In certain medical applications, transmitting an ultrasound beam through the skin to manipulate a solid object within the human body would be beneficial. Such applications include, for example, controlling an ingestible camera or expelling a kidney stone. In this paper, ultrasound beams of specific shapes were designed by numerical modeling and produced using a phased array. These beams were shown to levitate and electronically steer solid objects (3-mm-diameter glass spheres), along preprogrammed paths, in a water bath, and in the urinary bladders of live pigs. Deviation from the intended path was on average <10%. No injury was found on the bladder wall or intervening tissue.

Theoretical models allow design of acoustic traps to manipulate objects with radiation force. Here, a model of the acoustic radiation force by an arbitrary beam on a solid object was validated against measurement. The lateral force in water of different acoustic beams was measured and calculated for spheres of different diameter (2-6 wavelengths λ in water) and composition. This is the first effort to validate a general model, to quantify the lateral force on a range of objects, and to electronically steer large or dense objects with a single-sided transducer. Vortex beams and two other beam shapes having a ring-shaped pressure field in the focal plane were synthesized in water by a 1.5-MHz, 256-element focused array. Spherical targets (glass, brass, ceramic, 2-6 mm dia.) were placed on an acoustically transparent plastic plate that was normal to the acoustic beam axis and rigidly attached to the array. Each sphere was trapped in the beam as the array with the attached plate was rotated until the bead fell from the acoustic trap because of gravity. Calculated and measured maximum obtained angles agreed on average to within 22%. The maximum lateral force occurred when the target diameter equaled the beam width; however, objects up to 40% larger than the beam width were trapped. The lateral force was comparable to the gravitation force on spheres up to 90 mg (0.0009 N) at beam powers on the order of 10 W. As a step toward manipulating objects, the beams were used to trap and electronically steer the spheres along a twodimensional path.

Multielement focused ultrasound phased arrays have been used in therapeutic applications to treat large tissue volumes by electronic steering of the focus, to target multiple simultaneous foci, and to correct aberration caused by inhomogeneous tissue pathways. There is an increasing interest in using arrays to generate more complex beam shapes and corresponding acoustic radiation force patterns for manipulation of particles such as kidney stones. Toward this end, experimental and computational tools are needed to enable accurate delivery of desired transducer vibrations and corresponding ultrasound fields. The purpose of this paper was to characterize the vibrations of a 256-element array at 1.5 MHz, implement strategies to compensate for variability, and test the ability to generate specified vortex beams that are relevant to particle manipulation. The characterization of the array output was performed in water using both element-by-element measurements at the focus of the array and holography measurements for which all the elements were excited simultaneously. Both methods were used to quantify each element's output so that the power of each element could be equalized. Vortex beams generated using both compensation strategies were measured and compared to the Rayleigh integral simulations of fields generated by an idealized array based on the manufacturer's specifications. Although both approaches improved beam axisymmetry, compensation based on holography measurements had half the error relative to the simulation results in comparison to the element-by-element method.

Burst wave lithotripsy (BWL) is a technology for comminuting urinary stones. A BWL transducer's requirements of high-pressure output, limited acoustic window, specific focal depth, and frequency to produce fragments of passable size constrain focal beamwidth. However, BWL is most effective with a beam wider than the stone. To produce a broad-beam, an iterative angular spectrum approach was used to calculate a phase screen that was realized with a rapid prototyped lens. The technique did not accurately replicate a target beam profile when an axisymmetric profile was chosen. Adding asymmetric weighting functions to the target profile achieved appropriate beamwidth. Lenses were designed to create a spherically focused narrow-beam (6 mm) and a broad-beam (11 mm) with a 350-kHz transducer and 84-mm focal depth. Both lenses were used to fragment artificial stones (11 mm long) in a water bath, and fragmentation rates were compared. The linearly simulated and measured broad beamwidths that were 12 mm and 11 mm, respectively, with a 2-mm-wide null at center. The broad-beam and the narrow-beam lenses fragmented 44 ± 9% and 16 ± 4% (p = 0.007, N = 3) of a stone by weight, respectively, in the same duration at the same peak negative pressure. The method broadened the focus and improved the BWL rate of fragmentation of large stones.

