**4. Radiofrequency in medicine**

The specificity of RF energy in medicine is that it acts as an electrical current flowing through the tissue but differently than radiation. RF energy is associated with electro-surgical devices and can be defined as high frequency alternating electrical current heating soft tissue without significant electrical nerve stimulation. It is critical to minimize nerve impact to avoid electric shock which may cause muscle spasm and cardiac arrest.

It is important to remember that tissue has ion conductivity with the most prominent varieties being Na+, K+, and Cl– (sodium, potassium, and chlorine ions respectively). Nerves are affected as a result of ion penetration through the membrane of neuron. Under normal conditions the nerve is surrounded by electrically neutral liquid where ions with positive and negative charge compensate each other and bound by Coulomb force preventing free diffusion of the electrical charge. As an electrical field is applied the ion starts to move and the nerve stimulating effect depends on ion displacement (D) in alternating electrical field that can be presented as following:

$$D \sim \frac{\mu E}{f} \tag{1}$$

where *μ* is mobility of ions which is proportional to conductivity of tissue *σ*; *E* is electrical field strength; *f* is frequency of electric field.

**Figure 3.**

*Ione displacement for a) low amplitude and high frequency of electric field; b) low amplitude and low frequency; c) high amplitude and low frequency.*

It is obvious the displacement of the ions is higher when electrical field is stronger and it is applied for longer time (**Figure 3**).

In general, polarity of RF voltage is changed so fast that ions vibrate in the same place without significant movement. However, users of RF may occasionally observe small muscle tweaking when high RF parameters are used. Therefore, RF energy used in electrosurgery is limited by lowest frequency of 100 kHz, while the recently developed esthetic devices operate at frequencies above 300 kHz.

The typical range of RF is 100 kHz to 5 MHz according to the FDA guidance [11]. This is intended to exclude other frequencies that may technically fall within the RF portion of the electromagnetic spectrum, but operate in a fundamentally different manner. However, there are few products with higher RF frequency of up to 40 MHz. If RF is higher than 5 MHz there is significant radiative component with reduced capability to predict the distribution in the patient's body and can even potentially affect the treatment attendant.

The ions oscillating in RF field interact with the surrounding tissue, losing its kinetic energy and generating the heat. The heat generated by electrical current in conductive media is described by Joule's law:

$$\mathbf{H} = \sigma \mathbf{E}^2 \tag{2}$$

The heat generated in each point of tissue is proportional to tissue conductivity (*σ*Þ and square of electric field (*E*).

The Ohm's low in vector form allows to calculate the density of RF current ( *j*) in each point of tissue:

$$
\mathbf{j} = \sigma \mathbf{E} \tag{3}
$$

While continuity equation allows to analyze RF current distribution in the tissue

$$\nabla \mathbf{\dot{q}} = \mathbf{0} \tag{4}$$

The Eq. 4 states that electrical current coming into any volume of tissue is equal to the current going out of the same volume (**Figure 4**).

*The Basic Science of Radiofrequency-Based Devices DOI: http://dx.doi.org/10.5772/intechopen.96652*

**Figure 4.** *Schematic illustration of continuity law.*

The other conclusion from the charge continuity equation is that all RF current emanating from one electrode into the tissue flows to the other electrode. The current density on the electrode surface depends on the size of the electrode.
