Electrode-Proximal 3D Particle Focusing via Acoustic Streaming for High-Sensitivity Sheathless Impedance Flow Cytometry

The nonuniform electric field generated by coplanar electrodes in microfluidic impedance flow cytometry (MIFC) introduces a position-dependent factor, which is a critical challenge for the sensitivity and accuracy of measured signals. To address this challenge, we put forward a novel MIFC configuration by monolithic on-chip integration of a bulk acoustic wave resonator with a pair of coplanar electrodes. This design utilizes the acoustic streaming effect to achieve three-dimensional (3D) particle focusing, enabling the particles to traverse the downstream impedance sensing region along a consistent trajectory in the electrode near-field region. This not only mitigates the position-dependent variability but also leverages the inherent electric field distribution characteristics of coplanar electrodes to enhance detection sensitivity. The experimental results demonstrated efficient focusing performance across various sample flow rates, with a coefficient of variation (C.V.) of 1.7% for 5 μm diameter particles. Furthermore, the substantial improvement in impedance measurement precision and sensitivity not only enhances the accuracy of size-based detection and discrimination at low frequencies but also supports more reliable multifrequency impedance analysis, thereby providing a more precise insight into the electrophysiology of the cell interior. In conclusion, this work presents a promising methodology for overcoming the position-dependent factor and enhancing detection sensitivity without the necessity for an additional sheath flow system or complex signal processing.