Abstract:
A method for medical treatment includes bringing an applicator, having an array of protrusions disposed thereon, into proximity with a skin surface of a patient so as to maintain a space between a plurality of the protrusions and the skin surface. Radio-frequency (RF) electrical power is applied to the applicator so as to cause electrical discharges to be generated, in response to the RF electrical power, between the plurality of the protrusions and respective points on the skin surface via a fluid medium in the space.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 60/887,352, filed Jan. 31, 2007, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to apparatus and methods for medical treatment, and specifically to devices for treatment of tissues using radio-frequency (RF) electrical energy. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various techniques and devices are known in the art for medical treatment of skin layers by application of electromagnetic energy, and in particular radio-frequency (RF) energy. The term “RF” is used broadly in the present patent application and in the claims to refer to electromagnetic waves at any frequency in the range between several kilohertz and several hundred gigahertz. 
         [0004]    For example, PCT International Publication WO 2006/077567, whose disclosure is incorporated herein by reference, describes a system and method for heating biological tissue using RF energy. The system includes a RF source, phase shifter, impedance matching network and resonator connected to an applicator, which may be used in selective heating of cellulite bodies. 
         [0005]    Some treatment techniques use a RF plasma discharge to deliver energy to the skin. For example, U.S. Patent Application Publication 2006/0189976, whose disclosure is incorporated herein by reference, describes a system in which a path of current from an electrode to the surface of the biological tissue to be treated is formed, thereby igniting a gas flow and forming a plasma gas-discharge. Electromagnetic interactions between the treated biological tissue and the plasma gas discharge traversing an electromagnetic interaction gap shape the profile of the plasma gas discharge. Other treatment systems using RF plasma generation are described in U.S. Pat. Nos. 6,105,581, 6,518,538, 6,629,974, and 6,723,091, whose disclosures are likewise incorporated herein by reference. 
         [0006]    Some techniques and devices for treatment of skin layers use multi-electrode configurations to apply electrical energy to the skin at multiple points simultaneously. For example, U.S. Pat. No. 6,277,166, whose disclosure is incorporated herein by reference, describes a system and method for shrinking collagen in the dermis using a carrier with an array of electrodes. A microporous pad on the carrier, overlying the electrodes, contacts the patient&#39;s skin and ionically transports applied electromagnetic energy to ohmically heat dermal tissue beneath the epidermis. U.S. Patent Application Publication 2002/0120260, whose disclosure is incorporated herein by reference, likewise describes a tissue surface treatment apparatus in which a tissue contact surface has a plurality of apertures. Electrodes are advanced through the apertures and penetrate the skin in order to ablate tissue using RF energy. 
         [0007]    As another example, U.S. Pat. No. 6,148,232, whose disclosure is incorporated herein by reference, describes a device for ablating the stratum corneum epidermidis of a subject. The device includes a plurality of electrodes, which are applied to the subject&#39;s skin at respective points. A power source applies electrical energy between two or more of the electrodes, in order to cause ablation of the stratum corneum for purposes of transdermal drug delivery or analyte extraction. Similarly, PCT International Publication WO 2005/096980, whose disclosure is incorporated herein by references, uses a device with a plurality of electrodes to create a pattern of perforations in one or more tissue layers. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention that are described hereinbelow provide devices and methods for treatment of biological tissue. In these methods, multiple RF plasma discharges are created in a gap between an applicator and the surface of the tissue, such as the skin surface. The discharges are typically channeling-type discharges, having the form of streamers, arcs or sparks. The discharge parameters may be controlled so that the RF energy in each discharge is delivered to a respective treatment region below the tissue surface, thus creating a pattern of treated regions. This sort of pattern is particularly useful, for example, in ablating sub-surface skin tissue for purposes of cosmetic treatment. 
         [0009]    In some embodiments, the RF discharges are created using a device with an applicator that comprises an applicator body, with a terminal for receiving RF electrical power. An array of protrusions is disposed on the front side of the applicator body. The front side is brought into proximity with the skin, while maintaining a space between at least a majority of the protrusions and the skin surface. As a result, RF electrical discharges are generated between the majority of the protrusions and respective points on the skin via a fluid medium (gas or liquid) in the space between the protrusions and the skin. When a gaseous medium is used, it may comprise either ambient air or another gas that is introduced into the space. The applicator is capable of creating the discharges while operating in a unipolar mode, without a dedicated ground electrode. 
