Ultrasonic Particle Manipulation Research Articles

Piezoelectric metasurface for high-frequency ultrasonic transducer application around 50 MHz

High-frequency ultrasound plays a vital role in ultrasonic imaging and particle manipulation. However, the control of high-frequency ultrasound fields with traditional acoustic lens presents challenges in impedance matching, precise control, material selection. Therefore, piezoelectric ultrasonic transducers prepared based on traditional methods are difficult to achieve high frequency and efficient sound beam control. To address these challenges, we have developed a piezoelectric metasurface-based ultrasonic transducer, using the encode binary information method to modulate acoustic waves. Simulation results reveal that at a frequency of 50 MHz, the system exhibits efficient acoustic focusing capabilities, which have been successfully applied in the experimental trapping of microspheres with acoustic tweezers. The design and development of high-frequency ultrasound transducers have emerged as a burgeoning research area at the confluence of acoustics and numerous interdisciplinary fields.

Acoustofluidics in an equilateral triangular channel

Acoustofluidics is of great interest in many fields, such as physics, biology, chemistry, medicine, and manufacturing. Numerous research has been conducted on the basic physics of acoustofluidics in common (e.g., rectangular and circular) fluid channels. Recently, it has been reported that the resonant acoustic field in a triangular channel can be applied for rapid acoustofluidic sedimentation of microparticles. In this paper, we examine in detail the fundamentals of acoustofluidics in triangular channels and show their potential for ultrasonic particle manipulation. The variations of acoustofluidic fields, including acoustic resonances, acoustic radiation force, and outer acoustic streaming, in an equilateral triangular channel are investigated both experimentally and numerically. We develop a generalized limiting velocity solution for predicting the outer acoustic streaming fields in triangular channels of arbitrary shape. We show that it can generate both symmetric and antisymmetric acoustic resonances in triangular channels, which lead to outer acoustic streaming fields that are similar or distinct to those seen in common fluid channels. Boundary vibration-dependent asymmetric standing wave profiles can typically be excited, where spatial separation of acoustic pressure and streaming patterns is observed, which is consistent with the recent prediction of acoustofluidics in asymmetric acoustic resonances. The results demonstrate the richness of acoustic resonances and boundary-driven acoustic streaming patterns that arise in standing acoustic wave fields and open perspectives for acoustomicrofluidic applications such as heat and mass transfer enhancement and dynamic manipulation of micro- and nanoparticles.

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.

Acoustic black hole profiles for high-performance ultrasonic tweezers

Ultrasonic tweezers have provided significant capabilities for manipulating and handling objects in fluids across the fields of biomedical engineering, material preparation, micro/nano manufacturing, micro/nano robots, and so forth. However, most ultrasonic tweezers require precise design and professional operation, and an ultrasonic tweezer with more versatile, robust, and easy-to-use features is still scarce. In this study, through incorporating a one-dimensional acoustic black hole (ABH) structure, we propose and develop an innovative acoustic black hole tweezer (ABHT), which can easily realize multiple manipulation functionalities such as trapping of particles in various fluidic environments and cross-media extraction of particles. Numerical simulation results show that the ABH-enabled focused flexural waves at the tweezer’s tip can generate strong acoustofluidic effects including acoustic radiation forces on particles and acoustic streaming for particle trapping. We experimentally evaluate the performance of our ABHT, and also propose and demonstrate the topological device design with the high-throughput configurations. This work indicates that besides the conventional vibro-acoustic applications such as vibration mitigation and energy harvesting, ABHs can also play an effective role in ultrasonic particle manipulation devices. We believe the employment of ABHs would spur the emergence of more novel and useful ultrasonic particle manipulation and acoustofluidic devices in the future.

