Interdisciplinary Applied Mathematics

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of small, yet similar-size, particles with different biological properties, including chromosomes, viruses, DNA, and other macromolecules. For example, Gascoyne et al. were able to separate human breast cancer cells from blood using (AC electric field) dielectrophoretic separation (Gascoyne and Wang, 1997). Their technique utilized the frequency dependence of dielectrophoretic (DEP) properties of blood and cancer cells and worked in the following sequence: First, trapping and accumulation of both blood and cancer cells on microfabricated dielectric affinity chambers (electrodes) using DEP collection at 500 kHz. Second, reducing the DEP collection to 50 kHz,    where the    blood    cells    are    released and only    the    cancer    cells    are

trapped on the electrodes. This is followed by washing the released blood cells with pressure-driven flow, where the blood cells are convected downstream, while the cancer cells remained on the electrode tips (Gascoyne and Wang, 1997).

In a somewhat different subsequent design, Gascoyne and Wang used four spiral microfabricated electrodes for dielectrophoretic separation of human leukemia cells from the normal cells. Figure 7.24 shows the leukemia cells concentrated toward the center of four spiral electrodes. In this design the normal cells are trapped on the electrode surfaces, and human leukemia cells are washed toward the center. Several other applications of dielectrophoresis can be found in (Gascoyne et al., 1992; Markx and Pethig, 1995; Markx et al., 1996; Fiedler et al., 1998; Cheng et al., 1998; Morgan et al., 1999).

In a series of papers Gascoyne and coworkers have also utilized combined dielectrophoretic/gravitational field flow fractionation for cell separation on microfabricated electrodes (DeGasperis et al., 1999; Yang et al., 1999a; Yang et al., 2000). The gravitational field flow fractionation utilizes balance between the vertically applied dielectrophoretic forces and the gravitational forces for levitation of different particles to different heights in a miniaturized channel flow system. The bulk flow is pressure-driven in the axial direction, and it splits into two different outlet ports at a desired channel height, separating the heavier particles in the bottom exit port from the lighter ones in the top exit port. This particle separation system is free from any moving mechanical components.

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