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

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 in situ injection and photopolymerization that eliminates microchannel anechoic corners by augmenting acoustic energy transmission via 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.