Abstract:
Electrosurgical therapy is provided with an electrode array configured to ablate tissue during insertion of the electrode array into tissue being treated. Once the electrode array is fully inserted, deep heating of the treated tissue can be performed by applying an additional waveform to the tissue with the electrode array. Optionally, the electrical waveform can be varied continuously during insertion of the electrode array to control the extent of coagulation at the side walls and at the bottom of the channels produced by tissue ablation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. application Ser. No. 12/657,949, entitled “Electro-thermotherapy of tissue using penetrating microelectrode array”, filed Jan. 29, 2010, and hereby incorporated by reference in its entirety. Application Ser. No. 12/657,949 claims the benefit of U.S. provisional application 61/206,522, entitled “Electro-thermotherapy of tissue using penetrating microelectrode array”, filed Feb. 2, 2009, and hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention is in the general field of electrosurgery. One application is to electrosurgery for cosmetic procedures on skin. 
       BACKGROUND 
       [0003]    Various approaches have been considered for skin rejuvenation and other tissue/skin treatments based on delivery of energy to the tissue being treated. In non-ablative approaches, energy is delivered to tissue, but no tissue is thereby removed. In ablative approaches, energy is delivered to tissue such that some of the tissue is removed by ablation. 
         [0004]    Non-ablative skin rejuvenation (e.g., using radiofrequency, ultrasound, or light) is typically performed using application of the electrical current, ultrasound energy or light beam to the tissue surface, and heating tissue to temperatures not exceeding the vaporization threshold. Optical examples of this approach include U.S. Pat. No. 6,723,090. Electrical examples of this approach include U.S. Pat. Nos. 5,871,524; 6,662,054. 
         [0005]    Carbon dioxide (CO2) laser systems have been recently applied to ablative fractional resurfacing of human skin. In these procedures, a pulsed CO2 laser is used to drill channels of approximately 100 micrometers in diameter and 0.5-0.7 mm in depth. Using scanning mirror, these holes are applied in patterns with spacing of approximately 1 mm. The epidermis and part of the dermis demonstrate columns of thermal coagulation that surround tapering ablative zones lined by a thin eschar layer. Typically, a thermal coagulation zone at the edges of such laser channels in skin is on the order of 40 micrometers. Such ablation (tissue removal) and coagulation of skin leads to stimulation of its rejuvenation and tightening, and thus results in improved cosmetic appearance. This approach, called “fractional skin resurfacing” was found to be clinically very effective. An example of this approach is considered by Hantash et al. in an article titled “In vivo Histological Evaluation of a Novel Ablative Fractional Resurfacing Device” (Lasers in Surgery and Medicine 39:96-107 (2007)). 
         [0006]    Disadvantages of the CO2 laser systems include their relatively large size, somewhat cumbersome articulated arm beam delivery system, and relatively high cost. In addition, since ablation craters produced by lasers taper towards the bottom, there is a limit on the ratio of depth-to-width of the channels (so called aspect ratio) that can be produced by laser ablation. Typically this aspect ratio does not exceed 10, i.e. channels of 100 μm in diameter do not exceed 1 mm in depth. Another limitation of the laser-based tissue drilling approach is that the thermal damage zone at the side walls of the channels is typically similar or even larger than that at the bottom. 
         [0007]    Another non-ablative approach that has been considered for such skin treatment is the use of an array of needle electrodes that is first inserted into the skin, and then energized to provide therapeutic effects. Examples of this approach include US 2007/0142885 and US 2008/0091182. However, insertion of the needles and following tissue coagulation in this approach does not involve tissue removal by ablation (vaporization and ejection forming the empty channels or craters), and thus is not as effective in skin tightening as the ablative laser approach. 
         [0008]    It would be desirable to be able to ablate tissue and create channels of arbitrary aspect ratio and with an independent control over the width of the thermal damage zone at the side walls and at the bottom. 
       SUMMARY 
       [0009]    Electrosurgical therapy is provided with an electrode array configured to ablate tissue during insertion of the electrode array into tissue being treated, and to form coagulation zone at the edges of the channels. Once the electrode array is fully inserted, deep heating of the treated tissue can be performed by applying an additional waveform to the tissue with the electrode array. Optionally, the electrical waveform can be varied continuously during insertion of the electrode array for balancing both functions—ablation and coagulation at various depths of the channels. 