COMMUNICATION 1801609 (1 of 8) underneath, and the thickness of the film. [3] While this setting has been widely explored in quasi-static regimes, wherein inertial effects are ignored, the dynamic formation and propagation of such surface wrinkles in soft, viscoelastic, and layered elastomers subjected to high-speed impact has hitherto not been studied. Many works have explored the dynamics of related systems prone to elastic instabilities ranging from studies of dynamic buckling of thin bars [12][13][14] to Schallamach waves, [15] and studies (see ref.[16] and references therein) including the dynamics of wrinkling instabilities of thin, freestanding elastic sheets [17] and bands, [18] floating membranes, [19] and filaments in viscous fluids. [20] Recent works have also explored the dynamics of layered composites containing stiff inclusions in soft, viscoelastic matrices subjected to strain rates of up to 10 −1 s −1 , [21] and axial dynamic pulse buckling in sandwich composite plates. [22,23] In contexts directly related to wrinkling of stiff films on viscoelastomeric bases, recent studies have also theoretically explored the dynamics of wrinkle growth and coarsening, [24][25][26][27][28] as well as experimentally studied the slow (≈300 s) growth and reorganization of folds and wrinkles under biaxial compression [8] and changing compression direction, [29] respectively, slip dynamics of ripple dislocation, [30] evaporationdriven wrinkle growth, [31] and nanoscale anisotropic wrinkle growth under the presence of ion bombardment. [32] In this work, we study the dynamics of surface wrinkle formation and propagation in a soft elastomer block containing a stiff surface film due to high-speed plate impact (simulated strain rates over 500 s −1 and plate velocities approaching 25% of wave speeds observed in the block). The plate travels such that its velocity vector lies within the plane of the film, and the impact launches a large-deformation compression wave in the block that induces progressive wrinkle formation. We measure the evolution of the wrinkles using high-speed video and compare the measured dynamics with those of the opposite side of the same block (which does not contain a surface film). By tracking the out-of-plane motion of the surfaces and the 2D motion of several surface particles in each case, we see that inertial (wave propagation) effects play a major role in the substrate and drive the formation of wrinkling instabilities. We observe that the wrinkle formation speed correlates with, but is slightly slower than, the speed of the compression wave in the block, the trajectory of the tracked surface particles are nearly the same for both cases (with and without the film), andThe formation of periodic wrinkles in soft layered materials due to mechanical instabilities is prevalent in nature and has been proposed for use in multiple applications. However, such phenomena have been explored predominantly in quasi-static settings. Here, the dynamics of soft elastomeric blocks with stiff surface films subj...

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Acoustic radiation forces can remotely manipulate particles. Forces from a standing wave field align microscale particles along the nodal or anti-nodal locations of the field to form three-dimensional (3D) patterns. These patterns can be used to form 3D microstructures for tissue engineering applications. However, standing wave generation requires more than one transducer or a reflector, which is challenging to implement in vivo. Here, a method is developed and validated to manipulate microspheres using a travelling wave from a single transducer. Diffraction theory and an iterative angular spectrum approach are employed to design phase holograms to shape the acoustic field. The field replicates a standing wave and aligns polyethylene microspheres in water, which are analogous to cells in vivo, at pressure nodes. Using Gor’kov potential to calculate the radiation forces on the microspheres, axial forces are minimized, and transverse forces are maximized to create stable particle patterns. Pressure fields from the phase holograms and resulting particle aggregation patterns match predictions with a feature similarity index > 0.92, where 1 is a perfect match. The resulting radiation forces are comparable to those produced from a standing wave, which suggests opportunities for in vivo implementation of cell patterning toward tissue engineering applications.