Dielectrophoretic Motion and Assembly of Janus Liposomes.

As an electric field-operated and relatively noninvasive technique, dielectrophoresis (DEP) has been frequently employed in the manipulation, assembly and sorting of bioparticles, including cells, organelles and liposomes. Of the latter category, however, prior investigations have been mostly generic, treating liposomes as just another type of dielectric colloids and leaving specific lipid properties, such as their type and phase, largely unprobed. In this work, we subject microsized liposomes in either liquid-ordered or liquid-disordered phase, or their combination, under DEP, and follow their movement and assembly using fluorescence microscopy. Across the range of AC frequencies tested (10 Hz to 30 MHz), we have observed (1) frequency-dependent liposome accumulation, chain formation, size-based sorting, and levitation; (2) two DEP crossover frequency regimes, first from negative to positive mode DEP at ∼1 kHz and then back to negative-mode at ∼1 MHz; (3) lipid phase-dependent liposome transformation and assembly patterns, as manifested by orientation-dependent liposome deformation and domain penetration at low frequencies (10 Hz-300 Hz) and liposome field alignment at intermediate frequencies (10 kHz-300 kHz); and (4) lipid composition-dependent DEP mobility of liposomes. Our results demonstrate DEP as a suitable tool to study lipid phase separation in liposomes.