In-Petri-dish acoustic vortex tweezers†

Acoustic tweezers facilitate a noninvasive, contactless, and label-free method for the precise manipulation of micro objects, including biological cells. Although cells are exposed to mechanical and thermal stress, acoustic tweezers are usually considered as biocompatible. Here, we present a holistic experimental approach to reveal the correlation between acoustic fields, acoustophoretic motion and heating effects of particles induced by an acoustic tweezer setup. The system is based on surface acoustic waves and was characterized by applying laser Doppler vibrometry, astigmatism particle tracking velocimetry and luminescence lifetime imaging. In situ measurements with high spatial and temporal resolution reveal a three-dimensional particle patterning coinciding with the experimentally assisted numerical result of the acoustic radiation force distribution. In addition, a considerable and rapid heating up to 55 °C depending on specific parameters was observed. Although these temperatures may be harmful to living cells, counter-measures can be found as the time scales of patterning and heating are shown to be different.

Existed tuneable acoustic metasurfaces (AMSs) can only transfer high order orbital angular momentum by converting the incident wave into reflected acoustic vortices (AVs). Here, a continuously tuneable AMS is designed and fabricated to produce transmitted AVs with multiple orders. The proposed AMS is composed of multiple layers of reconfigurable sector-shaped resonator-based units. A new design mechanism of the tuneable AMS is proposed to ensure that the discrete precision of the phase shift is not affected by the number of the layer. The theoretical, numerical and experimental results prove that the proposed AMS is able to continuously generate high-order transmitted AVs, which is promising in applications of real-time acoustic rotational manipulations such as acoustic microfluidics, cell manipulation, acoustic tweezers and acoustic communication.

With the emergence of regenerative medicine, cell printers, and labs on chips, the contactless selective manipulation of microscopic objects such as particles, cells, or drops has become a key feature. To complete this task, acoustic tweezers appear as a tremendous alternative to their magnetic and optical counterpart. Indeed, they do not require pre-tagging of the manipulated object and they enable particles trapping with forces several orders of magnitude larger than optical tweezers at same input power. Recently, Baresh et al. [Phys. Rev. Lett., 116, 024301 (2016)] demonstrated the selective 3D manipulation of particles with a specific class of waves called acoustical vortices. Nevertheless, such manipulation was achieved with a complex transducer array coupled with a high end programmable electronics. This system is cumbersome, not compatible with microscopes and hardly miniaturizable. To overcome these difficulties, our team developed new tweezers [Riaud et al., Phys. Rev. Appl. 7, 024007 (2017)] based on spiraling interdigitated transducers (IDTs), some electrodes sputtered at the surface of piezoelectric substrates patterned by photolithography. The shape of the electrodes encodes the phase of the field like a hologram. For applications, these tweezers have many attractive features: they are selective, flat, easily integrable, and compatible with disposable substrates.

Recent studies on the acoustic radiation forces exerted by sound impinging spherical objects suggest the use of structured wavefronts for particle entrapment and controlled manipulation. In the scope of understanding why it is made possible to trap and manipulate small particles with sound, we present a general model for the acoustic radiation forces in three dimensions. A first generalization comes from the extension of well known results for the radiation pressure of plane waves to incident wavefields having arbitrary wavefronts. Secondly, the elastic spherical target of any dimension is allowed to be arbitrarily located within the wavefield. Introducing a new class of "single-beam" acoustical tweezers, we discuss the capabilities of different acoustical beams to achieve particle trapping and manipulation tasks. In addition, using an efficient experimental setup, we report the propagation of a peculiar beam carrying orbital angular momentum, namely an acoustical vortex, which is our selected candidate to achieve the first three-dimensional acoustic trap for elastic particles.

