Introduction
Passive electric properties describe how cells and tissues behave in an electric field that is too weak to excite them. A tissue can be pictured as conducting fluids separated by cell membranes, which are poor conductors and act as tiny capacitors. A membrane has a capacitance of about 1 µF/cm².
Conductivity for direct and alternating current
- Direct current (DC): the membranes block the current, so it travels mainly through the extracellular fluid. Tissue resistance is high, and ions building up at the electrodes and membranes polarize them.
- Alternating current (AC): membrane capacitance lets the current pass. As frequency rises, more current goes through the cells, so conductivity rises and impedance falls.
Polarization of dielectrics
In an electric field, charges in a material shift slightly, and this is called polarization. The main types are:
Each type needs a certain time to develop, so at high frequencies the slower types cannot follow the field.
Impedance and dispersion
Impedance (Z) is the total opposition to alternating current. It combines resistance (R) with capacitive reactance (X), which falls as frequency (f) rises:
The dielectric permittivity of tissue falls in steps as frequency rises. This is called dispersion:
Beta dispersion disappears when cell membranes are damaged, so impedance can show whether tissue is alive.