Abstract:
Methods and apparatus for generating an electric field inside a patient include at least a first electrode and a second electrode on a common implantable substrate. Each of the electrodes has a respective proximal end and a respective distal end. Electrical energy can be applied that causes electrical current to flow simultaneously through the first and second electrodes preferentially from the proximal end of one electrode to the distal end of the other electrode.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation of U.S. application Ser. No. 11/133,741 (Attorney Docket No.: 021433-001700US), filed May 19, 2005, the full disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
     Field of the Invention  
       [0002]     The invention relates generally to medical devices and methods, and more particularly, to implantable electrodes for applying electrotherapy/electrostimulation that utilizes at least a pair of electrodes employing a reverse electrode configuration.  
         [0003]     Implantable electrode assemblies for electrotherapy or electrostimulation are well-known in the art. For example, various configurations of implantable electrodes are described in U.S. Patent Publication No. U.S. 2004/0010303, which is incorporated herein by reference in its entirety. One type of electrode assembly described therein is a surface-type stimulation electrode that generally includes a set of generally parallel elongate electrodes secured to, or formed on, a common substrate or base. Prior to implantation in a patient, the electrodes are generally electrically isolated from one another. Once the electrode assembly is implanted, one or more of the electrodes are utilized as a cathode(s), while one or more of the remaining electrodes are utilized as an anode(s). The implanted cathode(s) and anode(s) are electrically coupled via the target region of tissue to be treated or stimulated.  
         [0004]     One example of an application for this type of electrode assembly is for implantation onto a surface of tissue to be the target for electrotherapy or electrostimulation. The target tissue may have an irregular or complex shape, such as the outer surface of a blood vessel. The base or substrate and the electrodes of the electrode assembly can be sufficiently flexible to conform to the shape of the target tissue while maintaining a particular relative positioning of the electrodes. The geometry of the electrode assembly can be especially adapted for implantation at a particular site. For example, an electrode assembly can be sized and shaped to be implanted around the outside of the vascular wall such that the electrotherapy or electrostimulation can be focused on a particular target region.  
         [0005]     One known problem associated with state of the art implantable electrode arrangements of certain geometries is their tendency to produce non-uniform electric fields or currents in the target region under certain conditions. An example of such a condition is when the implantation site has a low enough impedance to approach that of the electrode materials. In such cases, the internal resistance of the implanted electrodes becomes a significant parameter in the electrical/electromagnetic model of the implanted electrode arrangements. This problem can become pronounced in electrode arrangements in which the size of the electrodes approaches or exceeds the general size of the target region, or in arrangements in which the electrodes have structural geometries other than merely point electrodes.  
         [0006]     Non-uniformity of electric field in the target region can result in sub-optimal electrotherapy or electrostimulation. Another consequence that occurs when the surface regions of implanted electrodes operate with disparate charge densities is an increased susceptibility of the electrodes to corrosion. Because corrosion is a charge density based phenomenon, increased concentrations of charge carriers in certain regions of the electrodes tends to focus faradaic processes responsible for corrosion at those regions.  
         [0007]     In U.S. Pat. No. 5,265,623, a defibrillation catheter is described having generally linear electrode geometries (elongate helical or spiral coils) that, when implanted, are both situated generally longitudinally in the heart. The electrodes of the defibrillation catheter are connected to the defibrillation energy source from a center point of each electrode, rather than at the ends thereof. This arrangement is intended to provide an improved field distribution around the catheter electrode and avoid high current densities at the electrode ends. The arrangement nevertheless operates with a charge density gradient in each electrode due to the construction and relative positioning of the electrodes. Thus, the applied electric signaling is not likely to be uniform along the length of the electrodes. Furthermore, the multi-layer or multi-axial construction of the disclosed catheter requires a complex and relatively expensive fabrication process. Moreover, the catheter electrode assembly is not suitable for the surface-type of implantation applications described above.  
