A 50 µm acoustic resonator microchannel enables focusing 100 nm polystyrene beads and sub-micron bioparticles

Abstract

The manipulation and focusing of submicron/nanometre-scale objects are becoming increasingly important in the fields of biology, chemistry, and materials science. Acoustofluidics provides for the high throughput and precise manipulation of cells and particles with simple equipment. However, application to submicron/nanoparticles is challenging due to the particle volume dependence of the acoustic radiation force and the relatively larger effect of the viscous drag force on small objects. Here, we present an acoustofluidic device that can focus submicron/nanoparticles, bacterial cells, and extracellular vesicles (EVs) using a high-frequency two-dimensional acoustic standing wave. We fabricate 50 µm × 50 µm square cross-section channels as acoustic resonators and generate a two-dimensional acoustic standing wave at an actuation frequency of 14.9 MHz. This configuration enables a strong acoustic radiation force towards the centre of the microchannel and a cross-section-centred single-vortex stream that does not counteract the acoustic radiation force. Using this device, polystyrene beads ranging in diameter from 50 to 500 nm were focused toward the channel centre, where a focused bead stream width of less than 5 µm was achieved at a flow rate of 3 µL min−1 for 100 nm beads and 9 µL min−1 for 200 nm beads. Furthermore, bacterial cells were successfully focused at a flow rate of 40 µL min−1, and the focusing of EVs was also demonstrated. Our acoustofluidic device can become a promising tool for a wide range of biological analyses targeting submicron cells, viruses, and EVs.