396
Electromagnetic Fields in Biological Systems
activate proteolysis mechanisms to degrade and remove the proteins (See Section 7.5.1
for more details on the CSR.).
In addition to directly damaging biomolecules, thermal effects to cellular organalles
are also observed for the low-temperature damage zone. A few examples include: cell
shrinkage, nanoporation of lipid membrane, disruption of mitochondria function, and
increased membrane permeability. Interestingly, if the magnitude of the thermal stress
is severe, such effects typically result in necrosis, a traumatic-type of cell death. In contrast, if the magnitude of the thermal stress is severe, yet the cell has adequate resources
available (i.e., Adenosine triphosphate (ATP), oxygen, etc.), then it preferentially activates apoptotic pathways, a means of controlled “cellular death.”
Several thermal effects associated with the low-temperature zone also manifest themselves at a tissue level. A few notable effects include structural protein denaturation,
birefringence loss, and collagen hyalinization (Henriques and Moritz 1947; Moritz 1947;
Moritz and Henriques 1947). Structural proteins, such as fibrillar collagen, have higher
energy bonds, are less labile than intracellular proteins, and are typically damaged when
tissue temperatures reach 60°C for ∼1 minute or longer (Vogel and Venugopalan 2003)
(See Figure 7.17). In contrast, structural membrane proteins can withstand slightly
higher temperatures and become compromised as the temperature rises to 70°C–80°C.
7.4.5.2 Medium- and High-Temperature Zones
THz sources that have the output power to cause biological effects associated with the
middle and higher temperature zones are currently not available. However, such sources
are expected to be developed in the near future. Therefore, knowledge of these effects
will become more relevant, especially as higher power THz sources are developed for
use in new therapeutic applications. Envisaged applications include therapeutic heating,
biostimulation, laser skin resurfacing, thermal preconditioning, laser surgical ablation
and/or cutting, or even tumor ablation. In brief, the medium-temperature damage zone
region is characterized by water-dominated effects (i.e., vaporization, irreversible fibrillar collagen damage, and the “popcorn effect”), whereas the high-temperature region
is characterized by tissue ablation, carbonization, and molecular dissociation. At these
elevated temperatures, structural proteins such as fibrillar collagen are irreversibly damaged leading to further coagulation and visible tissue whitening. Coagulation signifies a
lethal end point for the tissue. Another irreversible change is the birefringence loss that
occurs when the regular arrangement of collagen molecules is disrupted. Birefringence
is a rotation in the angle of polarization of the tissue. Alterations in collagen microstructure can be detected by transmission polarizing microscopy (TPM), TEM, and highresolution multiphoton microscopy (MPM) using second-harmonic generation.
7.4.6 Tissue Damage at Terahertz Frequencies
7.4.6.1 Arrhenius Integral and Characterizing
Terahertz-Induced Tissue Damage
The Arrhenius integral is a rate process that is commonly used to characterize tissue damage, coagulation, or birefringence loss as a function of temperature and time.
Electromagnetic Fields in Biological Systems
activate proteolysis mechanisms to degrade and remove the proteins (See Section 7.5.1
for more details on the CSR.).
In addition to directly damaging biomolecules, thermal effects to cellular organalles
are also observed for the low-temperature damage zone. A few examples include: cell
shrinkage, nanoporation of lipid membrane, disruption of mitochondria function, and
increased membrane permeability. Interestingly, if the magnitude of the thermal stress
is severe, such effects typically result in necrosis, a traumatic-type of cell death. In contrast, if the magnitude of the thermal stress is severe, yet the cell has adequate resources
available (i.e., Adenosine triphosphate (ATP), oxygen, etc.), then it preferentially activates apoptotic pathways, a means of controlled “cellular death.”
Several thermal effects associated with the low-temperature zone also manifest themselves at a tissue level. A few notable effects include structural protein denaturation,
birefringence loss, and collagen hyalinization (Henriques and Moritz 1947; Moritz 1947;
Moritz and Henriques 1947). Structural proteins, such as fibrillar collagen, have higher
energy bonds, are less labile than intracellular proteins, and are typically damaged when
tissue temperatures reach 60°C for ∼1 minute or longer (Vogel and Venugopalan 2003)
(See Figure 7.17). In contrast, structural membrane proteins can withstand slightly
higher temperatures and become compromised as the temperature rises to 70°C–80°C.
7.4.5.2 Medium- and High-Temperature Zones
THz sources that have the output power to cause biological effects associated with the
middle and higher temperature zones are currently not available. However, such sources
are expected to be developed in the near future. Therefore, knowledge of these effects
will become more relevant, especially as higher power THz sources are developed for
use in new therapeutic applications. Envisaged applications include therapeutic heating,
biostimulation, laser skin resurfacing, thermal preconditioning, laser surgical ablation
and/or cutting, or even tumor ablation. In brief, the medium-temperature damage zone
region is characterized by water-dominated effects (i.e., vaporization, irreversible fibrillar collagen damage, and the “popcorn effect”), whereas the high-temperature region
is characterized by tissue ablation, carbonization, and molecular dissociation. At these
elevated temperatures, structural proteins such as fibrillar collagen are irreversibly damaged leading to further coagulation and visible tissue whitening. Coagulation signifies a
lethal end point for the tissue. Another irreversible change is the birefringence loss that
occurs when the regular arrangement of collagen molecules is disrupted. Birefringence
is a rotation in the angle of polarization of the tissue. Alterations in collagen microstructure can be detected by transmission polarizing microscopy (TPM), TEM, and highresolution multiphoton microscopy (MPM) using second-harmonic generation.
7.4.6 Tissue Damage at Terahertz Frequencies
7.4.6.1 Arrhenius Integral and Characterizing
Terahertz-Induced Tissue Damage
The Arrhenius integral is a rate process that is commonly used to characterize tissue damage, coagulation, or birefringence loss as a function of temperature and time.