Acoustic Tweezers Research Articles

Acoustic trapping of sub-wavelength microparticles and cells in resonant cylindrical shells

Acoustic tweezers based on the sound beam systems hold the promise of contactless manipulation of microparticles. However, in these conventional diffraction-limited system, acoustic diffraction severely limits the trapping strength and the minimum size of the trapped particles. Here, we theoretically propose and experimentally demonstrates that a simple cylindrical shell based acoustic tweezers can be utilized for trapping of sub-wavelength particles and cells with a radius as small as 1/400 of the corresponding acoustic wavelength. This mechanism is attributed to the significantly enhanced acoustic radiation force originating from the resonant excitation of low order circumferential mode intrinsically existing in the cylindrical shell, which is a highly localized field around its surfaces and breaking the diffraction limit. We further demonstrate that the manipulation ability of these tweezers is significantly stronger than that of traditional standing wave based acoustic tweezers, which can significantly reduce physiological damage to cells or other biological objects arising from the thermal effects. Thus, the cylindrical shells based acoustic tweezers are simple, disposable, low cost, biocompatible, and functional, with applications including 3-D bio-printing, cell culturing and tissue engineering.

Extraordinary trapping by Gorkov potential of ordinary and vortex limited-diffraction beams

Trapping behaviors by using standing waves or focused beams have been investigated and widely used for acoustic levitation and tweezers. It is commonly understood that dense and stiff particles are repelled from pressure anti-nodes (pressure maximum) where the Gor'kov potential has a maximum. Here, the trapping behaviors of a small particle by using Gor'kov potential of ordinary and vortex non-diffracting beams are examined. Unlike the standing waves or strongly focused beams, the ordinary non-diffracting beam with proper parameters is found to have a Gor'kov potential minimum at its pressure maximum to trap a relatively dense and stiff particle therein [Fan and Zhang, Phys. Rev. Appl. 11(1), 014055 (2019)]. For a vortex beam to trap a dense and stiff particle at the pressure null, the paraxiality of the beam has to be larger than some values. These seemly to be extraordinary behaviors are interpreted by the contribution of axial velocity to the Gor'kov potential. The results are applicable to beams with weak diffraction or weak focusing. Following from the results, ways to simplify methods for particle manipulation and development of acoustic tweezers are suggested.

Flexible transportation of microparticles and cells in phononic crystal based acoustofluidic channel

As one of the prominent technologies for non-contact manipulations, acoustic tweezer devices have used both standing waves and sound beams to trap particles at a fixed point, i.e., static positioning. However, dynamic manipulations of microparticles, cells, and living organisms are demanded in various micro/nanotechnologies, such as biomedical sensors, imaging devices, diagnostic tools, etc. The dynamic manipulation of microparticles in real-time requires time-variant acoustic fields. Here, a novel way for three-dimensional dynamic manipulation of microparticles and cells in the acoustofluidic channel is proposed by using a phononic crystal plate that modulates the acoustic field in a desired manner. This approach is superior to the conventional dynamic manipulations based on tuning phases of standing waves or moving sound sources. The phononic crystal based manipulations benefit from the resonant excitation of specific modes in the phononic crystal plate. The switch between different resonant modes in the phononic crystal plate enables the flexible switching between trapping and transportation, sets up a basis for completely flexible manipulation of microparticles in acoustofluidic channel.

Generating multifunctional acoustic tweezers in Petri dishes for contactless, precise manipulation of bioparticles.