         [0008]     While techniques have been developed to improve the distribution of electric field densities with respect to axial defibrillation electrode leads, it would be desirable to provide designs for implantable electrodes that improve the distribution of electrical field densities with respect to surface-type implantable electrodes.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention provides methods and apparatus for generating an electric field inside a patient that enhances the effective distribution of the density of the electric field. In one embodiment, at least a first and a second electrode are arranged on a common implantable substrate such that a proximal end of the first electrode is closer to a distal end of the second electrode and vice-versa. The electrodes are arranged and electrically connected to an energy source via the proximal ends such that when electrical energy is applied across the first and second electrodes, electrical current preferentially flows simultaneously through the first and second electrodes from the proximal end of one electrode to the distal end of the other electrode. In one aspect of the invention, the first and second electrodes are situated proximate a surface of tissue to be stimulated such that the electrodes are generally uniformly electrically coupled along their lengths via the tissue.  
         [0010]     A method of evenly distributing an electric field between at least a pair of implanted electrodes according to another aspect of the invention includes providing an implantable electrode assembly that includes at least two electrodes on a common substrate, wherein each electrode has an elongate shape and is situated generally equidistantly along its length from at least one of the other electrodes in a configuration where the proximal and distal ends of the electrodes are reversed between an anode and a cathode. Electrical energy is applied through the at least two electrodes such that a generally uniform current density is established between the anode and the cathode.  
         [0011]     An electrode assembly according to another aspect of the invention includes a generally flexible base and at least three generally parallel elongate electrode structures secured over a surface of the base, each electrode structure having a proximal end and a distal end. An outer pair of the electrode structures is electrically isolated from an inner electrode structure, and the proximal end of each electrode structure is electrically coupled to a conductive lead adapted to carry electrical energy. The proximal ends of the outer electrode structures and the distal end of the inner electrode structure are proximately situated at a first surface region of the base, and the distal ends of the outer electrode structures and the proximal end of the inner electrode structure are proximately situated at a second surface region of the base that is separate from the first surface region.  
         [0012]     Another aspect of the invention is directed to an electrode assembly implanted at an electrotherapy site in a patient, the electrode assembly comprising a substrate; at least two elongate electrodes secured to the substrate and electrically coupled to the electrotherapy site, each electrode having opposing ends proximally and distally situated; and an electrotherapy signal generator circuit electrically coupled with the at least two electrodes and supplying electrotherapy signaling to the electrotherapy site via the at least two electrodes. The electrical coupling facilitates a generally uniform charge density in the at least two electrodes.  
         [0013]     An electrotherapy arrangement implanted at an electrotherapy site according to another aspect of the invention comprises an electrode assembly that includes a substrate; and at least two elongate electrodes secured to the substrate and electrically coupled to the electrotherapy site, wherein each electrode is situated such that its proximal ends are longitudinally opposing. The electrotherapy arrangement further comprises an electrotherapy signal generator circuit electrically coupled with the at least two electrodes and applying electrotherapy signaling to the electrotherapy site via the at least two electrodes. The electrotherapy signaling generates an electric field through the electrotherapy site that is generally uniformly distributed along a longitudinal reference axis located in the electrotherapy site between the anode and cathode of the at least two electrodes.  
         [0014]     According to another aspect of the invention, an implantable electrode assembly includes an implantable substrate and three elongate electrodes secured to the substrate. Each electrode has respective proximal and distal opposing ends, and the electrodes are relatively situated such that each electrode is generally uniformly spaced along its length with respect to its adjacent electrode(s). The electrodes are also relatively spaced such that an inner electrode is flanked on two sides by a pair of outer electrodes. A first lead is connected to the inner electrode at the inner electrode&#39;s proximal end positioned on one side of the substrate, and second and third leads are connected to the outer electrodes at the respective proximal ends positioned on an opposite side of the substrate. In one embodiment, each of the leads for the outer electrodes are positioned on one side and the lead for the inner electrode is positioned on the opposite side.  
         [0015]     In an alternate embodiment, all of the leads are arranged on one side and a portion of the lead to the inner electrode is routed across the substrate within an insulator for the distal end of the lead to connect to the proximal end of the electrode.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0017]      FIG. 1A  is a top-view diagram illustrating an electrode assembly according to one embodiment of the present invention.  
         [0018]      FIG. 1B  is a diagram illustrating one physical embodiment of the electrode assembly of  FIG. 1A .  
         [0019]      FIG. 1C  is a top-view diagram illustrating an electrode assembly having a particular wiring arrangement according to one embodiment of the invention.  
         [0020]      FIG. 1D  is a perspective view diagram illustrating an example physical layout of the electrode assembly of  FIG. 1C  in which the lead connecting one of the electrodes to the signal generator runs along the bottom surface of the substrate.  
