Acoustofluidic separation of cell-encapsulated droplets with co-encapsulated carrier microparticles

Droplet microfluidics can provide isolated environments for cells encapsulated within droplets for various biomedical applications such as single-cell analysis, cell-to-cell interaction, genetic sequencing, and drug discovery. Precise, on-chip manipulation of the cell-encapsulated droplets is essential for the droplet microfluidic applications. Various external force fields, including optical, magnetic, electric, and acoustic fields, have been utilized for on-chip manipulation of the cell-laden droplets. However, most previous methods require a priori labeling and consequent detection of the in-droplet sample for manipulation, which may cause undesirable effects, including damage or decline in cell growth. Here we propose an acoustofluidic approach for label-free, simultaneous, and selective separation of cell-encapsulated droplets with co-encapsulated carrier microparticles. The traveling surface acoustic wave-induced acoustic radiation force exerted on the carrier microparticle-encapsulated droplets varies significantly depending on the in-droplet particle size and compressibility. Based on the investigation of the carrier microparticle-laden droplet behaviors when exposed to a travelling surface acoustic wave field, we showcase label-free, detection-free separation of Escherichia coli and platelets-encapsulated droplets via co-encapsulated carrier polymer microparticles at high purity and recovery rate of above 92.5 %. We expect that the proposed acoustofluidic approach can serve as a promising next-generation tool for on-chip manipulation of cell-capsulated droplets and expand the boundaries of droplet-based microfluidic applications.