         [0010]    There is therefore provided, in accordance with an embodiment of the present invention, a medical device, including: 
         [0011]    an applicator body, including a terminal for receiving radio-frequency (RF) electrical power; and 
         [0012]    an array of protrusions disposed on a front side of the applicator body and configured to maintain a space between at least a majority of the protrusions and a skin surface of a patient when the applicator body is brought into proximity with the skin surface so as to cause electrical discharges to be generated, in response to the RF electrical power, between the majority of the protrusions and respective points on the skin surface via a fluid medium in the space. 
         [0013]    In one embodiment, the front side of the applicator body is convex. The protrusions are typically pointed. 
         [0014]    Alternatively or additionally, the applicator body includes a spacer for engaging the skin surface so as to maintain the space between the protrusions and the skin surface. The spacer may be configured to seal the space against ambient air, and the device may include a fluid port communicating with the space for at least one of evacuating the space and introducing a fluid into the space. 
         [0015]    Further alternatively or additionally, the array of protrusions is configured to generate the discharges via ambient air. 
         [0016]    There is also provided, in accordance with an embodiment of the present invention, apparatus for medical treatment, including: 
         [0017]    a radio-frequency (RF) power source, which is configured to generate RF electrical power; and 
         [0018]    a skin treatment device, which is coupled to receive the RF electrical power and includes an applicator, which includes an applicator body and an array of protrusions disposed on a front side of the applicator body, 
         [0019]    wherein the skin treatment device is configured to maintain a space between at least a majority of the protrusions and a skin surface of a patient when the applicator is brought into proximity with the skin surface, so as to cause electrical discharges to be generated, in response to the RF electrical power, between the majority of the protrusions and respective points on the skin surface via a fluid medium in the space. 
         [0020]    In some embodiments, the skin treatment device is configured to be held by an operator and moved by the operator over the skin surface, and wherein the apparatus includes a cable, which connects the RF power source to the skin treatment device. Typically, the RF power source is configured to generate the RF electrical power at a predetermined frequency, and the cable has a length equal to an integer number of half-waves at the predetermined frequency. The skin treatment device may include a resonant circuit, which is tuned to the predetermined frequency. 
         [0021]    In a disclosed embodiment, the RF power source has a predetermined impedance, which is different from a discharge impedance of the electrical discharges between the protrusions and the respective points on the skin, and the apparatus includes an impedance matching circuit, for matching the predetermined impedance to the discharge impedance. Additionally or alternatively, the RF power source includes a RF generator, which is configured to generate the RF electrical power at a constant amplitude, and a pulse-width modulation (PWM) controller, which is coupled to switch the RF generator with a variable duty cycle so as to adjust a power level of the RF electrical power that is supplied to the skin treatment device. Further additionally or alternatively, the RF power source includes a phase controller, which is operative to adjust a phase of the RF electrical power at the front side of the applicator. 
         [0022]    In one embodiment, the RF power source and the skin treatment device are configured to generate the electrical discharges while operating in a unipolar mode. 
         [0023]    There is additionally provided, in accordance with an embodiment of the present invention, a method for medical treatment, including: 
         [0024]    bringing an applicator, having an array of protrusions disposed thereon, into proximity with a skin surface of a patient so as to maintain a space between a plurality of the protrusions and the skin surface; and 
         [0025]    applying radio-frequency (RF) electrical power to the applicator so as to cause electrical discharges to be generated, in response to the RF electrical power, between the plurality of the protrusions and respective points on the skin surface via a fluid medium in the space. 
         [0026]    In one embodiment, the method includes sealing the space against ambient air, and performing at least one of evacuating the space and introducing a fluid into the space, wherein introducing the fluid may include filling the space with a gas selected from a group of gases consisting of inert gases and reactive gases. Alternatively, applying the RF electrical power includes generating the electrical discharges via ambient air. 
         [0027]    Typically, applying the RF electrical power includes creating multiple perforations in skin tissue of the patient at the respective points. Creating the multiple perforations may cause a tightening of the skin tissue upon healing of the perforations. 
         [0028]    There is further provided, in accordance with an embodiment of the present invention, a medical device, including: 
         [0029]    an applicator body, including a terminal for receiving radio-frequency (RF) electrical power; 
         [0030]    a roller, which has an axis coupled to the applicator body and has a radial surface for contacting a skin surface of a patient, and which is configured to rotate about the axis as the roller is advanced over the skin surface; and 
         [0031]    an array of protrusions disposed on the radial surface of the roller and configured to cause multiple electrical discharges to be generated, in response to the RF electrical power, in a space between the protrusions and respective points on the skin as the roller is advanced over the skin surface. 