The paper reports the intensity characteristics of fractional vortex beam, and controls the coupling between photon orbital angular momentum and spin angular momentum by coherently manipulating Laguerre–Gaussian vortex beam with linearly polarized plane light and its conjugate beam. By comparing theory with experiment, fractional vortex beams with different topological charges are generated by using pure phase liquid crystal spatial light modulator, and their intensity characteristics and interference characteristics are observed. The results show that the radial notch size of the spot is related to the fractional part of the topological charge value, and the closer the fractional part of the topological charge value is to the half integer, the more obvious the radial notch is. With the increase of integer part of topological charge, the spot radius also increases. In addition, interference with linearly polarized plane light reveals that the number of branches is equal to the number of topological charges. Interfere with its conjugate beam, and judge the topological charge number of vortex beam by the relationship between petal number and topological charge number. The petal number of integer vortex beam is twice the topological charge number, and fractional order vortex beam destroys the ring structure of Laguerre–Gaussian beam.

The optical tweezers has been a very useful tool in biological research. However, due to finite penetration depth in optics, it is only utilized for transparent particle. This limitation reduces its potential of in-vivo manipulation. The idea of acoustic tweezers with better penetration ability than optics was recently proposed. In this paper, a novel pulse-wave mode is theoretically demonstrated to implement this idea to reduce the risk of damage from thermal effect. The simulations are based on a time-varying acoustic field with a 100-MHz, 50% bandwidth, and 1.5 MPa transmitting Gaussian pulse. For simplicity, all calculations are simply done for the axis-symmetric case. The data of instantaneous intensity can be obtained from time tables of the simulated field. After applying it into the theory of acoustic tweezers, and integrating it over the spherical surface of a particle, net radiation forces at specific axis point can be obtained. Then, average forces at pulse durations along the beam axis are calculated, showing a transducer with F#/1, acoustic impedance of 1.38 MRayls, and particle size ranging from 120 mum to 210 mum are suitable for the existence of negative force induced by pulse waves. Furthermore, track of a particle versus time are simulated with 10 MPa pulse waves with PRF of 100 kHz, the results showing the average speed of the particle to be stably trapped is about 350 mum per second. This reveals the mobility of transducer should keep low to prevent particle escaping from the beam axis, but higher with both higher PRF and sound pressure. In conclusion, these results certainly show the feasibility of the acoustic tweezers in the pulse wave mode, indicating significant acoustic trapping induced in the pulse-wave mode depends on not only acoustic impedance mismatch and degree of focusing, but also adjustments of PRF and sound pressure.

Most of the reported vortex beam generators generate vortex beams at a fixed frequency, which limits the practical applications. Therefore, it is inevitable to explore a vortex beam generator, which can actively control the operating frequency. We propose a switchable frequency terahertz vortex beam metasurface, it is freely switchable under single-frequency mode and dual-frequency mode by changing the external temperature, the phase state of vanadium dioxide (VO<sub>2</sub>) is also changeable. External temperature changes will cause VO<sub>2</sub> to transform from insulating state to metallic state. Generally, VO<sub>2</sub> conductivity can increase by several orders of magnitude as operating temperature changes. By using the phase change property of VO<sub>2</sub>, we can obtain a metasurface with switchable operating frequencies. For operating at room temperature, the proposed metasurface behaves as a single-frequency terahertz vortex generator. When (left-handed circularly polarized, LCP) terahertz wave is vertically incident on the metasurface, it generates vortex beams with different topological charge numbers at a frequency of 1.1 THz, and the mode purity is above 85%. The simulation results show that the mode purity of the vortex beam with the topological charge <i>l</i> = 1 is 90%, and the mode purity is about 91.1% for the vortex beam with <i>l</i> = 2, and 85.4% for the vortex beam with <i>l</i> = 3. When the external temperature is of 68 ℃, the designed metasurface becomes a dual-frequency vortex beam generator. At this time, the operating frequencies of vortex beams with different topological charges (<i>l</i> = 1, 2, 3) are 0.7 and 1.23 THz, whose mode purities are both above 60%. That is to say, the corresponding mode purities at topological charge with<i> l</i> = 1 for two operating frequencies are 89.1% and 71.6%, respectively. The mode purities are 83.2% and 94.4% with topological charge <i>l</i> = 2, respectively. The mode purities are 62.4% and 68.2% with topological charge<i> l</i> = 3, respectively. Therefore, the proposed switchable frequency terahertz vortex generator provides a new design idea for working frequency modulation in wireless terahertz communication.