         [0021]      FIG. 1E  is a perspective view diagram illustrating another example physical layout of the electrode assembly of  FIG. 1C  in which at least one of the electrodes is formed around a portion of the lead connected with the electrode.  
         [0022]      FIG. 1F  is a perspective view diagram illustrating one example physical embodiment of a coiled outer electrode structure formed around an inner coiled lead portion that is connected to the electrode at a proximal end.  
         [0023]      FIG. 1G  is an end-view diagram of the electrode structure of  FIG. 1F .  
         [0024]      FIG. 1H  is a perspective view diagram illustrating another example physical embodiment of a coiled outer electrode structure formed around an inner coiled lead portion that is connected to the electrode at a proximal end, in which an insulator is positioned between the outer electrode and inner conductor portion.  
         [0025]      FIG. 1I  is a side-view diagram illustrating an example coiled electrode structure of the type illustrated in  FIG. 1H , in which the outer and inner coils are generally helical in geometry and in which the insulator is generally cylindrical in geometry.  
         [0026]      FIG. 1J  is a side view diagram illustrating an example coiled electrode structure of the type illustrated in  FIG. 1H , in which the inner coil has a spiral geometry wherein coil radius decreases towards the proximal end, and in which the insulator has a geometry wherein the walls have a profile of decreasing radius that corresponds to the decreasing radius profile of the inner coil.  
         [0027]      FIG. 1K  is a cross-sectional view diagram of the electrode structure of  FIG. 1J .  
         [0028]      FIG. 2  is a schematic diagram illustrating functional aspects of electrode assemblies known in the art.  
         [0029]      FIG. 3  is a chart illustrating the current distribution through target tissue during operation of the electrodes represented in the schematic of  FIG. 2 .  
         [0030]      FIG. 4  is a schematic diagram illustrating functional aspects of an electrode assembly according to one embodiment of the present invention.  
         [0031]      FIG. 5  is a chart illustrating the current distribution through target tissue during operation of the electrodes represented in the schematic of  FIG. 4 .  
         [0032]      FIG. 6  is a diagram illustrating an electrode assembly according to one embodiment of the present invention in which the electrodes are non-linear. 
     
    
       [0033]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0034]      FIG. 1A  is a diagram illustrating an implantable electrode assembly  100  according to one example embodiment of the invention. Electrode assembly  100  can be utilized in a variety of applications including, but not limited to, electrotherapy or stimulation of the patient. Tissue regions that are potential targets of electrotherapy/electrostimulation include the patient&#39;s nervous system (including nerve cells and synapses, and sensory receptors such as baroreceptors), muscle tissue, organs, and blood vessels. Electrode assembly  100  includes a base structure or substrate  102  that includes a flexible and electrically insulating material suitable for implantation, such as silicone, optionally reinforced with a flexible material such as polyester fabric. Base structure or substrate  102  can be sized and shaped according to the implantation site for the target tissue region (e.g., targeted blood vessels, muscles, nerves, skin, bone, organs, cells, etc.), and can have flexible and/or elastic properties. Thus, for example, base structure or substrate  102  can have a length suitable to wrap around all (360 degrees) or a portion (i.e., less than 360 degrees) of the circumference of one or more blood vessels.  
         [0035]     In one embodiment, electrode assembly  100  includes elongate electrodes  104   a - 104   c  for making contact with the target tissue region into which electrotherapy or electrostimulation is to be applied. The electrodes can be un-insulated portions of larger electrical conductors, dedicated un-insulated conductive structures, or a combination thereof. While the elongate electrodes  104   a - 104   c  generally extend along a longitudinal axis, it will be recognized that embodiments of the elongate electrodes can include nonlinear geometries such as serpentine, curved or zig-zag, for example, and that in some embodiments not all of an electrode structure need be considered as part of the elongate electrode geometry. In one example embodiment, as illustrated in  FIG. 1A , elongate electrodes  104   a - 104   c  are each about the same length, and are situated generally parallel to one another such that proximal ends  106   a  and  106   c  of outer electrodes  104   a  and  104   c  are positioned on the same side  102   a  of base  102  as distal end  108   b  of center electrode  104   b . On the other side  102   b  of base  102 , distal ends  108   a  and  108   c  of outer electrodes  104   a  and  104   c  are positioned proximate to proximal end  106   b  of center electrode  104   b  on side  102   b . For purposes of the present invention it will be understood that proximal is used to reference a region proximate an end of a structure that is electrically closer to the pulse generator and that distal references a region proximate an end of a structure that is further away electrically from the pulse generator as compared to the proximal portion.  
