Particle focusing in planar geometries is essentially required in order to develop cost-effective lab-on-a-chips, such as cell counting and point-of-care (POC) devices. In this study, a novel method for sheathless particle focusing, called "Elasto-Inertial Particle Focusing", was demonstrated in a straight microchannel. The particles were notably aligned along the centerline of the straight channel under a pressure-driven flow without any additional external force or apparatus after the addition of an elasticity enhancer: PEO (poly(ethylene oxide)) (∼O(100) ppm). As theoretically predicted (elasticity number: El≈O(100)), multiple equilibrium positions (centerline and corners) were observed for the viscoelastic flow without inertia, whereas three-dimensional particle focusing only occurred when neither the elasticity nor the inertia was negligible. Therefore, the three-dimensional particle focusing mechanism was attributed to the synergetic combination of the elasticity and the inertia (elasticity number: El≈O(1-10)). Furthermore, from the size dependence of the elastic force upon particles, we demonstrated that a mixture of 5.9 and 2.4 µm particles was separated at the exit of the channel in viscoelastic flows. We expect that this method can contribute to develop the miniaturized flow cytometry and microdevices for cell and particle manipulation.

Free-radical photopolymerization performed within PDMS microfluidic devices is now used for a variety of applications. We propose, through model and experiment, that atmospheric oxygen diffusing in through the porous PDMS is responsible for the presence, under UV light, of a thin, un-cross-linked film of oligomer abutting the walls of an all-PDMS device. After the advent of light exposure, an induction time τ i is required before the oxygen present in the oligomer is depleted, and cross-linking reactions can begin. A polymerized structure then grows from the center of the device outward, increasing sharply in height with time and leaving only a thin un-cross-linked film of thickness, δ i,c , close to the walls where oxygen can penetrate. Under suitable simplification of the reaction-diffusion model developed, scaling relationships were obtained for τ i (∼Da-1) and δ i,c (∼Da-1/2) as a function of a Damköhler number, Da. The relationships were successfully verified by comparison with both the full solution and experimental data. The analysis shows that control over particle height can be obtained more easily by changing initiator concentration, irradiation intensity, or channel height rather than exposure time.

No abstract

DNA is distinguished by both long length and structural rigidity. Classical polymer theories predict that DNA enhances the non-Newtonian elastic properties of its dilute solution more significantly than common synthetic flexible polymers because of its larger size and longer relaxation time. Here we exploit this property to report that under Poiseuille microflow, rigid spherical particles laterally migrate and form a tightly focused stream in an extremely dilute DNA solution (0.0005 (w/v)%). By the use of the DNA solution, we achieve highly efficient focusing (499.5%) over an unprecedented wide range of flow rates (ratio of maximum to minimum flow rates B400). This highly tunable particle-focusing technique can be used in the design of cost-effective portable flow cytometers, high-throughput cell analysis and also for cell sorting by size. We demonstrate that DNA is an efficient elasticity enhancer, which originates from its unique structural properties.

We present an efficient method for measuring cell stretching based on three-dimensional viscoelastic particle focusing. We suspended cells in a biocompatible viscoelastic medium [poly(vinylpyrrolidone) solution in phosphate-buffered saline]. The medium viscoelasticity significantly homogenized the trajectories of cells along the centerline of a simple straight channel, which could not be achieved in conventional Newtonian media. More than 95% of red blood cells (RBCs) were successfully delivered to the stagnation point of a cross-slot microchannel and stretched by extensional flow. By computational simulations, we proved that this method prevents inaccuracies due to random lateral distributions of cells and, further, guarantees rotational-free cell stretching along the shear-free channel centerline. As a demonstration, we characterized the differences in RBC deformabilities among various heat treatments. Furthermore, we monitored the decrease of deformability due to nutrient starvation in human mesenchymal stem cells. We envisage that our novel method can be extended to versatile applications such as the detection of pathophysiological evolution in impaired RBCs due to malaria or diabetes and the monitoring of cell quality in stem cell therapeutics.