Acoustic separation of cells and bacteria in open sessile droplets

Efficient and non-destructive separation of cells and pathogenic microorganisms is a core technical bottleneck in clinical point-of-care diagnostics. Traditional separation methods mostly rely on microchannel structures, external fluid actuation, or biochemical labeling, making it difficult to synergistically optimize separation efficiency, purity, cell viability, and system portability. To address this challenge, this study proposes a label-free separation platform based on traveling surface acoustic wave (TSAW) in an open sessile droplet. The platform innovatively places a droplet of only 2 μL asymmetrically at the edge of the acoustic aperture of an interdigital transducer (IDT), with approximately one-third of the droplet area lying within the acoustic propagation path. By exploiting the asymmetric leakage of TSAW at the solid-liquid interface, a gradient-distributed acoustic streaming vortex field is induced inside the droplet, enabling rapid separation of cell-bacteria systems without channels, pumps, or labels. To elucidate the separation mechanism, a multiphysics coupling model was established and a size-dependent force-balance criterion was established to analyze the motion behaviors of particles of different sizes. Experimental results showed that under 17.5 MHz TSAW excitation, human breast cancer cells (MCF-7, ∼20 μm in diameter) and Escherichia coli (E. coli, 2-3 μm) were successfully separated. The smaller bacteria were driven by the streaming-induced drag force and thus enriched at the droplet periphery, whereas the larger cells were dominated by the acoustic radiation force (ARF) and concentrated at the droplet center. A separation efficiency of 91.2  ± 1.2 % was achieved, and the cells retained high viability after separation. The platform requires only 2 μL of sample, combines simple operation, rapid separation, non-invasiveness, and portability, and can provide a lightweight, label-free paradigm for biological sample pretreatment in application scenarios, such as point-of-care screening of bloodstream infections, circulating tumor cell enrichment, and environmental microbial detection.