         [0036]     In a related type of embodiment, the electrodes are generally co-extensive. Among electrode assemblies of this type, the extent of co-extensiveness can vary according to the geometry of the implantation site. For example, in one example embodiment, the electrodes are co-extensive to within +/−25%. In another embodiment, the electrodes are co-extensive to within +/−5%. While this embodiment features one arrangement of three electrodes  104   a - 104   c  in accordance with the present invention, other arrangements and configurations of electrodes  104  as described hereinafter may also be utilized to enhance the uniform distribution of the electric field delivered through the electrodes to the target tissue region.  
         [0037]     Electrodes  104   a - 104   c  are made from a suitable implantable material, and are preferably adapted to have flexible and/or elastic properties. Electrodes  104   a - 104   c  can comprise round wire, rectangular ribbon or foil formed of an electrically conductive and radiopaque material such as platinum. In one embodiment, the base structure  102  substantially encapsulates the conductive material, leaving only exposed electrode  104   a - 104   c  portions for electrical connection to the target tissue. For example, each conductive structure can be partially recessed in the base  102  and can have one side exposed along all or a portion of its length for electrical connection to target tissue. The exposed portions constitute electrodes  104   a - 104   c . In another embodiment, the electrodes  104   a - 104   c  are made from conductive structures that can be adhesively attached to the base  102  or can be physically connected by straps, moldings or other forms of operably securing them to the base  102 . Electrical paths through the target tissue are defined by anode-cathode pairs of the elongate electrodes  104   a - 104   c . For example, in one embodiment, center electrode  104   b  is a cathode, and outer electrodes  104   a  and  104   c  are both anodes, or vice-versa. Thus, electrons of the electrotherapy or electrostimulus signaling will flow through the target region either into, or out of, electrode  104   b.    
         [0038]     Each electrode  104   a - 104   c  is connected at the corresponding proximal end  106   a - 106   c  to an electrotherapy/electrostimulus source, such as an implantable pulse generator (not shown) via a corresponding lead  110   a - 110   c . In one example embodiment, leads  110   a - 110   c  are each an insulated wire formed with, welded to, or suitably interconnected with each corresponding electrode  104   a - 104   c . Persons skilled in the art will appreciate that leads  110   a - 110   c  can be made of any suitable materials or geometries. Furthermore, leads  110   a - 110   c  can each include a combination of conductor types. Thus, for example, leads  110   a - 110   c  can each include an insulated stranded wire portion, an un-insulated solid wire portion, and/or a coiled wire portion having helical, spiral, or other such coiled geometry.  
         [0039]      FIG. 1B  illustrates a physical embodiment of the example electrode assembly  100  of  FIG. 1A . The shape of base structure or substrate  102  includes finger-type extensions  112 , and reinforced portions  114  for facilitating wrapping and securing the electrode assembly  100  to the implantation site during implantation. Because leads  110   a  and  110   c  are connected at opposite electrode ends from lead  110   b , leads  110   a  and  110   c  naturally extend in a different direction away from the electrodes  104  than the direction of lead  110   b . In certain applications, it may be desirable for the leads to extend in the same direction away from the electrodes  104 .  
         [0040]      FIG. 1C  is a diagram illustrating example electrode assembly  150  according to a related embodiment. Electrode assembly  150  includes a flexible and stretchable implantable substrate  152 , to which elongate electrodes  154   a - 154   c  are secured. Electrodes  154   a  and  154   c  are connected respectively to leads  160   a  and  160   c  at proximal ends  156   a  and  156   c  located on side  152   a  of substrate  152 . Electrode  154   b  is connected to lead  160   b  at proximal end  156   b  located on side  152   b  of substrate  152 . Distal ends  158   a  and  158   c  of electrodes  154   a  and  154   c , respectively, are located on side  152   b  and are not connected to any leads. Distal end  158   b  of electrode  154   b  is located on side  152   a  and is not connected to any lead. Lead  160   b  extends along the length of electrode  154   b  towards distal end  156   b , and further extends in the same direction as leads  160   a  and  160   c , as illustrated in  FIG. 1C . The leads  160   a - 160   c  are optionally bundled and secured together by wire tie  161 .  
