Acoustic planar concentration of suspended micro-objects for rapid microscopy

Abstract

Rapid microscopic examination of suspended microorganisms, cells, and micrometer-scale particles is often limited by the time required for samples to reach a stable and observable distribution. Conventional counting chambers rely mainly on passive sedimentation, which can require tens of minutes before reliable imaging and counting, whereas automated imaging or flow-based systems improve throughput at the cost of increased equipment complexity. Here, we introduce acoustic planar concentration (APC), a compact pulsed bulk-acoustic strategy for improving the single-plane observability of suspended micro-objects. APC is implemented in a conventional platelet-counting chamber with a 100 μm-high observation cavity, without redesigning the chamber into a microfluidic resonator. An approximately 4 MHz pulsed acoustic field is coupled into the standard slide-based chamber, where acoustically induced particle redistribution improves single-plane observation. Pulsed excitation was used to avoid prolonged uninterrupted acoustic driving, while an external acoustic waveguide defines the actuation region. Using 5 and 2 μm polystyrene fluorescent particles as model objects, we demonstrate improved single-plane observability within seconds under the tested conditions. For 2 μm particles, the fraction of detected particle images classified as in focus increased from 76.1 ± 6.5% to 90.6 ± 2.5% within approximately 5 s. APC was further demonstrated using Chlorella sorokiniana cells of comparable size, for which the detected cell count within the selected observation plane increased by approximately 2.6-fold after actuation. This approach provides a compact, contact-free route compatible with conventional counting chambers and an inverted fluorescence microscopy setup.