         [0032]    In disclosed embodiments, the roller has a shape that is selected from a group of shapes consisting of cylindrical, spherical, ellipsoidal and barrel shapes. 
         [0033]    There is moreover provided, in accordance with an embodiment of the present invention, apparatus for medical treatment, including: 
         [0034]    a radio-frequency (RF) power source, which is configured to generate RF electrical power; and 
         [0035]    a skin treatment device, which is coupled to receive the RF electrical power and which includes:
       a roller, which has an axis and has a radial surface for contacting a skin surface of a patient, and which is configured to rotate about the axis as the roller is advanced over the skin surface; and   an array of protrusions disposed on the radial surface of the roller and configured to cause multiple electrical discharges to be generated, in response to the RF electrical power, in a space between the protrusions and respective points on the skin as the roller is advanced over the skin surface.       
 
         [0038]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a schematic, pictorial illustration of a system for skin treatment, in accordance with an embodiment of the present invention; 
           [0040]      FIG. 2  is a schematic cutaway view of a device for skin treatment, in accordance with an embodiment of the present invention; 
           [0041]      FIGS. 3A and 3B  are schematic, pictorial and sectional views, respectively, of an applicator for use in the device of  FIG. 2 , in accordance with an embodiment of the present invention; 
           [0042]      FIG. 4A  is a schematic, sectional view of an applicator used in treating an area of skin, in accordance with an embodiment of the present invention; 
           [0043]      FIG. 4B  is a schematic, pictorial view of an applicator used in treating an area of skin, in accordance with another embodiment of the present invention; 
           [0044]      FIG. 5  is a block diagram that schematically shows electrical components of a system for skin treatment, in accordance with an embodiment of the present invention; 
           [0045]      FIG. 6  is a schematic plot showing a pattern of application of RF energy over time by a system for skin treatment, in accordance with an embodiment of the present invention; and 
           [0046]      FIG. 7  is an electrical schematic diagram of an impedance matching circuit, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0047]      FIG. 1  is a schematic, pictorial illustration of a system  20  for skin treatment, in accordance with an embodiment of the present invention. An operator  30 , such as a physician, brings an applicator  24  of a skin treatment device  22  into proximity with the skin of a patient  32 . A control console  26  supplies RF electrical energy to device  22  via a cable  28 . The RF energy causes electrical plasma gas discharges of the channeling type to be created in the space between the front side of the applicator and the patient&#39;s skin. The discharges convey the energy to the skin surface (stratum corneum) and, particularly, to sub-surface layers of the skin, particularly the epidermis and dermis. Details of the design and operation of device  22  and applicator  24  are presented hereinbelow with reference to the figures that follow. 
         [0048]    Application of RF energy to body tissues in system  20  is useful in a variety of medical applications. For example, device  22  may be used to ablate local regions of sub-skin tissue (dermis) for cosmetic applications, such as skin tightening and resurfacing, including treatment of fine lines, wrinkles, and acne scars. Alternatively or additionally, ablation of the outer skin layer by device  22  may be performed in conjunction with application of drug preparations to the skin for enhancing drug delivery. Further alternatively, although system  20  and the embodiments described herein are configured for skin treatment, device  22  and applicator  24  may be adapted for plasma-based treatment of the surface layers of other biological tissues. 
         [0049]      FIG. 2  is a schematic, cutaway view of device  22 , in accordance with an embodiment of the present invention. The RF energy carried by cable  28  is fed to RF delivery circuitry  40  in device  22  when the operator presses a trigger  42 . Circuitry  40  typically comprises a resonant circuit, tuned to the frequency of the RF energy. Details of these circuits are shown in  FIG. 5  and described hereinbelow with reference thereto. 
         [0050]    Careful design of circuitry  40  for the frequency and impedance characteristics of the discharges enhances the efficiency of delivery of the RF energy to the patient&#39;s skin and reduces heat dissipation. Nevertheless, to keep circuitry  40  and device  22  at a stable temperature, the device may comprise a cooler  44 , which removes heat by flow of a coolant through suitable tubing. The cooler may be connected to an external pump and heat exchanger, which are controlled by a suitable temperature sensor in device  22  (not shown in the figures). The inventors have found that a water cooling system, with a water temperature of about 10-15° C., gives good results. Alternatively, a thermo-electrical cooler (TEC) may be used for this purpose. Temperature feedback may be provided to the cooling system by a thermistor attached to the ground plate of device  22 . 
