Compact system for measuring the dielectric properties of biological tissues at extremely-low and ultra-low frequencies

The aim of this paper is to describe a system designed for measuring the dielectric properties of biological tissues at extremely-low frequencies (ELF) and ultra-low frequencies (ULF), in particular in the 0.1 Hz - 1 kHz range. In this frequency range, literature data are very limited or absent, since measurement techniques are strongly affected by systematic errors. In order to carry out the aforementioned measurements, the paper presents the system design and metrological tests for assessing accuracy in complex impedance measurements. The uncertainty of the measuring system was determined using reference R-C circuits, showing extremely low errors as compared to high-accuracy multimeters and LCR meters. In order to obtain the sample complex permittivity, the system was calibrated in saline solutions to determine the cell constant K. Then, experimental results on the bioimpedance and related complex permittivity of bovine liver are shown, performed with the 4-electrode measurement technique to limit the effect caused by the electrodes polarization. In particular, measurements were performed in 20 tissue samples, obtained from 5 different livers. Both conductivity and relative permittivity results have been compared with the few existing literature data, finding a satisfactory agreement between the values obtained from the literature and those achieved by measurements with the proposed system.