Acoustofluidic separation of cell-encapsulated droplets based on traveling surface acoustic wave-induced acoustic radiation force

Microscale droplets encapsulating various samples have been widely utilized in droplet microfluidics for single-cell analysis, cell biology, and drug screening. However, the random sample encapsulation within droplets, known as the Poisson distribution, inevitably renders many undesirable empty droplets, whose number far exceeds that of the sample-laden droplets. Precise separation of sample-encapsulated droplets from empty droplets is crucial for improved applicability. The previous methods for droplet separation usually depend on fluorescent, magnetic, or electrical labeling and require upstream detection. Here, we propose an acoustofluidic method for on-chip separation of the sample- encapsulated droplets in a label-free and detection-free manner based on traveling surface acoustic wave (TSAW)-induced acoustic radiation force (ARF). When exposed to an acoustic field, the droplets experience the TSAW-induced ARF, leading to lateral migration in the TSAW direction. The ARF acting on the sample-laden droplets is greater than that on the empty drops due to the additional ARF exerted on the encapsulated samples within the droplets. Our acoustofluidic device demonstrates simultaneous, label-free, detection-free isolation of the drops encapsulating polystyrene microparticles, Thalassiosira eccentrica microalgae and cardiomyocyte cells with high purity and recovery rate. The proposed acoustofluidic approach can open up new horizons for droplet microfluidic applications, especially for single-cell/bead assays.