Flow rate- and particle size-insensitive viscoelastic 3D sheathless focusing: A mechanistic study for refined understanding.

Three-dimensional (3D) cell focusing is a critical prerequisite for precise microfluidic-based cell manipulation and detection. Despite significant advances, devices capable of realizing flow rate- and particle size-insensitive 3D sheathless focusing remain scarce. To address this challenge, we propose a straight-channel viscoelastic microfluidics (VEM) and systematically investigate microparticle dynamics through theoretical modeling. The correlations of polyethylene oxide (PEO) concentration and flow rate with particle focusing ability and migration velocity are quantitatively characterized, elucidating the mechanism of viscoelastic 3D focusing. Experimental validation with 10 μm and 15 μm polystyrene microspheres (PSMSs) demonstrates equilibrium positions consistent with theoretical predictions. Reliable 3D focusing is achieved over 0.3 mLꞏh-1 - 3 mLꞏh-1 in 0.15% PEO, with positional deviations below 1.39 μm and efficiencies exceeding 94.04%. Mixed-particle experiments (10/15/20 μm) further confirm robust simultaneous focusing particles with multiple sizes, with deviations under 2.37 μm and efficiencies above 94.70%. Finally, tests with A549 and MCF-7 tumor cells confirm effective 3D focusing within the same flow range, with high focusing efficiencies (>94.25%). This study introduces a viscoelastic straight-channel 3D focusing technique and provides in-depth insights into the focusing mechanism through theoretical modeling. The approach overcomes the limitations of conventional methods, which typically exhibit low efficiency and poor stability due to sensitivity to flow rate and particle size, and demonstrates significant potential for applications in cell sorting and single-cell analysis.