the control of a uniform rotating magnetic field. Because of the fluid dynamics induced by the Janus microrobots, a highly efficient removal of Rhodamine B is accomplished. This smart robot system may open a door for pollutant purification.

Rigorous analysis of the axial acoustic radiation force on a spherical object for single-beam acoustic tweezing applications

Acoustic tweezers are increasingly utilized for the contactless manipulation of small particles. This paper provides a theoretical model demonstrating the acoustic manipulation capabilities of single-beam acoustic transducers. Analytical formulas are derived for the acoustic radiation force on an isotropic spherical object of arbitrary size, centered on a circular piston, simply supported and clamped radiator in an inviscid fluid. Using these results, the existence of a negative axial force pulling the object closer to the radiator is revealed and explored. These findings offer further insight into the feasibility of trapping objects in the near-field of a single-beam acoustic transducer. The calculations illustrate the trapping capabilities of the different emitters as a function of radiator size, particle size, and distance from the source and highlight the impact of radiator boundary conditions. Manipulation of a cell-like fluid sphere in water and an expanded polystyrene sphere in air are studied in more detail with results that are validated through finite element analysis. The developed theoretical model allows fast evaluation of acoustic radiation forces which could aid in the development of relatively simple and inexpensive contactless manipulation solutions.

Large amplitude vibration of a cantilever actuated by a high-frequency pulsed laser

Laser excitation based on the thermoelastic principle is effective for micro-scale actuation, enabled energy conversion from optical to mechanical. The major advantages lie in non-contact actuation, easy miniaturization, and integration. To avoid surface damage, the laser power per unit is limited, leading to several micrometers of the vibration. In this study, a pure nickel millimeter-sized cantilever is successfully actuated at a low-frequency resonance (around Hz) via a nanosecond pulsed laser. By modal interaction, the energy is transferred from a low-intensity, high-frequency (around kHz) excitation to a low-frequency response with millimeter amplitude. The stable low-frequency resonance of the cantilever was maintained by changing the laser pulse parameters and the illumination locations. We also present a method to control the vibration of the cantilever using a modulated wave (MW: the laser wave modulated by a rectangular wave). The cantilever's amplitude can be efficiently adjusted by changing the laser power or duty cycle of the MW. The resonance frequency of the cantilever also can be altered by optimizing the geometries to meet various actuation requirements. This study enables large actuation (up to tens of millimeters) by laser excitation, facilitating applications in precision manipulation, microfluidic mixing, lab-on-a-chip device, and other related micro actuation devices.

Numerical Study of Acoustic Holograms for Deep-Brain Targeting through the Temporal Bone Window

Acoustic holograms can encode complex wavefronts to compensate the aberrations of a therapeutical ultrasound beam propagating through heterogeneous tissues such as the skull, and simultaneously, they can generate diffraction-limited acoustic images, that is, arbitrary shaped focal spots. In this work, we numerically study the performance of acoustic holograms focusing at the thalamic nuclei when the source is located at the temporal bone window. The temporal window is the thinnest area of the lateral skull and it is mainly hairless, so it is a desirable area through which to transmit ultrasonic waves to the deep brain. However, in targeting from this area the bilateral thalamic nuclei are not aligned with the elongated focal spots of conventional focused transducers, and in addition, skull aberrations can distort the focal spot. We found that by using patient-specific holographic lenses coupled to a single-element 650-kHz-frequency 65-mm-aperture source, the focal spot can be sharply adapted to the thalamic nuclei in a bilateral way while skull aberrations are mitigated. Furthermore, the performance of these holograms was studied under misalignment errors between the source and the skull, concluding that for misalignments up to 5°, acoustic images are correctly restored. This work paves the way to designing clinical applications of transcranial ultrasound such as blood-brain barrier opening for drug delivery or deep-brain neuromodulation using this low-cost and personalized technology, presenting desirable aspects for long-term treatments because the patient's head does not need to be shaved completely and skull heating is low.

Microscale concert hall acoustics to produce uniform ultrasound stimulation for targeted sonogenetics in hsTRPA1-transfected cells

The field of ultrasound neuromodulation has rapidly developed over the past decade, a consequence of the discovery of strain-sensitive structures in the membrane and organelles of cells extending into the brain, heart, and other organs. Notably, clinical trials are underway for treating epilepsy using focused ultrasound to elicit an organized local electrical response. A key limitation to this approach is the formation of standing waves within the skull. In standing acoustic waves, the maximum ultrasound intensity spatially varies from near zero to double the mean in one half a wavelength, and has lead to localized tissue damage and disruption of normal brain function while attempting to evoke a broader response. This phenomenon also produces a large spatial variation in the actual ultrasound exposure in tissue, leading to heterogeneous results and challenges with interpreting these effects. One approach to overcome this limitation is presented herein: transducer-mounted diffusers that result in spatiotemporally incoherent ultrasound. Herein, we numerically and experimentally quantified the effect of a diffuser in an enclosed domain, and show that adding the diffuser leads to a two-fold increase in ultrasound responsiveness of hsTRPA1 transfected HEK cells. Furthermore, we demonstrate the diffuser allow us to produce an uniform spatial distribution of pressure in the rodent skull. Collectively, we propose that our approach leads to a means to deliver uniform ultrasound into irregular cavities for sonogenetics.

