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This manuscript presents high-throughput sorting of cellular-sized microparticles within a three-dimensional microfluidic channel by focused bulk acoustic wave (BAW) produced by a Self-Focusing Acoustic Transducer (SFAT). The focused ultrasound induces a substantially higher acoustic radiation force within the focal region, enabling sorting based on particle size and density. Unlike surface-acoustic-wave-based setups, the BAW-based technique uses a three-dimensional microfluidic channel through which a mixture of particles is transported, while SFAT(s) may be placed at multiple points along the channel for multi-stage sorting. The technique has been successfully used in sorting 50 μm microparticles, which are analogous to cancerous or differentiated Mesenchymal Stem Cells (MSC), from 30 μm microparticles, which are analogous to healthy MSC. The sorting results in 97.5% purity at the smaller microparticle outlet and a 97.2% recovery rate for the smaller particles. The technique allows sorting 650,000 smaller and 142,000 larger microparticles within a mere 10 minutes. 

This paper presents the mixing, trapping, and ejection of a single microparticle based on an acoustic tweezers. Finite Element Model (FEM) simulation, along with analytical modeling, is used to study the selectivity of particles based on size and material properties. The acoustic tweezers is optimized to have a single trapping zone, where particles are trapped due to acoustic radiation force (which is calculated for particle sizes exceeding the Rayleigh approximation). The tweezers is experimentally shown to lift microparticles from the tweezers surface, selectively trap a single particle based on size and material acoustic properties, and then eject it upwards for collection. All these are obtained with negligible heat generation.