RF/Microwave Interaction Mechanisms in Biological Materials - RF/Microwave Interaction with Biological Tissues - page 64

2.2.1 Ionization and Nonionization

Radio frequencies and microwaves are nonionizing radiation. To appreciate
the difference between nonionizing and ionizing radiation, one must note that
the energy of EM waves is quantified with the quantum of energy (in joules)
being equal to Planck's constant times frequency [Eqn. (1.50)]. This energy can
also be expressed in electron volts, that is, in multiples of the kinetic energy
acquired by an electron accelerated by a potential difference of 1 V, so that
1 eV 10 -19 J. The energy is proportional to frequency: While the energy

of one quantum of energy is 4.12

@ 1.6 ¥

¥ 10 -7 at 100 MHz, it is 4.12 ¥ 10 -5 at 10 GHz,

41.2 at 10 16 Hz (ionizing UV rays), and 4.12 ¥ 10 5 at 10 20 Hz (penetrating

X-rays) [10].
Ionization can be brought about not only by absorption of EM energy but
also either by collision with foreign (injected) elementary particles of the req-
uisite energy or by sufficiently violent collision among its own atoms. Ioniza-
tion by any outside agent of the complex compounds that make up a living
system leads to profound and often irreversible changes in the operation of
the system. Ionization is due to the possible coupling of appropriate frequen-
cies to vibrational and rotational oscillations. If the incident energy quantum
has sufficient magnitude, it can tear an electron away from one of the con-
stituent atoms. The “ionization potential” is the energy required to remove one
electron from the highest energy orbit. Typical ionization potentials are of the
order of 10 eV. Since chemical binding forces are essentially electrostatic,
ionization implies profound chemical changes.
At RF/microwaves, and even at millimeter waves, the quantum energies are
well below the ionization potential of any known substance. Excitation of
coherent vibrational and rotational modes requires considerably less energy
than ionization and it can occur at RF. Many other possible biological effects
require energies well below the level of ionizing potentials, such as heating,
dielectrophoresis, depolarization of cell membranes, and piezoelectric trans-
duction. In all these cases, one has of course to estimate the rates at which
energy has to be delivered to produce specific effects.

2.2.2 Dielectric Characterization

The dielectric properties of materials have been characterized in Sections 1.6
and 1.7. The characterization, however, was limited to bulk homogeneous
materials. The present section will investigate dielectric properties of tissues
in more detail, in particular for heterogeneous materials. The interested reader
may find more information in a number of topics, in particular in [8-10].
Three relaxation processes are mainly responsible for the dielectric prop-
erties of tissues: dipolar orientation, interfacial polarization, and ionic diffu-
sion. The theories summarized below apply to linear responses to weak fields.
As the field intensity is increased, at some level the response will no longer be
linear. The threshold at which nonlinearity becomes noticeable depends on the