fig1 a 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 . 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 fig1 a , 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 . 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 . 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 . 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 . fig1 b illustrates a physical embodiment of the example electrode assembly 100 of fig1 a . 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 . fig1 c 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 fig1 c . the leads 160 a - 160 c are optionally bundled and secured together by wire tie 161 . fig1 d 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 ). fig1 e 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 ′. fig1 f - 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 fig1 f 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 . 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 fig1 f . 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 . fig1 g 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 . 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 . in another type of embodiment , as illustrated in fig1 h , 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 fig1 i , 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 ′. fig1 j and 1k illustrate an example of a variation of the embodiment of fig1 i . fig1 j 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 fig1 j and 1k provides a benefit of securing in place the insulator 180 ′ by preventing it from sliding towards either end of the structure . in the configuration of electrodes 104 a - 104 c ( fig1 a and 1b ) and 154 a - 154 c ( fig1 c , 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 . fig2 - 5 , described in detail below , illustrate these principles . fig2 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 . 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 . 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 fig2 . 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 . fig3 illustrates an example current distribution through volume v over length l . the axis labeled l corresponds to the l dimension of fig2 , and the paths 1 - 6 correspond to the paths taken by example current components i 1 - i 6 . fig4 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 fig2 . the only difference in the arrangement between the example of fig2 and the example of fig4 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 . another result of the electrode arrangement of fig4 is that the cathode current i cathode ′ is directed along the same direction as the anode current i anode ′. fig5 illustrates the uniform distribution of aggregate current i ′ over length l for the example electrode assembly configuration of this embodiment . 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 . the present invention contemplates a variety of electrode forms or shapes , not necessarily limited to straight linear segments . fig6 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 . 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 .
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 .