Electric Field and Tissues - Interaction At Different Wavelengths
|Field Frequency (cycles/sec)||Energy Coupling Mechanism||Tissue Damage|
|D.C. to 103||Ionic Currents Forces on Cell Structures||Joule Heating Membrane Poration|
|103 to 107||Ionic Currents Field Energy Absorption by Cells||Joule Heating Cell Spinning|
|107 to 109||Field Energy Absorption by Proteins||Macromolecular Heating|
|109 to 1011||Field Energy Absorption by Water||Microwave Heating of Water|
|1011 to 1015||Field Energy Absorption by Atomic Bonds||Photo-optical Protein Damage|
The fundamental bioengineering perspective is that the human body is considered to be a compartmentalized (or lumped element) conducting dielectric: consisting of about 60% water by weight (33% intracellular and 27% extracellular). Body fluid in both the intracellular and extracellular compartments is highly electrolytic, and these two compartments are separated by a relatively impermeable, highly resistive plasma membrane. Current within the body is carried by mobile ions in the body fluid. While electrons are the charge carriers in metallic conductors or electrical arcs, in the human body the charge carriers are ions.
At low frequencies (i.e., below radio frequencies), the electric current passing across the body distributes such that the electric field strength is nearly uniform throughout any plane perpendicular to the current path. As a consequence, the current density distribution depends on the relative electrical conductivity of various tissues and the frequency of the current.
At a macroscopic scale, upper limbs are mostly involved in electric shocks, especially the right upper limb, as would be expected from dominant hand interactions with electrical sources. Computational models of human high-voltage electrical shock suggest that the induced tissue electric field strength in the extremities is high enough to electroporate skeletal muscle and peripheral nerve cell membranes and to possibly cause electroconformational denaturation of membrane proteins.
At a more microscopic scale, low frequency current distribution within tissue is determined by the density, shape and size of cells. The cell membrane acts as an insulating ion transport barrier that mostly shields the cytoplasmic fluid from low frequency electrical current. In addition, the presence of cells diminishes the area available for ionic current and, in effect, makes tissues less conductive. As cell size increases, the membrane has less impact on a cell’s electrical properties, because the volume fraction of the cell occupied by the membrane is inversely proportional to the total cell radius. Similarly, the conductivity of skeletal muscle parallel to the long axis of the muscle cells is greater than that perpendicular to this axis. Solid volume fraction of the extracellular matrix can also be important in certain tissues and anatomic locations.
At higher frequencies, in RF and microwave ranges, the current distribution is dependent on different parameters. The cell membrane is no longer an effective barrier to current passage, and capacitive coupling of power across the membrane readily permits current passage into the cytoplasm. Frequency-dependent factors like energy absorption and skin-depth effects govern the field distribution in tissues. At the highest frequency ranges, including light and shorter wavelengths, other effects such as scattering and quantum absorption effects become important in governing field distribution in tissues. Table above provides a categorization of frequency regimes, with the corresponding wavelength spectrum, their common applications, and their effects on tissues as a result of electrical injury.
Experts at ESR have studied tissue injury from exposures at different wavelengths. For more information, contact us.