         [0010]    Compared to laser treatment approaches and mechanical needle insertion treatment approaches, the present approach provides more flexible control of therapeutic parameters, such as channel aspect ratio, and the side wall to bottom energy dose ratio. 
         [0011]    For example, laser based drilling approaches tend to provide a side wall thermal damage zone that is similar to or larger than that at the bottom. However, the opposite dosage pattern (i.e., a larger coagulation zone at the bottom of the channels than on the sides) often provides more benefits for skin tightening than coagulation at the top of the channels. Reduced damage at the skin surface also helps to accelerate healing. With the present approach, this desirable dosage pattern (i.e., larger dose at channel bottoms than at channel sides) can be delivered. 
         [0012]    Conventional needle electrodes that are mechanically inserted do not form empty channels in the tissue being treated. Healing response is improved by the presence of empty channels, so the ability of the present approach to provide such empty channels by ablation is a significant advantage relative to conventional needle electrode approaches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1   a - e  show an exemplary tissue treatment sequence making use of an embodiment of the invention. 
           [0014]      FIG. 2  shows an alternate embodiment of the invention. 
           [0015]      FIG. 3  shows a top view of an arrangement of active and return electrode suitable for use in embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the present approach, channels are produced in skin (or any other kind of tissue) electrosurgically. More specifically, electrosurgical electrodes having a needle-like configuration can be employed to form channels in skin by electrically ablating tissue as the electrodes are inserted into the skin. Here and throughout this application, the term “ablation” refers to vaporization and removal of tissue. This electrosurgical system can produce patterns of channels in skin with predetermined depth and spacing. These channels are regions where tissue has been removed by ablation. The system can also adjust the extent of thermal coagulation in the surrounding tissue and at the edges of the channels. In addition, a coagulating waveform can be applied after partial or full insertion of the electrodes into the tissue in order to provide thermal treatment at the depth of the tissue, while sparing its surface. 
         [0017]    An exemplary system is shown on  FIG. 1   a  and includes the following components:
   1) Power supply  108  delivering electrosurgical waveforms.   2) Array of microelectrodes  104  with electrode diameter preferably in the range of 25-250 micrometers, and electrode length preferably in the range of 0.1-2 mm.   3) Compressible or deflectable return electrode pad  112 .   
 
         [0021]    As shown in the example of  FIGS. 1   a - e , the electrosurgical waveforms can be applied between the active microelectrodes  104   a - d , on the array and the large return electrode  106   a - b  placed on the surface of the body. The return electrode can be a metal film at the base of the array, and contact with the tissue being treated  102  (e.g., skin) can be achieved via a conductive fluid filling a foam or other compressible porous material  112  placed between the return electrode and the tissue surface. 
         [0022]    This example includes some optional features of preferred embodiments. One such feature is the presence of insulator  110  on the sides of the protruding electrodes  104   a - d . Another such feature is the disposition of a standoff plate  116  on electrode base  114 . This standoff plate ensures that only the protruding parts of electrode array  104  can make contact with tissue  102 . 
         [0023]    Upon application of pressure onto the array, the electrodes  104   a - d  will be pushed into the tissue  102 , while the foam  112  is getting compressed, thus allowing the electrodes to move into the tissue ( FIG. 1   b ). During insertion, the electrosurgical waveform is applied to the electrodes, producing plasma-mediated discharge predominantly at the tips of the electrodes. This discharge vaporizes the tissue in front of the electrode thus allowing for advancement. Additional energy can be deposited during or after the insertion for coagulation or thermal therapy of the skin. The resulting thermal damage zone at the channel sides is referenced as  120   a - d . The example of  FIGS. 1   a - e  shows an optional step of providing additional energy ( FIG. 1   d ) after the electrodes are fully inserted ( FIG. 1   c ). The resulting thermal damage zone at the channel bottoms is referenced as  130   a - d . After the channels  140   a - d  are produced and tissue is heated, the array is pulled back, as shown in  FIG. 1   e . The procedure can be repeated to cover larger areas of skin surface. 
