What Is Bioimpedance and Biopermittivity

Impedance is the ratio between voltage and current. It applies to both direct current (DC) and alternating current (AC). Admittance is the inverse of impedance—that is, not impede, but admit, current flow. Immittance is the combined term for impedance and admittance, so a better and more generic term than bioimpedance is bioimmittance. 

Note the difference between resistance, conductance, impedanee, admittance, immittance—and resistivity, eonductivity, rmpedivity, admittivity, immittivity, permittivity. The -ance parameters are dependent both on the eleelrieal properties of the sample and the measuring system geometry. The -ivity parameters are material eonstants dependent only on the electrical properties of the sample, not its geometry and dimensions (as discussed in Chapters 3 and 4). 

Bioimmittance is frequency dependent. In dielectric or electrolytic models there is a choice between a step (relaxational) and sinusoidal (single-frequency) waveform excitation. As long as the step response waveform is exponential and linear conditions prevail, the information gathered is the same. At high voltage and current levels, the system is nonlinear, and models and parameters must be chosen with care. Results obtained with one variable cannot necessarily be recalculated to other forms. In some cases, one single pulse may be the best waveform because it limits heat and sample destruction. 

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