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Focusing of particles/cells in the vertical direction inside a micromachined flow cytometer is a critical issue while using an embedded optical detection system aligned with microchannels. Even if the particles/cells have been focused centrally in the horizontal direction using coplanar sheath flows, appreciable errors may still arise if they are randomly distributed in the vertical direction. This work presents a vertical focusing device utilizing dielectrophoretic (DEP) forces and its application on micromachined flow cytometer. A pair of parallel microelectrodes is deposited on the upper and bottom surface of the microfluidic channel to drive particles/cells into the vertical center of the sample flow. This new microfluidic device is capable of three-dimensional (3-D) focusing of microparticles/cells and thus improves the uniformity of the optical detection signals. This 3-D focusing feature of the sample flow is realized utilizing the combination of dielectrophoretic and hydrodynamic forces. Initially, two sheath flows are used to focus the sample flow horizontally by means of hydrodynamic forces, and then two embedded planar electrodes apply negative DEP forces to focus the particles/cells vertically. A new micromachined flow cytometer integrated with an embedded optical detection mechanism is then demonstrated. Numerical simulation is used to analyze the operation conditions and the dimension of the microelectrodes for DEP manipulation. The dynamic trace of the moving particles/cells within a flow stream under the DEP manipulation is calculated numerically. Micro polystyrene beads and diluted human red blood cells (RBC) are used to test the performance of the proposed device. The experimental results confirm the suitability of the proposed device for applications requiring precise counting of particles or cells. Experimental data indicates the proposed method can provide more stable signals over the other types of micromachined flow cytometers that were previously reported.
Date of Publication: Dec. 2004