Dielectrophoretic Separation of Cancer Cells from Blood

Abstract

Recent measurements have demonstrated that the dielectric properties of cells depend on their type and physiological status. For example, MDA-231 human breast cancer cells were found to have a mean plasma membrane specific capacitance of 26 mF/m(2), more than double the value (11 mF/m(2)) observed for resting T-lymphocytes. When an inhomogeneous ac electric field is applied to a particle, a dielectrophoretic (DEP) force arises that depends on the particle dielectric properties. Therefore, cells having different dielectric characteristics will experience differential DEP forces when subjected to such a field. In this article, we demonstrate the use of differential DEP forces for the separation of several different cancerous cell types from blood in a dielectric affinity column. These separations were accomplished using thin, flat chambers having microelectrode arrays on the bottom wall. DEP forces generated by the application of ac fields to the electrodes were used to influence the rate of elution of cells from the chamber by hydrodynamic forces within a parabolic fluid flow profile. Electrorotation measurements were first made on the various cell types found within cell mixtures to be separated, and theoretical modeling was used to derive the cell dielectric parameters. Optimum separation conditions were then predicted from the frequency and suspension conductivity dependencies of cell DEP responses defined by these parameters. Cell separations were then undertaken for various ratios of cancerous to normal cells at different concentrations. Eluted cells were characterized in terms of separation efficiency, cell viability, and separation speed. For example, 100% efficiency was achieved for purging MDA-231 cells from blood at the tumor to normal cell ratio 1:1 x 10(5) or 1:3 x 10(5), cell viability was not compromised, and separation rates were at least 10(3) cells/s. Theoretical and experimental criteria for the design and operation of such separators are presented.

Fig. 1

The schematic drawing of a dielectric-affinity separation chamber. In operation, the cell mixture sample is loaded into the chamber through the cell sample port. Then, appropriate electrical signals are applied to the DEP electrode array. Fluid flow is then started by pumping eluate through the eluate inlet port. The eluted cells are collected at the chamber output port at the other end of the chamber.

Fig. 2

Sequence of cell separation by dielectric affinity chambers. (a) After DEP collection at 500 kHz, all cells were accumulated at electrode edges before the fluid flow was started. (b) The DEP frequency was dropped to 50 kHz and fluid flow started at 10 μL/min. Blood cells were released and carried off by the eluate (these cells appear as streaks between the electrodes), while human breast cancer cells were retained. (c) Cancer cells remained on the electrode tips after blood cells had been swept downstream.

Fig. 3

(a) Typical ROT spectra for the human breast cancer cell lines MDA-231 (◇), MDA-468 (△) and for T-lymphocytes (o) in isotonic sucrose suspension of conductivity 56 mS/m. Continuous curves are the best fits of single-shell dielectric model [8], [15], [25]. (b) DEP responses for the same cell types calculated from the dielectric parameters derived from the ROT spectra for the conditions used in cell separation experiments MDA-231 cells (—), MDA-468 (— –), and for T-lymphocytes (– – –).