Acoustic tweezers using bisymmetric coherent surface acoustic waves for dynamic and reconfigurable manipulation of particle multimers

The generation and application of the vortex beams are part of the hot topics in the optical field. In this paper, the phase structure of the four-step phase plates, analyzed by Fourier series expansion method, is composed of a series of spiral phase plates. When the phase plate is directly irradiated by linearly polarized light, multi-order diffraction waves with different topological charge numbers are generated. Unlike vortex waves, the intensity distribution of the multi-order diffraction has a deviation from the axial symmetry due to the interference with each other. On this basis, a new scheme is proposed to generate vortex beams by the four-step phase plates. With the help of Mach-Zehnder interferometer, the diffraction waves generated by two pieces of the four-step phase plates overlap each other. By adjusting the phase difference of the Mach-Zehnder interferometer, some orders of diffraction waves generate destructive interference while the others generate constructive interference. Thus the linear polarized light can be converted into vortex beams. The diffraction intensity and angular momentum distributions of the four-step phase plates with different cycle numbers are numerically simulated and compared with the spiral phase plates, we can provethat the vortex beams can be obtained by simple four-step phase plates which are the same as those obtained by spiral phase plates. In addition, the four-step phase plates with a small cycle number can generate vortex beams with a large topological charge number and the fabrication difficulty of the phase plates is reduced.

This article presents a continuous home telemonitoring system for chronic respiratory patients using 5G connectivity developed in partnership with Vodafone as a part of the 5G Trial in Milan established by the Italian Ministry of Economic Development. The system features a wearable respiratory and activity monitor, an environmental sensor and a pulse oximeter sending the data through a 5G router to a Multi-Edge Computing server, incorporated in the Vodafone 5G infrastructure, where they are stored and accessible for visualization. In particular, activity, respiratory and environmental data are continuously streamed and collected. The solution has been tested on 18 healthy volunteers during non-supervised recordings lasting at least 48 hours. The combination of recognized activities and associated respiratory parameters provided statistically significant variations in breathing patterns between one activity and the other, thus giving more complete information to the clinicians than previously studied telemedicine systems based on spot-checks. In particular, statistically significant differences are found in tidal volume and minute ventilation between horizontal and vertical postures (p < 0.001) and between vertical postures and dynamic activities (p < 0.001); the respiratory rate shows statistically significant differences between horizontal and vertical postures (p < 0.001). Some environmental parameters have different mean values between day and night, such as carbon dioxide (p < 0.001). Trials on patients are needed to further study this telemedicine solution and make it commercially available in the future. The main further technical development suggested is the use of commercial 5G smartphones as routers, in order to make the system usable outside of home settings.

Spatially concentrating and manipulating biotherapeutic agents within the circulatory system is a longstanding challenge in medical applications due to the high velocity of blood flow, which greatly limits drug leakage and retention of the drug in the targeted region. To circumvent the disadvantages of current methods for systemic drug delivery, we propose tornado-inspired acoustic vortex tweezer (AVT) that generates net forces for noninvasive intravascular trapping of lipid-shelled gaseous microbubbles (MBs). MBs are used in a diverse range of medical applications, including as ultrasound contrast agents, for permeabilizing vessels, and as drug/gene carriers. We demonstrate that AVT can be used to successfully trap MBs and increase their local concentration in both static and flow conditions. Furthermore, MBs signals within mouse capillaries could be locally improved 1.7-fold and the location of trapped MBs could still be manipulated during the initiation of AVT. The proposed AVT technique is a compact, easy-to-use, and biocompatible method that enables systemic drug administration with extremely low doses.

