This article reviews acoustic microfiuidics: the use of acoustic fields, principally ultrasonics, for application in microfiuidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.

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Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

The Journal of the Acoustical Society of America

Autonomously moving micro-objects, or micromotors, have attracted the attention of the scientific community over the past decade, but the incompatibility of phoretic motors with solutions of high ionic strength and the use of toxic fuels have limited their applications in biologically relevant media. In this letter we demonstrate that ultrasonic standing waves in the MHz frequency range can levitate, propel, rotate, align, and assemble metallic microrods (2 μm long and 330 nm diameter) in water as well as in solutions of high ionic strength. Metallic rods levitated to the midpoint plane of a cylindrical cell when the ultrasonic frequency was tuned to create a vertical standing wave. Fast axial motion of metallic microrods at ∼200 μm/s was observed at the resonant frequency using continuous or pulsed ultrasound. Segmented metal rods (AuRu or AuPt) were propelled unidirectionally with one end (Ru or Pt, respectively) consistently forward. A self-acoustophoresis mechanism based on the shape asymmetry of the ...

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Autonomous Motion of Metallic Microrods Propelled by Ultrasound

After 20 years of innovation in techniques that specifically image the biomechanical properties of tissue, the evolution of elastographic imaging can be viewed from its infancy, through a proliferation of approaches to the problem to incorporation on research and then clinical imaging platforms. Ultimately this activity has culminated in clinical trials and improved care for patients. This remarkable progression represents a leading example of translational research that begins with fundamentals of science and engineering and progresses to needed improvements in diagnostic and monitoring capabilities applied to major categories of disease, surgery and interventional procedures. This review summarizes the fundamental principles, the timeline of developments in major categories of elastographic imaging, and concludes with recent results from clinical trials and forward-looking issues.

https://iopscience.iop.org/article/10.1088/0031-9155/56/1/R01/pdf

Imaging the elastic properties of tissue: the 20 year perspective.

After X-radiography, ultrasound is now the most common of all the medical imaging technologies. For millennia, manual palpation has been used to assist in diagnosis, but it is subjective and restricted to larger and more superficial structures. Following an introduction to the subject of elasticity, the elasticity of biological soft tissues is discussed and published data are presented. The basic physical principles of pulse-echo and Doppler ultrasonic techniques are explained. The history of ultrasonic imaging of soft tissue strain and elasticity is summarized, together with a brief critique of previously published reviews. The relevant techniques—low-frequency vibration, step, freehand and physiological displacement, and radiation force (displacement, impulse, shear wave and acoustic emission)—are described. Tissue-mimicking materials are indispensible for the assessment of these techniques and their characteristics are reported. Emerging clinical applications in breast disease, cardiology, dermatology, gastroenterology, gynaecology, minimally invasive surgery, musculoskeletal studies, radiotherapy, tissue engineering, urology and vascular disease are critically discussed. It is concluded that ultrasonic imaging of soft tissue strain and elasticity is now sufficiently well developed to have clinical utility. The potential for further research is examined and it is anticipated that the technology will become a powerful mainstream investigative tool.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2011.0054

Medical ultrasound: imaging of soft tissue strain and elasticity

An analytical solution for the scattering of an acoustic Bessel beam of any order by a deformable sphere centered on the beam is used to calculate the acoustic radiation force acting along the wave propagation axis. Situations are noted where, even in the absence of absorption, the radiation force of a high-order Bessel beam acting on the sphere is opposite to the direction of beam propagation. In the present research, the cases of a fluid and solid elastic spheres are considered with particular emphasis on how the mechanical properties and resonances of spheres as well as the beam parameters affect the negative radiation force. Conditions for the negative attracting force on hexane (fluid), aluminum and gold spheres are established, which help in designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.

https://iopscience.iop.org/article/10.1088/1751-8113/42/24/245202/pdf

Negative axial radiation force on a fluid and elastic spheres illuminated by a high-order Bessel beam of progressive waves

Vibro-acoustography method is explored for detecting and imaging brachytherapy metal seeds in gel phantoms. In a previous paper, we have shown that some immersed objects' resonance frequencies could be detected by vibro-acoustography. Here, we use this idea to optimize the vibro-acoustic excitation of two different sized brass seeds implanted in an agar gel phantom. In the experiments, the best excitation vibration frequencies were determined either by calculating fundamental resonance frequencies for each of the seeds or the experimental optimal resonance frequency of the gel. The resulting vibro-acoustography images demonstrate remarkable contrast in acoustic emission amplitude compared with images obtained at nonresonance frequencies. Results suggest the possible application of vibro-acoustography for directing prostate brachytherapy seed implantation treatment.

Improving the use of vibro-acoustography for brachytherapy metal seed imaging: A feasibility study

The acoustic radiation force of Langevin type resulting from the interaction of a high-order Bessel beam with a rigid immovable sphere in an ideal fluid is theoretically investigated. The analysis is based on applying the generalized Rayleigh series used in the near-field acoustic scattering problem to calculate the force. With appropriate selection of specific Bessel beam parameters, results for the rigid sphere unexpectedly reveal a negative radiation force caused by the Lagrangean energy density. Specifically, the negative force on the rigid sphere arises when the kinematic energy density is larger than the potential energy density. This condition provides an impetus for further designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

Acoustic waves may force a suspended object in the wavepath to spin by exerting a radiation torque. Generally, this torque depends on how the incident wave is scattered and absorbed by the object. We derive a general formula for the Cartesian components of the acoustic radiation torque produced by an arbitrary incident beam on an object of any geometrical shape in a nonviscous fluid. To illustrate the method, we calculate the acoustic radiation torque produced by a zero- and a first-order Bessel beam on an absorbing sphere in the off-axis configuration. Unexpectedly, the results show that some radiation torque components reverse their directions depending on the beam offset and the sphere size factor.

https://iopscience.iop.org/article/10.1209/0295-5075/97/54003/pdf

Radiation torque produced by an arbitrary acoustic wave

This paper concerns the ultrasonic characterization of human cancellous bone samples by solving the inverse problem using experimental transmitted signals. The ultrasonic propagation in cancellous bone is modeled using the Biot theory modified by the Johnson et al. model for viscous exchange between fluid and structure. The sensitivity of the Young modulus and the Poisson ratio of the skeletal frame is studied showing their effect on the fast and slow wave forms. The inverse problem is solved numerically by the least squares method. Five parameters are inverted: the porosity, tortuosity, viscous characteristic length, Young modulus, and Poisson ratio of the skeletal frame. The minimization of the discrepancy between experiment and theory is made in the time domain. The inverse problem is shown to be well posed, and its solution to be unique. Experimental results for slow and fast waves transmitted through human cancellous bone samples are given and compared with theoretical predictions.

Farid G. Mitri

Papers

Negative axial radiation force on a fluid and elastic spheres illuminated by a high-order Bessel beam of progressive waves

Improving the use of vibro-acoustography for brachytherapy metal seed imaging: A feasibility study

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

Radiation torque produced by an arbitrary acoustic wave

Ultrasonic characterization of human cancellous bone using the Biot theory: Inverse problem