Nanogap Capacitors Used for Impedance Characterization of Living Cells
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Nanogap Capacitors Used for Impedance Characterization of Living Cells Divya Padmaraj1,2, Wanda Zagozdzon-Wosik1,2, John H. Miller2,3, Joe Charlson1, and Len Trombetta1 1 Electrical and Computer Eng., University of Houston, 4800 Calhoun Rd., Houston, TX, 77205 2 Texas Center for Superconductivity, Houston, TX, 77205 3 Physics Department, University of Houston, Houston, TX, 77205 ABSTRACT Nano-gap metal oxide semiconductor (MOS) capacitors were studied to evaluate their limitations in applications of dielectric spectroscopy in living cells. The purpose was to optimize the design of a transducer to avoid interfacial polarization at the electrodes. Silicon IC technology was selected for designing processes in which we could limit electric double layer impedance by precisely controlling dielectric thickness of the capacitors in the range of 17 to 150 nm. The working capacitance was defined by lateral oxide etching of capacitor structures of various configuration to ensure high perimeter to area ratios. Highly doped n+ polysilicon and n+ implanted Si substrate were acting as capacitors electrodes. Restrictions known from CMOS circuits regarding oxide leakage current, which depends on geometry and increases with the gate area were taken into account. To allow for testing cells (yeasts), which have larger dimensions than nano structures it was necessary to include cell manipulation using dielectrophoresis (DEP). Entrapment of cells at the electrode perimeter preceded electrical measurements. Our focus in analyses was on the frequency dependence of impedance parameters. INTRODUCTION Electrical characterization of biological samples as a function of frequency has a well recognized importance, especially in search for clarification of basic mechanisms of cells behavior. Electromagnetic field can be used for interrogation of live particles (cells, membranes, or internal organelle such as mitochondria). Capacitive interrogation of cells indicates changes of dielectric/charge properties (polarization, conductivity etc.) with frequency. Information about cell size, conductivity of cytoplasm and membrane capacitance, structural information as well as data on functional and metabolic behavior in cell membranes, mitochondria, chloroplasts, motor proteins, and cytoskeletal proteins can be then derived and dielectric properties of cells can be also deduced. Electromagnetic field can also be used for manipulation of the cells. Micro and nano fluidics that accommodate probes designed for manipulation and interrogation of individual cells became possible with advances of nanotechnology. Controlled positioning and entrapment of individual biological cells as well as their electrical stimulation and measurements is complemented by improved electrical detection systems that can process small electrical signals originating from living organisms. All that contributed to the rapid development of electrophysiology that increasingly becomes the media for acquiring information about basic bio-mechanism at the cellular level. In diele
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