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CAREER: Dynamics of Holographic Acoustic Lenses for Nonlinear Ultrasound Focusing

Focused ultrasound (FU) is a transformative technology with the potential to treat many medical disorders noninvasively. Like using a convex lens to focus light beams on a single point, in FU, acoustic lenses are used to concentrate acoustic energy onto the desired target deep in the body. FU can be used to heat up, destroy, or change the target tissue, and has been projected as an effective tool for non-invasive brain tumor ablation, transient blood-brain barrier disruption, and neuromodulation, potentially leading to novel treatments of brain tumors, epilepsy, and Alzheimer’s and Parkinson’s diseases. However, in these applications, the inhomogeneous medium with non-flat geometry strongly attenuates, reflects, and distorts ultrasound waves, which could lead to inefficient and inaccurate delivery of acoustic energy. This Faculty Early Career Development Program (CAREER) project will enhance the state-of-the-art wave focusing capabilities by introducing a new generation of acoustic lenses capable of generating specified high-intensity FU fields. The lens design is based on characterizing the shape and acoustic properties of the target, imposing the desired acoustic field by a backward propagation model, calculating the unique thickness map of the lens design, and finally using a forward propagation model for reconstructing the target acoustic field. This research has the potential to lead the progress of science in emerging therapeutic applications of FU, enabling patient-specific medicine, where lenses are customized and 3D printed for each specific patient. Along with the research activities, the educational plan includes summer camps that will serve, mentor, and empower underrepresented students from historically black colleges and universities on the topic of ultrasound haptics using acoustic lenses. The student interns will acquire unique skills, build professional networks, and gain cross-cultural experiences.

The research introduces the concept of computer-generated holographic techniques to nonlinear acoustics. The experiments and modeling approaches aim to extend the capabilities of the acoustic holographic lenses to generate high-intensity scalable acoustic fields from a single element transducer. At higher excitation amplitudes, the nonlinear effects are exhibited by the generation of harmonics, distortion of the acoustic waveform, and possibly the formation of shock fronts. Such phenomena influence the pressure distribution and the diffraction pattern of the sound field. Therefore, new mix-domain algorithms that incorporate nonlinearities, for the forward and backward wave propagation, will be introduced to achieve efficient and precise patterning of high-intensity fields. The outcome of the research includes a mathematical framework for nonlinear wavefront shaping that advances the knowledge of inverse problems of nonlinear acoustics.

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Dynamics of Ultrasound-Responsive Polymeric Systems

Shape memory polymers are an emerging class of smart materials that have the ability to return from a deformed temporary shape to their original permanent shape when subjected to an external stimulus such as heat, light, and a magnetic field. These polymers have recently gained substantial interest in many applications including robotics, biomedical devices, and soft electronics. In many of these applications, there is an immediate industrial need for replacing conventional triggering methods for actuating the polymers with a more efficient and flexible method. This project will investigate high-intensity focused ultrasound as a novel and promising stimulus with unique capabilities to actuate the controlled shape recovery of shape memory polymers. Focused ultrasound actuates the polymer remotely and locally, is noninvasive, and is bio-compatible. These properties make the methodology a superior candidate, particularly for biomedical applications.

 

This project will support the experiments and multiscale modeling of the dynamics of shape memory polymers under high-intensity focused ultrasound fields. The research aims at filling a knowledge gap in terms of considering time-variant and nonlinear effects associated with high excitation levels in acoustic-responsive polymers. A multiphysics framework will be established to bridge the dynamical deformation mechanisms at the atomistic scale to the response of the polymer at the macroscale. This framework will then be combined with experiments to efficiently design the chemical composition and crystalline structure of ultrasound-responsive polymers, based on extrinsic length scales and intrinsic material properties. The output of the research effort will unravel the unknown mechanisms of acoustic-induced thermal actuation, by which ultrasound waves heat polymers, and help in optimizing the dynamic processes of shape fixation and recovery of shape memory polymer structures in high-intensity focused ultrasound fields. The findings will also uncover how the geometrical aspects of the additively manufactured shape memory polymers will affect the dynamics of the polymer in various ultrasound fields. In collaboration with the industrial partner, MedShape Inc., the approach developed in the research will be utilized to design and fabricate novel ultrasound-responsive polymer-based devices with medical applications. This project is performed in collaboraiton with Future Materials Laboratory.