Theoretical framework to predict the acoustophoresis of axisymmetric irregular objects above an ultrasound transducer array

Acoustic radiation force and torque arising from wave scattering are able to translate and rotate matter without contact. However, the existing research mainly focused on manipulating simple symmetrical geometries, neglecting the significance of geometric features. For the nonspherical geometries, the shape of the object strongly affects its scattering properties, and thus the radiation force and torque as well as the acoustophoretic process. Here, we develop an analytical framework to calculate the radiation force and torque exerted on the sound-hard or the sound-soft, axisymmetric particles excited by a user-customized transducer array based on a conformal transformation approach, capturing the significance of the geometric features. The derivation framework is established under the computation coordinate system (CCS), whereas the particle is assumed to be static. For the dynamic processes, the rotation of particle is converted as the opposite rotation of transducer array, achieved by employing a rotation transformation to tune the incident driving field in the CCS. Later, the obtained radiation force and torque in the CCS should be transformed back to the observation coordinate system for force and torque analysis. The radiation force and torque exerted on particles with different orientations are validated by comparing the full three-dimensional numerical solution in different phase distributions. It is found that the proposed method presents superior computational accuracy, high geometric adaptivity, and good robustness to various geometric features, while the computational efficiency is more than 100 times higher than that of the full numerical method. Furthermore, it is found that the dynamic trajectories of particles with different geometric features are completely different, indicating that the geometric features can be a potential degree of freedom to tune acoustophoretic process. The ability to predict the acoustophoretic process of nonspherical particles above a user-customized transducer array has improved our understanding of the effect of shape asymmetry, which can also be used to verify the effectiveness of acoustic tweezers in manipulating nonspherical objects.

Harmonic acoustics for dynamic and selective particle manipulation

Precise and selective manipulation of colloids and biological cells has long been motivated by applications in materials science, physics and the life sciences. Here we introduce our harmonic acoustics for a non-contact, dynamic, selective (HANDS) particle manipulation platform, which enables the reversible assembly of colloidal crystals or cells via the modulation of acoustic trapping positions with subwavelength resolution. We compose Fourier-synthesized harmonic waves to create soft acoustic lattices and colloidal crystals without using surface treatment or modifying their material properties. We have achieved active control of the lattice constant to dynamically modulate the interparticle distance in a high-throughput (>100 pairs), precise, selective and reversible manner. Furthermore, we apply this HANDS platform to quantify the intercellular adhesion forces among various cancer cell lines. Our biocompatible HANDS platform provides a highly versatile particle manipulation method that can handle soft matter and measure the interaction forces between living cells with high sensitivity.

Vector-Controlled Wheel-Like Magnetic Swarms With Multimodal Locomotion and Reconfigurable Capabilities

Inspired by the biological collective behaviors of nature, artificial microrobotic swarms have exhibited environmental adaptability and tasking capabilities for biomedicine and micromanipulation. Complex environments are extremely relevant to the applications of microswarms, which are expected to travel in blood vessels, reproductive and digestive tracts, and microfluidic chips. Here we present a strategy that reconfigures paramagnetic nanoparticles into a vector-controlled microswarm with 3D collective motions by programming sawtooth magnetic fields. Horizontal swarms can be manipulated to stand vertically and swim like a wheel by adjusting the direction of magnetic-field plane. Compared with horizontal swarms, vertical wheel-like swarms were evaluated to be of approximately 15-fold speed increase and enhanced maneuverability, which was exhibited by striding across complex 3D confinements. Based on analysis of collective behavior of magnetic particles in flow field using molecular dynamics methods, a rotary stepping mechanism was proposed to address the formation and locomotion mechanisms of wheel-like swarm. we present a strategy that actuates swarms to stand and hover in situ under a programming swing magnetic fields, which provides suitable solutions to travel across confined space with unexpected changes, such as stepped pipes. By biomimetic design from fin motion of fish, wheel-like swarms were endowed with multi-modal locomotion and load-carrying capabilities. This design of intelligent microswarms that adapt to complicated biological environments can promote the applications ranging from the construction of smart and multifunctional materials to biomedical engineering.

