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Surface AC electro-osmotic flow or non-uniform DC electro-osmotic flow can produce converging stagnation flows. If a body force like DEP, gravity or magnetic field is applied at bacteria and blood cells at the stagnation point or line in the direction of the surface, these bioparticles can avoid being resuspended by the flow and are trapped at the stagnation line/point. Such concentration can be used to enhance the speed and sensitivity of biosensors and can be used to separate bioparticles by size via the body force.

Wu, J., Ben, Y., Battigelli, D. and Chang, H.-C., “Long-Range AC Electroosmotic Trapping and Detection of Bioparticles”, Ind. Eng. Chem. Res ., 44 , 2815(2005).

Gagnon, Z. and Chang, H.-C., “Aligning fast alternating current electroosmotic flow fields

and characteristic frequencies with dielectrophoretic traps to achieve rapid

Bacteria trapped at the stagnation lines of a planar electrode array with converging AC EO flows.

Because the shape, size, elasticity and surface protein/ion-channel densities of blood cells, cancer cells and bacteria affect their migration speed due to hydrodynamic shear, linear electrophoresis and nonlinear electrokinetics such as dielectrophoresis, the group is exploiting these mobility differences to separate and detect bioparticles. AC dielectrophoresis and Impedance Spectroscopy are especially promising as the polarization of bioparticles due to ion migration are highly frequency dependent.

Figures 1 and 2 are the AC dielectrophoresis of red blood cells under a non-uniform electric field. The red blood cells are repelled from the electrodes due to electrode polarization at the high frequency 700KHz and then form linear aggregates along the field lines. In Figure 3, Impedance Spectroscopy is used to measure the complex impedance of different red blood cell solutions with different degrees of blood cell aggregation (no aggregate in the first frame and severe aggregation in the third). The measured capacitance (phase lag) is a strong function of the size distribution of the blood cell aggregates, which reflects propensity for coagulation.The roll-off frequency can be correlated to the average aggregate size.

However, a major obstacle to electrokinetic separation in microfluidic devices is its low throughput due to the low electrokinetic velocity (<1mm/s). We are designing separation chips that combine the high-throughput feature of pressure-driven flow and high selectivity of electrokinetic separation.We are also investigating the use of magnetic nanoparticles with anchored antibodies. Once attached to the bacteria, the complex can be easily manipulated by magnetic fields and traps.

Relevant publication: Minerick, A., Takhistov, P., Zhou, R. amd Chang, H.-C., “ Manipulation and Characterization of Red Blood Cells with AC Fields in Micro-Devices”, Electrophoresis , 24 , 3703 – 3717 (2003).

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