**13. Tissue modification by RF energy**

The thermal effect of RF on tissue is not different from laser or any other heating method. Multiple studies [21, 22] discuss the temperature effect on tissue. Since treatment effect is not only a function of temperature, but also of the period of time (when this temperature is applied), it is known that in the millisecond range the coagulation temperature is 70-90 °C, while if temperature is applied for a few seconds, the temperature of 45 °C causes irreversible damage. Hyperthermia studied intensively for treatment of cancer confirms strong dependance of tissue vitality on time that temperature is maintained [23]. RF induced hyperthermia was measured for adipocytes in a clinical study [24]. The fat cell viability was 89% after RF heating for 1 min at 45 °C while after heating during 3 min the vitality dropped down to 40% (**Table 2**).

There is extensive data on the correlation between tissue temperature, pulse duration and treatment effect. Moritz and Henriques demonstrated that the skin thermal damage threshold is a function of temperature and time [25]. Later it was demonstrated that skin damage function can be described by Arrhenius equation where pre-exponential factor is a linear function of pulse duration [22].

$$D = At\ \exp\left(\frac{-\Delta E}{RT}\right) \tag{17}$$

Pulse duration is one of the most critical parameters when utilizing RF energy in order to achieve a clinical response. It affects treatment results because timing influences the thermo-chemical process in tissue. The other effect of pulse duration is energy dissipation away from the treatment zone due to heat conductivity from the exposed area to the surrounding tissue.

In other words, the degree of damage is a linear function of pulse duration and an exponential factor of tissue temperature. This means that tissue temperature is more influential on the degree of damage than the pulse duration.

It is well known that sustained hyperthermia at 42 °C for tens of minutes causes death of most sensitive cells such as in the brain [26]. In laser medicine the pulse duration in the millisecond range causes tissue to burn at a temperature above 60-70 °C.

Dehydration and carbonization of the ablated treated tissue may cause the accumulation of a non-conductive tissue layer on the electrode surface. This tissue is sometimes called eschar and if it accumulates on the surface of the treatment electrode, it may affect significantly the energy delivery to the electrode and hence


**Table 2.** *Tissue thermal effect.*

**Figure 17.** *Cutting Bovie electrocautery, with an eschar built upon the fine needle tip.*

the treatment zone or even damage the hand piece. Carbonization or Eschar reduces or totally blocks the working area of electrodes and affects treatment efficiency, reducing the electrical current flow to the tissue (**Figure 17**).

Usually, the surgeon must clean an electro-surgical instrument periodically during the treatment to remove any eschar from the treatment electrode. Alternatively, companies, like InMode created a technological solution avoiding this problem. In InMode devices, impedance monitoring measures the increase resistance to flow (increased impendence) caused by eschar on one of the electrodes and cuts off the RF energy and flow of RF current briefly, minimizing the risk of the eschar built up at all.

The most important tissue modification induced by RF heating is a contraction of collagenous tissue. This effect is known for decades and is used intensively in orthopedy [27, 28] and ophthalmology [29].

Skin contraction was a primary focus for the first RF devices in esthetic medicine [9, 15, 17, 19]. Only in the last decade there is the understanding that the skin appearance is more affected by collagen in the reticular dermis and fibro septal network (FSN) binding skin with superficial fascia and muscles. A study published in 2011 [30] showed that skin has very dense collagenous tissue and shrinkage of collagen fibers is limited, while connective tissue in the subdermal space may contract above 30% during a few seconds of heating. The threshold temperature for collagen contraction was measured in the range of 60-70 °C.

In the experiments in our facility, the contraction of FSN was quantified on ex-vivo post abdominoplasty human tissue. The area was marked proximal to the RFAL cannula tip and monitored during RF energy application. The resulting measurements are presented in **Figure 18**.

One can see that thermal exposure of subcutaneous tissue with RF energy during three seconds resulted in area contraction by 42%.

## **14. Radiofrequency assisted lipolysis (RFAL)**

RFAL technology was developed by InMode Ltd. to improve treatment results during liposuction procedure. The thermal contraction of collagen in dermis and subdermal FSN allows treatment of patients with saggy skin and patients for whom previously excessive skin was a main concern [31].

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

#### **Figure 18.**

*Subcutaneous fat before and after application of RF energy.*

The uniqueness of RFAL technology is that it does not fall under any standard device definitions. It combines features of monopolar and bipolar technologies, minimally-invasive and non-invasive technologies, creating very specific energy profile treating simultaneously subcutaneous fat, connective tissue forming FSN and dermis. Each of these tissue components requires different thermal exposure. Adipose tissue should be destroyed, FSN should be remodeled without denaturation of collagen while skin should be exposed to sub-necrotic heat to modify it without superficial burn [31–33].

The RFAL device geometry is shown in **Figure 19**. The RF current flows back and forth from the internal electrode (cannula tip), where the thermal effect is coagulative, to a larger, external electrode. The external electrode moves along the skin surface, in tandem with the internal electrode and creates a gentle, nonablative bulk heating effect on the dermis. Ratio between size of internal and external electrode is selected to limit skin heating at sub-necrotic heating while temperature in the fat should reach 50-70 °C.

Moving the hand piece back and forth through the intended treatment area, uniform coagulation of adipose and vascular tissue is achieved. While the external electrode is always moved over the skin surface, the internal electrode should pass through the deep, intermediate and/or superficial fat layers to treat the adipose

**Figure 19.** *Schematic depiction of RFAL treatment geometry.*