Reconfigurable assembly of colloidal motors towards interactive soft materials and systems

Due to the highly flexible reconfiguration of swarms, collective behaviors have provided various natural organisms with a powerful adaptivity to the complex environment. To mimic these natural systems and construct artificial intelligent soft materials, self-propelled colloidal motors that can convert diverse forms of energy into swimming-like movement in fluids afford an ideal model system at the micro-/nanoscales. Through the coupling of local gradient fields, colloidal motors driven by chemical reactions or externally physical fields can assembly into swarms with adaptivity. Here, we summarize the progress on reconfigurable assembly of colloidal motors which is driven and modulated by chemical reactions and external fields (e.g., light, ultrasonic, electric, and magnetic fields). The adaptive reconfiguration behaviors and the corresponding mechanisms are discussed in detail. The future directions and challenges are also addressed for developing colloidal motor-based interactive soft matter materials and systems with adaptation and interactive functions comparable to that of natural systems.

Direct sound printing

Photo- and thermo-activated reactions are dominant in Additive Manufacturing (AM) processes for polymerization or melting/deposition of polymers. However, ultrasound activated sonochemical reactions present a unique way to generate hotspots in cavitation bubbles with extraordinary high temperature and pressure along with high heating and cooling rates which are out of reach for the current AM technologies. Here, we demonstrate 3D printing of structures using acoustic cavitation produced directly by focused ultrasound which creates sonochemical reactions in highly localized cavitation regions. Complex geometries with zero to varying porosities and 280 μm feature size are printed by our method, Direct Sound Printing (DSP), in a heat curing thermoset, Poly(dimethylsiloxane) that cannot be printed directly so far by any method. Sonochemiluminescnce, high speed imaging and process characterization experiments of DSP and potential applications such as remote distance printing are presented. Our method establishes an alternative route in AM using ultrasound as the energy source.

Photo- and thermo-activated polymerization and melting processes are dominant in Additive Manufacturing (AM) while ultrasound activated sonochemical reactions have not been explored for AM so far. Here, the authors demonstrate 3D printing of structures using acoustic cavitation produced directly by focused ultrasound which creates sonochemical reactions in highly localized cavitation regions.

Acoustofluidic black holes for multifunctional in-droplet particle manipulation

Acoustic black holes offer superior capabilities for slowing down and trapping acoustic waves for various applications such as metastructures, energy harvesting, and vibration and noise control. However, no studies have considered the linear and nonlinear effects of acoustic black holes on micro/nanoparticles in fluids. This study presents acoustofluidic black holes (AFBHs) that leverage controlled interactions between AFBH-trapped acoustic wave energy and particles in droplets to enable versatile particle manipulation functionalities, such as translation, concentration, and patterning of particles. We investigated the AFBH-enabled wave energy trapping and wavelength shrinking effects, as well as the trapped wave energy-induced acoustic radiation forces on particles and acoustic streaming in droplets. This study not only fills the gap between the emerging fields of acoustofluidics and acoustic black holes but also leads to a class of AFBH-based in-droplet particle manipulation toolsets with great potential for many applications, such as biosensing, point-of-care testing, and drug screening.

Acoustic levitation and rotation of thin films and their application for room temperature protein crystallography

Acoustic levitation has attracted attention in terms of chemical and biochemical analysis in combination with various analytical methods because of its unique container-less environment for samples that is not reliant on specific material characteristics. However, loading samples with very high viscosity is difficult. To expand the scope, we propose the use of polymer thin films as sample holders, whereby the sample is dispensed on a film that is subsequently loaded onto an acoustic levitator. When applied for protein crystallography experiments, rotation controllability and positional stability are important prerequisites. We therefore study the acoustic levitation and rotation of thin films with an aspect ratio (the diameter-to-thickness ratio) of 80-240, which is an order of magnitude larger than those reported previously. For films with empirically optimized shapes, we find that it is possible to control the rotation speed in the range of 1-4 rotations per second while maintaining a positional stability of 12 ± 5 µm. The acoustic radiation force acting on the films is found to be a factor of 26-30 higher than that for same-volume water droplets. We propose use cases of the developed films for protein crystallography experiments and demonstrate data collections for large single crystal samples at room temperature.

Ultra-Broadband Bending Beam and Bottle Beam Based on Acoustic Metamaterials

We report the realization of an ultra-broadband bending beam based on acoustic metamaterials by the theoretical prediction and the numerical validation. The proposed structure is composed of a series of straight tubes with spatially modulated depths. We analytically derive the depth profile required for the generation of an ultra-broadband bending beam, and examine the performance of the metastructure numerically. The design is then extended for the generation of a three-dimensional bottle beam. The transverse trapping behaviours on small rigid objects by the bottle beam are investigated based on the force potential. Our work will help the further study of broadband acoustic meta-structures, and may also find applications in a variety of fields such as ultrasound imaging, health monitoring and particle manipulations.

