FIELD OF THE INVENTION
This invention relates to methods and systems for treating skin.
BACKGROUND OF THE INVENTION
The term “target” is used herein to denote a skin defect such as a vascular lesion, pigmented lesion, acne, unwanted hair or wrinkle. Selective thermal treatment of skin is commonly used in aesthetic medicine to remove skin targets. In order to be destroyed, the target must be raised to a temperature of about 70° C. without raising the temperature of the surrounding epidermis or dermis to damaging levels. The most popular method of thermal skin treatment is selective photo-thermolysis in which light energy produced by a laser or flash lamp is selectively absorbed by a pigmented portion of the target. However, with this method it is often not possible to heat the entire target to a temperature necessary for destroying it without heating the surrounding skin to damaging levels. The main problem is that the optical contrast between the target and the surrounding skin tissue is not high enough to obtain a significant difference in temperature between the target and the surrounding skin tissue.
U.S. Pat. No. 5,755,753 disclose use of the radio-frequency (RF) range of electro-magnetic energy for skin tightening, where RF energy is applied to a pre-cooled skin surface. U.S. Pat. No. 5,846,252 discloses treating hairs to reduce their electrical resistance and then applying RF current.
SUMMARY OF THE INVENTION
The present invention is based upon the finding that selective heating of a skin target by RF energy is enhanced if prior to the application of the RF energy the skin is treated to make the temperature of the target (Tt) higher than the temperature of the surrounding skin tissue (Ts). The initial temperature gradient (Tt−Ts>0) between tie target and surrounding tissue may be achieved either by preheating the target or pre-cooling the surrounding tissue.
The invention thus provides a system for treating a skin target comprising:
(a) one or more RF electrodes configured to be attached to the skin, so as to apply an RF current to the skin;
(b) a temperature effector configured to create a temperature gradient between the target and skin surrounding the target such that the target is at a higher temperature than the surrounding skin.
The invention still further provides a method for treating a skin target comprising:
a) creating a temperature gradient between the target and skin surrounding the target such that he target is at a higher temperature than the surrounding skin; and
b) applying RF energy to the skin.
The system and method of the invention may be used for such skin targets as a vascular lesion, pigmented lesion, hair follicle, wrinkle and acne.
While not wishing to be bound by a particular theory, it is believed that selective thermolysis of a target by RF energy is enhanced when Tt−Ts>0 due to an increase in the electrical conductivity in the RF range of tissues when thee tissue temperature is increased [Frances A. Duck, Physical Properties of Tissue, a Comprehensive Reference Book, Academic Press, 1990, p.173]. Accordingly, the dependence of the conductivity a of a tissue on temperature T is given by:
σ=σ0(1+α(T−T 0)) (1)
where σ0 is the conductivity at the reference temperature T0 and α is a constant known as the temperature coefficient.
Heat generation by RF current can be estimated by Joule's Law:
H=σE 2 (2)
and the change in temperature in the tissue is obtained using the heat conductivity equation:
where c is the heat capacity of the tissue, ρ is the mass density and E is the intensity of the electric field.
Inserting Equations 1 and 2 into (3),
and integrating Equation 4, the result is
where Ti is the initial temperature of the tissue before the application of RF energy, t is the duration of the application of RF energy, and T′ is the formal temperate of tie tissue at the end of the application of RF energy.
If the initial temperatures of the target and sounding skin tissue are Tt
>0), then Equation 6 becomes for the target:
and for the surrounding skin,
subtracting Equation (8) from Equation (7) yields
T′ t −T′ s=(T ti −T si)e At (9)
where Tti−Tsi is the initial temperate gradient between the target and the surrounding skin, and T′t−T′s is the final temperature gradient. Equation (9) shows that as the RF current is applied, the temperature gradient increases exponentially. Therefore, by creating an initial relatively small temperature gradient Tti−Tsi>0, and applying RF energy, a larger temperature gradient is obtained. This allows heating of the target to a sufficiently high temperature to destroy the target without heating the surrounding skin tissues to damaging levels.
Assuming a typical RF fluence (F) in the skin of 20 J/cm2
, α=0.03 (C°)−1
and a heat capacitance cρ=3.6 J/cm3
°K the factor eAt
in Equation (9) is
Thus, the temperature gradient increases by a factor of about 2.3 during the application of the RF energy.
Referring now to FIGS. 4 and 5, a system for creating a temperature gradient between a skin target and the surrounding ski, in accordance with another embodiment of the invention is shown. An applicator 803, to be described in detail below, contains a pair of RF electrodes 401 and 402. The applicator 803 is adapted to be applied to the skin of an individual 805 in the region of a target. The control unit 801 includes a power source 808. The power source 808 is connected to an RF generator 815 that is connected to the RF electrodes in the applicator 803 via wires in the cable 802. The control unit 801 controls a refrigeration unit 812 that cools a fluid such as ethanol or water for cooling the applicator 803. The cooled fluid flows 10 from the refrigeration unit 812 to the applicator via a first tube in the cable 802, and flows from the applicator 803 back to the refrigeration unit via a second tube in the cable 802. The control unit 801 has an input dice such as a keypad 810 that allows an operator to input selected values of parameters of the treatment, such as the frequency, pulse duration and intensity of the RF energy or the temperature of the coating fluid. The control unit 801 optionally contains a processor 809 for monitoring and controlling various functions of the device. For example, the processor 809 may monitor the electrical impedance between the electrodes in the applicator 803, and determine the temperature distribution in the vicinity of the target. The processor 809 may also determine the parameters of the treatment based upon the impedance measurements.