Dielectrophoretic investigations of internal cell properties
Dielectrophoresis (DEP) is a term which describes the motion of polarisable particles induced by a non-uniform electric field. It has been the subject of research into a variety of fields including nanoassembly, particle filtration and biomedicine. The application of DEP to the latter has gained significant interest in recent years, driven by the development of microfluidic “Lab-on-a-chip” devices designed to perform sophisticated biochemical processes. It provides the ability to characterise and selectively manipulate cells based on their distinct dielectric properties in a manner which is non-invasive and label free, by using electrodes which can be readily integrated with microfluidic channels. Under appropriate conditions a biological cell will experience a DEP force directing it either towards or away from concentrations in the electric field. At a so-called “crossover frequency” the cell is effectively invisible to the field resulting in no DEP force, a response typically observed in the 1 kHz to 1 MHz range. Its value is a function of cell membrane dielectric properties and has been the subject of research directed at devices capable of using it to both characterise and sort cells. The aim of this work was to investigate the behaviour of a higher frequency crossover referred to as fxo2, predicted to occur in the 1 MHz to 1 GHz range. At these frequencies the electric field is expected to penetrate the cell membrane and behave as a function of intracellular dielectric properties. Standard lithography techniques have been used to fabricate electrodes carefully designed to operate at these frequencies. The existence of fxo2 was then confirmed in murine myeloma cells, in good agreement with dielectric models derived from impedance spectroscopy. A temperature dependent decrease in its value was observed with respect to the time that cells were suspended in a DEP solution. This decrease is consistent with previous studies which indicated an efflux of intracellular ions under similar conditions. An analytical derivation of fxo2 demonstrates its direct proportionality to intracellular conductivity. Direct control of the crossover was achieved by using osmotic stress to dilute the intracellular compartment and thereby alter its conductivity. By using a fluorophore which selectively binds to potassium, a strong relationship has been demonstrated between the value of fxo2 and the concentration of intracellular potassium. Measurements of fxo2 for an unfed culture demonstrated a correlation with viability and subtle shifts in its distribution were caused by the early stages of chemically induced apoptosis.