various embodiments , including preferred embodiments , will now be described in detail below with reference to the drawings : fig1 illustrates a preferred embodiment of multi - electrode lead 10 . while the leads illustrated and generally discussed here have eight electrodes , lead 10 of the present invention may be constructed having any number of electrodes ( i . e ., one or more ): lead 10 includes a proximal end 12 and a distal end 14 . the proximal end 12 includes a plurality of electrically conductive terminals 16 , and the distal end 14 includes a plurality of electrically conductive electrodes 18 . while typically each terminal 16 is electrically connected to a single electrode 18 via a conductor 20 ( fig3 ), a terminal 16 can be connected to two or more electrodes 18 . terminals 16 and electrodes 18 are preferably formed of a non - corrosive , highly conductive material . examples of such material include stainless steel , mp35n , platinum , and platinum alloys . in a preferred embodiment , terminals 16 and electrodes 18 are formed of a platinum - iridium alloy . spanning between electrodes 18 of the distal end 14 and terminals 16 of the proximal end 12 , body 22 is formed from a medical grade , substantially inert material , for example , polyurethane , silicone , or the like . while the specific material used for body 22 is not critical to the present invention , body 22 must be non - reactive to the environment of the human body , provide a flexible and durable ( i . e ., fatigue resistant ) exterior structure for the components of lead 10 , and insulate adjacent terminals 16 and / or electrodes 18 . serving as a sheath , body 22 substantially provides the exterior structure that contains the internalized elements of lead 10 . specifically , body 22 provides an enclosure for each conductor 20 that connects a terminal 16 with one or more electrodes 18 . each conductor 20 is formed of a conductive material that exhibits the desired mechanical properties of low resistance , corrosion resistance , flexibility , and strength . for consideration , however , it should be appreciated that in the context of a multiple electrode lead 10 , a plurality of conductors 20 are required to fit within the interior of body 22 . accordingly , the cross - sectional area of each conductor 20 is restricted . as but one example , for a lead in accordance with the present invention that has an outer diameter of approximately 0 . 055 inches , conductor 20 could be on the order of approximately 0 . 0065 inches . while stranded bundles of stainless steel , mp35n , platinum , platinum - iridium alloy , drawn - brazed silver ( dbs ) or the like can be used , the preferred embodiment of conductors 20 utilizes wires formed of multi - strands of drawn - filled tubes ( dft ), as illustrated in fig4 . each strand is formed of a low resistance material 20 a and is encased in a high strength material 20 b ( preferably , metal ). a selected number of strands ( seven , for this example ) are wound and coated with an insulative material 20 c . with regard to the operating environment of the present invention , insulative material 20 c protects the individual conductors 20 if body 22 were breached during use . wire formed of multi - strands of drawn - filled tubes to form conductors 20 , as discussed here , is available from temp - flex cable , inc . ( city , state ). in addition to providing the requisite strength , flexibility , and resistance to fatigue , conductors 20 formed of multi - strands of drawn - filled tubes , in accordance with the preferred embodiment , provide a low resistance alternative to other conventional materials . specifically , a stranded wire , or even coiled wire , of approximately 60 cm and formed of mp35n or stainless steel or the like would have a measured resistance in excess of 30 ohms . in contrast , for the same length , a wire formed of multi - strands of drawn - filled tubes , as illustrated in fig4 , could have a resistance less than 4 ohms . accordingly , in a preferred embodiment , each conductor 20 , having a length equal to or less than 60 cm , has a resistance of less than 25 ohms . in a more preferred embodiment , each conductor 20 , having a length equal to or less than 60 cm , has a resistance equal to or less than 10 ohms . in a most preferred embodiment , each conductor 20 , having a length equal to or less than 60 cm , has a resistance of less than 4 ohms . as an alternative embodiment , body 22 can further encompass stylet tubing 24 ( fig3 ). stylet tubing 24 extends from the proximal end 12 to a point within a distal portion of lead 10 ; however , in a preferred embodiment , stylet tubing 24 extends to cap electrode 34 . in cooperative reference to fig2 , stylet tubing 24 operatively receives stylet 100 for purposes of allowing better control over lead 10 during placement . while the following discussion provides but one example of a sequence of steps to form a lead similar to that illustrated in fig2 and 3 . one having ordinary skill in this art shall appreciate that the following steps may be performed in a differing order or otherwise inconsequentially modified to still yield the present invention . consequently , such minor variations are still regarded as being within the scope of the present invention and should be construed in such manner . furthermore , for purposes of illustration , the following example includes certain physical dimensions to illustrate the relationship between elements as well as effects of differing processes . accordingly , the provided physical dimensions are used merely for example and shall not restrict the scope of the present invention . the following illustrative example concerns the construction of an eight electrode , epidural lead that accommodates a stylet . one skilled in the art shall appreciate , however , that a lead in accordance with the present invention may have more than or less than eight electrodes and / or have a larger or smaller diameter than the following example and remain within the scope of this disclosure . in reference to fig5 , stylet tubing 24 is positioned over mandrel 150 . stylet tubing 24 has an outer diameter of approximately 0 . 