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.

Dynamic Ultrasound Projector Controlled by Light

Dynamic acoustic wavefront control is essential for many acoustic applications, including biomedical imaging and particle manipulation. Conventional methods are either static or in the case of phased transducer arrays are limited to a few elements and hence limited control. Here, a dynamic acoustic wavefront control method based on light patterns that locally trigger the generation of microbubbles is introduced. As a small gas bubble can effectively stop ultrasound transmission in a liquid, the optical images are used to drive a short electrolysis and form microbubble patterns. The generation of microbubbles is controlled by structured light projection at a low intensity of 65 mW cm^-2 and only requires about 100 ms. The bubble pattern is thus able to modify the wavefront of acoustic waves from a single transducer. The method is employed to realize an acoustic projector that can generate various acoustic images and patterns, including multiple foci and acoustic phase gradients. Hydrophone scans show that the acoustic field after the modulation by the microbubble pattern forms according to the prediction. It is believed that combining a versatile optical projector to realize an ultrasound projector is a general scheme, which can benefit a multitude of applications based on dynamic acoustic fields.

Acoustic particle trapping driven by axial primary radiation force in shaped traps

We study particle trapping driven by the axial primary radiation force (A-PRF) in shaped traps exposed to standing bulk acoustic waves (S-BAW) using numerical simulations and experiments. The utilization of the stronger A-PRF as the main retention force is a consequence of standing-wave formation along the flow direction, instead of the orthogonal direction as in the case of traditionally used lateral-PRF S-BAW trapping setups. The study of particle dynamics reveals that the competition between A-PRF and viscous drag force governs particle trajectory. The ratio of the acoustic energy to the viscous work (β) provides a general criterion for particle trapping at a distinctive off-node site that is spatially controllable. Particles get trapped for β≥β_{cr} at some distance away from the nodal plane and the distance varies as β^{-c} (c=0.6-1.0). The use of A-PRF as the retention force could potentially allow traditional S-BAW trapping systems to envisage high-throughput advancements surpassing the current standards in cell-handling unit operations.

Achromatic metasurfaces by dispersion customization for ultra-broadband acoustic beam engineering

Metasurfaces, the ultra-thin media with extraordinary wavefront modulation ability, have shown great promise for many potential applications. However, most of the existing metasurfaces are limited by narrow-band and strong dispersive modulation, which complicates their real-world applications and, therefore require strict customized dispersion. To address this issue, we report a general methodology for generating ultra-broadband achromatic metasurfaces with prescribed ultra-broadband achromatic properties in a bottom-up inverse-design paradigm. We demonstrate three ultra-broadband functionalities, including acoustic beam deflection, focusing and levitation, with relative bandwidths of 93.3%, 120% and 118.9%, respectively. In addition, we reveal a relationship between broadband achromatic functionality and element dispersion. All metasurface elements have anisotropic and asymmetric geometries with multiple scatterers and local cavities that synthetically support internal resonances, bi-anisotropy and multiple scattering for ultra-broadband customized dispersion. Our study opens new horizons for ultra-broadband highly efficient achromatic functional devices, with promising extension to optical and elastic metamaterials.

Focusing in on the Future of Focused Ultrasound as a Translational Tool

This article summarizes the field of focused ultrasound for use in neuromodulation and discusses different ways of targeting, delivering, and validating focused ultrasound. A discussion is focused on parameter space and different ongoing theories of ultrasonic neuromodulation. Current and future applications of the technique are discussed.

Maskless Fourier transform holography

Fourier transform holography is a lensless imaging technique that retrieves an object's exit-wave function with high fidelity. It has been used to study nanoscale phenomena and spatio-temporal dynamics in solids, with sensitivity to the phase component of electronic and magnetic textures. However, the method requires an invasive and labor-intensive nanopatterning of a holography mask directly onto the sample, which can alter the sample properties, forces a fixed field-of-view, and leads to a low signal-to-noise ratio at high resolution. In this work, we propose using wavefront-shaping diffractive optics to create a structured probe with full control of its phase at the sample plane, circumventing the need for a mask. We demonstrate in silico that the method can image nanostructures and magnetic textures and validate our approach with a visible light-based experiment. The method enables investigation of a plethora of phenomena at the nanoscale including magnetic and electronic phase coexistence in solids, with further uses in soft and biological matter research.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.

