24. Passive electric properties of cells and tissues: electroconductivity of cells and tissues for DC and AC. Types of polarization of dielectrics and heterogeneous systems. Electroporation and electrofusion. Impedance and dispersion of the dielectric permittivity. Applications in medicine
Explains how cells and tissues conduct direct and alternating current, then covers the types of dielectric polarization, with a table listing electronic, ionic, orientational and interfacial types. It goes on to impedance and the alpha, beta and gamma dispersion of permittivity, shown in a second table, followed by electroporation, electrofusion and medical applications such as bioimpedance analysis, electrochemotherapy and diathermy.
Inside: 6 sections · 2 tables · 1 formula · 16 flashcards · 3 min
Topic 24 of 26 in the MU Sofia syllabus for Biophysics (Dental Medicine, Year 2). The free AceMed note covers it in 6 sections, about 293 words (3 min of reading), with 16 flashcards.
In this note
Introduction
Conductivity for direct and alternating current
Polarization of dielectrics
Impedance and dispersion
Electroporation and electrofusion
Applications in medicine
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.
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