         [0041]      FIG. 1D  illustrates a physical embodiment of example electrode assembly  150 . Electrodes  154   a - 154   c  are secured to substrate  152 , and are oriented along the reference z-axis. Insulated leads  160   a - 160   c  are attached to their respective electrodes as shown. Leads  160   a  and  160   b  are connected to respective electrodes  154   a  and  154   c  at respective proximal ends  156   a  and  156   c . Lead  160   b  is connected to electrode  154   b  at proximal end  156   b . Leads  160   a  and  160   c  extend in the −z direction away from their corresponding electrodes  154   a  and  154   c , and proceed in the −x direction. Lead  160   b  extends away from electrode  154   b  in the +z direction, then loops around substrate  152 , and further proceeds in the −z along the underside of substrate  152 . Leads  160   a - 160   c  are secured to substrate at various points by anchors  162  as shown. Lead  162   b  is also secured at points along the underside surface of substrate  152  by anchors  162  (not shown).  
         [0042]      FIG. 1E  illustrates another example physical layout of the electrode assembly  150 . In this embodiment, electrode  154   b  is formed from an elongate structure having a hollow core  154   b ′. Lead  160   b  enters core  154   b ′ at an opening at distal end  158   b  and passes through core  154   b ′ of the elongate structure to proximal end  156   b , at which point lead  160   b  connects with electrode  154   b . In one example embodiment, lead  160   b  includes a portion  160   b ′ that is specially adapted to be situated within core  154   b ′. Optionally, electrodes  154   a  and  154   c  are adapted to be pliably compatible with the structure of electrode  154   b  having a portion of lead  160   b  in its core  154   b′.    
         [0043]      FIGS. 1F-1H  illustrate various example electrode structures that each include an elongate electrode portion in the shape of a coil formed around a portion of a lead that is connected to the electrode portion at the proximal end. Structure  164  of  FIG. 1F  is an elongate structure having a length l. Structure  164  has an outer coiled portion  166  made of non-insulated wire and generally helical in its geometry. At least a portion of structure  164  can operate as an electrode when in contact with target tissue. Structure  164  further includes a generally helical inner coiled portion  168  passing through the core  170  defined by the wire of outer coiled portion  166 . Inner coiled portion  168  is thus circumscribed along at least a portion of its length by outer coil portion  166 . One type of wire material that can be suitable for certain implantable applications is 80/20 Pt/Ir. However, persons skilled in the art will recognize that other suitable materials may be used. Inner coiled portion  168  enters core  170  at distal end  172 , and helically extends through core  170  towards proximal end  174 , at which point inner coiled portion  168  makes contact with outer coiled portion  166 .  
         [0044]     In one example embodiment, near proximal end  174 , one or more windings of outer coiled portion  166  have a progressively reducing radius as they approach proximal end  174  such that, at proximal end  174 , windings of outer coiled portion  166  have approximately the same radius as the windings of inner coiled portion  168 . This embodiment is illustrated in  FIG. 1F . Outer coiled portion  166  includes windings  167  having a relatively larger radius, and windings  176  having a relatively smaller radius. Reduced radius windings  176  are situated in a bifilar arrangement at proximal end  174  with the windings of inner coiled portion  168 .  FIG. 1G  is an end view of structure  164  illustrating this embodiment. Outer coiled portion  166  includes windings  167  having larger radius r 1 , and windings  176  near the proximal end  174  having smaller radius r 2 . Winding  178  of outer coiled portion  166  integrally bridges the larger radius windings  167  with the smaller radius windings  176 . In one embodiment, at proximal end  174 , the wire forming outer coiled portion  166  is welded to the wire forming inner coiled portion  168 . Persons skilled in the art will appreciate that other suitable mechanisms of creating an electrical contact between these wires, including, but not limited to, soldering, crimping, twisting, or conductively adhesively bonding, may be utilized.  
