Patent Publication Number: US-2011066029-A1

Title: Electromagnetic Medical Device

Description:
FIELD 
     This invention relates to catheters and other insertable or implantable medical devices. 
     BACKGROUND 
     Various specialized insertable or implantable medical devices, including catheters (e.g., ablation catheters, electrophysiological diagnostic catheters, pressure monitoring catheters and delivery catheters), leads (e.g., cardiac and neurological leads) and other elongated medical devices, are sometimes equipped with location sensors for determining the location of the device within a patient. Multiple location sensors may be arrayed along a distal segment of an elongated medical device to provide a more intuitive indication of the device location than would be provided by a single location sensor. 
     One type of location sensor employs an electromagnetic coil in which current is induced by an externally applied electromagnetic field. The location of the coil relative to the field may be determined by measuring the induced current and performing appropriate calculations. Some elongated insertable or implantable medical devices include an extruded polymeric covering, lumen or tube, and in such devices an electromagnetic location sensor may be formed by wrapping wire (e.g., copper wire) in a helical coil around the covering, lumen or tube. Other devices may include an electromagnetic location sensor formed by wrapping wire in a helical coil around a core made from solid or powdered magnetically permeable material. 
     SUMMARY 
     Electromagnetic location sensors formed by wrapping wire around a polymeric covering, lumen or tube do not receive the amplification benefit of being wrapped around a high permeability core. This can limit the induced current signal and impair sensitivity, signal to noise ratio or accuracy. Electromagnetic location sensors formed by wrapping wire around a solid or powdered magnetically permeable core may have greater magnetic permeability than sensors formed around a polymeric covering, lumen or tube, but also have high stiffness. This can make it difficult to insert or implant a medical device equipped with such sensors, especially if the medical device also includes other inflexible or not very flexible elements such as electrical conductors, guide wires or steering wires. Sensors formed using such cores may provide improved results if the core is lengthened appreciably (viz., in the axial direction) so that it extends beyond the wire coil length, but this may further limit flexibility compared to a sensor made on a shorter core. 
     The present invention provides, in one aspect, an insertable or implantable medical device comprising an elongated member having a proximal end, a distal end, at least one conductive coil near the distal end, and electrical conductors which carry current from the coil towards the proximal end, wherein the coil surrounds or is surrounded by a flexible magnetic polymeric composite. 
     The invention provides, in another aspect, a location sensor bobbin comprising a conductive coil surrounding or surrounded by a flexible magnetic polymeric composite, the bobbin being hollow and being sized and shaped to fit on or into an elongated insertable or implantable medical device. 
     The invention provides, in another aspect, a method for making an insertable or implantable medical device, which method comprises forming an elongated member having a proximal end and a distal end, forming at least one conductive coil surrounding or surrounded by a flexible magnetic polymeric composite near the distal end, and connecting electrical conductors to the coil to carry current from the coil towards the proximal end 
     The invention provides, in another aspect, a method for locating an elongated insertable or implantable medical device in a patient, which method comprises exposing at least one electromagnetic coil in such device to an external magnetic field and measuring current induced in such coil, wherein the coil surrounds or is surrounded by a flexible magnetic polymeric composite. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a plan view of a navigable guide catheter provided with a plurality of electromagnetic location sensors on a single core; 
         FIG. 2  is a sectional view of an electromagnetic location sensor taken along line 2-2′ in  FIG. 1 ; 
         FIG. 3  is a plan view of the distal end segment of the  FIG. 1  catheter in a bent position; 
         FIG. 4  is a plan view of a distal end segment of a navigable guide catheter provided with a plurality of electromagnetic location sensors on individual cores; 
