Comparison of Rectangular Wave Excitations in Broad Band Impedance Spectroscopy for Microfluidic Applications M. Min 1,2 , A. T. Giannitsis 1 , R. Land 1,2 , B. P. Cahill 1 , U. Pliquett 1 , T. Nacke 1 , D. Frense 1 , G. Gastrock 1 , and D. Beckmann 1 1 Institute for Bioprocessing and Analytical Measurement Techniques, 37308 Heilbad Heiligenstadt, Germany 2 Tallinn University of Technology, Department of Electronics, Ehitajate tee 5, 19086 Tallinn, Estonia Abstract— We present a potential application of time do- main impedance spectroscopy to the area of cell detection and characterizing in microfluidic devices. Digital pulses of pre- selected spectral properties were applied in the Hz-to-MHz frequency range. This method offers important diagnostic information about the dynamic and structural properties of biological cells. We investigate the possibility of using rectan- gular wave chirp excitations for fast measuring of impedance. Keywords— Impedance spectroscopy, signal processing, micro- fluidics, cell impedance I. INTRODUCTION Frequency domain impedance measurement is a common approach in assessing passive electrical properties of cells and tissues [1]. However, sweeping or hopping of a sine wave excitation over a wide frequency range is too slow to monitor fast impedance changes in cells or movements in high throughput microfluidic devices [2-5]. The use of short-time and broadband pulse excitation for monitoring the response as a function of time significantly reduces the measurement interval. The sine wave chirp excitation – a non-periodic one-shot signal with continuously changing frequency – has been found to be the most promising in terms of well defined frequency range, acceptable crest factor, and achievable signal-to-noise ratio [6]. Signal processing becomes simpler when the rectangular wave signals with only +1 and −1 value are used. More- over, the rectangular waveforms have the minimal crest factor (equal to 1). A known method is to generate a pseu- do-random maximum length sequence (MLS) of rectangular pulses [4, 5]. In this paper, we propose another method: generation of a sequence of regularly shortening or widen- ing rectangular pulses (Fig.1). In other words we have a rectangular chirp excitation, which can be described as a signum-chirp function instead of the sine chirp [7]. Besides the simplest rectangular chirp having non-return-to-zero (NRZ) pulses, some versions of return-to-zero (RZ) rectan- gular chirp function (has +1, 0, and −1 values [8]) are con- sidered and compared. Fig. 1 Rectangular excitation signals: (a) signum-chirp, NRZ pulses; (b) maximum length sequence MLS, NRZ pulses; (c) rectangular chirp, 18° shortened RZ pulses; (d) rectangular chirp, 30° shortened RZ pulses Figure 2 depicts a simplified structure of an impedance spectrometer, in which the rectangular wave excitation is generated during the time interval T exc from t 1 to t 2 . Excita- tion signal covers a pre-selected excitation bandwidth BW from f 1 to f 2 , passes through the impedance Ż and gives a response, which will be cross-correlated with a reference pulse (the same signal as excitation, typically). The result is a time domain correlation function representing an impulse response g z (t) of the complex impedance Ż. Performing the Fourier transform, we will obtain a time dependent complex b c d a freq: f 1 to f 2 Generator of rectangular wave excitation Ż Cross correlation C{V z (t),V r (t,τ)} Fourier Transform (FFT) g z (t) S z (jω,t) time: t 1 to t 2 reference, Vr Excitation control Fig. 2 Basic structure of the system for performing road band impedance spectroscopy response, Vz Impedance spectrum O. Dössel and W.C. Schlegel (Eds.): WC 2009, IFMBE Proceedings 25/VII, pp. 85–88, 2009. www.springerlink.com