Dielectric models
A dielectric model of human erythrocytes
In a planar ultramicroelectrode chamber, we were able to measure the dielectric properties of human red blood cells in the frequency range from 2 kHz to 200 MHz up to physiological ion concentrations (conductivity range from 2 mS/m to 1.5 S/m). Complete electrorotation spectra and both critical frequencies of dielectrophoresis (DP) were measured. At low ionic strength, red cells exhibit a typical electrorotation spectrum with an anti-field rotation peak at low frequencies and a co-field rotation peak at higher ones. With increasing medium conductivity, both electrorotational peaks shift towards higher frequencies. The co-field peak becomes anti-field for conductivities higher than 0.5 S/m.
For erythrocytes, the ellipsoidal model, first introduced to describe DP and electrorotation (ER) in low external conductivity [50], is also valid at high external conductivities. Our new results can only be correctly described when a cytoplasmic dispersion is introduced. A single-shell erythrocyte model is proposed. It pictures the cell as an oblate spheroid with a long semi-axis of 3.3 µm and axial ratio of 1:2. Its membrane exhibits a capacitance of 1µF/cm2 and a specific conductance of 480 S/m2. The cytoplasmic parameters, a conductivity of 0.4 S/m at a dielectric constant of 212, disperse around 15 MHz to become 0.535 S/m and 50, respectively. We attribute this cytoplasmic dispersion to hemoglobin and cytoplasmic ion properties [39].
Introduction of a cytoplasmic dispersion would not have been necessary if measurements were carried out only at very low or at very high media conductivities. Only sweeping the whole conductivity range from below 2 mS/m to above 1.5 S/m revealed the drawbacks of a model with frequency independent parameters. Measurements in an external conductivity window around the cytoplasmic conductivity are especially sensitive to cytoplasmic dispersions, since forces arising from the polarization of the different cell structures become comparably small. The reason is that the polarizabilities of the external medium and the cytoplasm becomes similar.
More information: Jan Gimsa