         FIG. 5  and  FIG. 6  are sectional views of two additional electromagnetic location sensors; 
         FIG. 7  is a perspective view of a location sensor bobbin; and 
         FIG. 8  through  FIG. 9  are bar graphs showing mechanical and magnetic properties for an unfilled polymer and various flexible magnetic polymeric composites. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description describes certain embodiments and is not to be taken in a limiting sense. All weights, amounts and ratios herein are by weight, unless otherwise specifically noted. The terms shown below have the following meanings: 
     The term “elastomeric” when used in reference to a material means that the material, if stretched to at least 200% of its original length and released, will return with force to substantially its original length. 
     The term “flexible” means bendable. A flexible device may be resiliently bendable (viz., returning to or nearly to its original configuration when bent and then released) or deformably bendable (viz., remaining in or nearly in a bent configuration when bent and then released). 
       FIG. 1  is a plan view of a navigable, steerable open end guide catheter  10  including proximal end  12 , proximal end segment  14 , intermediate segment  16 , distal end segment  18  and distal end  20 . Proximal end segment  14  includes manipulative handle  22 , shielded connector  24 , access hub  26  and pull wire  28  with grip  30 . Handle  22  is joined to elongated member  32  which surrounds pull wire  28  and other elements discussed in more detail below, and whose outer wall  34  may be made for example from a polyurethane, silicone or other biocompatible polymer suitable for use on the exterior of an insertable or implantable medical device. Distal end segment  18  includes a single flexible polymeric composite core  36  provided with a plurality of electromagnetic location sensing coils  38 ,  40 ,  42  and  44  wound around core  36  and separated from one another by unwrapped core portions  46 ,  48  and  50 . More or fewer sensing coils than those shown in the embodiment depicted in  FIG. 1  may be employed, and the sensing coils may have similar or different constructions. In the embodiment shown in  FIG. 1 , the sensing coils all have a similar construction. Core  36  may be made from a medically acceptable polymer within which magnetizable particles (not shown in  FIG. 1 ) are dispersed. The type and loading level (viz., wt. %) of magnetizable particles desirably is sufficient to improve one or more performance-related sensor factors such the minimum required coil diameter, minimum required coil length, minimum required number of wire turns, the sensor or coil flexibility, or other factors influenced by the physical or electromagnetic characteristics of core  36  or sensing coils  38 ,  40 ,  42  and  44 . Core  36  desirably has greater magnetic permeability than outer wall  34 , thereby permitting a reduction in the required coil diameter or length or the required number of wire turns compared to sensing coils formed without such a core, e.g., sensing coils formed by wrapping wire around an outer wall  34  made from an unfilled polymer or from a polymer containing non-magnetically permeable material. Core  36  desirably is sufficiently flexible to facilitate insertion and navigation of catheter  10  through confined areas or tortuous paths within a patient undergoing surgery or treatment, and desirably has greater flexibility than a comparison device having sensing coils formed by wrapping wire around a solid or powdered magnetically permeable core. Flexing of core  36  may for example take place along any or all portions of core  36 , e.g., along lengths of core  36  covered by wire turns, along lengths of core  36  not covered by wire turns, or along all portions of core  36 . Core  36  desirably also is more flexible than intermediate segment  16 , as this may facilitate bending or otherwise flexing distal end segment  18  rather than intermediate segment  16  as catheter  10  is advanced into a patient. 
     Distal end segment  18  may also include a generally ring-shaped anchoring member  52  encircling the outer circumference of sleeve  54  near the distal end  20  of catheter  10 . Pull wire  28  may be fixedly attached to anchoring member  52 , using for example welding or other appropriate bonding or joining methods. Anchoring member  52  may optionally serve as an electrode with pull wire  28  serving as a conductive element to carry electrical current between anchoring member  52  and a contact or other fitting in proximal connector  24 . Distal end segment  18  also may include end cap member  58  equipped with central opening  60 . 