         [0051]      FIGS. 3A and 3B  are schematic, pictorial and sectional views, respectively, of applicator  24 , in accordance with an embodiment of the present invention. Typically, device  22  may be used with a variety of different, interchangeable applicators, having different geometrical, electrical and functional characteristics. A number of alternative applicator designs are described hereinbelow. 
         [0052]    Applicator  24  comprises a body  50 , having a terminal  52  that connects to circuitry  40  ( FIG. 2 ) in order to receive RF energy. An array of protrusions  56  is formed on a front side  54  of the applicator, i.e., the side adjacent to the patient&#39;s skin when device  22  is in use. Typically, the protrusions are conical or pyramidal, as shown in  FIGS. 3A and 3B , with pointed ends. The size of front side  54  and the density of protrusions  56  may be varied depending on the patient&#39;s skin type and treatment requirements. In typical dermatological applications, for example, front side  54  may have a diameter between 4 and 20 mm, while protrusions  56  are spaced between 0.5 and 2.0 mm apart. These dimensions, however, are given solely by way of example, and larger or smaller dimensions may alternatively be used. 
         [0053]    The points at the tips of protrusions  56  concentrate the electric field and thus facilitate ignition of the electrical discharges between the protrusions and the patient&#39;s skin. For this purpose, the protrusions may alternatively have the form of pins or wires, such as those as described in the above-mentioned PCT International Publication WO 2005/096980 or other publications cited in the Background section above. Further alternatively, the protrusions may have other shapes, such as round or rectangular shapes. Although applicator  24  is pictured in  FIGS. 3A and 3B  as a unitary component, the applicator may alternatively comprise internal circuitry, such as capacitors and/or resistors (not shown in the figures), arranged to cause the protrusions to function as separate electrodes, and thus to assist in sustaining multiple separate discharges, in a manner similar to types of multi-sectional electrodes that are used in some gas-discharge fast flow lasers. 
         [0054]    As can be seen in  FIGS. 3A and 3B , front side  54  is convex, with a radius of curvature that is typically in the range of 20-30 mm. As a result, when operator  30  brings applicator  24  into light contact with the patient&#39;s skin (without exerting pressure against the skin), protrusions  56  will contact the skin only over a small area of front side  54 . Thus, the majority of the protrusions will be spaced away from the skin by a small distance and will not be in electrical contact with the skin. Alternatively, the operator may hold applicator above the skin, so that none of the protrusions contact the skin. When RF energy is applied to the applicator, the protrusions that are not in electrical contact with the skin will ignite arc or spark discharges into the skin through the fluid (gas or liquid) medium between the protrusions and the skin. 
         [0055]    Typically, body  50  and protrusions  56  are made from a suitable biocompatible metal, such as stainless steel. To enhance the discharge qualities, the protrusions may be coated with an additional conductive layer and/or with a non-conductive layer, such as a ceramic material or a polymer or glass. Optionally, applicator  24  may be pre-heated to facilitate ignition of the discharges. 
         [0056]      FIG. 4A  is a schematic sectional view of an applicator  60 , which may be used in device  22  in place of applicator  24 , in accordance with another embodiment of the present invention. The figure shows applicator  60  in contact with a surface  64  of skin  70  of patient  32 . The applicator comprises protrusions  56  on the front side of applicator body  50 , as in applicator  24 . In this embodiment, however, applicator  60  comprises a spacer  62  in order to maintain the desired spacing between protrusions  56  and skin surface  64 . The spacer in this embodiment encloses the entire array of protrusions  56 , and comprises a seal  66 , such as an O-ring, which inhibits the flow of air between a space  76  enclosed by the spacer and the ambient environment. Alternatively, the spacer may be configured solely for the purpose of maintaining the desired distance between the protrusions and the skin surface, while allowing air flow through the space between the protrusions and skin surface. In either case, spacer  62  is configured to maintain a certain distance between the skin surface and the tips of protrusions  56 . Typically, this distance is between about 0.1 mm and 5 mm. 
         [0057]    Optionally, space  76  between protrusions  56  and skin surface  64  may be filled with a selected fluid (gas or liquid) to a desired pressure via a fluid port  74 . For example, the space may be filled with an inert gas that has suitable discharge-sustaining properties, such as a rare gas (such as He, Ar, Ne, Kr or Xe) or nitrogen. The gas may be filled to a pressure equal to or above atmospheric pressure, or the space may alternatively be evacuated and held at a sub-atmospheric pressure. Alternatively or additionally, a reactive gas, such as oxygen, water vapor, oxides of nitrogen or carbon, or volatile organic or inorganic gases, may be injected into space  76  for treating skin  70 . Regardless of the gas composition, the gas may be stationary or may flow through space  76  during the discharge. Further alternatively, space  76  may be filled with a liquid, particularly an electrolyte, such as saline solution. 
