Micro-electric impedance spectroscopy: Biological materials and cells

The passive dielectric properties and excitable electrical characteristics of cell membranes, organelles and cytoplasm are fundamental to basic cell physiology, yet current experimental techniques are often inadequate in spatial resolution and temporal response dynamics. Our long-term goal is to improve the spatiotemporal resolution of cellular dielectric measurements by developing microscale technologies capable of noninvasive single cell micro-electric impedance tomography (mu-EIT). The ability to fabricate mu-EI systems at the appropriate scale and to resolve variations in cellular dielectric properties was addressed through completion of the following studies: (1) Microfabrication technologies were employed to fabricate a mu-EI measurement system containing fluid filled microchannels lined with integrated gold electrodes. The mu-EI recording zones were designed to have cross-sectional dimensions similar to red blood cells (4 x 10 mum) and be capable of interrogation of femtoliter (10-15) volumes of liquid or gas. (2) The mu-EI systems were electrically characterized when the microchannels and recording zone were filled with samples of air, deionized water, and various saline solutions. Raw data showed statistically significant differences in impedance measured between physiologic saline solutions having as little as 5% difference in ionic concentration (tested between 100 Hz to 15 MHz). (3) Investigations were conducted to determine the mu-EI system sensitivity to measure dielectric changes associated with physiological gel/sol transitions. Differences in impedance at high frequency (>1MHz) between solutions containing free tubulin and microtubules (polymerized tubulin) were statistically significant. (4) Frequency domain mu-EI spectroscopy was conducted on isolated living cells to characterize their cytoplasmic and membrane dielectric properties. mu-EI spectra of teleost fish red blood cells (RBCs) and vital and nonvital human polymorphonuclear leukocytes (PMNs) were distinctly different. High frequency magnitude and phase data enabled the determination of lumped-parameter circuit values for apparent membrane capacitance (Cm) and cytoplasmic resistance (R c). Results also show a frequency dependence of the cell membrane/solution interface in the 10k to 15MHz range.; These results suggest the feasibility of microscale EI systems to noninvasively image isolated living cells with single cell spatial resolution and 10 -7 second temporal response. Results also demonstrated the ability to interrogate cell membrane capacitance in the radio-frequency (RF) range.