Acoustic tweezers are a promising technology for the biocompatible, precise manipulation of delicate bioparticles ranging from nanometer-sized exosomes to millimeter-sized zebrafish larva. However, their widespread usage is hindered by their low compatibility with the workflows in biological laboratories. Here, we present multifunctional acoustic tweezers that can manipulate bioparticles in a disposable Petri dish. Various functionalities including cell patterning, tissue engineering, concentrating particles, translating cells, stimulating cells, and cell lysis are demonstrated. Moreover, leaky surface acoustic wave-based holography is achieved by encoding required phases in electrode profiles of interdigitated transducers. This overcomes the frequency and resolution limits of previous holographic techniques to control three-dimensional acoustic beams in microscale. This study presents a favorable technique for noncontact and label-free manipulation of bioparticles in commonly used Petri dishes. It can be readily adopted by the biological and medical communities for cell studies, tissue generation, and regenerative medicine.

Modulation of Orbital-Angular-Momentum Symmetry of Nondiffractive Acoustic Vortex Beams and Realization Using a Metasurface

Orbital angular momentum (OAM) carried by acoustic vortex attracts extensive research interests due to the possible application such as acoustic tweezer and communications. The OAM in the conventional symmetric acoustic vortex is generally determined by topological charge l. Here, we provide the beam symmetry as an alternative factor to manipulate the OAM beyond the topological charge. We propose a nondiffractive asymmetric acoustic vortex beam, which can be generated by an acoustic metasurface and modulate the OAM to arbitrary values. Rather than the radially symmetric distribution, the intensity of the asymmetric acoustic vortex is characterized with a crescent shape controlled by the asymmetry parameter $\ensuremath{\alpha}$. The ratio between axial OAM and sound energy on the transverse plane depends on the asymmetry parameter and thus is not quantized by the ratio of topological charge l to wave angular frequency $\ensuremath{\omega}$ as in conventional vortex beam. Moreover, we present the realization of the proposed vortex beam by designed metasurface, and reveal the nondiffractive nature of the asymmetric acoustic vortex beam. Our work expands the family of acoustic vortex and demonstrates an alternative route to control acoustic OAM.

Standing surface acoustic waves, and the mechanics of acoustic tweezer manipulation of eukaryotic cells

Manipulation by focused ultrasound is an emerging technology with much promise for non-contact handling of microscale objects. A particularly promising approach for achieving this with living cells involves incorporating standing surface acoustic waves (SSAWs) into a microfluidic device. SSAWs must be tuned to provide the necessary range of acoustic radiation force (ARF), but models enabling this tuning have neglected the mechanics of the cells themselves, treating cells as rigid or homogenous spheres, and have also neglected energy transfer from the substrate to the fluid at the Rayleigh angle. We therefore applied Mie scattering theory to develop a model of the ARF arising from a SSAW impacting an idealized eukaryotic cell in an inviscid fluid. The cell was treated as a three-layered body with a nucleus, cytoplasm, and cortical layer. Results showed strong dependence on cell structures and the Rayleigh angle that can be harnessed to develop novel applications for cell manipulation and sorting. ARF can be tuned using the new model to both push away and pull back a cell towards the sound source. The proposed analytical model provides a foundation for design of microfluidic systems that manipulate and sort cells based upon their mechanical properties.

Efficient Detection and Single-Cell Extraction of Circulating Tumor Cells in Peripheral Blood.

Circulating tumor cells (CTCs) play an important role in cancer biology studies. To further elucidate the role of single CTCs in tumor metastasis and prognosis, effective methods must be developed to isolate and encapsulate single CTCs. In this work, a single CTC capture and encapsulation platform based on ZnO nanofibers and surface acoustic waves was constructed. We also demonstrated that this platform can capture and encapsulate single CTCs efficiently. We first validated that the dense ZnO nanofibers provide additional binding sites, resulting in an increased capture efficiency of up to 93.3%. We then demonstrated that the release efficiency was increased to 100% by dissolving the substrate with ultralow concentration (25 mM) phosphoric acid, and the activity of the released cells was not affected. Furthermore, the released cells were suspended in alginate solution and encapsulated single CTCs via droplet-based surface acoustic wave devices. The encapsulating rate of a single cell is up to 13% under a pulse width duration of 520 μs. Few technology based on nanofibers and acoustic tweezers for single-cell encapsulation via acoustic droplet vaporization has been reported. It has a wide range of potential applications to acquire single target cells, which may facilitate further early clinical diagnosis and treatment of cancer patients.