         [0058]    When the RF energy is applied to applicator  60  (or similarly, to the other types of applicators that are described herein), a spark is generated at each of protrusions  56 , breaking down the gas in space  76  and generating a plasma discharge  68  that extends into skin  70 . The discharge ablates the skin, creating a perforation  72  that extends into the sub-surface layers of the skin (typically the epidermis and dermis). For effective, well-regulated ablation, it is desirable that most of the energy transferred by the applicator to the skin be conveyed by the discharge, rather than by thermal or conductive energy transfer (as would occur if protrusions  56  were in contact with the skin surface). 
         [0059]    The depth and shape of the perforations (for example, rounded or flat bottom) may be controlled by appropriate selection of the RF discharge parameters, such as amplitude, power, frequency, phase and duration, as well as the distance of protrusions  56  from the skin. A pulse-width modulation (PWM) scheme may be used to control the delivery of RF power to the skin, as described below with reference to  FIG. 6 . In typical cosmetic treatment applications, the perforations are made about 100-150 μm deep and 80-120 μm in diameter, but the inventors have found that clean, well-controlled perforations may be created in the manner described above to a depth of at least 300 μm. Alternatively, the perforations may be made wider and shallower. 
         [0060]    In operation of system  20 , operator  30  typically moves device  22  so that the applicator scans over an area of the skin that is to be treated. At each location in the area, the operator actuates the device for a short time, typically on the order of 100-200 ms at a RF power level between 30 and 80 W. Alternatively, longer exposure times at lower power may be used, and vice versa. Depending on the skin type and condition being treated, the operator may make only a single pass or several passes over the area. 
         [0061]      FIG. 4B  is a schematic, pictorial illustration of an applicator  73 , in accordance with another embodiment of the present invention. Applicator  73  comprises a roller  78 , which has multiple protrusions  56  on its radial surface. The roller may be cylindrical, as shown in  FIG. 4B , or may have any other suitable shape, such as a spherical, ellipsoidal or barrel shape. Roller  78  is mounted to rotate on an axis  79 , so as to roll over the area under treatment. Typically, the roller is advanced over the skin at a speed between 1 and 10 mm/sec. As the roller advances, protrusions  56  alternately contact the skin and move away from the skin. When a given protrusion is near the skin, but not in direct contact, a discharge is ignited between that protrusion and the skin. 
         [0062]      FIG. 5  is a block diagram that schematically shows electrical components of system  20 , in accordance with an embodiment of the present invention. Console  26  comprises a RF generator  82 , which generates RF energy at a desired frequency. For skin treatment, the inventors have found that RF energy at about 40 MHz (typically at 40.68 MHz) gives good results. Alternatively, the components of system  20  may be adapted to operate at higher or lower frequencies, from the kilohertz to the gigahertz range. RF generator  82  typically comprises an oscillator and amplifier, which may be of any suitable type, such as a cascade amplifier with a Pierce-type oscillator, as described, for example, in U.S. Pat. No. 4,626,767, whose disclosure is incorporated herein by reference. To give a pure sinusoidal output, the final stage of the RF generator may comprise a matched pair of transistors in a push/pull configuration. 
         [0063]    In one embodiment, RF generator  82  outputs approximately 300-400 W into a 50Ω resistive load, with minimal reflected RF power. To maintain good impedance matching, the amplitude of the RF wave output by the RF generator should be held roughly constant. As noted above, however, it is often necessary to adjust the amount of RF power this is delivered to the patient&#39;s skin to a considerably lower level. For this purpose, the RF generator is switched by a pulse-width modulation (PWM) controller  80 , and the average power output of the RF generator is thus controlled by modulating the duty cycle while maintaining constant RF amplitude during the RF pulses. 