         [0045]     In one embodiment, inner coiled portion  168  is positioned relative to larger outer coiled portion  166 &#39;s windings  167  such that, in operation, the inner coiled portion  168  and outer coiled portion  166  do not make contact at any point other than at the proximal end  174 . In one embodiment, inner coiled portion  168  is formed from insulated wire. In another embodiment, inner coiled portion  168  is formed from un-insulated wire, but inner surfaces of windings  167  are insulated. In another embodiment, the radius of the inner coiled portion&#39;s windings R 2  is sized relatively to the outer coiled portion&#39;s windings  167  having larger radius r 1  such that undesired contact points are not created when the structure  164  is elastically flexed to a maximum limit.  
         [0046]     In another type of embodiment, as illustrated in  FIG. 1H , structure  164 ′ includes an insulating material  180  that is coaxially situated between inner coiled portion  168 ′ and large-radius coils  167 ′ of outer coiled portion  166 ′. According to one example of this type of embodiment, as illustrated in the side view diagram of  FIG. 1I , the radii of inner coiled portion  168 ′ and of outer coiled portion  167 ′ remain generally constant over the length of the structure (disregarding the change in radius of the outer portion near the proximal end). Insulator  180  has a generally cylindrical outer wall that is adjacent to outer coiled portion coils  167 ′, and a generally cylindrical inner wall adjacent to inner coiled portion  168 ′.  
         [0047]      FIGS. 1J and 1K  illustrate an example of a variation of the embodiment of  FIG. 1I .  FIGS. 1J and 1K  are, respectively, side view diagrams of a structure in which large-radius coils  167 ″ of outer coiled portion  166 ″ have a constant radius over length l, but inner portion  168 ″ has a spiral geometry in which the coil radius decreases towards the proximal end. The radii of outer and inner walls of insulator  180 ′ also have a profile of decreasing radius that corresponds to the decreasing radius profile of inner coils  168 ″. According to one aspect of this embodiment, the geometry of the structure of  FIGS. 1J and 1K  provides a benefit of securing in place the insulator  180 ′ by preventing it from sliding towards either end of the structure.  
         [0048]     In the configuration of electrodes  104   a - 104   c  ( FIGS. 1A and 1B ) and  154   a - 154   c  ( FIGS. 1C, 1D , and  1 E), having the electrode/lead connections at opposite ends for electrodes of opposite polarity provides improved electric field uniformity and improved corrosion resistance as compared against equivalent configurations having the connections at the same end.  FIGS. 2-5 , described in detail below, illustrate these principles.  
         [0049]      FIG. 2  is a diagram illustrating an electrical circuit model of a state-of-the-art implanted electrode assembly. Distributed resistance  202  represents one or more cathodes  203  connected to the electrotherapy/electrostimulus signal generator. Likewise, distributed resistance  204  represents one or more anodes  205  connected to the opposite pole of the electrotherapy/electrostimulus generator. Distributed resistances  202  and  204  are each distributed over the length L and quantity of their corresponding electrode(s), and are not necessarily equal in magnitude. Target tissue impedance  206  represents the electrical properties of the target tissue interconnecting the electrodes. Target tissue impedance  206  is modeled as a set of parallel resistor-capacitor pairs  206   a - 206   f  distributed over the aggregate volume V that separates the cathode(s) from the anode(s). The resistance of each electrode&#39;s distributed resistance  202  and  204  is generally evenly distributed over length L. With increasing length L, the resistance component of impedance  206  decreases, whereas the capacitance component increases.  
         [0050]     Cathode  203  is connected to the signal generator at top end  208 ; anode  205  is connected to the opposite pole of the signal generator at top end  210 . When the electrotherapy/electrostimulus signal is applied across electrodes  203  and  205 , an aggregate current I generally passes through the resistive component of the target tissue having impedance  206 . Also, an aggregate electric field E generally exists across the electrodes  203  and  205  due to the capacitive component of impedance  206 . However, due to the distributed resistances of the electrodes  203  and  205 , as well as the distributed target tissue impedance  206 , the current I and electric field E are also distributed over the length L and volume V. The distribution of current I and electric field E depends on the distribution of tissue impedance  206 , and on the charge distribution over the length L of each electrode  203  and  205 .  
         [0051]     Within each of the distributed electrode resistances  202  and  204 , there exist a cathode current i cathode  and i anode , and corresponding voltage drops v cathode  and v anode . These currents and voltages occur within each elongate electrode because the electrode resistances  202  and  204  create voltage and current divisions with respect to signal paths through the target tissue. Because the signal generator connections are located at the top ends  208  and  210  of electrodes  203  and  205 , respectively, the cathode and anode currents and voltages have opposite directions and polarities, as illustrated in  FIG. 2 .  