Using acoustic vortex tweezers (AVTs) to spatially accumulate microbubbles (MBs) shows promise for enhancing drug delivery efficiency and reducing off-target effects. The strong echogenicity of accumulated MBs also improves diagnostics via conventional ultrasound (US) B-mode imaging. However, the annular high-pressure distribution of AVT inhibits MBs inflow at the inlet, reducing MBs collection. The spatial resolution of US B-mode imaging further limits theranostic applications of AVT-mediated MBs accumulation. To address these challenges, we integrated an AVT waveform with volumetric super-resolution imaging (VSRI) to monitor the dynamic growth of MBs cluster during accumulation. We used a 5-MHz 2-D array transducer for VSRI, employing plane wave pulses interleaved with accumulating pulses to retain MBs at a flow rate of 0.023-0.047 mL/s in a 3-mm vessel phantom. An asymmetrical AVT waveform (AVT ) was produced by modulating the pressure at the MBs inlet compared to the outlet. The effectiveness was validated in rat cerebral vessels for real-time volumetric tracking of MBs clusters. Microscopy observations showed that AVT could quickly gather flowing MBs into cluster without repelling them at a flow rate of 0.023 mL/s. Statistical results indicated that microscopic data correlated better with VSRI than with B-mode images, suggesting VSRI suffices to detect the dynamics of AVT -actuated MBs accumulation in real-time. Additionally, VSRI detected a significant increase in MBs cluster size over time during AVT in the superior sagittal sinus (SSS) of the rat brain. These findings demonstrate that the proposed strategy can accumulate the flowing MBs at a desired location and simultaneously observe this phenomenon.

Acoustic vortices (AV) are special beams with a screw-type dislocation, a pressure null along their principal axis and the ability to carry angular momentum. Mono-element sources, metamaterials, and phased arrays can be used to generate an AV. Among them, phased arrays are employed in the most of works that report particle manipulation with AV because of their ease to shape beams. However, special care must be taken at the design stage of the array because additional zones of constructive interference may appear, i.e., the so called grating lobes surrounding the main beam. When generating vortex beams using phased arrays, we have experimentally verified that these grating lobes have similar characteristics than the main beam, so we called them grating vortices (GV). However, GV exhibit certain level of distortion in both phase and magnitude. In this work, we present an experimental study of the angular momentum transfer capability of GV of different topological charge. We estimate the mechanical torque induced and the angular momentum transferred to disk-like samples of different size. The vortices are generated using a 30-element array operating at 40 kHz in air. A discussion on the potential of use of GV in the manipulation of particle/objects is given.

Acoustic vortex beams have attracted significant research interest in the last decade. The orbital angular momentum provides an additional degree-of-freedom, hence attracting attention in physics and technology. Generation and measurement are important parts of acoustic vortex research. For the production of acoustic vortices, it is convenient and less costly to use passive materials. Moreover, a point-by-point scanning procedure with a hydrophone still remains the commonly used method and is cumbersome to measure a three-dimensional acoustic field. However, an acoustic vortex field is usually three-dimensional, dynamic, and complex. Thus, the demand for imaging methods for complex pressure distributions has emerged. Herein, we introduced an improved hybrid single-arm coiling slit to generate an acoustic vortex with a deep potential well and infirm focusing. In addition, we proposed a method for holographic reconstruction and visualization of a three-dimensional acoustic field, which does not destroy the acoustic field information. The spatial-temporal properties of the acoustic vortex in the experiment closely match that of theoretical prediction. This study provides a reference for the manipulation and representation of a three-dimensional underwater acoustic wave.

Acoustic tweezer has a promising capability for object manipulation in vivo under ultrasound image guidance by another linear probe. Compared to linear probes, matrix array probes can be used for acoustical vortex generation and real-time imaging. It inspired us to achieve real-time imaging and object trapping simultaneously using one single matrix probe. However, common ultrasound systems only have 256 channels or even fewer, and imaging matrix probes, such as Vermon, have a large number of elements which makes the generation of acoustical vortex using full elements challenging. In this study, we designed a sparsity-controlled acoustic vortex tweezer using Vermon 1024-element matrix probe. Then, we verified the feasibility of the sparse acoustical vortex by simulation and experiments. In the vitro experiments, we trapped microbubbles (MBs) successfully and reconstructed three profiles (X-Y, X-Z, Y-Z) images in real-time.