Acoustic manipulation dynamics of levitated particle with screw-shaped reflecting surface

Existing single-axis acoustic levitation devices with an axisymmetric reflector can manipulate particles in a variety of ways. However, the mechanism by which particles are suspended in a single-axis acoustic levitator with a non-axisymmetric reflector remains poorly understood. This work addresses this issue by proposing a novel single-axis ultrasonic levitator design that includes a flat plane emitter and a screw-plane reflector. The node positions of the standing wave formed in this levitator were predicted by calculating the Gor'kov potential according to a numerical model. The analysis results demonstrate that the nodes were distributed off-axis and their positions varied in a spiral manner when changing the distance between the emitter and reflector. Corresponding experiments based on the proposed design were also conducted, and the results indicated that the distance changes between the emitter and reflector could induce some spiral trajectories of a polyethylene-foam particle placed in the ultrasonic field. Moreover, the trajectory of the suspended particle was found to distribute along a conical surface centered on the central axis of this device. This work provides a new approach for ultrasonic particle manipulation by changing the geometry of the reflector.

Particle Manipulation in 2D Space Using a Capacitive Micromachined Ultrasonic Transducer.

Ultrasonic particle manipulation is a noncontact method for controlling microscale objects, such as cells or microparticles, using an acoustic field. In this study, a 2D array of capacitive micromachined ultrasonic transducers (CMUTs), placed horizontally in immersion, generated ultrasonic waves in the vertical direction, and the oil’s surface increased due to the radiation force of the ultrasonic waves. In addition, the radiation force directly exerted a force on a floating particle. By measuring the movement of the reflected laser light by the moving oil surface, the height of the oil’s surface deformed by the acoustic radiation force (ARF) was measured. The ARF made a floating particle, as well as the oil’s surface, move. The particle moved radially away from the surface position above the transducer, and its velocity was determined by its position on the fluid’s surface. When a single channel was operated, it moved 0.4 mm at an average speed of 90 μm/s, and when two adjacent channels were operated, it moved 1.2 mm at a speed of 272 μm/s. The particles moved in any direction on the surface of the oil by controlling the actuation channel using an electrical switch.

Investigation of the effect of chip material on acoustofluidic microparticle manipulation

Ultrasonic particle manipulation in microchannels is a rapidly growing research field with significant potential applications in biological studies. Several factors are essential in achieving effective manipulation, such as obtaining resonance conditions in the actuator, chip, and/or the microchannel. The choice of chip material is an important design parameter since it has significant implications on acoustic performance, thermal performance, stability, ease of manufacturing and the overall cost of the acoustofluidic separation unit. Even though different materials are employed in acoustofluidic studies, most studies implement silicon as the chip material due to its superior acoustic properties. On the other hand, silicon has a complicated manufacturing procedure that requires cleanroom facilities. Therefore, from the manufacturing perspective, it is beneficial to use materials that enable mechanical machining. In this study, for the first time in the literature, we have compared the performance of acoustofluidic chips manufactured from silicon, glass, PMMA (polymethyl methacrylate) and PDMS (polydimethylsiloxane) materials in a comparison study. Furthermore, we propose using a new composite material, FR4, which seems to possess favorable acoustic properties of acoustically hard materials (such as silicon and glass) and the ease of manufacturing of acoustically soft materials such as polymers.

Ultrasonic Particle Manipulation in Glass Capillaries: A Concise Review.

Ultrasonic particle manipulation (UPM), a non-contact and label-free method that uses ultrasonic waves to manipulate micro- or nano-scale particles, has recently gained significant attention in the microfluidics community. Moreover, glass is optically transparent and has dimensional stability, distinct acoustic impedance to water and a high acoustic quality factor, making it an excellent material for constructing chambers for ultrasonic resonators. Over the past several decades, glass capillaries are increasingly designed for a variety of UPMs, e.g., patterning, focusing, trapping and transporting of micron or submicron particles. Herein, we review established and emerging glass capillary-transducer devices, describing their underlying mechanisms of operation, with special emphasis on the application of glass capillaries with fluid channels of various cross-sections (i.e., rectangular, square and circular) on UPM. We believe that this review will provide a superior guidance for the design of glass capillary-based UPM devices for acoustic tweezers-based research.

Acoustic Hologram Enhanced Phased Arrays for Ultrasonic Particle Manipulation

By combining phased-array technology with recent developments in acoustic holograms, the authors enhance the capabilities of ultrasonic particle manipulation while reducing the cost. A 3D-printed hologram enables them to create high-fidelity force fields and manipulate small objects along complex paths. Attached to this hologram is an array of independent sound sources; controlling the output delay (phase) of these sources allows movement of the force field in space, yielding noncontact ``tweezers'' that can trap particles of almost any shape. Applications include 3D-bioprinting of cells to create designer tissues, as well as holding microorganisms in place for careful biological study.

Generating Bessel beams with broad depth-of-field by using phase-only acoustic holograms

We report zero-th and high-order acoustic Bessel beams with broad depth-of-field generated using acoustic holograms. While the transverse field distribution of Bessel beams generated using traditional passive methods is correctly described by a Bessel function, these methods present a common drawback: the axial distribution of the field is not constant, as required for ideal Bessel beams. In this work, we experimentally, numerically and theoretically report acoustic truncated Bessel beams of flat-intensity along their axis in the ultrasound regime using phase-only holograms. In particular, the beams present a uniform field distribution showing an elongated focal length of about 40 wavelengths, while the transverse width of the beam remains smaller than 0.7 wavelengths. The proposed acoustic holograms were compared with 3D-printed fraxicons, a blazed version of axicons. The performance of both phase-only holograms and fraxicons is studied and we found that both lenses produce Bessel beams in a wide range of frequencies. In addition, high-order Bessel beam were generated. We report first order Bessel beams that show a clear phase dislocation along their axis and a vortex with single topological charge. The proposed method may have potential applications in ultrasonic imaging, biomedical ultrasound and particle manipulation applications using passive lenses.

Acoustic radiation and viscous torque for micromanipulation controlled rotation of particles in fluid cavities

Our investigations at ETH Zurich aimed at the theoretical analysis of the acoustic torque and its experimental realization of a controlled rotation of spherical and non-spherical particles by ultrasound. Ultrasonic manipulation of particles provides a contactless handling method for particles suspended in a fluid by acoustic streaming and radiation forces. In addition to the translation of particles in all three spatial directions, particles like functional beads, cells, clumps of cells, fibers, etc., can be rotated. Various methods for the rotation of non-spherical particles were developed with the acoustic radiation torque. The necessary varying pressure field, where the orientation of the nodal pressure lines was controlled by two orthogonal standing waves, was achieved by a modulation of one single parameter over time, e.g., amplitude, phase, and frequency. Stable particle and fiber rotations up to 40 rpm were reached. The rotation can be performed continuously or in a stepwise fashion. Moreover, the rotation of spherical objects was realized by the viscous torque. This torque is formed by acoustic streaming, due to two orthogonal standing waves shifted in phase at the same excitation frequency and amplitude. Angular rotations up to 1200 rpm for spherical 35.5 μm copolymer particles were reached.

Dynamic-field devices for the ultrasonic manipulation of microparticles.

The use of acoustic radiation forces in lab-on-a-chip environments has seen a rapid development in recent years. Operations such as particle sieving, sorting and characterisation are becoming increasingly common with a range of applications in the biomedical sciences. Traditionally, these applications rely on static patterns of ultrasonic pressure and are often collectively referred to as ultrasonic standing wave devices. Recent years have also seen the emergence of devices which capitalise on dynamic and reconfigurable ultrasonic fields and these are the subject of this review. Dynamic ultrasonic fields lead to acoustic radiation forces that change with time. They have opened up the possibility of performing a wide range of manipulations such as the transport and rotation of individual particles or agglomerates. In addition, they have led to device reconfigurability, i.e. the ability of a single lab-on-a-chip device to perform multiple functions. This opens up the possibility of channel-less microfluidic devices which would have many applications, for example in biosensing and microscale assembly. This paper reviews the current state of the field of dynamic and reconfigurable ultrasonic particle manipulation devices and then discusses the open problems and future possibilities.

