Contactless ultrasonic acoustic energy transfer (UAET) is a new technology that eliminates risks or impracticalities associated with wired electrical connections or batteries that need to be replaced on a regular basis. This technology, which is based on the reception of acoustic waves at ultrasonic frequencies by piezoelectric receivers, can be used to wirelessly charge low-power electronics. The execution and efficiency of this technology can be significantly enhanced through patterning and focusing of the transmitted acoustic energy in space to simultaneously power distributed sensors or devices. This work investigates the use of an acoustic hologram to create a multifocal pressure pattern in a plane where target receivers are located at specific focal points or regions. First, a phase-shifting hologram is designed using an iterative angular spectrum approach. Then, a multi-physics acoustic-electro-elastic model is presented for an axially vibrating cylindrical transmitter used to power multiple piezoelectric receivers, in conjunction with the hologram. Experiments are also performed to show the capability of an acoustic hologram to selectively power an array of the receivers. Both analytical and experimental results show a dramatic enhancement of power transfer to receivers exposed to a multi-focal pressure pattern created by the hologram. As a case study, a bi-focal receiver setup is investigated where one receiver is targeted. The UAET simulation predicts a doubling of the power transfer to the targeted receiver, which is verified by the proof-of-concept experiment. The effects of system parameters such as input frequency and hologram aperture size are also reported.

The acoustic impedance mismatch between transducer materials and medium in ultrasonic power transfer systems narrows the transduction bandwidth and causes losses through the back reflection of progressive pressure waves at the boundary between the transducers and medium. Capturing both resonances and losses due to impedance mismatch of interwoven elements is essential for advancing the development of these systems. We present a unified approach, based on the multiplication of a sequence of transfer matrices, to determine an equivalent acoustic impedance. The analytical model couples the properties of the transmitter and receiver with multiple matching layers and a single classical quarter-wave layer in controlled setups with the objective of minimizing reflections through acoustic impedance mismatch alleviation. Losses due to ultrasonic attenuation in the material layers and medium are also considered. The acoustic field at the receiver location constitutes the input to the coupled electro-elastic equations of the fluid-loaded and electrically-loaded piezoelectric receiver. Experiments are performed to identify the input acoustic pressure from a cylindrical transmitter to a receiver disk operating in the 33-mode of piezoelectricity. The results show significant enhancements in terms of the receiver’s electrical power output when implementing a two-layer matching structure. We present the results showing non-dimensional wave number variations versus characteristic impedance, which can be used to calculate the materials’ thicknesses for acoustically matching ultrasonic power transfer systems to an acoustic medium of interest at any desired resonant frequency while considering any type of glue or epoxy as the bonding layer. The derived physical models facilitate the development of high-fidelity matched systems with enhanced contactless power transmission.

Many biomedical applications such as ultrasonic targeted drug delivery, gene therapy, and molecular imaging entail the problems of manipulating microbubbles by means of a high-intensity focused ultrasound (HIFU) pressure field; namely stable cavitation. In high-intensity acoustic field, bubbles demonstrate translational instability, the well-known erratic dancing motion, which is caused by shape oscillations of the bubbles that are excited by their volume oscillations. The literature of bubble dynamics in the HIFU field is mainly centered on experiments, lacking a systematic study to determine the threshold for shape oscillations and translational motion. In this work, we extend the existing multiphysics mathematical modeling platform on bubble dynamics for taking account of (1) the liquid compressibility which allows us to apply a high-intensity acoustic field; (2) the mutual interactions of volume pulsation, shape modes, and translational motion; as well as (3) the effects of nonlinearity, diffraction, and absorption of HIFU to incorporate the acoustic nonlinearity due to wave kinematics or medium—all in one model. The effects of acoustic nonlinearity on the radial pulsations, axisymmetric modes of shape oscillations, and translational motion of a bubble, subjected to resonance and off-resonance excitation and various acoustic pressure, are examined. The results reveal the importance of considering all the involved harmonics and wave distortion in the bubble dynamics, to accurately predict the oscillations, translational trajectories, and the threshold for inertial (unstable) cavitation. This result is of interest for understanding the bubble dynamical behaviors observed experimentally in the HIFU field.

Ultrasound strain elastography (USE) is an imaging technology that enables us to detect changes in tissue stiffness resulting from cancer and other diseases. The objective of this study is to computationally model the application of USE for breast lesion characterization. We develop a well-defined simulation pipeline using open-source software to create in silico USE phantoms with one and two stiff targets. First, we use FreeCAD software for tissue 3D modeling and Gmsh software for finite element (FE) meshes. Second, we place randomly positioned point scatterers within the meshed models to form pre-deformation virtual ultrasound phantoms. Then, a simulated ultrasound transducer is used to compress and deform tissue in FE simulations using FEBio software to create a post-deformation virtual ultrasound phantom. Third, we use the k-Wave acoustics toolbox to generate pre- and post-deformation ultrasound echo signals and B-mode images. Finally, we estimate axial and lateral displacements using a speckle tracking method, and strain elastograms, using a least-squares method. Displacements from the USE simulation pipeline and phantom experiments were compared against true FEBio-simulated displacements for accuracy. We have also quantitatively compared the resultant strain elastograms obtained from FEBio simulations, USE simulation pipeline, and phantom experiments. Finally, model validation is performed by comparing the performance of the USE software platform and physical phantom experiments for a range of compression values (0.5%–5% axial strain). The results confirm the use of the well-validated USE simulation pipeline as a robust non-clinical assessment tool for USE system development.