Remote acoustic manipulation of objects in vivo*

Acoustic radiation forces can be converged onto an object to trap and manipulate it in three dimensions (3D). Recent advances in acoustic trapping have shown the capability to remotely manipulate small, lightweight objects in a benchtop experiment using a single or multiple transducers. Acoustic manipulation can also be used as a noninvasive medical tool to control foreign objects in the body, such as kidney stones. However, such technology requires the controlled manipulation of large solid objects, and the ability to transmit stable and robust beams through the skin; all while using a single acoustic source due to limited acoustic window in the body. A theoretical model using the Fourier spectrum was validated and used to quantify the radiation forces applied by arbitrary acoustic wavefields on heavy objects larger than the acoustic wavelength. A 1.5 MHz, focused, multi-element array was used to synthesize various acoustic beam traps to levitate and steer spheres in 3D. A characterization of the array was performed to equalize the complex vibrational output of each element to produce acoustic beams with uniform spatial pressure distribution. Beams generated for trapping had toroidal pressure fields such as vortex beams and other traps that were synthesized using an iterative angular spectrum method. The robustness of the trapping stability and steering range of the beams was achieved by controlling the pulsing directionality to eliminate spinning of the spheres, the relative size of the sphere to beam diameter, and the delivered acoustic power. We successfully demonstrated the remote acoustic levitation and manipulation of glass spheres along preprogrammed paths in the urinary bladders of live pigs placed under anesthesia at safe acoustic exposure levels. Other potential applications include controlling an ingestible camera, or cellular patterning for tissue generation in vivo using a simple source having a tailored phase mask.