     Opening  60  may communicate with one or more generally central lumens (such as the single central lumen  70  shown in  FIG. 2 ) extending axially within elongated member  32  and thence with access hub  26  such that a medical device or therapy may be delivered through hub  26  and the central lumen(s) and may exit opening  60 . End cap member  58  may be formed from a biocompatible polymeric material and may be over-molded onto distal end  20  of catheter  10 . End cap member  58  may if desired be formed from a conductive biocompatible metal or alloy, for example stainless steel, platinum, iridium, titanium, or alloys thereof, and may serve as an electrode for sensing cardiac or other electrophysiological signals or for delivering current to a treatment site. 
       FIG. 2  shows a sectional view taken along line  2 - 2 ′ in  FIG. 1 . Central lumen  70  is defined by inner wall  72  of core  36 . Magnetically permeable particles  74  are generally uniformly distributed throughout core  36 . Pull wire  28  may as noted above be connected at its distal end to anchoring member  52 . Electrode conductor  76  may for example be connected to conductive end cap member  58  or to another conductive surface (not shown in  FIG. 1 ) at or near-the distal end of catheter  10 . Conductors  80  and  82  may for example be connected to the respective distal and proximal ends of sensing coil  44 . Conductor  84  may for example be connected to the distal end of sensing coil  42 . Sensing coil  42  may as shown in  FIG. 2  have several layers of wire surrounding core  36 , or may have more, fewer or even a single layer of wire. 
       FIG. 3  is a plan view of a portion of the distal end of catheter  10  in a bent position. Bending may be restricted at coils  42  and  44 , and less restricted at unwrapped core portions  48  and  50 . 
       FIG. 4  is a plan view of a portion of the distal end of a closed end navigable guide catheter  400 . Catheter  400  includes a tip  408 , a generally ring-shaped anchoring member  410  encircling the outer circumference of sleeve  412 , and sensors  418 ,  420 ,  422  and  424 . Sensor  418  is formed by sensing coil  438  on core  458 . Sensor  420  is formed by sensing coil  440  on core  460 . Sensor  422  is formed by sensing coil  442  on core  462 . Sensor  424  is formed by sensing coil  444  on core  464 . More or fewer sensors than those shown in  FIG. 4  may be employed, and the sensors may have similar or different constructions. In the embodiment shown in  FIG. 4 , sensors  418 ,  420  and  422  have similar constructions and sensor  424  (the most distally-located sensor) has a different construction. A device having a group of sensors including one different sensor such as sensor  424  need not deploy the different sensor in the most distally-located sensor position, and may instead deploy the different sensor in the most proximally-located sensor position or anywhere in between the most distal and most proximal sensor locations. Although each of sensors  418 ,  420 ,  422  and  424  is flexible, sensor  424  may have a more flexible construction than sensors  418 ,  420  and  422 . Doing so may make it easier to bend sensor  424  and thereby aid in steering catheter  400  within a patient. Such more flexible construction may be accomplished in a variety of ways, including using fewer overlapping turns of wire (e.g., using a narrower or shorter core), more widely spaced turns of wire, thinner wire or more flexible wire in coil  444  compared to coils  438 ,  440  and  442 ; by using one or both of a lower loading of magnetically permeable particles or a more flexible polymer in core  464  compared to cores  458 ,  460  and  462 ; by using one or both of a larger inside diameter or smaller outside diameter for core  464  than for cores  458 ,  460  and  462 ; or by using a bellows-like construction, weakening lines, varying wall thickness or other flexibility-inducing measures to make core  464  more flexible than cores  458 ,  460  and  462 . 
       FIG. 5  shows a sectional view of a sensor  500  for use in the disclosed insertable or implantable medical devices. Sensor  500  includes coil  542  wound inside core  536 . Central lumen  570  is defined by the inner wall  572  of coil  542 . A protective polymeric coating (not shown in  FIG. 5 ) may be applied to inner wall  572  to prevent damage to the wire insulation in coil  542 . Magnetically permeable particles  574  are generally uniformly distributed throughout core  536 . Pull wire  528  and conductors  576 ,  580 ,  582  and  584  may all pass through core  536 . 