Design of Acoustic Bifocal Lenses Using a Fourier-Based Algorithm

In this work, we develop a new design method based on fast Fourier transform (FFT) for implementing zone plates (ZPs) with bifocal focusing profiles. We show that the FFT of the governing binary sequence provides a discrete sequence of the same length, which indicates the location of the main foci at the ZP focusing profile. Then, using reverse engineering and establishing a target focusing profile, we are capable of generating a binary sequence that provides a ZP with the desired focusing profile. We show that this design method, based on the inverse fast Fourier transform (IFFT), is very flexible and powerful and allows to tailor the design of bifocal ZPs to achieve focusing profiles with the desired foci locations and resolutions. The key advantage of our design algorithm, compared to other alternatives presented in previous works, is that our method provides bifocal focusing profiles with an absolute control of the foci locations. Moreover, although we analyze the performance of this novel design algorithm for underwater ultrasonics, it can also be successfully extended to different fields of physics, such as optics or microwaves, where ZPs are widely employed.

Reconfigurable Disk-like Microswarm under a Sawtooth Magnetic Field

Swarming robotic systems, which stem from insect swarms in nature, exhibit a high level of environmental adaptability and enhanced tasking capabilities for targeted delivery and micromanipulation. Here, we present a strategy that reconfigures paramagnetic nanoparticles into microswarms energized by a sawtooth magnetic field. A rotary-stepping magnetic-chain mechanism is proposed to address the forming principle of disk-like swarms. Based on programming the sawtooth field, the microswarm can perform reversible transformations between a disk, an ellipse and a ribbon, as well as splitting and merging. In addition, the swarms can be steered in any direction with excellent maneuverability and a high level of pattern stability. Under accurate manipulation of a magnetic microswarm, multiple microparts with complicated shapes were successfully combined into a complete assembly. This reconfigurable swarming microrobot may shed light on the understanding of complex morphological transformations in living systems and provide future practical applications of microfabrication and micromanipulation.

Sequential structured volumetric ultrasound holography for self-positioning using monaural recording

In this article, a structured acoustic holography technique in the self-positioning method of a single microphone from the monaurally recorded signals is proposed. A series of three-dimensional ultrasonic holograms, designed for positioning in a workspace, are sequentially projected. As a result, the microphone receives a position-dependent sequence of amplitude signals encoded with information on the observation position. Subsequently, the microphone position is determined by obtaining the peak position of the cross-correlation function between the received signal and the reference signal. Experiments were conducted using a custom-made phased array of 40-kHz ultrasound transducers to evaluate the positioning accuracy. It is demonstrated that when applied to a 100×100×50 mm³ workspace, the measurement error was less than 1 mm at all observation points in the numerical experiment, which was maintained for more than 96% of the points in the real-environment experiments. The proposed method is advantageous in that it does not use the phase information of the recorded signals, thus requiring no multiple synchronized recordings as the microphone-array-based methods. In addition, this scheme does not directly use the absolute value of the received amplitude as a positioning clue, which means that no amplitude-to-voltage calibration is required.

Potential of the acoustic micromanipulation technologies for biomedical research

Acoustic micromanipulation technologies are a set of versatile tools enabling unparalleled micromanipulation capabilities. Several characteristics put the acoustic micromanipulation technologies ahead of most of the other tweezing methods. For example, acoustic tweezers can be adapted as non-invasive platforms to handle single cells gently or as probes to stimulate or damage tissues. Besides, the nature of the interactions of acoustic waves with solids and liquids eliminates labeling requirements. Considering the importance of highly functional tools in biomedical research for empowering important discoveries, acoustic micromanipulation can be valuable for researchers in biology and medicine. Herein, we discuss the potential of acoustic micromanipulation technologies from technical and application points of view in biomedical research.

Study of Contact Electrification at Liquid-Gas Interface

It is known that the suspended liquid droplets in clouds can generate electrostatic charges, which finally results in the lightning. However, the detailed mechanism related to the contact-electrification process on the liquid-gas (L-G) interfaces is still poorly understood. Here, by introducing an acoustic levitation method for levitating a liquid droplet, we have studied the electrification mechanism at the L-G interface. The tribo-motion between water droplets and air induced by the ultrasound wave leads to the generation of positive charges on the surface of the droplets, and the charge amount of water droplets (20 μL) gradually reaches saturation within 30 s. The mixed solid particles in droplets can increase the amount of transferred charge, whereas the increase of ion concentration in the droplet can suppress the charge generation. This charge transfer phenomenon at L-G interfaces and the related analysis can be a guidance for the study in many fields, including anti-static, harvesting rainy energy, micro/nano fluidics, triboelectric power generator, surface engineering, and so on. Moreover, the surface charge generation due to L-G electrification is an inevitable effect during ultrasonic levitation, and thus, this study can also work for the applications of the ultrasonic technique.

