Acoustofluidic separation of oblate spheroids from spheres using acoustic radiation torque and force

Cells, bacteria, and other bioparticles exist in diverse shapes, and their morphology plays a pivotal role in biological functions and clinical significance. Various microfluidic approaches have been developed for shape-based separation of bioparticles with the same or similar volume; however, most of them were limited to separation of prolate spheroids from spheres or required a priori labeling and consequent detection. Here, we propose a vertical-type acoustofluidic method for the first separation of oblate spheroids from spheres in a label-free manner. The acoustic radiation torque suppresses the rotational motion of oblate micro-objects, leading to horizontal alignment with an increase in projected surface area compared to that of isovolumetric spheres. The enhanced acoustic radiation force, proportional to the projected surface area normal to the wave propagation, allows the oblate spheroids to have greater vertical migration inside a microchannel, resulting in distinct trajectories for shape-based separation. We conduct numerical simulations of asymmetric wave scattering to elucidate the working principle and experiments to demonstrate the separation of polystyrene microparticles and red blood cells of spherical and oblate shapes at high purity and recovery rate. The proposed acoustofluidic approach holds promise for label-free, shape-based manipulation of bioparticles in cell biology and microbiology.