Rare cell sorting is a critical step in bioprocessing as cells are often exposed to various environmental factors that can affect their growth, differentiation, and overall behaviour. Acoustophoresis, a non-invasive technique using acoustic radiation forces, offers a gentler and more precise alternative to conventional cell manipulation methods.

In a millifluidic channel, acoustic radiation forces act on cells within a continuous flow, guiding them toward the centerline of the channel, where they can be efficiently collected. Notably, this directional cell movement can be reversed, redirecting cells to lateral outlets if their density is lower than the surrounding medium, enabling density-based sorting.

This density inversion can be achieved by attaching tailor-made low-density microbubbles (MB) or acoustic beads to the cells. Of notice, non-functionalized lipid-shelled gas-encapsulated microbubbles—which are clinically approved and commercially available as contrast agents in ultrasound medical imaging—make these acoustic beads a practical and scalable tool for advanced cell sorting applications.