Front-end electronics for impedimetric microfluidic devices

Abstract

Impedance spectroscopy is a common approach in assessing passive electrical properties of biological matter, however, serious problems appear in microfluidic devices in connection with distortion free signal acquisition from microelectrodes. The quality of impedance measurements highly depends on the presence of stray capacitances, signal distortions, and accompanying noises. Measurement deficiencies may be minimized with optimized electronics and sensing electrodes. The quality can further be improved with appropriate selection of measuring signals and also with selection of measuring methods such as a choice between current or voltage sources and between differential or singleended techniques. The microfluidic device that we present here incorporates an impedance sensor, which consists of an array of two sequential pairs of parallel microelectrodes, embedded in a microfluidic channel. All electronics and fluidic components are placed inside a metal holder, which ensures electric and fluidic connections to peripheral instruments. This configuration provides short electric connections and proper shielding. The method that we are using to evaluate the sample's impedance is the differential measurement technique, capable of suppressing the common mode signals and interferences, appearing in the signal-conditioning front-end circuit. Besides, it opens the possibility for compensating stray effects of the electrodes. For excitation we employ wideband signals, such as chirps or multifreqyency signals, which allow fast measurements, essential in the most impedimetric experiments in biology. The impedance spectra cover the frequency range between 10kHz -10MHz. This is essential for accessing information relating to β-dispersion, which characterizes the cell's structural properties. We present two measurement schemes: (i) an in-phase differential method, which employs two transimpedance amplifiers, and (ii) an anti-phase method, which uses one transimpedance amplifier. In this study we analyze and compare the sensitivity, signal-to-noise-ratio, and operational bandwidths of these two methods against other commonly used related circuits.

Multichannel Electrical Impedance Spectroscopy Analyzer with Microfluidic Sensors

“…Microfluidic sensors are to be connected directly with analog front-end connectors, as illustrated in Figure 4. The shortest connections provide minimal stray inductances and capacitances that ensure minimal distortion of the impedance spectra, lower noises of the input TIA stages [11], and lesser susceptibility to external disturbances.…”

“…For the better SNR of the following AD-converters, it is desirable to keep the signal level near their full scale. However, the choice of R F value also influences the noise and bandwidth properties of TIA [11], which limits the overall performance of the EIS analyzer. For the full range of frequencies (10 MHz), the R F value should not exceed 10 kΩ.…”

“…In the case of using current excitation, the amplitude of the current must not produce a voltage drop above 50 mV on the double-layer impedance of electrodes to avoid significant nonlinearity of current-to-voltage relationship [3,4,11]. At higher frequencies, the use of a fixed (constant) current produces a low and decaying amplitude of the response voltage that decreases the signal to noise (SNR) of measurements.…”

“…The sensitivity of detection of properties of small particles depends on the ratio of the volumes of particles and measurement cell and the placement of electrodes [10,11,12]. As shown in a previous study [13], the parallel plate electrodes provide better sensitivity and more uniform sensing area in comparison to other shapes.…”

“…In this case, a reference signal compensates the basal value of the response signal, and the changes of the remaining signal are amplified, typically up to the second half of the full scale of the measuring device. The basal value may be obtained from the same dual microfluidic sensor, as shown in previous studies [4,11], from another (reference) sensor, or the results of modeling or previous measurements.…”