Sheathless prefocusing in SU-8/PDMS hybrid microchannels <i>via</i> sidewall-assisted bimodal acoustic field cascading.

Abstract

Sheathless prefocusing of particles and cells is a critical preparatory step in biomedical assays, while focusing performance directly impacts processing throughput and detection sensitivity. Although standing surface acoustic wave (SSAW)-based acoustofluidics has been widely used across nano- and micro-scale applications, reliance on sheath flow remains a major constraint. Here, we introduce an SU-8/PDMS hybrid microchannel fabricated through <i>in situ</i> injection and photopolymerization that eliminates microchannel anechoic corners by augmenting acoustic energy transmission <i>via</i> hybrid sidewalls. The influence of sidewall dimensions on acoustic field distribution was systematically investigated using a two-dimensional finite element model, guiding the design of a cascaded hybrid microchannel that achieved over tenfold focusing of 2.5 and 5 μm polystyrene (PS) beads without sheath flow. Comparable performance was demonstrated with human promyelocytic leukaemia (HL-60) and human umbilical vein endothelial cells (HUVECs) at processing rates up to 3 × 10⁴ cells per min, confirming broad biological applicability. The modular and fabrication-friendly architecture presents a versatile sheathless prefocusing solution for integrated acoustofluidic systems.