Using Microelectrode Models for Real Time Cell-Culture Monitoring Alberto Yúfera 1 Member, IEEE, Paula Daza 2 , Daniel Cañete 3 Abstract: This paper proposes a cell-microelectrode model for cell biometry applications, based on the area overlap as main parameter. The model can be applied to cell size identification, cell count, and their extension to cell growth and dosimetry protocols. Experiments performed with comercial electrodes are presented, illustrating a procedure to obtain cell number in both growth and dosimetry processes. Results obtained for the AA8 cell line are promising. Keywords- Microelectrode; ECIS; bio-impedance; impedance sensor; cell culture; dosimetry. I. INTRODUCTION any biological parameters and processes can be sensed and monitored using its impedance as marker [1-5], with the advantage of being a non-invasive and relatively cheap technique. Cell growth, cell activity, changes in cell composition and shape or in cell location are examples of processes which can be detected with microelectrode-cell impedance sensors [6-9]. Among Impedance Spectroscopy (IS) techniques, Electrical Cell-substrate Impedance Spectroscopy (ECIS) [7.8], based on two-electrode setups, allows the measure of cell-culture impedance and the definition of the biological nature (material, internal activity, motility and size) of a kind of cell and its relationship with the environment [11]. One of the main drawbacks of ECIS technique is the need of efficient models to decode the electrical performance of the full system composed by the electrodes, medium and cells. Several works have been developped in this field. In [8], magnitude and phase impedance are deduced from electric field equation solution at the cell-electrode interface, giving a three parameter based model. h, the cell-electrode distance, R b , barrier resistance and r cell , cell radius. In [9,10], finite element simulation (FEM) are executed for solving electrical field considering the whole structure. This method gives one parameter model (R gap ) for describing the gap or cell-electrode region resistance. In both, the derived model considers the cell confluent phase [7] or a fixed area covered by cells [9]. The latest was extended in [10] to several cell sizes, allowing to define the cell-electrode covered area as the main model parameter. Manuscript received March 26, 2011. This work was supported in part by the Spanish founded Project: Auto-calibración y auto-test en circuitos analógicos, mixtos y de radio frecuencia: Andalusian Government project P0-TIC-5386, co-financed with FEDER program. 1 A. Yúfera is with the Seville Microelectronics Institute (IMSE-CNM), Seville Uni.. Av. Americo Vespucio, sn, 41092, Seville. Spain. Phone: +34954466666. yufera@imse-cnm.csic.es. 2 P. Daza is with the Dpt. of Cell Biology, Biology Faculty. Seville Uni. Av. Reina Mercedes, sn, 41012, Seville. Spain. pdaza@us.es. 3 D. Cañete is with the Dpt. of Electronic Technology, ETSII, Seville Uni.. Av. Reina Mercedes, sn, 41012, Seville. Spain. dani@zariweyo.es. This work considers a modification of model in [9], to incorporate the cell-microelectrode area overlap [10]. Impedance sensor sensitivity curves based on the cell size and density will be presented and applied to measure the growth-tax in cell-cultures and to describe cell toxicity experiments. Section II resumes the electrode solution model useful for cell- electrode characterization. The process to extract useful cell- microelectrode models is included at section III, illustrating the simulations on a simplified system for cell size detection. Section IV relies on real time cell culture monitoring and the application of the proposed model to dosimetry experiments. Conclusions will be highlighted at section V. II. ELECTRODE-ELECTROLYTE MODEL The impedance of electrodes in ionic liquids has been rather extensively investigated. An excellent review can be found at [6]. The main componets describing the electrical performance of an electrode metal inside a solution are four: the double layer capacitance, C I , the current flowing through the electrified interface will encounter a resistance R ct caused by the electron transfer at the electrode surface and Warburg impedance Z W due to limited mass diffusion from the electrode surface to the solution. The electron transfer resistance R ct is in series with the mass diffusion limited impedance Z W . As the current spreads out to the bulk solution, the electrode has a solution conductivity determined by series resistance, represented as spreading resistance R S in the equivalent circuit. These four parameters depend on technology, medium and geometry. Figure 1. Circuit for the electrode-solution interface. CI is the double layer capacitance, Faradic impedance includes Zw, the Warburg impedance and Rct, the charge-transfer resistance. Rs is the spreading resistance. III. CELL-ELECTRODE MODEL Figure 2 illustrates a two-electrode impedance sensor useful for ECIS technique: e 1 is the sensing electrode and e 2 the reference one. Electrodes can be manufactured in CMOS process with metal layers [9] or using post-processing steps [13]. The cell location and size on e 1 top must be detected. The model in Fig. 3 considers the sensing surface of e 1 could be total or partially filled by cells. For the two-electrode sensor in Fig. 2, e 1 is the sensing area A, and Z( ) the impedance by unit area of the empty electrode (without cells on top). When e 1 is partially covered by cells in a surface A c , Z( )/(A-A c ) is the electrode impedance associated to non-covered area by cells, and Z( )/A c is the impedance of the covered area. R gap models the current flowing laterally through the electrode-cell interface, which depends on the electrode-cell distance at the M 978-1-4244-4122-8/11/$26.00 ©2011 IEEE 3983 33rd Annual International Conference of the IEEE EMBS Boston, Massachusetts USA, August 30 - September 3, 2011