Impedance-based Biosensors

Impedance measurements on arrays of microelectrodes can provide information about the growth, motility, and physiology of cells growing on the electrodes. In this talk, we report recent results obtained for the growth of 3T3 mouse fibroblasts and HCT116 human cancer cells on gold electrodes approximately 0.4 mm in area. Cells produce a characteristic peak in the impedance change plotted as a function of frequency. With the aid of electrical modeling of the cell-electrode system, the details of the changes in the measured impedance can be correlated to the cell size, fractional electrode coverage, and cell-electrode gap. In particular, comparison of impedance measurements of these two cell types show clear differences in the growth rate and the ratio of the cell-electrode gap to the cell size. In addition to presenting these experimental results illustrating the utility of electrode impedance measurements, we will outline the issues encountered when electrodes are scaled to cell size and incorporated into a matrix-addressed array. INTRODUCTION Impedance-based microelectrode sensor arrays are potentially useful for performing drug screening experiments and also for studies of cell adhesion and micromotion. The use of impedance sensors to study cell behavior was first reported by Giaever and his coworkers in 1986 who monitored cell proliferation, morphology, and motility [1]. The impedance sensor consists of two metal electrodes: one large common reference electrode and one small working electrode. When cells are cultured on the electrodes the measured AC impedance changes in a way which depends on the frequency of measurement, the cell coverage, and the cell-electrode gap. We are developing arrays of cell-sensing electrodes which can be used to monitor many individual cells or many individual clusters. In the following, we first briefly outline the impedance changes which result from cell growth and the relation between these impedance changes and the cell coverage and cell-electrode gap. We then present data on the spreading and growth of normal mouse fibroblasts and human cancer cells. Finally, we describe the design of an electrode array fabricated using a CMOS technology and discuss some of the design issues. Figure 1 shows the basic sensor configuration. A small sensing electrode and a larger counterelectrode are immersed in cell growth medium. Except at very high frequencies, the measured impedance is dominated by the surface impedance of the smaller electrode. That surface impedance depends on transport through the electrical double layer and monotonically decreases in magnitude with increasing frequency. Cells growing on the electrode adhere to the electrode only at focal adhesion regions which represent a small (5-10%) fraction of their area. At low frequencies, current can flow from regions beneath the cell and through the medium in the cell-electrode gap and there is no change in the measured impedance. At moderate frequencies, however, cells obstruct the current flow and there is an increase in the measured impedance. Analytic theory [2] and simulations [3] show that cells lead to a characteristic peak in the impedance change as a function of frequency. The magnitude of the impedance change depends on the cell coverage and the cell-electrode gap.