Dynamics of Multi-functional Acoustic Holograms in Contactless Ultrasonic Energy Transfer Systems

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Virginia Tech

Contactless ultrasonic power transfer (UPT), using piezoelectric transducers, is based on transferring energy using acoustic waves, in which the waves are generated by an acoustic source or transmitter and then transferred through an acoustic medium such as water or human tissue to a sensor or receiver. The receiver then converts the mechanical strain induced by the incident acoustic waves to electricity and delivers to an electrical load, in which the electrical power output of the system can be determined. The execution and efficiency of this technology can be significantly enhanced through patterning, focusing, and localization of the transmitted acoustic energy in space to simultaneously power pre-determined distributed sensors or devices. A passive 3D-printed acoustic hologram plate alongside a single transducer can generate arbitrary and pre-designed ultrasound fields in a particular distance from the hologram mounted on the transmitter, i.e., a target plane. This dissertation presents the use of these simple, cost-effective, and high-fidelity acoustic holograms in UPT systems to selectively enhance and pattern the electrical power output from the receivers. Different holograms are numerically designed to create single and multi-focal pressure patterns in a target plane where an array of receivers are placed. The incident sound wave from a transmitter, after passing through the hologram, is manipulated, hence, the output field is the desired pressure field, which excites the receivers located at the pre-determined focal points more significantly. Furthermore, multi-functional holograms are designed to generate multiple images at different target planes and driving frequencies, called, respectively, multi-image-plane and multi-frequency patterning holograms. The multiple desired pressure distributions are encoded on the single hologram plate and each is reconstructed by changing the axial distance and by switching the frequency. Several proof-of-concept experiments are performed to verify the functionality of the computationally designed holograms, which are fabricated using modern 3D-printers, i.e., the desired wavefronts are encoded in the hologram plates' thickness profile, being input to the 3D-printer. The experiments include measurement of output pressure fields in water using needle hydrophones and acquisition of receivers' voltage output in UPT systems.

Another technique investigated in this dissertation is the implementation of acoustic impedance matching layers deposited on the front leading surface of the transmitter and receiver transducers. Current UPT systems suffer from significant acoustic losses through the transmission line from a piezoelectric transmitter to an acoustic medium and then to a piezoelectric receiver. This is due to the unfavorable acoustic impedance mismatch between the transducers and the medium, which causes a narrow transducer bandwidth and a considerable reflection of the acoustic pressure waves at the boundary layers. Using matching layers enhance the acoustic power transmission into the medium and then reinforce the input as an excitation into the 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. Significant enhancements are obtained in terms of the receiver's electrical power output when implementing a two-layer matching structure. A design platform is also developed that can facilitate the construction of high-fidelity acoustically matched transducers, that is, the material layers' selection and determination of their thicknesses. Furthermore, this dissertation presents a numerical analysis for the dynamical motions of a high-intensity focused ultrasound (HIFU)-excited microbubble or stable acoustic cavitation, which includes the effects of acoustic nonlinearity, diffraction, and absorption of the medium, and entails the problem of several biomedical ultrasound applications. Finally, the design and use of acoustic holograms in microfluidic channels are addressed which opens the door of acoustic patterning in particle and cell sorting for medical ultrasound systems.

acoustic hologram, acoustic patterning, ultrasonic power transfer, acoustic impedance matching layers, high-intensity focused ultrasound, nonlinear acoustics, piezoelectric transducers, stable cavitation, ultrasound microbubble dynamics