         [0052]     As a result, the distributed current I and electric field E through the target tissue region are not evenly distributed. By way of example, discrete current components i 1 -i 6  located successively at greater distances from the top ends  208  and  210  are successively lower in amplitude such that i 1  has the greatest amplitude while i 6  has the lowest amplitude.  FIG. 3  illustrates an example current distribution through volume V over length L. The axis labeled L corresponds to the L dimension of  FIG. 2 , and the paths  1 - 6  correspond to the paths taken by example current components i 1 -i 6 .  
         [0053]      FIG. 4  illustrates an electrical diagram of an example implanted electrotherapy/electrostimulation electrode assembly according to one embodiment of the present invention. Distributed resistances  402  and  404  represent aggregate elongate electrodes  403  and  405 , respectively. Electrodes  403  and  405 , and the target tissue having impedance  406  are all correspondingly similar to their respective analogues described above with reference to  FIG. 2 . The only difference in the arrangement between the example of  FIG. 2  and the example of  FIG. 4  is the connection of the signal generator to the cathode and anode. Cathode  403  is connected to the signal generator at top end  408 ; whereas anode  405  is connected at bottom end  411 . As a result of this reversal, the bottom end  411  is more negatively charged than the top end  410  of the anode  405 . This causes the charge density of the electrodes  403  and  405  to be evenly distributed along the length L, which results in an even distribution of aggregate current I′ and aggregate electric field E′. Thus, example discrete current components i 1 ′-i 6 ′ each pass through an equivalent impedance. For instance, current component i 2 ′ passes through a smaller portion of cathode impedance  402 , through target tissue impedance component  406   b , and through a larger portion of anode impedance  404 ; whereas current component i 5 ′ passes through a larger portion of cathode impedance  402 , an equivalent target tissue impedance component  406   e , and through a smaller portion of anode impedance  404 .  
         [0054]     Another result of the electrode arrangement of  FIG. 4  is that the cathode current i cathode ′ is directed along the same direction as the anode current i anode ′.  FIG. 5  illustrates the uniform distribution of aggregate current I′ over length L for the example electrode assembly configuration of this embodiment.  
         [0055]     By distributing the charge density evenly over each of the electrodes  403  and  405 , any faradaic processes are also distributed over the surfaces of the electrodes. This effect results in an increased corrosion threshold because electrode corrosion is based on the charge density. Another effect is an increase in capacitance seen by the electrotherapy or electrostimulation signaling. Because the charge density is evenly distributed along length L, the signaling sees a greater overall target tissue capacitance. With an increased overall capacitance created by the charge balancing, more of the activation current is used to charge the electrode double layer and less is available for faradaic processes. The charging of the electrode double layer results in an induced current in the target tissue resulting in the desired stimulation or therapeutic effect.  
         [0056]     The present invention contemplates a variety of electrode forms or shapes, not necessarily limited to straight linear segments.  FIG. 6  is a diagram illustrating an example electrode assembly  600  according to another embodiment of the present invention. Electrode assembly  600  includes a flexible substrate  602  to which arcing elongate electrodes  604   a - 604   c  are secured. Arcing elongate electrodes  604   a - 604   c  each have a first end in a first region A and a second end in a second region B such that the electrodes  604   a - 604   c  arcuately extend between regions A and B. Electrode  604   a  has a lead  610   a  electrically connected to its first end  606   a . Electrode  604   b  has a lead  610   b  electrically connected to its second end  608   b . Electrode  604   c  has a lead  610   c  electrically connected to its first end  606   c . For electrodes  604   a  and  604   c , respective second ends  608   a  and  608   c  have no leads connected thereto. Conversely, for electrode  604   b , first end  606   b  is free of any lead connection. When implanted in a patient, electrodes  604   a - 604   c  are electrically interconnected by the target tissue. In operation, the charge distribution in each of the electrodes is approximately uniform, resulting in electric fields and currents approximately uniformly distributed through the interconnecting target tissue.  
         [0057]     The invention may be embodied in other specific forms without departing from the essential attributes thereof, therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive.