Abstract:
An electrode system is provided that generates a current flow that envelops and permeates an entire wound site. The electrode system includes two electrodes that are shaped and oriented to cause the current to flow from one electrode through the wound to the other electrode. A first electrode is applied to the wound site and the second electrode encircles the first electrode and is applied to the skin surrounding the wound cite. The two electrodes may be mounted to an oxygen-permeable layer that provides support for the electrodes and allows the wound site to breathe. An electrically insulative element may be disposed between the two electrodes. A power supply, which may be local to or remote from the electrode system, is provided for applying a voltage potential across the electrodes. In another suitable embodiment, the two electrodes are comprised of oppositely charged polymers.

Description:
This is a divisional of U.S. patent application Ser. No. 09/872,956, filed Jun. 1, 2001, now U.S. Pat. No. 6,631,294, which is hereby incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to apparatus and methods for facilitating wound healing through the use of electrical stimulation, and more particularly to apparatus and methods for providing a voltage gradient and a pattern of current flow that envelopes and permeates the wound. 
   Connective tissue wound healing typically occurs in three distinct phases. Although these phases intertwine and overlap, each has a specific sequence of events that distinguishes it. During the initial, or inflammatory phase, the body begins to clean away bacteria and initiate hemostasis. The inflammatory phase has three subphases: hemostasis; leukocyte and macrophage migration; and epithelialization. This phase typically lasts for about four days. 
   The second phase, the proliferative phase, is characterized by a proliferation of fibroblasts, collagen synthesis, granulation, and wound contraction. The proliferative phase typically begins about 48 hours after the wound is inflicted and can extend anywhere from two hours up to a week. In this phase, the fibroblast cells begin the synthesis and deposition of the protein collagen, which will form the main structural matrix for the successful healing of the wound. 
   In the third phase, the remodeling phase, the collagen production slows. The collagen that is formed in this stage is more highly organized than the collagen formed in the proliferative phase. Eventually, the remodeled collagen increases the tensile strength in the wound and returns the wound to about 80% of the skin&#39;s original strength. 
   This is the general process that occurs in healthy human beings. Patients that suffer from conditions which limit the flow of blood to the wound site are unfortunately not able to exhibit the normal wound healing process as described. In some patients this process can be halted. Factors that can negatively affect this normal wound healing process include diabetes, impaired circulation, infection, malnutrition, medication, and reduced mobility. Other factors such as traumatic injuries and burns can also impair the natural wound healing process. 
   Poor circulation, for varying reasons, is the primary cause of chronic wounds such as venous stasis ulcers, diabetic ulcers, and decubitus foot ulcers. Venous stasis ulcers typically form just above the patient&#39;s ankles. The blood flow in this region of the legs in elderly or incapacitated patients can be very sluggish, leading to drying skin cells. These skin cells are thus oxygen starved and poisoned by their own waste products and begin to die. As they do so, they leave behind an open leg wound with an extremely poor chance of healing on its own. Diabetic foot ulcers form below the ankle, in regions of the foot that have very low levels of circulation. 
   Similarly, decubitus ulcers form when skin is subjected to constant compressive force without movement to allow for blood flow. The lack of blood flow leads to the same degenerative process as described above. Paraplegics and severely immobile elderly patients which lack the ability to toss and turn while in bed are the main candidates for this problem. 
   Traditional approaches to the care and management of these types of chronic non-healing wounds have included passive techniques that attempt to increase the rate of repair and decrease the rate of tissue destruction. Examples of these techniques include antibiotics, protective wound dressings, removal of mechanical stresses from the affected areas, and the use of various debridement techniques or agents to remove wound exudate and necrotic tissue. 
   For the most part, these treatment approaches are not very successful. The ulcers can take many months to heal and in some cases they may never heal or they may partially heal only to recur at some later time. 
   Active approaches have been employed to decrease the healing time and increase the healing rates of these ulcers. These approaches may include surgical treatment as well as alterations to the wound environment. These alterations may include the application of a skin substitute impregnated with specific growth factors or other agents, the use of hyperbaric oxygen treatments, or the use of electrical stimulation. It has also been shown experimentally (both in animal and clinical trials) that specific types of electrical stimulation will alter the wound environment in a positive way so that the normal wound healing process can occur or in some cases occur in an accelerated fashion. 
   Therapeutic Electrostimulation 
   The relationship between direct current electricity and cellular mitosis and cellular growth has become better understood during the latter half of the twentieth century. Weiss, in Weiss, Daryl S., et. al., Electrical Stimulation and Wound Healing, Arch Dermatology, 126:222 (February 1990), points out that living tissues naturally possess direct current electropotentials that regulate, at least in part, the wound healing process. Following tissue damage, a current of injury is generated that is thought to trigger biological repair. This current of injury has been extensively documented in scientific studies. It is believed that this current of injury is instrumental in ensuring that the necessary cells are drawn to the wound location at the appropriate times during the various stages of wound healing. Localized exposure to low levels of electrical current that mimic this naturally occurring current of injury has been shown to enhance the healing of soft tissue wounds in both human subjects and animals. It is thought that these externally applied fields enhance, augment, or take the place of the naturally occurring biological field in the wound environment, thus fostering the wound healing process. 
   Weiss continues to explain, in a summary of the scientific literature, that intractable ulcers have demonstrated accelerated healing and skin wounds have resurfaced faster and with better tensile properties following exposure to electrical currents. Dayton and Palladino, in Dayton, Paul D., and Palladino, Steven J., Electrical Stimulation of Cutaneous Ulcerations—A Literature Review, Journal of the American Podiatric Medical Association, 79(7):318 (July 1989), also state that the alteration of cellular activity with externally applied currents can positively or negatively influence the status of a healing tissue, thereby directing the healing process to a desired outcome. 
   Furthermore, research conducted by Rafael Andino during his graduate tenure at the University of Alabama at Birmingham, also demonstrated that the presence of electrical fields (in this case induced by the application of pulsating electromagnetic fields) dramatically accelerated the healing rates of wounds created in an animal model. This research found that the onset and duration of the first two phases of the wound healing process, the inflammatory and proliferative phases, had been markedly accelerated in the treated wounds while the volume of collagen which had been synthesized by the fibroblasts was also markedly increased in the treated wounds. This resulted in the wounds healing in a much shorter amount of time. Similar findings from other researchers can be found in other wound healing literature. 
   U.S. Pat. No. 5,433,735 to Zanakis et al. and U.S. Pat. No. 4,982,742 to Claude describe various electro-stimulation apparatus and techniques for facilitating the regeneration and repair of damaged tissue. However, each of these references suffers from the disadvantage that the pattern of current flow generated with these electrode devices does not pass through all portions of the wound and thus, certain portions of the wound site may not be exposed to the beneficial effects of electrostimulation. 
   U.S. Pat. No. 4,911,688 to Jones describes a wound cover that includes a chamber that encloses fluid around the wound. One electrode is located in the chamber and another electrode is placed away from the wound on the skin. By using conductive liquid within the chamber, a circuit is completed allowing current to flow from the electrode in the chamber, through the liquid, wound, and surrounding tissue and skin to the other electrode. The liquid is introduced into the chamber and replaced using two ports, one port is used to introduce the liquid while at the same time the other port is used to remove the gas (when the wound cover is originally applied to the wound) or fluid within the chamber. This wound cover, however, is complicated to use and involves a delicate process of adding and replacing the conductive liquid. 
   In view of the foregoing, it is an object of the present invention to provide improved apparatus and methods for easily providing a voltage gradient and a pattern of current flow that envelops and permeates the entire wound site. 
   SUMMARY OF THE INVENTION 
   This and other objects of the invention are accomplished in accordance with the principles of the present invention by providing an electrode system that includes-two electrodes that are adapted for connection to a power source sufficient to cause a current to flow between them. The electrodes are shaped and oriented to cause a pattern of current flow that envelops and permeates the entire wound site. Such shapes and orientations may include a circular first electrode located at and covering the wound site and a second electrode shaped as a ring fully encircling the first electrode. The second electrode may be located outside or partially within the wound site. Other suitable shapes of the electrodes may include electrodes that are ovally shaped, rectangularly shaped, triangularly shaped or any other suitable shape where one electrode encircles the other electrode. The shape of the electrode may conform to the shape of the wound. 
   The two electrodes of the electrode system may be mounted to an oxygen-permeable top layer that is impermeable to water and water vapor. The top layer may provide support for the electrodes and may allow the wound site to breathe. 
   The electrode system may also include an electrically insulative element that is disposed between the two electrodes. The insulative element may ensure that most if not all of the current flow between the electrodes passes through the damaged and healthy surrounding tissue. 
   The power supply for applying a voltage potential across the electrodes may be local to or remote from the electrode system. In one suitable arrangement, the power supply is attached to the top layer of the electrode system. The power supply can be configured to provide a constant or varying voltage, a constant or varying current, or any other suitable electrical output to the electrodes to facilitate wound healing. For example, the power supply may be configured to provide the desired current or voltage to the electrodes at different time intervals with the same electrode system in place. In one suitable embodiment, the power supply is a battery. In another suitable embodiment, the power supply is electronic circuitry that is configured to provide the desired current or voltage. 
   In another suitable embodiment of the invention, the two electrodes of the electrode system are comprised of oppositely charged polymers of sufficient voltage differential and charge capacity to cause a current to flow from the first electrode to the second electrode through the wound. 
   The electrode system can be designed and fabricated to be either disposable or reusable. 
   The electrode system according to the various embodiments described herein is capable of generating a voltage gradient and a pattern of current flow that envelops and permeates the entire wound site. Such a pattern of current flow maximizes the recruitment of the necessary cells to the wound location at the appropriate times during the various stages of wound healing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
       FIG. 1  is a cross-sectional view of an illustrative electrode system in accordance with the present invention taken generally along the line  1 — 1  of FIG.  2 . 
       FIG. 2  is a cross-sectional view of the electrode system of  FIG. 1  taken generally along the line  2 — 2  of  FIG. 1   
       FIG. 3  is a cross-sectional view of the electrode system of  FIG. 1  as applied to a wound that illustrates the pattern of current flow generated by the electrode system in accordance the present invention. 
       FIG. 4  is a perspective view of an illustrative electrode system placed over a wound site in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a cross-sectional view of electrode system  10 . The view in  FIG. 1  is taken along the line  1 — 1  of FIG.  2 .  FIG. 2  shows a simplified cross-sectional view of electrode system  10  taken alone the line  2 — 2  of FIG.  1 . As illustrated in  FIG. 1 , electrode system  10  includes top overlay layer  20  to which electrodes  22  and  24 , electrically insulative element  26 , and end material  28  are attached. Electrode  22  is located towards the center of top overlay layer  20 . Electrically insulative element  26  surrounds electrode  22  and electrode  24  surrounds electrically insulative element  26 . Attached to the other side of electrodes  22  and  24 , electrically insulative element  26 , and end material  28  are adhesive layers  52  and  54 . As illustrated in  FIG. 2 , electrically conductive lead  32  connects electrode  22  to terminal  42  of power supply  40  and electrically conductive lead  34  connects electrode  24  to terminal  44  of the power supply  40 . 
   Top overlay layer  20  may serve several different purposes. First, top overlay layer  20  provides the mechanical integrity of electrode system  10 , thus providing structural support for electrodes  22  and  24 . Second, top overlay layer  20  should be flexible enough to allow electrode system  10  to conform to the contours of the skin surface to which it is adhered. Third, top overlay layer  20  should be oxygen permeable to allow the wound site to breathe. Finally, top overlay layer  20  should be water impermeable so that the wound site remains moist. In some embodiments, all of these characteristics may not be necessary. For example, a separate water impermeable layer may be used to keep the wound site moist. Top overlay layer  20  may be comprised of any suitable material or structure that exhibits these characteristics. For example, top overlay layer  20  may be comprised of a mesh structure of polypropylene, polyethylene, polyurethane, polytetrafluoroethylene (PTFE), or any other suitable material. In one embodiment, top overlay layer  20  can be electrically insulative to prevent current from flowing between electrodes  22  and  24 , which are attached to top overlay layer  20 . In another suitable embodiment, the adhesive or binding agent (not shown) used to adhere electrodes  22  and  24  to top overlay layer  20  can be electrically insulative to prevent current from flowing between electrodes  22  and  24 . 
   Electrodes  22  and  24  may be thin metal, metallic paint or pigment deposition, metallic foil, conductive hydrogels, or any other suitable conductive material. Hydrogels are generally clear, viscous gels that protect the wound from dessicating. In one suitable approach, conductive hydrogels may be used as the material for electrodes  22  and  24  because of their permeability to oxygen and ability to retain water. Both oxygen and a humid environment is required for the cells in a wound to be viable. In addition, hydrogels can be easily cast into any shape and size. Various types of conductive hydrogels may be employed, including cellulose, gelatin, polyacrylamide, polymethacrylamide, poly(ethylene-co-vinyl acetate), poly(N-vinyl pyrrolidone), poly(vinyl alcohol), HEMA, HEEMA, HDEEMA, MEMA, MEEMA, MDEEMA, EGDMA, mathacrylic acid based materials, and siliconized hydrogels. PVA-based hydrogels are inexpensive and easy to form. The conductivity of such hydrogels can be changed by varying the salt concentration within the hydrogels. By increasing the salt concentration within a hydrogel, the conductivity of the hydrogel increases. 
   Insulative element  26  prevents the flow of current between electrodes  22  and  24  above the wound surface such as by moisture trapped under the top overlay layer. Insulative element  26  may be composed of any high resistance material such as polythylene, poly(tetrafluoroethylene) (TEFLON), polyurethane, polyester, a hydrogel made to be an insulator or any other suitable insulative material. In addition, insulative element  26  may be formed of a material or designed to have gaps or openings within its body to prevent the flow of current or greatly increase the current resistance above the wound surface. 
   End material  28  surrounds electrode  24 . End material  28 , in combination with the outer edge of top overlay layer  20 , forms the outer edge of electrode system  10 . End material  28  may be comprised of any suitable material flexible enough to allow electrode system  10  to conform to the contours of the skin surface to which it is adhered. In one embodiment, end material  28  may be composed of the same material as top overlay layer  20 . In one suitable approach, end material  28  may be a part of and seamless with top overlay layer  20 . 
   Conductive adhesive layers  52  and  54  are attached to the underside of electrode system  10 , contacting electrodes  22  and  24 , respectively and electrically insulative element  26 . Adhesive layers  52  and  54  should be separated from each other by a suitable space or gap  58  to prevent short-circuiting of the electrodes. Adhesive layers  52  and  54  may be a hydrogel, fibrin, conductively transformed cyanoacrylates or can be comprised of any suitable electrically conductive material capable of attaching electrode system  10  to the skin and wound surfaces. Adhesive layer  52  can be arranged to distribute substantially the same voltage of electrode  22  to the entire surface of the wound. Similarly, adhesive layer  54  can be arranged to distribute substantially the same voltage of electrode  24  to the skin surrounding the wound. In another suitable approach, adhesive layer  52  can be arranged so that the center of adhesive layer  52  applies a voltage substantially similar to electrode  22  to the center of the wound and that the outer edge of adhesive layer  52  applies a voltage that is between the voltages of electrodes  52  and  54  to the outer edge of the wound. The voltage applied to the wound may be varied, for example, by varying the thickness of adhesive  52  or by any other suitable method. 
   As illustrated in  FIG. 1 , adhesive layer  52  extends beyond electrode  22 . In another suitable arrangement, adhesive layer  52  may be the same size as or smaller than electrode  22 . Adhesive layer  54  as illustrated is larger than electrode  24 . In another suitable arrangement, adhesive layer  54  may be the same size as or smaller than electrode  24 . 
   In another suitable embodiment, conductive adhesive layers  52  and  54  may be omitted from electrode system  10 . In this embodiment, electrodes  22  and  24  are themselves adhesive and capable of attaching electrode system  10  to the wound site. Conductive hydrogels can be fashioned to have the requisite adhesive properties, thereby eliminating the need for separate adhesive layers. One type of highly conductive hydrogel that is sufficiently tacky and adhesive to adhere to the skin is described in U.S. Pat. No. 4,989,607 to Keusch et al. Electrodes  22  and  24  may be comprised of any suitable conductive adhesive material capable of attaching electrode system  10  to the wound site. 
