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ELECTRICAL CHARACTERISTICS OF TISSUE

The medium where it acts, or the type of tissue on which the electrical component is applied is an important variable. Each tissue has resistance to the passing of the current (it is a variable of the content in polar molecules –water- of the tissue). Structures rich in water will be affected more intensely by higher radio frequencies.

Delivery is homogenously spread over the whole surface of the electrode, causing volumetric heating of the dermis and adjacent tissue. As well as producing immediate contraction of the collagen and progressive remodelling of the collagen and fibrous septa, it prevents peak heating of the dermis.

Penetrability of the Radiofrequency on the tissues is conditioned by:

• There is greater penetrability with less resistance on passing the electric current: the impendence is inversely proportional to the resistance to the electric pass, accordingly, at higher impedances, greater penetrability.
• According to the Ohm’s Law, the intensity of the electric current that circulates through the conductor materials is directly proportional to the difference of power applied and inversely proportional to it resistance: impendence Z to the movement of the electrons creates heat relative to the amount of current (amps) and the time (seconds).

Radiofrequency can generate up to 144 J/cm2. The adipose tissue has a high impendence and this generates a large amount of heat which favours a deep thermal effect. The greater the tissue’s resistance as the RF passes, the higher temperature generated (fat has a resistance of 2,180 Ohms, 10 times higher than that of the skin).

Electrical resistance has a direct relationship with the availability of very loose electrons (continuous region), in the considered atoms. If they are well balanced and in very deep orbits (close to the nucleus), resistance to the capture can be so great that we could talk about true electric isolation. This property is also related to the temperature, with the vibration of the atoms and with the dimensions of the substance. If the section electron flux decreases, or the distance to cover increases, then resistance also increases. The resistance of the substances can vary from almost zero ohms, known as superconductors, to several million, called insulators. The human body, which is our target has, taking two electrodes in each hand and with dry skin, a resistance equal to 5,000 and 10,000 ohms, but this value drops greatly in moist mouth tissue (100 to 500 ohms) and drastically when it crosses the skin; this should always be taken into account.

Resistance transforms the energy of radiofrequency into heat. The organism has an equivalent resistance of 5,000 to 10,000 ohms trough dry skin and is also heated on passing a current. Although we have spoken of insulators, we should not forget that there are possible energy differences for which a given substance can cease being an insulator. Electrical energy becomes calorific. This conversion is quantitatively equal to the product of the resistance by the square of the intensity (Joule’s Law).

Currents of around 0.1 amps, too small to generate major heating, interfere with essential nervous processes for vital functions such as the heartbeat. Smaller currents, around 0.01 amps, produce convulsive actions in the muscles and great pain. With 0.02 amps, for example, a person could not let go of the conductor and would lead to shock. We can see how large currents, but also some as small as 0.001 amps can cause ventricular fibrillation.

Radiofrequency applied on tissue produces:

• Ion current (Na+, Cl-, K+, Ca+2) with electron liberation
• Ion displacement, rotation of bipolar molecules (water, amino acids, proteins, polysaccharides).
• Heating results from the friction between the molecules in movement

Tissue heating mechanisms are based on the generation of heat joules through electric current:

• H = J2 / σ (J equals the density of electric current and σ is the conductivity); the value opposite to conductivity is known as resistance or impendence.
• H = J2 x R (R equals impendence)
• The tissues with greater resistance to the passing of radiofrequency absorb greater amounts of energy.
• Subcutaneous fat absorbs 10 times more than skin.

Tissue	Resistance
Muscle	110
Heart	132
Skin	289
Fat	2180

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