Spatially selective manipulation of cells with single-beam acoustical tweezers

Acoustical tweezers open major prospects in microbiology for cells and microorganisms contactless manipulation, organization and mechanical properties testing since they are biocompatible, label-free and have the potential to exert forces several orders of magnitude larger than their optical counterpart at equivalent power. Yet, these perspectives have so far been hindered by the absence of spatial selectivity of existing acoustical tweezers - i.e., the ability to select and move objects individually - and/or their limited resolution restricting their use to large particle manipulation only and/or finally the limited forces that they could apply. Here, we report precise selective manipulation and positioning of individual human cells in a standard microscopy environment with trapping forces up to ~200 pN without altering their viability. These results are obtained with miniaturized acoustical tweezers combining holography with active materials to synthesize specific wavefields called focused acoustical vortices designed to produce stiff localized traps with reduced acoustic power.

Trapping of sub-wavelength microparticles and cells in resonant cylindrical shells

Acoustic tweezers based on the focused field hold the promise of contactless manipulation of microparticles. However, acoustic diffraction severely limits the trapping strength and the minimum size of the trapped particles in conventional diffraction-limited systems. Here, we propose and demonstrate a simple cylindrical shell structure for the trapping of microparticles with a radius as small as 1/400 of the corresponding acoustic wavelength, and its trapping ability is much stronger than that of the standing wave. This mechanism is attributed to the significantly enhanced acoustic radiation force originating from the resonant excitation of low order circumferential modes intrinsically existing in the cylindrical shell, which is a highly localized field around its surfaces. Cylindrical shell-based acoustic tweezers are simple, disposable, low cost, biocompatible, and functional, which may be of interest for nano-scale manufacturing and biomedical applications such as bio-printing, cell culturing, and tissue engineering.

Beamforming with transformation acoustics in anisotropic media

Transformation acoustics correlates complex material properties in physical space with distorted wave manipulations in virtual space, such that wave propagation patterns can be determined by mathematical coordinate transformations. These transformations allow for accurate modeling of acoustic propagation in complex materials. Such models are relevant for both biomedical ultrasound therapies and integrated on-chip systems, where muscle fibers and piezoelectric substrates act as effective anisotropic media, respectively. Without considering the anisotropic density of these sophisticated media, attempts to beamform acoustic patterns by phase engineering result in a heavily distorted signal. This distortion is detrimental to the performance of high intensity focused ultrasound acoustic tweezers for noninvasive surgeries, cell trapping, and cell sorting. Here, we demonstrate that the distortion effects can be corrected by transformation acoustics in which the phased array profile is adjusted to account for the corresponding anisotropy. We perform experiments to verify this transformation acoustic correction for arbitrary focused and self-bending beams with two-dimensional anisotropic spoof surface acoustic waves. The benefit of transformation acoustics in suppressing undesired anisotropic effects on beamformed waves improves the precision and efficacy of medical treatments that facilitate noninvasive ultrasound therapies and integrated on-chip applications.

Heat and mass transfer characteristics of binary droplets in acoustic levitation

The complex relationships between the flow field and heat transfer phenomena of acoustically levitated droplets under evaporation were investigated. To explain these correlations, binary droplets of ethanol and water were used as test fluids. Immediately after droplet levitation, the droplet external flow field direction was toward the droplet, with a circulating vortex forming near the droplet surface. As evaporation progressed, the external flow transitioned toward the opposite direction, while the circulation vortex expanded. To better understand the transition process of the droplet thermal boundary layer, the heat transfer coefficient time series changes were calculated by assuming that the transitions of the ethanol and water binary droplets occurred in three stages: (1) preferential evaporation of ethanol, (2) transition (evaporation of ethanol and condensation of water), and (3) evaporation and condensation of water. Finally, by comparing the flow field and thermal boundary transitions, the transition mechanism for flow structures and heat transport phenomena of acoustically levitated droplets with evaporation was considered. Our experimental and analytical results provide deeper physical insights into noncontact fluid manipulation and suggest potential future applications, such as in acoustic tweezers and microreactors.