         [0024]    The depth of the thermal damage zone  120   a - d  at the edges of the channels can be controlled by the structure of the electrode and by electrosurgical waveform. For very low damage, the waveforms should consist of bursts with duration shorter than 100 microseconds, so that the thermal diffusion zone will not exceed approximately 10 micrometers. Bursts should be applied with repetition rate not exceeding 1 kHz in order to allow tissue cooling between the bursts, and thus prevent heat accumulation. To achieve deeper penetration of heat, the waveform should have higher duty cycle to provide for deeper heat diffusion into tissue. 
         [0025]    To minimize electric current, the side walls of the wire electrodes can be partially or completely covered with a thin layer of insulator  110 , as shown in  FIGS. 1   a - e . The insulator should be thin enough in order to not interfere with advancement of the electrodes into tissue. Such insulation can also help reducing the effects of electrical stimulation of nerves and muscles in the treated tissue. 
         [0026]    Even though electric current will be flowing from the microelectrodes to the return electrode on the surface through the bulk of tissue, due to enhancement of electric field around the tips of the electrodes, the thermal effects such as ablation and coagulation will be localized in these areas, and thus the damage to the bulk of the tissue between the microelectrodes can be minimized. 
         [0027]    Placement of the return pad electrode above the array can be employed to further minimize the current spread across the tissue, and reduce muscle and nerve stimulation. However, the return electrode can also be placed peripherally to the array (e.g., as in the example of  FIG. 2 ), where compressible pad  202  laterally surrounds array  104  and effectively defines a peripheral return electrode, or even at remote part of the body. 
         [0028]    Additional control of the extent of electric field penetration into tissue, and associated thermal effects can be provided by varying the length of the exposed fraction of the wire. The electric field decreases with distance from the electrode, with a characteristic penetration depth on the order of the exposed electrode size. Thus if only the tip of the wire is exposed, then penetration of electric field will be minimal—on the order of the wire diameter, and associated thermal damage will be relatively small. If a longer section of the wire will be exposed, then the electric field will expand accordingly. 
         [0029]    There are several variations and modifications of this general approach. Preferably, the length of the protruding electrodes is in the range of 0.1-2 mm. Preferably, the diameter of the protruding electrodes is in the range of 0.03-0.5 mm, and is more preferably in the range of 0.05-0.1 mm. 
         [0030]    The side walls of the electrodes in the array can be coated with insulator having a thickness that preferably does not exceed the electrode radius. Suitable insulators for this purpose include, but are not limited to glass, ceramics, and polymers. The protruding electrodes can be more or less completely insulated, as indicated above. 
         [0031]    The return electrode can make contact to the surface of the body via a compressible material filled with conductive fluid. The electrosurgical waveform can include RF bursts, with burst duration preferably in the range from 10 to 1000 microseconds, and more preferably from 20 to 200 microseconds. The repetition rate of the bursts preferably does not exceed 10 kHz. 
         [0032]    After the insertion is complete, an additional energy deposition can be applied to enhance tissue heating at the depth. 
         [0033]    Any number of electrodes can be in the array: e.g., from 1×2 to 10×10 (one dimensional and 2-dimensional arrays). 
         [0034]    In application of this system to skin treatment, the electrodes length should not exceed the thickness of skin. 
         [0035]    The extent of tissue coagulation along the channel can be controlled by the ablative waveform. The extent of additional coagulation at the bottom of the array (i.e., at the bottom of the channels in the treated tissue) can be controlled by a second waveform that is activated after insertion is complete. 
         [0036]    The waveform can vary with depth (i.e. with time during insertion), providing a continuous transition from the limited coagulation at the walls of the channel at the top surface of tissue to extended ablation/coagulation at the bottom of the channels. 
         [0037]    The several electrodes in an array can be activated simultaneously and/or sequentially. 
         [0038]    An electrode array can include one or more return electrodes, e.g., as in the example of  FIG. 3 . In this example, an electrode array  302  has half of the electrodes  308  connected to the active side  304  of the power supply, and has the other half of the electrodes  310  connected to the return side  306  of the power supply. With part of the penetrating electrodes connected to the return, the return pad electrode may not be needed. 
         [0039]    The present approach provides significant advantages. The use of variable waveforms during insertion provides flexibility for controlling and adjusting the extent of heating and coagulation at the sides and bottoms of the channels, while removing tissue inside the channels. Electrode length, diameter and array spacing can be selected according to the type/location of the skin and/or the skin condition being treated.