Screen-printed ultrasonic 2-D matrix array transducers for microparticle manipulation

This paper reports the development of a two-dimensional thick film lead zirconate titanate (PZT) ultrasonic transducer array, operating at frequency approximately 7.5MHz, to demonstrate the potential of this fabrication technique for microparticle manipulation. All layers of the array are screen-printed then sintered on an alumina substrate without any subsequent patterning processes. The thickness of the thick film PZT is 139±2μm, the element pitch of the array is 2.3mm, and the dimension of each individual PZT element is 2×2mm2 with top electrode 1.7×1.7mm2. The measured relative dielectric constant of the PZT is 2250±100 and the dielectric loss is 0.09±0.005 at 10kHz. Finite element analysis was used to predict the behaviour of the array and to optimise its configuration. Electrical impedance spectroscopy and laser vibrometry were used to characterise the array experimentally. The measured surface motion of a single element is on the order of tens of nanometres with a 10Vpeak continuous sinusoidal excitation. Particle manipulation experiments have been demonstrated with the array by manipulating Ø10μm polystyrene microspheres in degassed water. The simplified array fabrication process and the bulk production capability of screen-printing suggest potential for the commercialisation of multilayer planar resonant devices for ultrasonic particle manipulation.

Ultrasonic manipulation of magnetic particles in a microfluidic channel

In microfludic biosensor applications, many kinds of magnetic particles have been employed due to their bioaffinity property, so the appropriate manipulation of magnetic microparticles is important. In this work, we demonstrated the alignment of magnetic beads (diameter 2.8 μm) and nanowires (length ≤ 6 μm & diameter ≈ 50 nm) using ultrasonic standing wave (USW) in a steel-acrylic microfluidic channel (660 μm width) in the aim of concentrating them to a sensor area to enhance the performance of a sensor. Ultrasonic standing wave field generates the radiation force to concentrate microparticles at the pressure nodal planes. To generate a standing wave inside the channel, an ultrasound of 2.25 MHz resonance frequency was applied as a resonance frequency of the channel. Moreover, it was demonstrated here that the position of the aligned magnetic particles could be controlled by changing the applied frequency. These successful alignment and position control can be a milestone for future research which targets to improve the sensitivity of microfluidic sensors using magnetic particles.

Numerical modeling of ultrasonic particle manipulation for microfluidic applications

A numerical simulation methodology for ultrasonic particle/cell separation and cell washing processes is introduced and validated by comparing with the results from the literature. In this study, a finite element approach is used for modeling fluid flow in a microchannel and analytical relations are utilized for the calculation of the ultrasonic radiation forces. The solutions in acoustic and fluidic domains are coupled, and the particle separation under the influence of ultrasonic waves is numerically simulated. In order to simulate the cell washing process, diffusion and fluid dynamics solutions are coupled and solved. A Monte Carlo approach is chosen where statistical distributions are implemented in the simulations. Uniform distributions for the starting locations of particles/cells in the microchannel and normal distributions for the size of the particles are used in numerical simulations. In each case, 750 particles are used for the simulation, and the performance of separation process is evaluated by checking how many microparticles resulted in the targeted outlet channels. Channel geometries for the numerical simulations are adapted from the experimental studies in literature, and comparison between the reported experimental results and the numerical estimations is performed. It has been observed that the numerical estimations and experimental results from the literature are in good agreement, and the proposed methodology may be implemented as a design tool for ultrasonic particle manipulation for microfluidic applications.

Ultrasound-Induced Cell–Cell Interaction Studies in a Multi-Well Microplate

This review describes the use of ultrasound for inducing and retaining cell-cell contact in multi-well microplates combined with live-cell fluorescence microscopy. This platform has been used for studying the interaction between natural killer (NK) cells and cancer cells at the level of individual cells. The review includes basic principles of ultrasonic particle manipulation, design criteria when building a multi-well microplate device for this purpose, biocompatibility aspects, and finally, two examples of biological applications: Dynamic imaging of the inhibitory immune synapse, and studies of the heterogeneity in killing dynamics of NK cells interacting with cancer cells.

