**10. Bipolar RF systems**

According to FDA [11], bipolar is an electrosurgical device in which the current flows between two active electrodes placed in close proximity. In bipolar devices both electrodes create a similar thermal effect and are applied to the tissue treatment area (**Figure 14**). Bipolar devices create larger thermal zones and this circuit is used in electro-coagulators. The advantage of bipolar systems is the localization of all RF energy in the treatment zone (**Figure 14**).

Bipolar devices concentrate all RF energy between electrodes in the treatment area. This geometry is more suitable than a monopolar system to create uniform heating in larger volume of tissue. In order to understand heat distribution between electrodes the following three rules should be taken into the account:


A schematic distribution of electrical currents in uniform media in bipolar device is shown in **Figure 15**.

In bipolar devices, both electrodes create an equal thermal effect near each of the electrodes and the divergence of RF current is not as strong because of the small

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

**Figure 15.** *Electrical current distribution for bipolar system.*

#### **Figure 16.**

*Thermal image of heat distribution created in the skin folded between two parallel electrodes.*

distance between the electrodes. For bipolar systems shown in **Figure 15**, most of the heat is concentrated between the electrodes.

Penetration depth of RF for bipolar devices is a function of electrode size and the distance between them. By increasing the distance between the electrodes, the electrical current can go deeper, but divergence is also increased. In case the distance between the electrodes is much larger than the electrode size, the heating profile will be similar to two monopolar electrodes. Schematically, bipolar current distribution and measured thermal effect are presented in **Figures 5b** and **6b**, respectively.

The most uniform distribution of RF current is obtained in planar geometry when tissue is placed between two large parallel electrodes. This can be realized when negative pressure forces the tissue to fill the cavity between the parallel electrodes. Measured RF energy distribution for the cavity filled with the tissue is shown in **Figure 16**.

#### **11. Capacitive coupling of RF energy**

High frequency current is able to penetrate through the dielectric material which behaves as capacitor. This effect is used to isolate metal electrode from patient. This method is called capacitive coupling. There are a number of devices in the medical esthetic market that use this technology for RF delivery [18, 19].

The capacitance of planar dielectric layer is described by the following equation:

$$\mathbf{C} = \frac{\mathbf{e} \mathbf{e}\_0 \mathbf{S}}{\mathbf{L}} \tag{12}$$

Where *ε* is dielectric constant of dielectric material, *ε*<sup>0</sup> is the vacuum permittivity, *S* is area of dielectric and L is thickness of the layer.

Impedance of the dielectric layer depends on frequency of current (f)

$$R = \frac{1}{2\pi fC} \tag{13}$$

For example, polyimide layer with area of 4cm2 and thickness of 100micron has capacitance of about 106 pF and impedance of this layer is 1.5 kOhm at 1 MHz and 375 Ohm at 4 MHz.

For cylindrical geometry capacitance is represented by the following equation

$$\mathbf{C} = \frac{2\pi\varepsilon\varepsilon\_0 L}{\ln\left(\frac{b}{a}\right)}\tag{14}$$

Where *a* is inner diameter and *b* is outer diameter of dielectric coating.

The leakage of RF current through the dielectric coating should be taken into the account at design of electro-surgical instruments.

#### **12. Thermal relaxation time**

The temperature dissipation is characterized by thermal relaxation time (TRT) of the targeted area. For localized treatment, in order to avoid significant heat transfer, the pulse duration should be less than the TRT.

The TRT is a function of tissue thermal properties, heated volume shape and size. Soft tissue has thermal properties close to the water.

For the planar object the TRT can be estimated as [20].

$$TRT = \frac{d^2}{4\pi} \tag{15}$$

Where *d* is thickness of layer, and *a* is tissue diffusivity. Diffusivity is equal to tissue conductivity divided by the heat capacitance and measured in cm2 s �1 .

For a cylindrical object, such as a blood vessel or hair, a similar equation can be used with different geometrical factors.

$$TRT = \frac{d^2}{16\,\mu} \tag{16}$$

where *d* is object diameter; one can see that cooling time is square function of the size.

Thermal relaxation time should be taken in to the account when thermal effect should be localized. It is critical in fractional RF technologies when thermal coagulation should by limited by small zones around the needle electrodes.
