Raul Land

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.

A sampling multichannel bioimpedance analyzer for tissue monitoring

The paper focuses on principles of designing of a multichannel bioimpedance analyzer based on simultaneous multisine measurement. The measurement task arises due to the need to monitor patients during and after heart surgery operation performing MIMO (multiple-input-multiple-output) bioimpedance measurement. Frequencies of the simultaneously applied sinusoidal excitations must be close but simultaneously varied in a larger range (e.g. from 1 kHz up to 10 MHz). The main idea of the proposed approach is that the use of a rather specific signal system (frequencies of sinusoidal excitations are related as integers and sampling frequencies are properly related/adapted to them) makes it possible to separate responses to different excitations from the measured summary signals by means of a quite simple filter and different (under) sampling rates.

Modification of pulse wave signals in electrical bioimpedance analyzers for implantable medical devices

The problems of application of pulse wave signals in electrical bioimpedance analyzers foreseen for using in implantable medical devices as diagnostical means are discussed in this paper. The main problem arises at measurement of phasor parameters by the aid of rectangular pulse wave signals. The specific measurement errors appear due to presence of higher harmonics in the spectra of pulse waveforms. These errors are discussed in two cases, in the case of full cycle rectangular waveform, and in the case of using the shortened pulses introduced specially for reduction of errors.

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.

Broadband spectroscopy of dynamic impedances with short chirp pulses

An impedance spectrum of dynamic systems is time dependent. Fast impedance changes take place, for example, in high throughput microfluidic devices and in operating cardiovascular systems. Measurements must be as short as possible to avoid significant impedance changes during the spectrum analysis, and as long as possible for enlarging the excitation energy and obtaining a better signal-to-noise ratio (SNR). The authors propose to use specific short chirp pulses for excitation. Thanks to the specific properties of the chirp function, it is possible to meet the needs for a spectrum bandwidth, measurement time and SNR so that the most accurate impedance spectrogram can be obtained. The chirp wave excitation can include thousands of cycles when the impedance changes slowly, but in the case of very high speed changes it can be shorter than a single cycle, preserving the same excitation bandwidth. For example, a 100 kHz bandwidth can be covered by the chirp pulse with durations from 10 µs to 1 s; only its excitation energy differs also 10(5) times. After discussing theoretical short chirp properties in detail, the authors show how to generate short chirps in the microsecond range with a bandwidth up to a few MHz by using digital synthesis architectures developed inside a low-cost standard field programmable gate array.

Rectangular wave excitation in wideband bioimpedance spectroscopy

Method of time domain impedance spectroscopy using of rectangular wave chirp excitation is presented in the paper. Effectiveness of the excitation is high - more than 85% of the generated energy lies in the useful excitation bandwidth, which can cover several decades of frequency. Fast measurement and joint time-frequency analysis of dynamic impedances is envisaged. Implementations in impedance spectroscopy for cell detection and characterizing in different lab-on-a-chip type high-throughput microfluidic devices can be one recommended area for the proposed method. This method offers important diagnostic information about the dynamic and structural properties of biological cells, organs like beating heart, and the whole cardiovascular system.

Binary signals in impedance spectroscopy

Using of binary waveforms in the fast impedance spectroscopy of biological objects is discussed in the paper. There is shown that the energy of binary waveforms can be concentrated onto selected separate frequencies. We can optimize the binary excitation waveform depending on the shape of frequency response of the impedance under study to maximize the levels of signal components with certain selected frequencies. As a result, we are able to receive maximal amount of information about the properties and behavior of the impedance to be studied. We have designed and prototyped the impedance spectroscopy device operating in the frequency range from 100 mHz to 500 kHz to cover α- and β-regions of the bio-impedance spectrum of time-varying subjects as, for example, fast moving cells in micro-fluidic devices, beating heart and breathing lungs or the whole cardiovascular system.

Wideband object identification with rectangular wave chirp excitation

Using of chirp-type excitations for the fast broadband measurement and estimation of the complex bioimpedance is discussed in this paper. The proposed method includes cross-correlation of the excitation (reference) and response signals with a subsequent Fourier analysis of the obtained correlation function. It is shown that the phase response can be especially convenient for estimating of the state of a biological object and for monitoring the processes taking place in it. The results of this study stand for elaborating the respective off-chip and on-chip measurement devices with rectangular wave chirp pulses as excitation.

Multisine and Binary Multifrequency Waveforms in Impedance Spectrum Measurement - A Comparative Study

The multisine and binary waveforms are usable as multifrequency excitation signals in measurement of impedance spectrum of biological objects. A comparative study of these waveforms is given for the case of multifrequency signals covering 3 frequency decades with 11 spectral components with octave based frequency distribution. It is shown in the paper that the multisine excitation signal can be optimized by adjustment of phases of the desired spectral components. Moreover, it is concluded that the binary multifrequency signal formed from the corresponding multisine waveform as the signum function of it, is much more effective than the optimized multisine signal.

Broadband spectroscopy of a dynamic impedance

Impedance spectrum of dynamic systems is time dependent. For example, fast impedance changes take place in high throuput microfluidic devices. Also, the impedance of cardiovascular system is dynamic. Measurements must be as short as possible to avoid significant impedance changes during the spectrum analysis and, at the same time, as long as possible for enlarging the excitation energy. The authors propose to use specific short chirp pulses for excitation. Thanks to unique properties of the chirp function, it is possible to meet the needs for spectrum bandwidth, measurement time, and signal-to-noise ratio so that the most accurate impedance spectrogram is obtained. The chirp wave excitation pulse can include thousands of cycles when the impedance changes slowly, but in the case of very high-speed changes it can be even shorter than a single cycle, preserving the same excitation bandwidth. For example, 100 kHz bandwidth can be covered by the chirp pulse with duration from 10 μs to 1 s, only its excitation energy differs also 105 times.