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I have concerned myself with bioimpedance spectroscopy (BIS) from a medical standpoint for some time now, however there is one concept evading my grasp until now:

When measuring complex impedance of biological tissue in AC in the beta-dispersion range (1kHz to 100MHz), capacitive reactance Xc plotted against resistance R usually follows an arc-shaped impedance locus.

Why does Xc increase up to the tissue specific frequency fc and then decrease again?

From my understanding, the decrease in AC frequencies > fc is because the tissue specific dielectric relaxation time is larger than the relaxation time possible within this AC, therefore less and less polarization is taking place - phase angle and Xc decrease.

Why does Xc then increase when 0 < AC frequency < fc?

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  • \$\begingroup\$ What is the equivalent circuit they are trying to model? For example, to me, it looks like they are treating the tissue as a parallel arrangement of R and C and, as frequency rises skin-effect dominates and reactance effects become zero. Hence, we need to know what electrical model of the tissue they are assuming, \$\endgroup\$
    – Andy aka
    Commented May 12, 2023 at 9:59
  • \$\begingroup\$ @Andyaka: typically, electrode polarization and the electrode-skin interface is irrelevant as a tetrapolar setup is used. The simplest equivalent electrical circuit puts a resistor (for the extracellular volume) in parallel to a resistor (intracellular volume) + a capacitor. \$\endgroup\$
    – elmisterioso
    Commented May 12, 2023 at 10:16
  • \$\begingroup\$ Draw that circuit to avoid ambiguity (I read ambiguity in what you have written). \$\endgroup\$
    – Andy aka
    Commented May 12, 2023 at 15:30

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