Most reported studies of the dielectrophoresis (DEP) of biological cells have employed voltage signals of frequency that extend from 10 kHz to around 10 MHz. In this range the DEP response is determined by the dielectric properties of the suspending medium, as well as the size and shape of the cell and the dielectric properties (capacitance, conductivity) of its plasma membrane. An important parameter to determine is the so-called cross-over frequency f_x01, at which a transition from negative to positive DEP occurs on increasing the frequency. Theoretical and experimental evaluations [e.g., 1-5] of f_xo1 have been extensive and exploited in practical applications of DEP, such as the manipulation, separation, and isolation of target cells from mixtures in suspension.

DEP measurements up to 400 MHz have recently been reported [6] for murine myeloma cells. This provided, for the first time, the ability to determine under controlled conditions the value of the cross-over frequency f_x02, where the transition from positive back to negative DEP occurs. In agreement with theory, it was found that f_x02 is independent of the cell parameters that control f_x01, and is predominantly determined by the conductivity of the intracellular medium. The myeloma cells were suspended in buffer media of different osmolarities, with measurements taken of cell volume as well as both f_x01 and f_x02. Flow cytometry was employed with a potassium-sensitive fluorophore to monitor relative changes in the intracellular potassium concentration. This revealed that the value of f_x02 was highly correlated to the concentration of the most dominant cytoplasmic ion, namely potassium.

The implications of these new high-frequency results will be discussed, particularly with regards to the potential of DEP being adopted as a biomarker-free method for sorting and characterizing cells based upon their dielectric properties.

1. Lei U., Lo, Y. J., IET Nanobiotechnology 2011, 5, 86-106.
2. Cetin, B., Li, D., Electrophoresis 2011, 32, 2410-2427.
3. Gagnon, Z. R., Electrophoresis 2011, 32, 2466-2487.
4. Vahey, M. D., Pesudo, L. Q., Svenssson, J. P., Samson, L. D., Voldman, J., Lab Chip 2013, 2754-2763.
5. Pethig, R., Dielectrophoresis: Theory, Methodology and Biological Applications, John Wiley & Sons, Chichester 2017.
6. Chung, C., Pethig, R., Smith, S., Waterfall, M., Electrophoresis, doi: 10.1002/201700433