Dielectrophoresis (DEP) is a term commonly used to describe the field induced polarization and translational motion of a polarizable particle in a non-uniform AC field. Traditional DEP detection, particle sorting and concentration schemes are typically based on the determination and exploitation of particle specific DEP crossover frequencies (cof).

It has been shown previously [1,2] that bovine red blood cells (bRBCs) exhibit two cof's, a low frequency (~200 kHz) cof being sensitive to the properties of the cell membrane, and the high frequency cof (~ 2 MHz) being sensitive to properties of the cell cytoplasm. This dual cof behavior is due to the shell-like nature of the RBC and hence allows one to analyze and separate cells based on their unique membrane and interior properties.

In this work we carry the above analysis further using human red blood cells (hRBCs). We show that due to differences in membrane and cytoplasm properties hRBCS have largely different DEP behavior than bRBCs. By utilizing a glutaraldehyde crosslinking cell fixation reaction that is sensitive to cell membrane protein concentration, we demonstrate how species-specific protein and lipid cell membrane surface concentrations affect the dielectric response of blood. Our results reveal that, after cross-linking to produce conductive and polarized double bonds along their backbones, the lipid and amino-terminal protein concentrations in the cell membrane are essentially what determine the membrane conductivity and dielectric constant.

Such observations are exploited in order to separate a suspension of malaria infected red blood cells (~ 5% infected) from their healthy counterpart. A novel closed-loop electrothermal micro-pump is fabricated such that approximately 20 nL of infected blood is forced to circulate around a closed loop. By controlling the separation distance of the working electrodes, the device is engineered such that the field penetration (20 ìm) does not exceed the height of the microchannel (75 ìm). By crosslinking the infected cell suspension it becomes possible to separate healthy cells based on the reaction-induced differences in DEP mobility. It is shown by selectively staining the infected cells that healthy cells are concentrated at specific high field trapping regions, while infected cells are forced into fluid streamlines above the trap and swept away to continue around the closed loop via electrothermal fluid flow.

[1] Z. Gagnon, J Gordon, S Sengupta and H.-C. Chang Electrophoresis 2008, 29 (In press)

[2] Gordon, J.G., Gagnon, Z., Chang, H.-C., Biomicrofludics 2007, 1, 044102