A portable nucleic acid extraction system based on gigahertz acoustic tweezers.

In this study, a portable and compatible system for extraction of DNA based on gigahertz acoustic was proposed and verified. The system is based on tunable multiple acoustic tweezers which can switch between the mixing and separation mode to enable the efficient DNA extraction with a relatively small and portable setup. Using this system, we extracted DNA from the rat's whole blood and achieved about 80% recovery. By combining with real-time qPCR, this proposed detection method was able to detect adenovirus (ADV) DNA as low as 102 IU/mL.Clinical Relevance-The whole system is only in centimeter scale and can be applied for portable DNA extraction. There is great potential application in molecular diagnostics and point of care test (POCT).

Smartphone-enabled Dynamic Chemiluminescence Biomarker Quantitation Using Acoustic Tweezers Approach.

Quantitation of protein biomarker featured with portability, rapidity, high sensitivity is critical for the point-of-care testing (POCT) application. Herein, a novel smartphone-enabled microfluidic chemiluminescence platform for the quantitation of prostate specific antigen (PSA) was proposed based on acoustic tweezers approach. The primary antibodies labeled polystyrene microparticles (Ab1-PSs), target samples, the horseradish peroxidase labeled secondary antibodies (Ab2-HRP) were injected into the microfluidics simultaneously. Under the actuation of Lamb Wave Resonator (LWR), they were dynamically trapped and concentrated in the acoustic streaming; meanwhile, the biomolecular binding was enhanced. After the injection of chemiluminescent substrate, the concentrated immuno-particles catalyzed hydrogen peroxide (H2O2) reaction so that the emitted blue light was directly captured by smartphone. Besides, the flow rate and the applied power of LWR were optimized for the signal amplification. The chemiluminescence immunoassay exhibited a dynamic linear range from 0.5 ng/mL to 10 ng/mL with a limit of detection of 0.1 ng/mL in PBS buffer. The portable immunosensor will be utilized for the quantitation of PSA in serum samples to demonstrate the clinical significance.Clinical Relevance-The smartphone-enabled detection platform realizes the quantitation of biomarker within 10 min, which reveals a valuable potential tool for the early diagnosis of various diseases, even in resource-limited regions.

Acoustic tweezing for both Rayleigh and Mie particles based on acoustic focused petal beams

Acoustic tweezers (ATs) have been extensively exploited in physics, biology, chemistry, and medical medicine. However, previous ATs are limited by complex designs and cumbersome configurations, and the stable manipulation of Mie particles remains challenging. Here, an AT based on acoustic focused petal beams (AFPBs) is proposed to realize 2D stable manipulations of both Rayleigh and Mie particles in water. The AFPBs are generated by artificial structure plates (ASPs) engraved with two kinds of discrete curved slits. It is found that the bright petals of AFPBs are flexibly modulated by arranging the sectors of curved slits on ASP, and the central zero-intensity region encircled by bright petals is increased with the number of petals. Then, the acoustic radiation forces of the AFPBs with 2 and 10 petals acting on the Rayleigh and Mie particles are further studied, respectively, and a force equilibrium position is found in both cases. Finally, two ASP samples are fabricated to experimentally verify the generations of AFPBs, and the 2D stable trappings and movements of both Rayleigh and Mie particles are realized by AFPBs. This miniaturized AT is beneficial to practical applications in material fabrication, drug delivery, and tissue engineering.

