Dielectrophoresis of cell-size liposomes

The dielectrophoresis (DEP) behavior of cell size liposomes were studied in the frequency range from 20 kHz to 3 MHz. Liposomes in the size of about 10 microns in diameter were made from egg phosphatidylcholine (PC), egg phosphatidylethanolamine (PE), egg phosphatidylglycerol (PG) and brain phosphatidylserine (PS). These liposomes, having an internal conductivity of 58 microS/cm, were suspended in either PEG or Ficoll solutions of conductivities 9 and 20 microS/cm, respectively. The liposomes were induced to form pearl chains in an electric field gradient in specially designed chambers with either coaxial or flat electrodes. Liposome rotation and convection (mixed positive and negative dielectrophoresis) were observed just beyond the experimental frequency range, within which positive dielectrophoresis leading to pearl chain formation were found. The threshold voltages that caused pearl chain formation were recorded within the experimental frequency range. The threshold voltages remained more or less constant within the positive DEP frequency range, and increased at both ends of the range. Charged liposomes (PS, PG) were found to have lower threshold than uncharged ones (PC, PE). Theoretical DEP spectra were calculated using a model proposed by Kaler and Jones ((1990) Biophys. J. 57, 173-182), for uncharged liposomes. The surface current effect in charged liposomes was accounted for, using an approximation proposed by Schwartz ((1962) J. Phys. Chem. 66, 2636-42). The experimental data agreed with theoretical prediction in general. The higher cutoff frequencies observed experimentally were thought to represent a slight lowering of internal conductivity due to leakage. The higher than predicted ratio between threshold voltages of charged and uncharged liposomes was interpreted to be due to the slight difference in size of these two types of liposomes. The agreement between theory and experiment showed that the available theory was adequate, and that liposomes provide a good model to study the nature of dielectrophoretic forces.