       FIG. 6  shows a sectional view of a sensor  600  for use in the disclosed insertable or implantable medical devices. Sensor  600  includes coils  642  and  644  which are respectively wound inside and wrapped outside core  636 . Central lumen  670  is defined by the inner wall  672  of coil  642 . As in sensor  500 , a protective polymeric coating (not shown in  FIG. 6 ) may be applied to inner wall  672  to prevent damage to the wire insulation in coil  642 . Magnetically permeable particles  674  are generally uniformly distributed throughout core  636 . Pull wire  628  and conductors  676 ,  680 ,  682  and  684  may all pass through core  636 . 
       FIG. 7  is a perspective view of a location sensor bobbin  700  for use in manufacturing insertable or implantable medical devices. Bobbin  700  includes a discrete hollow cylindrical core  736  made from the disclosed flexible magnetic polymeric composite. Coil  742  is formed from fine-gauge insulated wire  780  wrapped around core  736 . Coil ends  790  and  792  may be cut to an appropriate length and soldered or otherwise connected to suitable conductors, or may simply be left longer than shown in  FIG. 7  and used as conductors in a later-formed insertable or implantable medical device (not shown in  FIG. 7 ). Bobbin  700  is flexible and depending on the nature of the chosen magnetic polymeric composite may be resiliently or deformably bent with respect to its main axis of symmetry  7 - 7 ′. 
     A variety of polymers may be employed in the disclosed flexible magnetic polymeric composite, including polyamides (e.g., nylon rubbers), polyether amides (e.g., PEBAX™ block copolymer from Arkema), polyethylenes, fluoropolymers (e.g., polytetrafluoroethylene, polyvinylidene fluoride, and other polymers and copolymers of fluorinated monomers including DYNEON™ fluoropolymers from Dyneon LLC and TEFLON™ fluoropolymers from E. I DuPont de Nemours and Co.), polyimides, organosilicones and other silicone rubbers (e.g., SILASTIC™ elastomers from Dow Corning Corp.), polyurethanes (e.g., PELLETHANE™ thermoplastic polyurethane elastomers from Dow Chemical Co.), polyvinyl chloride, mixtures thereof, and other flexible polymeric materials which will be familiar to persons skilled in the field of insertable or implantable medical devices. Resiliently bendable cores may more readily be made by using elastomeric polymers, and deformably bendable cores may more readily be made by using elongatable but non-elastomeric polymers. The bending characteristics of a finished core may also be influenced by the chosen type and amount of magnetically permeable particulate materials. 
     A variety of magnetically permeable particulate materials may be employed in the disclosed flexible magnetic polymeric composite. The magnetically permeable material may for example be paramagnetic, ferromagnetic or ferrimagnetic, and may for example contain metals including iron, cobalt, nickel or gadolinium, used as is, alloyed with other metals, or used in oxide form and optionally combined with other oxides to form a variety of magnetically permeable ceramics. Desirably the magnetizable particles cause low or no hysteresis loss when the completed devices are used in a patient. Exemplary magnetically permeable materials include iron powder, carbonyl iron, magnetite, iron-silicon alloys, aluminum-nickel-cobalt (alnico) alloys, samarium-cobalt alloys, neodymium-iron-boron (NdFeB) alloys, ferrites, and other finely-divided magnetic particulates which will be familiar to persons skilled in the field of magnetically permeable materials. Exemplary commercially available magnetically permeable materials include HIPERCO™ 50 iron-cobalt-vanadium soft magnetic alloy, PERMALLOY™ nickel-iron magnetic alloys, PERNENDUR™ cobalt-iron and cobalt-iron-vanadium alloys and SUPERMALLOY™ nickel-iron-molybdenum alloys. The particles may for example have an average particle diameter of about 1 to about 100, about 2 to about 70 or about 10 to about 50 micrometers. Larger or smaller particles, including submicron particles or nanoparticles, may be used if desired for particular applications. The particles desirably have an average particle diameter less than about 20% of the core wall thickness. The particles may be surface-treated to improve their dispersibility in the magnetic polymeric composite. The magnetic polymeric composite desirably contains sufficient particulate material to increase induced current in the disclosed coil, compared to a device that does not contain such particulate material, when the coil is exposed to a fluctuating applied external magnetic field. The magnetic polymeric composite may for example contain about 2 to about 60, about 5 to