Effect of Media Conductivity on High Frequency Dielectrophoretic Response

A method was developed for analyzing the high frequency dielectrophoresis (>20 MHz) response of microalgal cells with different lipid content on the basis of media conductivities. At these frequencies, the DEP response is expected to depend primarily on the dielectric properties of the cytoplasm. The medium used to perform DEP experiments has an important influence on their outcome since all properties are measured in relation to the medium, therefore its properties must be tightly controlled. This method is label free and only uses electric fields, and thus it is suitable for many other applications in the medical field. Chlamydomonas reinhardtii cells were cultured in regular medium and then the same cells were cultured under nitrogen-free conditions to accumulate neutral (non-polar) lipids. To measure the upper crossover frequency, two thin, needle shaped electrodes (100 nm thick) were patterned onto a glass slide using standard micro-technology processes and the motion of the cells was observed as an AC signal was swept from 20–80 MHz at ∼30Vpp (Volt peak to peak). Cells were injected to the microelectrode array in different medium conductivities that were adjusted by sodium chloride. The upper crossover frequencies of N-free cells in different media conductivities were measured. It was found that when the solution conductivity was between 4 ± 2 μs/cm and 106 ± 1 μS/cm and the operating frequency was between 20 and 80 MHz, the upper crossover frequency of the microalgae increased as the medium conductivity increased. The lipid content was verified via fluorescence microscopy and MATLAB image processing.