Forces on biological cells due to applied alternating (AC) electric fields. II. Electro-rotation

Measurements are presented of angular velocities of rotation of mammalian cells of K562 (human) and SP2 (mouse) in external alternating electric fields over a frequency range of 0.5 kHz to 12 MHz. Electro-rotation of the cells was observed for the case of "two cells in contact' using two parallel, cylindrical electrodes; only one cell was located on the electrode. A theoretical analysis is also presented which shows that the cell rotation arises from a torque produced by the interaction between the primary electric dipole moment induced in the spinning cell and the secondary electric fields, generated by the primary dipole induced in the adjacent cell. These secondary fields are out of phase with the applied electric field. The results show that (a) only the cell not located on the electrode rotates, (b) maximal electro-rotation occurs at two different excitation field frequency domains for the frequency range employed here, (c) the spin speed of the rotating cell at each frequency domain is much less than the excitation frequency, (d) the rotation direction of the cell depends on the angle (theta) between the external electric field and the line joining the centres of the two cells and (e) for a given angle theta, the rotation direction is the same for both excitation frequency domains. The experimental measurements allowed us to estimate the conductivities of the cytoplasms and membrane capacitances of the cells of K562 and SP2. The conductivities of the cytoplasms of the cells of K562 and SP2 were estimated to be 0.2 and 0.3 Sm-(1), respectively, whereas the membrane capacitances of these cells were found to be 2.7 +/- 0.8 and 9.8 +/- 0.6 mFm-(2), respectively.