about 50 or about 10 to about 50 volume % particles. The addition of magnetically permeable particles may also affect, sometimes adversely, other composite physical properties (for example, ultimate tensile strength, strain at yield or elongation at yield) and accordingly it may be desirable to strike a balance between an increase in magnetic permeability and a potential decrease in other physical properties. Relatively small additions of magnetically permeable particles can provide very desirable overall performance. For example, an addition of about 20 volume % of 10 micrometer average diameter SUPERMALLOY nickel-iron-molybdenum alloy particles to PEBAX block copolymer can provide an appreciable increase in magnetic permeability while maintaining other desirable physical properties such as ultimate tensile strength and strain at yield. 
     The core may comprise, consist essentially of or consist of the disclosed polymer and magnetically permeable particles. The core may if desired contain a variety of adjuvants, including fillers, extenders, radioopacifying agents (e.g., radioopacifying fillers), surface-active agents, polymer processing aids, pigments, and other ingredients which may improve the performance or processability of the magnetic polymeric composite. 
     The magnetic polymeric composite may be processed to form cores in a variety of ways including extrusion, pressure molding, dip coating and other techniques including those discussed in U.S. Pat. No. 5,817,017 to Young et al., for example by extrusion at or above the polymer melt flow temperature. The resulting cores may have a variety of shapes. For example, the core may have a cylindrical shape with coils wrapped around the outside of all or part of the cylinder sidewall, or with coils wound inside all or part of the cylinder sidewall. The core may also have a toroidal shape with coils wrapped entirely or partially around the toroid surface. 
     The wire in the disclosed coils may be made from a variety of materials including copper, gold and other medically acceptable metals or alloys which will be familiar to persons skilled in the field of insertable or implantable medical devices. The wire may be any type and diameter suitable for formation of sufficiently compact and durable coils, e.g., varnish-or otherwise-insulated wire in American Wire Gauge (AWG) sizes 58 (0.01 mm or 0.0004 in) to 38 (0.1 mm or 0.004 in). Larger or smaller diameter wire may be used if desired for particular applications. The coil may have a variety of lengths, for example a length of about 1.27 mm (0.05 in) to about 6.35 mm (0.25 in). The coil length desirably is less than about 2.5 mm (0.1 in). The coil may cover all or only a portion of the core. In one exemplary embodiment the core is about two to about three times (e.g., about 2½ times) as long as the coil along the central core axis. The number of coil turns may vary, and may for example be about 33 to about 167 turns per layer (e.g., about 66 turns per layer) for a four layer coil having a 2.5 mm length. The coil may be wound in a single layer or in a plurality of layers, with a low number of layers being desirable where reduced outer diameter or increased inner diameter are desired, for example to permit use of a smaller device in small blood vessels, to reduce recovery time, or to accommodate space for additional features in an existing device. In some embodiments, coils having fewer than 100 turns may be employed. Other numbers of turns and wire diameters may be employed depending on the desired sensor application. Exemplary coil configurations include those shown in U.S. Pat. No. 5,727,552 to Saad, U.S. Pat. No. 6,385,471 B2 to Hall et al. and U.S. Pat. No. 7,130,700 B2 to Gardeski et al., in U.S. Patent Application Publication No. US 2004/0097806 A1 to Hunter et al. and in published International Patent Application No. WO 99/40957 A1. A suitably thin and optionally flexible coating may be applied to the finished coil to help hold the wire in place when the core is bent or to help prevent damage to insulation on the coil wire. 
     The outermost portion of the core and coil may have a variety of diameters. Exemplary maximum diameters for the core, coil or for the distal end of the disclosed devices are for example at least about 1 French (0.33 mm or 0.013 in) and less than or equal to about 10 French (3.3 mm or 0.131 in), 9 French (3 mm or 0.118 in), 8 French (2.7 mm or 0.105 in), 7 French (2.3 mm or 0.092 in), 6 French (2 mm or 0.079 in), 5 French (1.67 mm or 0.066 in), 4 French (1.35 mm or 0.053 in) or 3 French (1 mm or 0.039 in). 
     The core and coil may be designed with the aid of equation I shown below: 
     