Acoustic trapping of microbubbles in complex environments and controlled payload release

Contactless manipulation of microparticles using acoustic waves holds promise for applications ranging from cell sorting to three-dimensional (3D) printing and tissue engineering. However, the unique potential of acoustic trapping to be applied in biomedical settings remains largely untapped. In particular, the main advantage of acoustic trapping over optical trapping, namely the ability of sound to propagate through thick and opaque media, has not yet been exploited in full. Here we demonstrate experimentally the use of the recently developed technique of single-beam acoustical tweezers to trap microbubbles, an important class of biomedically relevant microparticles. We show that the region of vanishing pressure of a propagating vortex beam can confine a microbubble by forcing low-amplitude, nonspherical, shape oscillations, enabling its full 3D positioning. Our interpretation is validated by the absolute calibration of the acoustic trapping force and the direct spatial mapping of isolated bubble echos, for which both find excellent agreement with our theoretical model. Furthermore, we prove the stability of the trap through centimeter-thick layers of bio-mimicking, elastic materials. Finally, we demonstrate the simultaneous trapping of nanoparticle-loaded microbubbles and activation with an independent acoustic field to trigger the release of the nanoparticles. Overall, using exclusively acoustic powering to position and actuate microbubbles paves the way toward controlled delivery of drug payloads in confined, hard-to-reach locations, with potential in vivo applications.

Gallium Nitride: A Versatile Compound Semiconductor as Novel Piezoelectric Film for Acoustic Tweezer in Manipulation of Cancer Cells

Gallium nitride (GaN) is a compound semiconductor which has advantages to generate new functionalities and applications due to its piezoelectric, pyroelectric, and piezo-resistive properties. Recently, surface acoustic wave (SAW)-based acoustic tweezers were developed as an efficient and versatile tool to manipulate nano- and microparticles aiming for patterning, separating, and mixing biological and chemical components. Conventional piezoelectric materials to fabricate SAW devices such as lithium niobate suffer from its low thermal conductivity and incapability of fabricating multiphysical and integrated devices. This article piloted the development of a GaN-based acoustic tweezer (GaNAT) and its application in manipulating microparticles and biological cells. For the first time, the GaN SAW device was integrated with a microfluidic channel to form an acoustofluidic chip for biological applications. The GaNAT demonstrated its ability to work on high power (up to 10 W) with minimal cooling requirement while maintaining the device temperature below 32°C. Acoustofluidic modeling was successfully applied to numerically study and predict acoustic pressure field and particle trajectories within the GaNAT, which agree well with the experimental results on patterning polystyrene microspheres and two types of biological cells including fibroblast and renal tumor cells. The GaNAT allowed both cell types to maintain high viabilities of 84.5% and 92.1%, respectively.

Thin film Gallium nitride (GaN) based acoustofluidic Tweezer: Modelling and microparticle manipulation

Gallium nitride (GaN) is a compound semiconductor which shows advantages in new functionalities and applications due to its piezoelectric, optoelectronic, and piezo-resistive properties. This study develops a thin film GaN-based acoustic tweezer (GaNAT) using surface acoustic waves (SAWs) and demonstrates its acoustofluidic ability to pattern and manipulate microparticles. Although the piezoelectric performance of the GaNAT is compromised compared with conventional lithium niobate-based SAW devices, the inherited properties of GaN allow higher input powers and superior thermal stability. This study shows for the first time that thin film GaN is suitable for the fabrication of the acoustofluidic devices to manipulate microparticles with excellent performance. Numerical modelling of the acoustic pressure fields and the trajectories of mixtures of microparticles driven by the GaNAT was performed and the results were verified from the experimental studies using samples of polystyrene microspheres. The work has proved the robustness of thin film GaN as a candidate material to develop high-power acoustic tweezers, with the potential of monolithical integration with electronics to offer diverse microsystem applications.