Metamachines of pluripotent colloids

Machines enabled the Industrial Revolution and are central to modern technological progress: A machine's parts transmit forces, motion, and energy to one another in a predetermined manner. Today's engineering frontier, building artificial micromachines that emulate the biological machinery of living organisms, requires faithful assembly and energy consumption at the microscale. Here, we demonstrate the programmable assembly of active particles into autonomous metamachines using optical templates. Metamachines, or machines made of machines, are stable, mobile and autonomous architectures, whose dynamics stems from the geometry. We use the interplay between anisotropic force generation of the active colloids with the control of their orientation by local geometry. This allows autonomous reprogramming of active particles of the metamachines to achieve multiple functions. It permits the modular assembly of metamachines by fusion, reconfiguration of metamachines and, we anticipate, a shift in focus of self-assembly towards active matter and reprogrammable materials.

Multichannel Piezo-Ultrasound Implant with Hybrid Waterborne Acoustic Metastructure for Selective Wireless Energy Transfer at Megahertz Frequencies

Ultrasound energy transfer (UET) is developed and integrated into various bioelectronics with diagnostic, therapeutic, and monitoring capabilities. However, existing UET platforms generally enable one function at a time due to the single ultrasound channel architecture, limiting the full potential of bioelectronics that requires multicontrol modes. Here, a multichannel piezo-ultrasound implant (MC-PUI) is presented that integrates a hybrid waterborne acoustic metastructure (HWAM), multiple piezo-harvesters, and a miniaturized circuit with electronic components for selective wireless control via ultrasound frequency switching. The HWAM that utilizes both a 3D-printed air-diffraction matrix and a half-lambda Fabry-Perot resonator is optimized to provide the advantage of ultrasound selectivity at megahertz frequencies. Complying with U.S. Food and Drug Administration regulations, frequency-controlled multifunctional operations, such as wireless charging (≈11.08 µW) at 3.3 MHz and high-sensitivity wireless switch/control (threshold ≈0.55 MPa) of micro-light-emitting diode/motor at 1 MHz, are demonstrated ex vivo using porcine tissue and in vivo in a rat. The developed MC-PUI enhances UET versatility and opens up a new pathway for wireless implant design.

Acoustofluidic Droplet Sorter Based on Single Phase Focused Transducers

Droplet microfluidics has revolutionized the biomedical and drug development fields by allowing for independent microenvironments to conduct drug screening at the single cell level. However, current microfluidic sorting devices suffer from drawbacks such as high voltage requirements (e.g., >200 Vpp), low biocompatibility, and/or low throughput. In this article, a single-phase focused transducer (SPFT)-based acoustofluidic chip is introduced, which outperforms many microfluidic droplet sorting devices through high energy transmission efficiency, high accuracy, and high biocompatibility. The SPFT-based sorter can be driven with an input power lower than 20 Vpp and maintain a postsorting cell viability of 93.5%. The SPFT sorter can achieve a throughput over 1000 events per second and a sorting purity up to 99.2%. The SPFT sorter is utilized here for the screening of doxorubicin cytotoxicity on cancer and noncancer cells, proving its drug screening capability. Overall, the SPFT droplet sorting device shows great potential for fast, precise, and biocompatible drug screening.

Evolvable Acoustic Field Generated by a Transducer with 3D-Printed Fresnel Lens

Evolvable acoustic fields are considered an effective method for solving technical problems related to fields such as biological imaging, particle manipulation, drug therapy and intervention. However, because of technical difficulties and the limited technology available for realizing flexible adjustments of sound fields, few studies have reported on this aspect in recent years. Herein, we propose a novel solution, using a Fresnel lens-focused ultrasonic transducer for generating excited-signal-dependent acoustic pressure patterns. Finite element analysis (FEA) is used to predict the performance of a transducer with a Fresnel lens. The Fresnel lens is printed using 3D additive manufacturing. Normalized intensity maps of the acoustic pressure fields are characterized from the Fresnel lens-focused transducer under various numbers of excited-signal cycles. The results demonstrate that under different cycle excitations, a temporal evolution acoustic intensity can be generated and regulated by an ultrasound transducer with a 3D Fresnel lens. This acoustical pattern control method is not only simple to realize but also has considerable application prospects.

Three-dimensional spiral motion of microparticles by a binary-phase logarithmic-spiral zone plate

Acoustic vortex beams, which have both linear and angular momentum, can be used to make precise acoustic tweezers. Limited by the symmetry of a normal vortex beam, these tweezers are usually used for trapping or rotating particles in two dimensions. Here, the three-dimensional spiral motion of two soft particles of different sizes was realized using a vortex beam with a twisted focus, which was synthesized by a silicone binary-phase logarithmic-spiral zone plate. Numerical simulations and experimental measurements demonstrated that the beam had anisotropic focuses of crescent transverse intensity profiles and a screw phase dislocation with a singularity at the center. Experiments showed that a small particle (k₀r ≈ 1.3) can follow the twisted intensity of the beam, but a large particle (k₀r ≈ 4.7) spirals up away from the twisted field pattern. This is attributed to the dominant gradient force for the small particle, whereas the scattering effect induced a scattering force combined with a gradient force for the large particle. This focused twisted beam, which was generated with a structured silicone plate, and the three-dimensional spiral motion of microparticles, advance the development of simple, compact, and disposable acoustic devices for the precise and diverse manipulation of microparticles.

