Point-of-care (POC) fast diagnostic devices have recently received increasing interest worldwide since the Covid-19 pandemic started. On-chip self-separation of blood plasma became critical to expanding the POC device applications to better detect the biomarkers in the plasma for multiple disease diagnoses. A spiral microchannel is employed to separate the blood cells and the blood plasma using inertial migration and Dean vortex effects on the blood cells. The inertial migration of the particles (blood cells here) happens in the microchannel when the Reynolds number is bigger than 1 and the particle size to channel hydraulic diameter ratio is bigger than 0.07. The net lift force applied to the blood cells will migrate the cells laterally to the equilibrium positions near the channel wall. The lift forces due to the shear gradient and wall effect will be balanced at the equilibrium positions so that the cells will stop lateral migration. The Dean vortices, a pair of counter-rotating vortices, will further focus the blood cells using the drag force to the cells by the secondary flow. The microchannel in this study is designed as a rectangular cross-sectional profile with 50 μm height and 120 μm width and five spiral turns with a radius from 0.3 mm to 1.9 mm that provides a significant lift force to migrate the red blood cells (8 μm in diameter at thickest part) and generates sufficient Dean vortices to additionally focus the blood cells to the inner side of the microchannel with Dean drag force in the secondary flow. As a technology target for POC applications, sophisticated equipment should be avoided. Different from many other microfluidic devices, this technology uses no external pumping systems to drive the blood flow in the microchannel but uses a capillary effect. The microchannel is made of Polydimethylsiloxane (PDMS), which is naturally hydrophobic. Using oxygen plasma to treat the microchannel surface could reduce the surface water contact angle from 107° to 20°, which can trigger a significant capillary effect in the microchannel that can provide a maximum flow rate of 1.8 μl/s. With the well-controlled combination of the flow velocity, microchannel cross-sectional dimension, and spiral microchannel radius of curvature, the clean plasma is obtained at the outer branch at the bifurcation outlet of the spiral microchannel.

Blood plasma separation enhanced by Dean vortex during the capillary flow in the spiral microchannel for point-of-care(POC) applications

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