       
         
           
             
               
                 
                   L 
                   = 
                   
                     π 
                     × 
                     μ 
                     × 
                     
                       
                         ( 
                         
                           
                             n 
                             2 
                           
                           × 
                           
                             r 
                             ave 
                             2 
                           
                         
                         ) 
                       
                       l 
                     
                   
                 
               
               
                 I 
               
             
           
         
       
     
     where: L is the induced current,
         μ is the magnetic permeability of the core material,   n is the number of wire turns,   r ave  is the effective radius of the coil, and       

     l is the length of the coil. 
     In general, it is desirable to produce the largest signal possible in the coil so that the coil position in space can be determined with less error or less signal-to-noise ratio. However, as r ave  decreases, the current induced in the coil decreases exponentially. Increasing the number of wire turns can provide an offsetting exponential increase in induced current. However, this may increase the coil length l and thereby undesirably increase coil rigidity. Through appropriate selection of the magnetic polymeric composite and the type and loading level of magnetically permeable particles, the core permeability μ may be increased sufficiently to permit downsizing the coil radius or changing the core or coil construction in other ways without sacrificing flexibility, minimum turn radius or other relevant steering or navigation properties for an insertable or implantable medical device. 
     The disclosed coils and cores may be used in a variety of insertable or implantable medical devices, including catheters (e.g., open-ended or close-ended ablation catheters, balloon catheters, stent delivery catheters, electrophysiological diagnostic catheters, pressure monitoring catheters, biologic delivery systems, and intravascular imaging devices such as intravascular ultrasound or IVUS and intracardiac echocardiography or ICE), leads (e.g., cardiac pacing, cardiac defibrillation cardiac or neurological leads), endoscopes, biopsy tools and other elongated medical devices. The distal ends of such devices may have a variety of shapes including ball ends, tapered ends and blunt ends. The devices may have no lumen, a single lumen or multiple lumens. The devices may include splined bodies (e.g., as shown in the above-mentioned U.S. Pat. No. 7,130,700 B2), and if desired all or a portion of such splined bodies may be made from the disclosed magnetic polymeric composite. The devices may include other components employed in insertable or implantable medical devices, for example pull wires, guide wires or stylets, electrodes, conductors, fluid delivery or other needles, additional sensors, deflection members, selectively activated shape memory devices and other components such as those discussed in the above-mentioned U.S. Pat. No. 7,130,700 B2. The disclosed devices may be steered or located in a patient using a variety of equipment and techniques including those discussed in U.S. Pat. No. 5,983,126 to Wittkarnpf and published International Patent Application No. WO 01/24685 A2. 
     The invention is further illustrated in the following non-limiting examples in which all parts and percentages are by weight unless otherwise indicated. 
     Example 1  
     A sample of PEBAX 55D block copolymer from Arkema was compounded in a batch mixer with 22 micrometer average diameter SUPERMALLOY particles (from Ultrafine Powder Technology, Inc., Woonsocket, R.I.) at 0 and 20% loading levels. The ultimate tensile strength, strain at yield and magnetic permeability for the resulting composites are shown in  FIG. 8  together with the results obtained for the unfilled copolymer. The magnetic permeability value increased from 1 to more than 1.4 as the loading level increased from 0 to 20%. The increased permeability observed at a 20% loading level should enable r ave  , the effective coil radius, to be reduced by about 18% or more while still maintaining a comparable level for L, the induced current. 
     Example 2 
     Using the method of Example 1, PEBAX block copolymer was compounded with 22 micrometer average diameter iron particles (from Atlantic Equipment Engineers, Bergenfield, N.J.), PERMENDUR cobalt-iron-vanadium alloy particles (from Ultrafine Powder Technology, Inc.) or with SUPERMALLOY nickel-iron-molybdenum alloy particles (from Ultrafine Powder Technology, Inc.), at a 20 volume % loading level. The ultimate tensile strength, strain at yield and magnetic permeability for each of the resulting composites are shown in  FIG. 9  together with the results obtained for the unfilled copolymer. The magnetic permeability value when using iron particles was more than 1.7, and the magnetic permeability value when using the alloys was about 1.3. These increases in magnetic permeability should enable r ave  to be reduced by 30% when using iron or by about 14% when using the alloys while still maintaining a comparable level for L. 
     Example 3  
     Using the method of Example 1, PEBAX block copolymer was compounded with 10 and 22 micrometer average diameter SUPERMALLOY particles (from Ultrafine Powder Technology, Inc.), at a 20 volume % loading level. The ultimate tensile strength, strain at yield and magnetic permeability for each of the resulting composites are shown in  FIG. 10  together with the results obtained for the unfilled copolymer. 
     Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiments, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.