Ultrasonic manipulation of particles in an open fluid film

Ultrasonic manipulation is a noncontact method of trapping and holding particles in suspension, and has found many applications in microfluidic systems. Typically, ultrasonic standing waves are used; this approach is well established in fully enclosed microfluidic systems consisting of channels or chambers with an attached piezoelectric actuator. In this work, we examine the use of ultrasonic manipulation in open fluid films, which offer a high degree of accessibility. A piezoelectric actuator is presented which can be lowered into a separate fluid tray. This two-part system offers a high degree of flexibility; indeed the actuator can be removed with little disturbance to the particle patterns, so manipulation could potentially be periodically applied as required. Particle manipulation is shown to be possible over a distance many times the size of the actuator. Furthermore, particle manipulation can also be achieved in a tilted fluid film, so alignment between the two parts of the system is not critical to its operation.

Acoustophoresis of hollow and core-shell particles in two-dimensional resonance modes

Motivated by the applications of ultrasonic particle manipulation in a biotechnological context, a study on acoustophoresis of hollow and core-shell particles is presented with analytical derivations, numerical simulations and confirming experiments. For a long-wavelength calculation of the acoustic radiation forces, the Gor’kov potential of hollow, air-filled particles and particles with solid or fluid core and shell is derived. The validity as well as the applicable range of the long-wavelength calculation is evaluated with numerical simulations in Comsol Multiphysics®. The results are experimentally verified in the acoustic field of an intrinsically two-dimensional fluid resonance mode, which allows for a more complex analysis than the common one-dimensional ultrasonic standing waves or their superposition to two-dimensional fields. Experiments were conducted with hollow glass particles (13.9 μm diameter) in a microfluidic chamber of 1.2 mm × 1.2 mm × 0.2 mm on a silicon-based device with piezoelectric excitation around 870 kHz. The described resonance mode is of additional interest for particle trapping and medium exchange on certain particle types, and it reveals a novel approach for particle characterization or separation.

Ultrasonic Manipulation of Yeast Cells in Suspension for Absorption Spectroscopy with an Immersible Mid-Infrared Fiberoptic Probe

Recent advances in combining ultrasonic particle manipulation with attenuated total reflection infrared spectroscopy of yeast suspensions are presented. Infrared spectroscopy provides highly specific molecular information about the sample. It has not been applicable to in-line monitoring of cells during fermentation, however, because positioning cells in the micron-thin measurement region of the attenuated total reflection probe was not possible. Ultrasonic radiation forces exerted on suspended particles by an ultrasonic standing wave can result in the buildup of agglomerates in the nodal planes, hence enabling the manipulation of suspended cells on the microscopic scale. When a chamber setup and a prototype in-line applicable probe were used, successful control over the position of the yeast cells relative to the attenuated total reflection sensor surface could be proven. Both rate of increase and maximum mid-infrared absorption of yeast-specific bands during application of a pushing frequency (chamber setup: 1.863 MHz, in-line probe: 1.990 MHz) were found to correlate with yeast cell concentration.

Influence of acoustic streaming on ultrasonic particle manipulation in a 100-well ring-transducer microplate

We characterize and quantify the performance of ultrasonic particle aggregation and positioning in a 100-well microplate. We analyze the result when operating a planar ultrasonic ring transducer at different single actuation frequencies in the range 2.20–2.40 MHz, and compare with the result obtained from different schemes of frequency-modulated actuation. Compared to our previously used wedge transducer design, the ring transducer has a larger contact area facing the microplate, resulting in lower temperature increase for a given actuation voltage. Furthermore, we analyze the dynamics of acoustic streaming occurring simultaneously with the particle trapping in the wells of the microplate, and we define an adaptive ultrasonic actuation scheme for optimizing both efficiency and robustness of the method. The device is designed as a tool for ultrasound-mediated cell aggregation and positioning. This is a method for high-resolution optical characterization of time-dependent cellular processes at the level of single cells. In this paper, we demonstrate how to operate our device in order to optimize the scanning time of 3D confocal microscopy with the aim to perform high-resolution time-lapse imaging of cells or cell–cell interactions in a highly parallel manner.