02 inches . depending on the quantity of conductors 20 required ( e . g ., for this illustration , eight ) and the size ( i . e ., diameter ) of such conductors 20 , arranging and securing conductors 20 can be problematic when they are being arranged and secured about an element having the dimensions of stylet tubing 24 . while any number of techniques may be used to achieve such arrangement of conductors 20 relative to stylet tubing 24 , fig1 illustrates an example of a fixture 200 that can assist in this task . specifically , fixture 200 includes first rotary clamp 202 , iris 204 , iris 206 , second rotary clamp 208 , and clamp 210 . rotary clamps 202 and 208 each include a corresponding plurality of conductor clamps 203 . while not required , it is preferred that the plurality of conductor clamps 203 of each rotary claim 202 and 208 be positioned within an arbitrary perimeter 205 , whereas perimeter 205 should be equal to or greater than a fully - opened inner diameter of either iris 204 or 206 . as illustrated , mandrel 150 , including stylet tubing 24 , passes through irises 204 and 206 and second rotary clamp 208 and is secured between clamps 202 and 210 . each conductor 20 similarly passes through irises 204 and 206 and is secured between respective clamps 203 of rotary clamps 202 and 208 . conductors 20 secured within fixture 200 are prepared for assembly in that a prescribed amount of insulative material 20 c is removed at or about the proximal and distal ends of each conductor 20 to expose conductive material 20 a and 20 b . as will be discussed later , this exposed conductive material 20 a and 20 b of the proximal and distal ends of each conductor 20 is eventually joined to an electrode 18 and a terminal 16 . accordingly , the exposed conductive material 20 a and 20 b is arranged at differing positions relative to stylet tubing 24 to accommodate the serial arrangement of terminals 16 and electrodes 18 . the rotational nature of rotary clamps 202 and 208 provides unobstructed access to the in - process lead 10 . specifically , upon securing a single conductor 20 between opposing ( or non - opposing ) clamps 203 , the rotary clamps 202 and 210 are simply rotated to allow access to unoccupied clamps 203 . when all of the conductors 20 are strung between claims 202 and 208 , irises 204 and 206 are actuated to close and draw conductor ( s ) 20 closely about the outer diameter of stylet tubing 24 . when conductor ( s ) 20 are resting against the outer diameter of stylet tubing 24 , conductor ( s ) 20 are secured in place . conductor ( s ) 20 may be secured using adhesive and / or subjected to a force applied through use of a temporary or permanent restraint , for example , one or more crimped collars . while the illustration of fig1 shows but one embodiment of fixture 200 , one skilled in the art should appreciate that other techniques / structures may be employed to position conductors 20 adjacent an exterior surface of stylet tubing 24 . specifically , clamps 203 of each rotary clamp 202 and 208 could be moveable along respective radial paths ( not shown ) that would allow strung conductors 20 to be moved from a first position to a second position adjacent the exterior surface of stylet tubing 24 . alternatively , conductors 20 could initially be secured to one end of stylet tubing 24 and only a single iris could be used to draw the unsecured portions of conductors 20 toward stylet tubing 24 . as yet another alternative , while the various alternatives offered provide some mechanism to control the rate of movement and relative positioning of conductors 20 , an operator could simply manipulate the conductor ( s ) 20 to manually position and secure them relative to stylet tubing 24 . once all conductors 20 are secured to stylet tubing 24 , transitional element 26 , electrode ( s ) 18 , electrode spacer ( s ) 28 , outer tubing 23 , terminal spacer ( s ) 30 , terminal ( s ) 16 , and stylet guide 32 are positioned over , and concentrically arranged with , stylet tubing 24 . the arrangement of these elements is in accordance with that illustrated in fig5 . transitional element 26 is illustrated in fig6 . as will be discussed later , transitional element 26 provides a platform to receive cap electrode 34 ( fig1 ). transitional element 26 further provides a durable guide 26 a to direct a distal end ( not shown ) of stylet 100 to cap electrode 34 via passage 26 b . transitional element 26 is preferably formed of a conductive material , for example , the same material used to form electrodes 18 . electrode spacer 28 is illustrated in fig7 . similarly , terminal spacer 30 is illustrated in fig8 . functionally , electrode spacer 28 and terminal spacer 30 accurately defines a space between adjacent electrodes 18 and terminals 16 , respectively . electrode spacer 28 and terminal spacer 30 are preferably formed of the same material as outer tubing 23 . however , spacers 28 and 30 may be formed of a material that differs from