Trapping of a mie sphere by acoustic pulses: effects of pulse length

Single-Beam Acoustic Tweezers for Cell Biology: Molecular to In Vivo Level

Acoustic tweezers have attracted attention in various fields of cell biology, including in vitro single-cell and intercellular mechanics. Compared with other tweezing technologies such as optical and magnetic tweezers, acoustic tweezers possess stronger forces and are safer for use in biological systems. However, due to the limited spatial resolution or limited size of target objects, acoustic tweezers have primarily been used to manipulate cells in vitro. To extend the advantages of acoustic tweezers to other levels (e.g., molecular and in vivo levels), researchers have recently developed various types of acoustic tweezers such as single-beam acoustic tweezers (SBATs), surface acoustic wave (SAW) tweezers, and acoustic-streaming tweezers. Among these, SBATs utilize a single-focused beam, making the transducer and system simple, noninvasive, and capable of producing strong forces compared with other types of tweezers. Depending on the acoustic beam pattern, SBATs can be classified into Rayleigh regime, Mie regime, and acoustic vortex with different trapping dynamics and application levels. In this review, we provide an overview of the principles and configuration of each type of SBAT, their applications ranging from molecular to in vivo studies, and their limitations and prospects. Thus, this review demonstrates the significance and potential of SBAT technology in biophysics and biomedical engineering.

Red blood cell trapping using single-beam acoustic tweezers in the Rayleigh regime

Acoustic tweezers (ATs) are a promising technology that can trap and manipulate microparticles or cells with the focused ultrasound beam without physical contact. Unlike optical tweezers, ATs may be used for in vivo studies because they can manipulate cells through tissues. However, in previous non-invasive microparticle trapping studies, ATs could only trap spherical particles, such as beads. Here, we present a theoretical analysis of how the acoustic beam traps red blood cells (RBCs) with experimental demonstration. The proposed modeling shows that the trapping of a non-spherical, biconcave-shaped RBC could be successfully done by single-beam acoustic tweezers (SBATs). We demonstrate this by trapping RBCs using SBATs in the Rayleigh regime, where the cell size is smaller than the wavelength of the beam. Suggested SBAT is a promising tool for cell transportation and sorting.

Tornado-inspired acoustic vortex tweezer for trapping and manipulating microbubbles

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.

Energy, momentum, and angular momentum of sound pulses

Pulse solutions of the wave equation can be expressed as superpositions of scalar monochromatic beam wavefunctions (solutions of the Helmholtz equation). This formulation leads to causal (unidirectional) propagation, in contrast to all currently known closed-form solutions of the wave equation. Application is made to the evaluation of the energy, momentum, and angular momentum of acoustic pulses, as integrals over the beam and pulse weight functions. Equivalence is established between integration over space of the energy, momentum, and angular momentum densities, and integration over the wavevector weight function. The inequality linking the total energy and the total momentum is made explicit in terms of the weight function formulation. It is shown that a general pulse can be viewed as a superposition of phonons, each with energy ℏck, z component of momentum ℏq, and z component of angular momentum ℏm. A closed-form solution of the wave equation is found, which is localized and causal, and its energy and momentum are evaluated explicitly.

Acoustical tweezers using single spherically focused piston, X-cut, and Gaussian beams

Partial-wave series expansions (PWSEs) satisfying the Helmholtz equation in spherical coordinates are derived for circular spherically focused piston (i.e., apodized by a uniform velocity amplitude normal to its surface), X-cut (i.e., apodized by a velocity amplitude parallel to the axis of wave propagation), and Gaussian (i.e., apodized by a Gaussian distribution of the velocity amplitude) beams. The Rayleigh-Sommerfeld diffraction integral and the addition theorems for the Legendre and spherical wave functions are used to obtain the PWSEs assuming weakly focused beams (with focusing angle α ⩽ 20°) in the Fresnel-Kirchhoff (parabolic) approximation. In contrast with previous analytical models, the derived expressions allow computing the scattering and acoustic radiation force from a sphere of radius a without restriction to either the Rayleigh (a ≪ λ, where λ is the wavelength of the incident radiation) or the ray acoustics (a ≫λ) regimes. The analytical formulations are valid for wavelengths largely exceeding the radius of the focused acoustic radiator, when the viscosity of the surrounding fluid can be neglected, and when the sphere is translated along the axis of wave propagation. Computational results illustrate the analysis with particular emphasis on the sphere's elastic properties and the axial distance to the center of the concave surface, with close connection of the emergence of negative trapping forces. Potential applications are in single-beam acoustical tweezers, acoustic levitation, and particle manipulation.

Exploring the physics of efficient optical trapping of dielectric nanoparticles with ultrafast pulsed excitation

Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

Feasibility of multiple micro-particle trapping--a simulation study

Both optical tweezers and acoustic tweezers have been demonstrated for trapping small particles in diverse biomedical applications. Compared to the optical tweezers, acoustic tweezers have deeper penetration, lower intensity, and are more useful in light opaque media. These advantages enable the potential utility of acoustic tweezers in biological science. Since the first demonstration of acoustic tweezers, various applications have required the trapping of not only one, but more particles simultaneously in both the axial and lateral direction. In this research, a method is proposed to create multiple trapping patterns, to prove the feasibility of trapping micro-particles. It has potential ability to electronically control the location and movement of the particles in real-time. A multiple-focus acoustic field can be generated by controlling the excitation of the transducer elements. The pressure and intensity of the field are obtained by modeling phased array transducer. Moreover, scattering force and gradient force at various positions are also evaluated to analyze their relative components to the effect of the acoustic tweezers. Besides, the axial and lateral radiation force and the trapping trajectory are computed based on ray acoustic approach. The results obtained demonstrate that the acoustic tweezers are capable of multiple trapping in both the axial and lateral directions.

Calculation of acoustical radiation force on microsphere by spherically-focused source

Based on the ray acoustics approach, the trapping effects on a microsphere by an ideally spherically-focused ultrasound are discussed. The acoustical radiation force from a focused ultrasound beam on a spherical particle in a three-dimensional sound field is calculated considering the effect of the attenuation of the ultrasound beam both inside the particle and in the surrounding medium. The results show that as long as the particle is in the range of the ultrasound beam and as long as the appropriate parameters of the transducer are selected, the particle will be captured in the vicinity of the focus of the ultrasound beam. Also, the particle radius and different parameters of the transducer are analyzed for their affect on the radiation force.