   Backing layer  60  is attached to conductive adhesives  52  and  54  to protect the adhesive layer prior to the use of electrode system  10 . Backing layer  60  may be peeled off of adhesives  52  and  54  to expose the adhesive layer prior to contacting electrode system  10  to the wound site. Backing layer  60  may protrude out from underneath top overlay layer  20  in one area, such as area  60 ′ as shown in  FIG. 2 , to allow the user to easily remove backing layer  60  from electrode system  10 . 
   In use, electrode system  10  is positioned over the wound site such that electrode  22  is located at approximate the center of the wound site and adhesive layer  52  can be sized to cover the entire wound. Electrode system  10  is provided in a family of sizes appropriate for wounds of various sizes. Electrode  24  and adhesive layer  54  are generally in the shape of a ring and are located a distance away from electrode  22 . In one arrangement, the diameters of the inner edges of electrode  24  and adhesive layer  54  are greater than the diameter of the wound. In another words, the size of the wound determines the minimum inner diameter of electrode  24  and adhesive layer  54 . In another suitable arrangement, adhesive layer  52  can be sized to cover the inner portion of the wound and the inner diameters of the inner edges of electrode  24  and adhesive layer  54  may be the same or less than the size of the wound. 
     FIG. 3  is a cross-sectional view of electrode system  10  as applied to wound  60 . As shown in  FIG. 3 , the pattern of current flow generated by electrode system  10  is toroidal in shape. A toroid is generally formed by rotating a circular disk about an axis, where the axis lies in the plane of the disk, but outside of the disk. Here, the pattern of current flow is similar to a semicircle rotated about an axis, where the axis lies in the plane of the semicircle and the axis is near the edge of the semicircle. The current generally flows tangential to the radial lines of the semicircle. Because electrode  24  surrounds electrode  22 , the pattern of current flow is similar to the semicircular disk rotated completely around the axis. Therefore, the pattern of current flow is toroidal in shape. The pattern of current flow as illustrated in  FIG. 3  would therefore generally be the same regardless of the angle of the cross-section cut through electrode system  10  with respect to reference direction  65  of FIG.  2 . More specifically, as illustrated, electrode  22  is negatively charged and electrode  24  is positively charged. The lines of current flow extend from adhesive  54  through wound  60  to adhesive  52  in an arcuate shape. The lines of current pass through the entire wound  60 , thereby enveloping and permeating the entire wound and the adjoining unwounded tissue. If the voltage that is applied to the wound from adhesive  52  is varied, as described above, then the current density at different portions of wound  60  can be increased or decreased accordingly. Electrode system  10  can produce a current density within the wound that is generally between 1 μA/cm 2  and 10,000 μA/cm 2 . Depending on the size and nature of the wound, electrode system  10  may be configured to produce a current density within the wound that is less than of 1 μA/cm 2  or greater 10,000 μA/cm 2 . 
   Referring to  FIG. 2 , conductive leads  32  and  34 , which connect electrodes  22  and  24  respectively to power supply  40 , may be comprised of metal, conductive ink or any other suitable conductive material. In one suitable arrangement, leads  32  and  34  are comprised of conductive carbon ink that is screened onto top overlay layer  20 . In such an arrangement, electrodes  22  and  24  are formed in place over conductive leads  32  and  34 , respectively. 
   Power supply  40  generates a voltage that is applied to electrodes  22  and  24  through leads  32  and  34 , respectively. Power supply  40  may be configured to apply a voltage that is anywhere between 1 mV and 9 V. The resulting current flow that flows through the wound may be between 1 μA and 50 mA. Depending on the size and nature of the wound, power supply  40  may be configured to apply a voltage that is less than 1 mV or greater than 9 V. The resulting current flow may therefore be less than 1 μA or greater than 50 mA. Power supply  40  may be attached to the upper portion of top overlay layer  20  or any other suitable location on electrode system  10  or may be located remote from electrode system  10 . In one suitable embodiment, power supply  40  is a battery. Power supply  40  may be any suitable battery such as an alkaline, nickel cadmium, or lithium battery. In one suitable arrangement, power supply  40  is a lithium polymer stack. The battery may be arranged so that terminal  42  is negative and terminal  44  is positive. Thus, electrode  22  functions as an anode and electrode  24  functions as a cathode. As described above, current will flow along outward radial lines from electrode  24  through the wound to electrode  22 . In another suitable approach, the battery can be arranged so that terminal  42  is positive and terminal  44  in negative. In such an approach, the lines of current are reversed and directed outward from electrode  22  to electrode  24 . 