that of outer tubing 23 ; provided however , any differing material used for electrode spacer 28 and / or terminal spacer 30 must be compatible with and possess largely the same mechanical properties ( e . g ., non - reactive to the environment of the human body , flexible and durable ) as outer tubing 23 . at least for purposes of this example , spacers 28 , and 30 are formed of a polyurethane material , for example , bionate 75d ( polymer tech . group , city , state ). as is noted in fig5 , spacers 28 and 30 should have an outer diameter greater than lead 10 . outer tubing 23 separates electrodes 18 from terminals 16 . in a preferred embodiment , outer tubing 23 has a diameter substantially equal to a diameter of lead 10 . alternatively , outer tubing 23 may have a diameter less than lead 10 , or a diameter greater than lead 10 . in regard to the latter alternative , outer tubing 23 must have a wall thickness greater than a differential between a radius of lead 10 and a radius ( to the outer diameter ) of outer tubing 23 . for this particular example , outer tubing 23 has a nominal outer diameter of approximate 0 . 055 inches . stylet guide 32 is illustrated in fig9 . stylet guide 32 provides an inlet to stylet tubing 24 . stylet guide 32 is preferably formed of conductive material , for example , the same material used to form electrodes 18 . stylet guide 32 , as well as terminals 16 , electrodes 18 , and transitional element 26 , preferably each have an outer diameter equal to or greater than a nominal diameter of lead 10 . in a more preferred embodiment , these elements each have an outer diameter greater than a nominal diameter of lead 10 . following the assembly of each of the elements described above , terminals 16 and electrodes 18 are joined to their respective conductors 20 . generally , each terminal 16 ( and each electrode 18 ) is positioned relative to exposed conductive material 20 a and 20 b of a conductor 20 and is joined in a manner that facilitates a transfer of electrical energy , for example , resistance weld or laser weld . once all terminals 16 and electrodes 18 are secured , stylet guide 32 is secured to a proximal - most terminal 16 , and transitional element 26 is secured to a distal - most electrode 18 . provided transitional element 26 and stylet guide 32 are formed a conductive material , these elements may be secured using a process consistent with that used to join terminals 16 and electrodes 18 with conductors 20 . otherwise , transitional element 26 and stylet guide 32 can be joined using an adhesive , cement or the like . the completed assembly ( fig5 ) is then over - molded , using well known injection molding techniques , using a material having mechanical properties consistent with a material ( s ) used to form outer tubing 23 , electrode spacer 28 , and terminal spacer 30 . in a preferred embodiment , the over - molding material and the material of outer tubing 23 , electrode spacer 28 , and terminal 28 are the same . this process has the beneficial effect of unitizing the element assembly to form lead 10 . moreover , electrode spacers 28 and terminal spacers 30 are placed in a state of flow , which , at least in part , results in a filling of regions between terminals 16 / electrodes 18 and stylet guide 24 . consequently , terminals 16 and electrodes 18 are partially surrounded ( i . e ., along an interior surface ) and supported by a fused matrix of ; material . importantly , as electrode spacers 28 and terminal spacers 30 are formed of a material mechanically equivalent to that of body 22 / outer tubing 23 , the stimulation / sensing portion and terminal portion of lead 10 are stabilized and strengthened while also retaining their flexible properties . the over - molded assembly ( not shown ) is then subjected to a grinding process to remove all excess material . in a preferred process , the over - molded assembly is subject to centerless grinding , wherein excessive material , including over - molded material , electrode material , terminal material , and the like , is removed . pursuant to the described over - molding and grinding of the entire lead assembly , an isodiametric lead is obtained , which is further free of any gaps or voids between insulative material and conductive material that may otherwise exist in conventional devices . following the grinding process , cap electrode 34 is affixed to transitional element 26 using conventional means , for example , resistance welding , laser welding , or the like . while addressed in part above , as the invention has been described herein relative to a number of particularized embodiments , it is understood that modifications of , and alternatives to , these embodiments , such modifications and alternatives realizing the advantages and benefits of this invention , will be apparent to those of ordinary skill in the art having reference to this specification and its drawings . it is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein , and it is intended that the scope of this invention claimed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled .
an implantable , substantially isodiametric , low resistance implantable lead having at least one electrode positioned in a stimulation / sensing portion of the lead as well as a method of manufacturing the same . at least the stimulation / sensing portion is unitized through partially surrounding and supporting insulation and conductive element of the stimulation / sensing portion with a fused matrix of material having mechanical properties consistent with a body of the lead .