Investigation of cell mechanics using single-beam acoustic tweezers as a versatile tool for the diagnosis and treatment of highly invasive breast cancer cell lines: an in vitro study

Advancements in diagnostic systems for metastatic cancer over the last few decades have played a significant role in providing patients with effective treatment by evaluating the characteristics of cancer cells. Despite the progress made in cancer prognosis, we still rely on the visual analysis of tissues or cells from histopathologists, where the subjectivity of traditional manual interpretation persists. This paper presents the development of a dual diagnosis and treatment tool using an in vitro acoustic tweezers platform with a 50 MHz ultrasonic transducer for label-free trapping and bursting of human breast cancer cells. For cancer cell detection and classification, the mechanical properties of a single cancer cell were quantified by single-beam acoustic tweezers (SBAT), a noncontact assessment tool using a focused acoustic beam. Cell-mimicking phantoms and agarose hydrogel spheres (AHSs) served to standardize the biomechanical characteristics of the cells. Based on the analytical comparison of deformability levels between the cells and the AHSs, the mechanical properties of the cells could be indirectly measured by interpolating the Young’s moduli of the AHSs. As a result, the calculated Young’s moduli, i.e., 1.527 kPa for MDA-MB-231 (highly invasive breast cancer cells), 2.650 kPa for MCF-7 (weakly invasive breast cancer cells), and 2.772 kPa for SKBR-3 (weakly invasive breast cancer cells), indicate that highly invasive cancer cells exhibited a lower Young’s moduli than weakly invasive cells, which indicates a higher deformability of highly invasive cancer cells, leading to a higher metastasis rate. Single-cell treatment may also be carried out by bursting a highly invasive cell with high-intensity, focused ultrasound.

Classification of Breast Cancer Cells Using the Integration of High-Frequency Single-Beam Acoustic Tweezers and Convolutional Neural Networks.

Single-beam acoustic tweezers (SBAT) is a widely used trapping technique to manipulate microscopic particles or cells. Recently, the characterization of a single cancer cell using high-frequency (>30 MHz) SBAT has been reported to determine its invasiveness and metastatic potential. Investigation of cell elasticity and invasiveness is based on the deformability of cells under SBAT’s radiation forces, and in general, more physically deformed cells exhibit higher levels of invasiveness and therefore higher metastatic potential. However, previous imaging analysis to determine substantial differences in cell deformation, where the SBAT is turned ON or OFF, relies on the subjective observation that may vary and requires follow-up evaluations from experts. In this study, we propose an automatic and reliable cancer cell classification method based on SBAT and a convolutional neural network (CNN), which provides objective and accurate quantitative measurement results. We used a custom-designed 50 MHz SBAT transducer to obtain a series of images of deformed human breast cancer cells. CNN-based classification methods with data augmentation applied to collected images determined and validated the metastatic potential of cancer cells. As a result, with the selected optimizers, precision, and recall of the model were found to be greater than 0.95, which highly validates the classification performance of our integrated method. CNN-guided cancer cell deformation analysis using SBAT may be a promising alternative to current histological image analysis, and this pretrained model will significantly reduce the evaluation time for a larger population of cells.

Magnetically controlled multifunctional membrane acoustic metasurface

Acoustic artificial structures have attracted great interest due to their unique capacity in manipulating acoustic waves. Among them, acoustic metasurfaces are highlighted for tuning acoustic waves in the subwavelength scale, which is expected for realizing acoustic device miniaturization. However, traditional acoustic metasurfaces are passive and non-multifunctional, which limits their further practical applications. In this paper, a magnetically controlled approach is investigated for achieving a multifunctional acoustic metasurface. The properties of the proposed acoustic metasurface, consisting of elastic films and additional mass, could be continuously modulated by magnetic force. Through switching the magnetic forces, the transmitted acoustic wave is easily tailored and different functions such as focusing, beam-splitting-like, and other near-field acoustic displays are switched. This work extends the research of multifunctional metasurfaces and has excellent potential in a wide range of applications including acoustic imaging, communications, and particle manipulation (such as suspension and acoustic tweezers).