Uwe Pliquett

Broadband excitation for short-time impedance spectroscopy

Frequency domain impedance measurements are still the common approach in assessing passive electrical properties of cells and tissues. However, due to the time requirements for sweeping over a frequency range for performing spectroscopy, they are not suited for recovering fast impedance changes of biological objects. The use of broad bandwidth excitation and monitoring the response as a function of time will greatly reduce the measurement time. The widespread usage of a square wave excitation is simple but not always the best choice. Here we consider different waveforms for excitation and discuss not only the advantages but also their limitations. Measurements in a miniaturized chamber where frequency and time domain measurements are compared show the suitability of different waveforms as excitation signals for the measurements of bio-impedance. The chirp excitation has been found to be most promising in terms of frequency range, signal-to-noise ratio and crest factor.

Continuous process monitoring for biogas plants using microwave sensors

The efficiency of biogas production during anaerobic digestion depends heavily on optimal dosing ratios and stable operations which can not be achieved without accurate and reliable monitoring and control of the dry matter (DM) and organic dry matter (oDM) content. The materials in biogas processes to be measured are either stored in a vessel flowing in a pipe, either as a liquid or as solid particles in pneumatic or liquid assisted transportation with a wide area of the content of DM (1% 90). In such cases microwave sensors provide an attractive solution, because microwaves penetrate most materials allowing the non destructive measurement to be representative for a special volume or the cross section of the pipe. The setup of the measuring system for materials with high water content and first results are presented.

Testing miniaturized electrodes for impedance measurements within the beta-dispersion – a practical approach

Abstract Miniaturized electrodes are introduced in life sciences in a great number and variety. They are often designed for a special purpose without the need of quantitative analysis, such as for detecting cells or water droplets in a fluid channel. Other developments aim in monitoring a single quantity in a process where all other factors held constant. To use miniaturized electrodes for quantitative measurements, their behavior should be known in detail and stable over time in order to allow a mathematical correction of the data measured. Here we show test procedures for evaluating macroscopic but also microscopic electrodes. The most important quality parameters for electrode systems used in life science are the electrode impedance, its stability, the useful frequency range as well as the limits for applied stimulus without driving the electrode system into a non-linear region of the current/voltage relation. Proper electrode design allows a bandwidth from 100 Hz up to some MHz for impedances ranging over decades from 50 Ω up to several MΩ.

Signals in bioimpedance measurement: different waveforms for different tasks

Alternatives to the traditional sine wave excitation are studied in the paper. Impedance measurements can be performed much faster by using a broad bandwidth signal for excitation. Using of square wave pulses, Gaussian function and its derivative, also modifications of sinc and chirp signals, is analysed. Carefully designed pulse wave excitation can become to an alternative to established excitation waveforms, especially, when fast measurements with exact timing are required, and when the energy consumption is important.

Front end with offset-free symmetrical current source optimized for time domain impedance spectroscopy

Fast impedance measurements are often performed in time domain utilizing broad bandwidth excitation signals. Other than in frequency domain measurements harmonic distortion cannot be compensated which requires careful design of the analog front end. In order to minimize the influence of electrode polarization and noise, especially in low-frequency measurements, current injection shows several advantages compared to voltage application. Here, we show an active front end based on a voltage-controlled current source for a wide range of impedances. Using proper feedback, the majority of the parasitic capacitances are compensated. The bandwidth ranges from dc to 20 MHz for impedance magnitude below 5 kΩ. The output is a symmetric signal without dc-offset which is accomplished by combination of a current conveyor and a voltage inverter. An independent feedback loop compensates the offset arising from asymmetries within the circuitry. We focused especially on the stability of the current source for usage with small metal electrodes in aqueous solutions. At the monitor side two identical, high input impedance difference amplifiers convert the net current through the object and the voltage dropping across into a 50 Ω symmetric output. The entire circuitry is optimized for step response making it suitable for fast time domain measurements.

Interfacing the AD5933 for bio-impedance measurements with front ends providing galvanostatic or potentiostatic excitation

The AD5933 [1], a specialized single chip impedance analyzer, made by Analog Devices, is basically not intended for use with four electrode interface. Due to electrochemical phenomena at the electrodes connecting the material under test (MUT), especially in the low frequency region below 100 kHz, a two electrode interface generates considerable errors during the measurement. Thus, for most application in bio-impedance measurement only a four electrode interface can guarantee reliable results. Here we show how a four electrode interface with galvanostatic excitation but also for potentiostatic excitation can be realized by just a few external components.

Comparison of Rectangular Wave Excitations in Broad Band Impedance Spectroscopy for Microfluidic Applications

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 rectangular wave chirp excitations for fast measuring of impedance.

Assessment of suspension medium conductivity by means of micro electrodes

Electrolytic conductivity of cell-based suspensions is an important parameter which can be easily and non-destructively measured as part of the electrical impedance. The low frequency conductivity of a cell suspension with very low cell density equals nearly the medium conductivity. However, a high cell density decreases the low frequency conductivity due to the insulating behaviour of the cell membranes. Here we use miniaturized electrode structures, smaller than the size of typical cells for impedometric conductivity measurement which allows an impedance measurement independent of the electrical properties of suspended cells or particles.

Electrodes – the challenge in electrical characterization of biological material

Electrodes are an important part of the impedance measurement chain but their influence is often underestimated. Electrochemical reactions or polarization effects, especially for galvanically coupled systems are sometimes completely neglected. The most important features of electrodes to be considered are the geometry with respect to the structure of the material to be tested and the electrochemistry at the electrode surface. Especially drift and corrosion effects may yield misleading results. If applicable, sophisticated electrode systems should be used in order to prevent the distorting influence of electrode polarization, for extending the useful frequency range of the electrodes or for enhancing the signal-to-noise ratio.

Messsystem für die impedanzspektroskopische Breitband-Prozessmesstechnik

Zusammenfassung Robuste Sensoren für die zerstörungsfreie Materialcharakterisierung im Labor und unter rauen industriellen Bedingungen sind von zunehmendem technischen und ökonomischen Interesse. Dieser Artikel behandelt die Entwicklung und Anwendung eines prozesstauglichen, modularen und breitbandigen impedanzspektroskopischen Hard- und Softwarekonzepts für den Feld- und Laboreinsatz. Besondere Beachtung finden weiterhin auch technische und biophysikalische Aspekte der Bio-Impedanzspektroskopie. Abstract Robust sensors for non-destructive contactless measurements in the laboratory and under industrial conditions are of increasing technical and economical interest. This article deals with the development and application of modular broadband impedance spectroscopic hardware and software concepts suitable for process measurement in the field and the laboratory. Special attention is paid to technical and biophysical aspects of bioimpedance spectroscopy.