IMPEDANCE EFFECTS DURING HIGH-FREQUENCY DIELECTROPHORESIS

Dielectrophoresis refers to the motion of electrically neutral particles in a spatially non-uniform electric field as a function of frequency. Analytical models predict a second or upper crossover frequency, but there is little experimental evidence to supporting this prediction. One reason for this is that standard bench top function generators are typically limited to maximum frequency of 20 MHz, and those with higher bandwidths are often unable to generate higher frequency electric fields with sufficient amplitude to induce DEP motion. Our experimental set-up seeks to sort cells on the basis of the upper crossover frequency in the range of 10-160 MHz. At these frequencies, the DEP response is expected to depend primarily on the dielectric properties of the cytoplasm. A needle pattern was used due to the ease of making spatially non-uniform electric fields. Sine waves were generated with a signal generator capable of generating sine waves with frequencies up to 160 MHz which was amplified with an RF amplifier. In order to help match the load to the amplifier and reduce the amount of reflected power an attenuator was placed between the amplifier and the electrodes. Two different amplifiers and attenuators were tested. Conductive copper tape and alligator clips were used to connect the glass slide to the electronic equipment. These systems were capable of 30 Volts peak-to-peak (Vpp) amplitudes in the given frequency range as monitored by an oscilloscope probing the electrodes. With this set-up, the amplitude of the signal was observed by oscilloscope to vary as much as 40 Vpp across the range of frequencies which is supplied by signal generator. We suspect that the impedance of the experimental set-up was highly frequency dependent at the high frequencies used in this study. Furthermore, the oscilloscope may also be enough of a load on the system to affect the impedance. To maintain the 30 Vpp output, the input amplitude to the RF amplifier was adjusted at each individual frequency across all frequencies of the DEP experiment.