   In another suitable embodiment, power supply  40  is comprised of electronic circuitry that is configured to provide a constant or varying voltage, a constant or varying current, or any other suitable electrical output. The current density within the wound site may therefore be constant or time varying. When power supply  40  varies the voltage or current, electrodes  22  and  24  may change polarities at a constant or at a time varying frequency. In another suitable electrical output, power supply  40  can be configured to pulse electrodes  22  and  24  to provide other possible therapeutic benefits. 
   In one suitable arrangement, the electrical circuitry can be configured to provide a constant current source using a current-to-voltage converter. The current to voltage converter may be probed at test points to check the current accuracy. The constant current source may be implemented with an operational amplifier (Op-amp). The Op-amp compares a precision voltage reference source to the output of a current-to-voltage converter and adjusts the output current until the reference and the converter are equal. The output voltage is limited to the battery voltage minus a certain predetermined amount used for operational purposes. 
   The circuit may be built with surface mount integrated circuits and other surface mount components and may be powered, for example, by lithium coin cell batteries. 
   The electrode system  10  herein described may not require a switch to be activated for current to commence flowing between electrodes  22  and  24 . Rather, current may begin to flow following conductive contact of electrodes  22  and  24  to the wound site. Such contact completes a circuit between the electrodes and results in current flow between the electrodes. In another suitable embodiment, a switch may be located on electrode system  10  that may allow the user to engage and disengage power supply  40  to electrodes  22  and  24 . 
   Electrode system  10  may contain within its circuitry a visual indicator to allow the user to determine whether or how well the electrode system is functioning. The visual indicator may be a light emitting diode (LED), a series of LEDs, a basic current meter, or any other suitable visual indicator. 
     FIG. 4  demonstrates a view of electrode system  10  placed over wound  60 . In this embodiment, electrode system  10  is a disposable, one-time-use bandage that uses a battery and associated circuitry as power supply  40 , which is attached to electrode system  10 . Appropriate electrical parameters may be selected such that the current generated by the internal circuitry will last for a desired period of time. For example, the desired period of time may be at least as long as the typical amount of time a normal bandage is used on the wound. For users with chronic ulcers, this amount of time may typically be 1 to 2 days. Therefore, after electrode system  10  is activated by placement over the wound, an electrical current may last for 1 to 2 days. When it is time for electrode system  10  to be replaced, a new electrode system will be applied and the treatment will continue as required by the individual user and the type of wound present. 
   While electrode system  10  has been described as being generally circular in shape, it is understood that electrode system  10  may also be provided in other shapes as well. For example, electrode system  10  may be provided in an oval shape, rectangular shape, triangular shape, or any other suitable shape. The resulting pattern of current flow would therefore be similar to the toroidal shape described above which has been stretch from a circle to an oval shape, rectangular shape, triangular shape, or any other suitable shape of electrode system  10 . Electrode system  10  is preferably provided in different shapes appropriate for wounds of different shapes. For example, if the wound is a long gash wound, a rectangular or oval shaped electrode system may be the appropriate shape for the wound. In one suitable approach, a preferred electrode system shape for a wound is a shape that will allow adhesive  52  to cover the entire wound and that will minimize the amount of area that adhesive  52  covers exterior to the wound. This will maximize the current flow through the wound. 
   In another suitable electrode system embodiment, electrodes  22  and  24  are electrically charged polymers. In this embodiment, power supply  40  and leads  32  and  34 , as illustrated in  FIGS. 1 and 2  are not required. In addition, top overlay layer  20  may not be required and electrodes  22  and  24  may be separately applied. Electrodes  22  and  24  can be oppositely charged polymers (e.g., hydrogel or any other suitable material for holding a charge) of sufficient differential voltage potential and of sufficient charge densities to cause a current to flow between the electrodes. In one suitable arrangement, electrode  22  is negatively charged and electrode  24  is positively charged. This would cause current to flow through the wound to negative electrode  22  from positive electrode  24 . In another suitable arrangement, electrode  22  is positively charged and electrode  24  is negatively charged. This would cause current to flow from positive electrode  22  through the wound to negative electrode  24 . 
   The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.