Recently, much work has been reported on the use of dielectrophoresis (DEP) in portable microfluidic diagnostic devices to trap, separate and analyze cells and other micron and sub-micron particle suspensions. Important information for such micro-fluidic designs is whether the particle exhibits positive or negative DEP mobility towards the high and low field regions, respectively. A cross-over from positive DEP to negative DEP beyond a cross-over frequency has been observed for most cells. The traditional (DEP) theory, however, is based on the classical Maxwell-Wagner polarization mechanism whose field induced particle dipole moment is due either to dielectric polarization or accumulation of free charge at the interface between a particle and its surrounding liquid domain. Because both domains are assumed electro-neutral, free charge accumulation at the particle-liquid interface occurs because the magnitude of induced polarization is not equal in the liquid and particle domains, thus resulting in a non-zero charge accumulation at the domains interface. While such a model qualitatively describes the experimentally observed DEP behavior of low conductivity (10^-6 S/m) polymer spheres suspended in a low conductive dielectric medium, it does not accurately predict the observed behaviors of various types of realistic biological suspensions, such as cells or micron and sub-micron particles suspended in highly conductive (~50 mS/m) media. Moreover, it fails to even predict the existence of a cross-over frequency. Previous attempts to remedy this distinct failure of the DEP theory is to include a conducting particle shell (Garcia et al., J. Phys. Chem.1985, 91, 6415 ). However, the resulting theory is largely empirical in nature and cannot capture the proper scaling of the cross-over frequency with respect to particle size and other cell parameters (Ermolina and Morgan, Journal of Colloid and Interface Science 2005, 285, 419). It has been speculated for a number of years that double layer polarization at the particle-liquid interface may play a large role during the formation of a field-induced dipole moment (Miles J B, Robertson H P, Phys Rev. 1932, 40, 583). Because the double layer results from a balance between electro-migration and diffusion, it introduces a new nm-sized Debye screening length scale with very large capacitance. Because of this large capacitance, the charging and polarization of these double layers dominate over the classical polarization mechanisms of the Maxwell-Wagner theory for the most common frequency ranges. Additionally, it is possible that double layers form both inside and outside the cellular interface and both double layers can contribute to the DEP particle dipole. A new DEP theory, with no empirical parameters, that takes into account the polarization contribution from both the interior and exterior double layers of a red blood cell has been developed. Effective electro-static boundary conditions are derived in the complex Fourier frequency space to include the fine-scale charging dynamics within the double layers. Based on this theory, the cross-over frequency of a healthy red blood cell as a function of medium conductivity is predicted to scale linearly with respect to the particle size and shown to compare well to measured experimental values. The new theory is used to design a DEP separator for blood cells from serum and one for malaria-infected blood cells from healthy cells. Effective separation will be reported for both DEP separators.