Measurement of the ultrasound attenuation and dispersion in 3D-printed photopolymer materials from 1 to 3.5 MHz

Over the past decade, the range of applications in biomedical ultrasound exploiting 3D printing has rapidly expanded. For wavefront shaping specifically, 3D printing has enabled a diverse range of new, low-cost approaches for controlling acoustic fields. These methods rely on accurate knowledge of the bulk acoustic properties of the materials; however, to date, robust knowledge of these parameters is lacking for many materials that are commonly used. In this work, the acoustic properties of eight 3D-printed photopolymer materials were characterised over a frequency range from 1 to 3.5 MHz. The properties measured were the frequency-dependent phase velocity and attenuation, group velocity, signal velocity, and mass density. The materials were fabricated using two separate techniques [PolyJet and stereolithograph (SLA)], and included Agilus30, FLXA9960, FLXA9995, Formlabs Clear, RGDA8625, RGDA8630, VeroClear, and VeroWhite. The range of measured density values across all eight materials was 1120-1180 kg  · m^-3, while the sound speed values were between 2020 to 2630 m  · s^-1, and attenuation values typically in the range 3-9 dB  ·  MHz^-1· cm^-1.

Materials and Schemes of Multimodal Reconfigurable Micro/Nanomachines and Robots: Review and Perspective

Mechanically programmable, reconfigurable micro/nanoscale materials that can dynamically change their mechanical properties or behaviors, or morph into distinct assemblies or swarms in response to stimuli have greatly piqued the interest of the science community due to their unprecedented potentials in both fundamental research and technological applications. To date, a variety of designs of hard and soft materials, as well as actuation schemes based on mechanisms including chemical reactions and magnetic, acoustic, optical, and electric stimuli, have been reported. Herein, state-of-the-art micro/nanostructures and operation schemes for multimodal reconfigurable micro/nanomachines and swarms, as well as potential new materials and working principles, challenges, and future perspectives are discussed.

Additive manufacturing of channeled acoustic topological insulators

We propose and fabricate an acoustic topological insulator to channel sound along statically reconfigurable pathways. The proposed topological insulator exploits additive manufacturing to create unit cells with complex geometry designed to introduce topological behavior while reducing attenuation. We break spatial symmetry in a hexagonal honeycomb lattice structure composed of a unit cell with two rounded cylindrical chambers by altering the volume of each chamber, and thus, observe the quantum valley Hall effect when the Dirac cone at the K-point lifts to form a topologically protected bandgap. Moderately protected edge states arise at the boundary between two regions with opposite orientations. The resulting propagation of a topologically protected wave along the interface is predicted computationally and validated experimentally. This represents a first step towards creating reconfigurable, airborne topological insulators that can lead to promising applications, such as four-dimensional sound projection, acoustic filtering devices, or multiplexing in harsh environments.

Acoustic holograms for bilateral blood-brain barrier opening in a mouse model

Transcranial focused ultrasound (FUS) in conjunction with circulating microbubbles injection is the sole non-invasive technique that temporally and locally opens the blood-brain barrier (BBB), allowing targeted drug delivery into the central nervous system (CNS). However, single-element FUS technologies do not allow the simultaneous targeting of several brain structures with high-resolution, and multi-element devices are required to compensate the aberrations introduced by the skull. In this work, we present the first preclinical application of acoustic holograms to perform a bilateral BBB opening in two mirrored regions in mice. The system consisted of a single-element focused transducer working at 1.68~MHz, coupled to a 3D-printed acoustic hologram designed to produce two symmetric foci in anesthetized mice \textit{in vivo} and, simultaneously, compensate the aberrations of the wavefront caused by the skull bones. T1-weighed MR images showed gadolinium extravasation at two symmetric quasi-spherical focal spots. By encoding time-reversed fields, holograms are capable of focusing acoustic energy with a resolution near the diffraction limit at multiple spots inside the skull of small preclinical animals. This work demonstrates the feasibility of hologram-assisted BBB opening for low-cost and highly-localized targeted drug delivery in the CNS in symmetric regions of separate hemispheres.

Programmable assembly of particles on a Chladni plate

In nature, simple building units can be assembled into complex shapes through long-term time-varying external stimuli that are often spatially nonlinear. In contrast, most artificial methods of externally directed assembly rely on field- or template-based energy minimization. However, methods directing the assembly process by controlling time-varying external stimuli instead of attaining the lowest-energy state remain largely unexplored. In this study, we introduce a method that applies time-varying and spatially nonlinear vibration fields to assemble particles into a desired two-dimensional shape. Our assembly method predicts, controls, and monitors the vibration-induced particle motion to iteratively minimize the difference between the desired shape and the actual particle distribution. We applied our method to a centrally actuated vibrating plate, also known as a Chladni plate, and assembled up to a hundred submillimeter particles into complex recognizable shapes. The method allows programmable formation of shapes beyond the intrinsic limits of periodic patterning of the plate.