         [0064]      FIG. 6  is a schematic plot showing a pattern  90  of output of RF energy over time by RF generator  82 , in accordance with an embodiment of the present invention. PWM controller  80  modulates the RF generator on and off with rectangular pulses at a frequency that is considerable less than the RF frequency, thus generating a sequence of RF pulses  92 . The PWM controller may be used to generate substantially any desired duty cycle (and thus any desired average power output from the RF generator, up to the full output power at 100% duty cycle). The inventors have found that operating PWM controller  80  at a modulation frequency between 10 and 100 Hz, with duty cycle between 5% and 30%, gives good results in typical skin treatment applications. For example, the PWM controller may be operated at a frequency of 20-25 Hz, and the PWM duty cycle controlled so that average RF power between about 30 and 70 W is applied to the skin. Power settings between 45 and 55 W, with application of the RF energy for about 0.1 sec at each location that is treated, have been found to give good results in cosmetic skin treatment. 
         [0065]    Returning now to  FIG. 5 , console  26  optionally comprises a phase controller  84 , for applying a variable phase shift to the output RF energy. The phase controller may comprise, for example, a trombone line, which is a transmission line that may be mechanically lengthened and shortened to vary the output phase. The phase controller may be used to determine the phase of the RF wave at the output from applicator  24 , and thus to vary the depth beneath the patient&#39;s skin at which the RF electric field is maximized. As a result, it may be possible to selectively ablate, denature or coagulate tissue in the collagen at multiple points beneath the skin surface, while minimizing damage to the outer skin layer. Subsequent healing in the collagen tightens and smoothens the skin. 
         [0066]    Alternatively or additionally, the length of cable  28  may be calibrated to give a known, fixed relation between the phase of the RF wave at the output from RF generator  82  and the phase of the wave received over the cable at device  22 . For example, the cable may be cut to a length that is equal to an integer number of half-waves at the RF frequency, i.e., L=nλ/2, wherein L is the length of the cable, λ is the RF wavelength, and n is an integer. 
         [0067]    Console  26  outputs the RF energy to cable  28  via an impedance matching circuit  86 . As noted earlier, the characteristic impedance of applicator  24  (together with resonant circuit  88  and the body tissue adjacent to the applicator) is typically in the range of 260-320Ω. Circuit  86  is thus designed to match the 50Ω output impedance of the RF generator to a 300Ω load impedance, so that the RF energy is delivered efficiently to the patient&#39;s skin with minimal reflection. 
         [0068]      FIG. 7  is an electrical schematic diagram of impedance matching circuit  86 , in accordance with an embodiment of the present invention. Circuit  86  in this example comprises a network  104  of capacitors (C) and a variable inductor (L) arranged between coaxial input and output terminals  100  and  102 . To match the 50Ω input impedance to the 300Ω load impedance at the 40.68 MHz operating frequency of system  20 , C=33 pF for all four capacitors, and L=400 nH. Alternatively, other network types and component values may be used depending on application requirements. 
         [0069]    When cable  28  is cut to a length that is equal to an integer number of half-waves (L=nλ/2, as explained above), the cable is “impedance-transparent,” and the output of impedance matching circuit  86  is effectively coupled directly to device  22 , as though there were no intervening cable. The real impedance (active resistance) of the cable at 40.68 MHz is typically about 2 kΩ, which is significantly higher than the impedance of the skin tissue (260-320Ω). The cable thus serves as a ballast resistor, dissipating 5-8% of the energy delivered from the RF generator. 
         [0070]    Returning to  FIG. 5 , the RF energy from cable  28  is conveyed to applicator  24  via a parallel resonant circuit  88 , which is tuned to the frequency of RF generator  82 . The resonant circuit comprises a variable inductor, with typical inductance of 2200 nH, coupled with the parasitic capacitance of the structure of device  22 , which is roughly 7 pF. The real impedance of resonant circuit  88  is typically in the range of 5-15 kΩ. The capacitance (and hence the resonant frequency of circuit  88 ) varies with temperature. Cooling of the circuits in device  22 , as described above, is thus useful in preventing changes in the resonant frequency during use of the device. 
         [0071]    As noted earlier, the therapeutic effects of system  20  depend on the characteristics of the discharge created by applicator  24 , which in turn depend on parameters such as the RF power, phase, exposure duration and distance of the applicator from the skin. These parameters may be controlled directly by operator  30 . Alternatively or additionally, console  26  may comprise a computerized controller (not shown), which sets the parameters automatically based on treatment information provided by the operator. Optionally, the computerized controller may receive feedback from the treatment site, such as a laser measurement of the depth of the perforations created in the patient&#39;s skin, and may adjust the treatment parameters accordingly. 
         [0072]    Optionally, the multi-point electrical discharge generated by device  22  may be used in conjunction with other treatment modalities. For example, the discharge-based methods described above may be used in conjunction with optical and/or ultrasonic therapies for enhanced therapeutic results. 
         [0073]    It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.