A One - Sided View of Acoustic Traps

by Martyn Hill∗ B iotechnology and automated assembly often involve the manipulation of tiny particles, such as cells, small organisms, and other submillimeter objects. In many cases, this manipulation needs to be performed without touching the particles. Yet doing so accurately is a challenge, particularly in biocompatible setups. Two groups, one in France, the other in the UK, have now made a significant step forward in the remote manipulation of particles using sound waves. Following different approaches, they have demonstrated an acoustic trap that levitates and constrains one or more particles in three dimensions using a single acoustic source [1, 2]. Compared to acoustic traps that require multiple sources or reflecting elements, these new “single beam” traps offer greater flexibility because they can access a collection of particles from just one side. This capability could, in the long term, help to maneuver cells or tissue in situ. Optical trapping is the dominant method of manipulating small particles without contact. But traps based on sound waves, as opposed to light, can trap larger particles (or collections of particles) with stronger forces and do not require the particles to have particular optical properties or to be accessible via an optically transparent path. In addition, ultrasound waves can be less disruptive to biomaterials than light. When an acoustic wave interacts with a particle, it exerts both an oscillatory force and a much smaller steady-state “radiation” force. This latter force is the one used for trapping and manipulation. Radiation forces are generated by the scattering of a traveling sound wave, or by energy gradients within the sound field. These gradients result from variations in the pressure and in the acoustic particle velocity—that is, the oscillatory velocity of the surrounding medium caused by the acoustic field. For particles much smaller than the sound wavelength, energy gradients tend to produce the largest radiation forces, drawing particles to low/high regions of pressure and velocity (depending on the particles’ relative density/stiffness). Thus a beam with