On the Design of Soret Zone Plates Based on Binary Sequences Using Directional Transducers

In this work, we analyze the effect of the distribution of transparent Fresnel regions over the focusing profile of Soret Zone Plates (SZP) based on binary sequences. It is shown that this effect becomes very significant in those fields where directional transducers are employed, such as microwaves or acoustics. A thorough analysis of both the SZP transmission efficiency and the focusing enhancement factor is presented. Moreover, experimental measurements are also carried out for a particular type of binary sequence, the Cantor ternary set, validating the theoretical model and demonstrating that the distribution of transparent Fresnel regions becomes a critical parameter in applications requiring directional emitters.

Valley Vortex Assisted and Topological Protected Microparticles Manipulation with Complicated 2D Patterns in a Star-like Sonic Crystal

Arranging microparticles into desired patterns, especially in a complicated pattern with a reliable and tunable manner, is challenging but highly desirable in the fields such as biomedicine and tissue engineering. To overcome these limitations, here, by using the concept of topology in acoustics, the valley vortex is utilized to manipulate particles on a large scale with complicated 2D patterns in the star-like sonic crystals at different frequencies. A topologically protected edge state is obtained at the interface of the crystals with different valley Hall phases, which shows the ability of reliable microparticles control along the sharp corner and the capability of robust particles cluster aggregation in a defective system. The results may provide intriguing resources for future microfluidic systems in a complicated and brittle environment.

Unconventional acoustic approaches for localized and designed micromanipulation

Acoustic fields are ideal for micromanipulation, being biocompatible and with force gradients approaching the scale of single cells. They have accordingly found use in a variety of microfluidic devices, including for microscale patterning, separation, and mixing. The bulk of work in acoustofluidics has been predicated on the formation of standing waves that form periodic nodal positions along which suspended particles and cells are aligned. An evolving range of applications, however, requires more targeted micromanipulation to create unique patterns and effects. To this end, recent work has made important advances in improving the flexibility with which acoustic fields can be applied, impressively demonstrating generating arbitrary arrangements of pressure fields, spatially localizing acoustic fields and selectively translating individual particles in ways that are not achievable via traditional approaches. In this critical review we categorize and examine these advances, each of which open the door to a wide range of applications in which single-cell fidelity and flexible micromanipulation are advantageous, including for tissue engineering, diagnostic devices, high-throughput sorting and microfabrication.

Directed Assembly of Particles for Additive Manufacturing of Particle-Polymer Composites

Particle-polymer dispersions are ubiquitous in additive manufacturing (AM), where they are used as inks to create composite materials with applications to wearable sensors, energy storage materials, and actuation elements. It has been observed that directional alignment of the particle phase in the polymer dispersion can imbue the resulting composite material with enhanced mechanical, electrical, thermal or optical properties. Thus, external field-driven particle alignment during the AM process is one approach to tailoring the properties of composites for end-use applications. This review article provides an overview of externally directed field mechanisms (e.g., electric, magnetic, and acoustic) that are used for particle alignment. Illustrative examples from the AM literature show how these mechanisms are used to create structured composites with unique properties that can only be achieved through alignment. This article closes with a discussion of how particle distribution (i.e., microstructure) affects mechanical properties. A fundamental description of particle phase transport in polymers could lead to the development of AM process control for particle-polymer composite fabrication. This would ultimately create opportunities to explore the fundamental impact that alignment has on particle-polymer composite properties, which opens up the possibility of tailoring these materials for specific applications.

Acoustic tweezer with complex boundary-free trapping and transport channel controlled by shadow waveguides

Acoustic tweezers use ultrasound for contact-free, bio-compatible, and precise manipulation of particles from millimeter to submicrometer scale. In microfluidics, acoustic tweezers typically use an array of sources to create standing wave patterns that can trap and move objects in ways constrained by the limited complexity of the acoustic wave field. Here, we demonstrate spatially complex particle trapping and manipulation inside a boundary-free chamber using a single pair of sources and an engineered structure outside the chamber that we call a shadow waveguide. The shadow waveguide creates a tightly confined, spatially complex acoustic field inside the chamber without requiring any interior structure that would interfere with net flow or transport. Altering the input signals to the two sources creates trapped particle motion along an arbitrary path defined by the shadow waveguide. Particle trapping, particle manipulation and transport, and Thouless pumping are experimentally demonstrated.