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Acoustic Energy Transfer for Wireless Charging of Low-Power Sensors

Energy transfer without the use of physical plugs or wires, referred to as contactless energy transfer, is a transformative technology with potentially endless applications. It is particularly relevant in applications where wired electrical contact is dangerous or impractical. Furthermore, it would enhance the development, use, and reliability of low-power sensors, control devices, and communication networks in applications where wired connections or changing batteries are not practical or may not be a viable option (e.g. medical implants), add to the complexities associated in the system's design and operation (e.g. homes, cars, and airplanes) or may pose their own hazards (e.g. fire hazard). As an alternative to the relatively well-studied method of contactless energy transfer, namely the inductive method, ultrasonic acoustic energy transfer, which is based on the propagation of acoustic waves at ultrasonic frequencies to a piezoelectric receiver, offers the potential for wireless charging systems over increased transmitter-receiver distances, with reduced power losses, and elimination of hazards associated with electromagnetic fields.

In this project, we establish an experimentally-validated mathematical framework for the acoustic-electroelastic dynamics of piezoelectric transmission and reception when subjected to an acoustic medium over broad ranges of low-to-high electrical and acoustic excitation levels. The performed experiments and analytical multiphysics modeling approach aim at filling a knowledge gap in terms of considering nonlinear effects associated with high excitation levels in ultrasound acoustic energy transfer systems. These effects include the coupled nonlinear acoustic field with non-conservative electroelastic structural responses along with reflections due to impedance mismatch that lead to spatial resonances, energy loss, and appearance of higher harmonics during wave propagation in a nonlinear dispersive medium. This research will lead to a new understanding of resonant acoustic-piezoelectric systems that will influence the design of ultrasonic acoustic energy transfer systems for various applications -- through innovative mechanisms for wireless charging of low-power sensors by an acoustic-based novel power delivering system, selectively and noninvasively. We will provide recommendations at the fundamental level for enhancing system performance through addressing power output for different transducer designs, acoustic impedance matching layers, and the material and nonlinearity effects. The output of the research effort will help in the design of optimal acoustic systems that can transfer higher power levels at higher efficiencies in various applications. See the Research Highlight by MInDS graduate researcher.

We also investigate the monolithic passive holographic lenses for spatial control over the sound field. These enable precise focusing of acoustic energy or high-resolution elaborate pressure patterning in ultrasonic contactless energy transfer. The 3-D-printed polymer-based transmission and metal reflective acoustic hologram are used. The modeling and design process of such holograms as well as their capabilities for constructing single and multifocal spatial distributions of acoustic energy in a target plane are investigated.

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Liquid-Coated Vibrating Meshes for Controlled Particle Collection

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Concentrated airborne dust is a longstanding safety and health concern for underground mining operations. Ultrafine dust is the main reason for incurable lung disease, and dust can also propagate devastating underground explosions. To address these concerns and meet regulatory requirements, mining operations implement a number of dust control measures, with the flooded-bed dust scrubber being one of the key technologies in continuous miner applications. Despite the proficiency of the current dust scrubber technology, several technical challenges limit the performance efficiency, and operability of this dust collection system. This project  investigates the impact of mesh vibration on the collection efficiency of this system as compared with the traditional static mesh units. CFD simulations are carried out to evaluate the importance of mesh vibration on dust particle-mesh interaction under various vibrational and flow conditions.

We study the vibration responses of the mesh-like structure in both time and frequency domains.  The fluid-structure interface interface combines fluid flow with solid mechanics to capture the interaction between the fluid and the vibrating solid structure. Using the structural mechanics interface and a single-phase flow interface we model the fluid and solid, respectively. The model geometry consists of a horizontal flow channel in the middle of which is the mesh while it is under vertical excitation from the shaker. The fluid flows from left to right and it imposes a distributed force on the mesh walls resulting from the viscous drag and fluid pressure. By system modeling, multiscale simulations will be performed to characterize the key parameters in the structure design through scale, ranging from small, mid, to full scale sizes.

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Modeling and Testing of Ultrasonic Drying Systems

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Researchers at Oak Ridge National Laboratory proposed an alternative mechanic for the drying of clothes which circumvents the need for thermal energy. This method is called direct-contact ultrasonic clothes drying, utilizing atomization through direct mechanical coupling between mesh piezoelectric transducers and wet fabric. 

 

In this project, we investigate the physical processes related to the direct contact ultrasonic drying process. Beginning with the electrical actuation of the transducer used in the world's first prototype dryer, we develop an electro-mechanical model for predicting the resulting deformation. Various considerations for the material properties and geometry of the transducer are made for optimizing the output acceleration of the device. Next, the drying rates of fabrics in contact with the transducer are modeled for identification of parameters which will facilitate timely and energy efficient drying. Future considerations and recommendations for the development of ultrasonic drying are made as a result of the insight gained by this investigation.

Using Dance to Visualize Complex Acoustic Phenomena

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