Microminiature EM Coil Sensor Pull Ring For Catheter

A microminiature electro-magnetic coil sensor pull ring with a pull wire attached thereto is used in changing the angle of a distal end of a medical catheter. The tubular pull ring has a connection recess with a flat bottom machined into the full wall thickness and located proximal to a coil wrap area. Circuit wires are electrically connected to the two lead ends of the coil within the connection recess, such that neither the circuit wires nor the lead ends stand proud of the full wall thickness. The coil wrap area is also recessed, and can have side walls defining an offset angle for the turns of the coil. In another aspect, a coil is wound around one of the pull wires for the pull ring.

BACKGROUND OF THE INVENTION

Microminiature electrical coils are used in various types of electronic and medical equipment, with an example being the AURORA electromagnetic tracking system provided by Northern Digital Inc. d/b/a NDI. Such electromagnetic tracking systems utilize a sensor coil to read and/or respond to electromagnetic fields, with a microprocessor based system interpreting the electrical or magnetic response to determine a location of the coil in three-dimensional space. U.S. Pat. Nos. 6,288,785, 6,385,482, 6,553,326, 6,625,465, 6,836,745, 7,353,125, 7,469,187, 7,783,441 and 7,957,925 describe such systems, incorporated by reference.

A preferred prior art coil used in the electromagnetic tracking system uses an extremely thin copper wire (such as 58 American Wire Gauge (AWG), i.e., 0.00039″ in diameter) wound around a core. The core may be a solid cylinder or a hollow tube or lumen. The core is typically formed of a ferrite-based or soft magnetic material, with a preferred core material being mu-metal. The core may be coated with a parylene layer to provide insulation. The electro-magnetic (EM) sensor coil is typically quite small, and is placed in the catheter shaft wall or interior of catheter. An application of such systems is with the coil configured as part of a catheter, to electromagnetically track the location of the catheter coil within the human body during a medical procedure. For instance, example applications include the use of the sensor coil in pulmonary bronchoscopy, mapping catheters, ablation catheters, diagnostic catheters and electrophysiology (EP) catheters.

In the prior art manufacturing assembly process for creating the EM sensor coil, two wires are used as leads for the coil, with the two leadwires being twisted into a twisted pair. The leadwires are typically thicker than the coil wire, such as 40 AWG (i.e., 0.003145″, or about eight times the diameter of the coil wire) leadwires encased in insulation but with their ends stripped. Since the coil wire is very tiny, it is difficult to attach the larger 40 AWG lead wires to the smaller 58 AWG coil wire ends. The typical connection between the coil wire and the leadwires involves crudely wrapping the coil wire ends around each leadwire end and then soldering. The sensor coil is encapsulated, such as with a biocompatible ultra-violet adhesive over the top of the coil windings, termination points, and a minimum of three twists of sensor leadwires.

Prior art EM sensor coils are typically somewhat small and fragile, and problems can occur with prior art EM sensor coils when being handled assembled into the catheter structure. One or both of the flexible ends of the coil wires may break, as well as one or both leadwires, or one or both ends of the coil wire, pulling out of the adhesive encapsulation. Additionally, because the EM sensor coil diameter is generally somewhat smaller than the diameter of the catheter it is a component of, a location offset can be introduced with the EM sensor coil axis being different from (and possibly skewed relative to) the catheter axis.

Separately, pull ring assemblies can be utilized in medical catheters to provide catheter steering capabilities. A pull ring with steering wire assembly can incorporate a single pull-wire attached to the pull ring or a plurality of pull wires attached to the pull ring to accommodate bi-directional or multi-directional steering. An example of a 0.1″ diameter stainless steel pull ring using two 0.004×0.012″ flat (equivalent to about 32 AWG) stainless steel pull wires is disclosed in U.S. Pat. Pub. No. 2007/0299424, incorporated by reference.

These various prior art structures have their own cost and space requirements and introduce potential failure locations into the final catheter product. Better solutions are needed.

BRIEF SUMMARY OF THE INVENTION

The present invention is a microminiature electro-magnetic coil sensor pull ring for use in changing the angle of a distal end of a medical catheter for navigation of the catheter through human tissue, and a method of manufacturing such a microminiature electro-magnetic coil sensor pull ring. The one aspect, the tubular pull ring has geometric features which facilitate having the coil formed about the tubular pull ring. One such geometric feature is a connection recess into the full wall thickness and located proximal to the coil wrap area. Circuit wires are electrically connected to the two lead ends of the coil within the connection recess, such that neither the circuit wires nor the lead ends stand proud of the full wall thickness. In another aspect, a coil is wound around one of the pull wires for the pull ring, and electrical connections can still be made within one or more connection recesses of the pull ring.

While the above-identified drawing figures set forth preferred embodiments, other embodiments of the present invention are also contemplated, some of which are noted in the discussion. In all cases, this disclosure presents the illustrated embodiments of the present invention by way of representation and not limitation. Numerous other minor modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

FIG. 1shows a first preferred embodiment of a microminiature electrical coil sensor pull ring10of the present invention, intended for use as a component in a catheter assembly (not shown). The coil sensor pull ring10includes components which are primarily structural of a rigid tubular pull ring12guided by at least one but more commonly a plurality of pull wires14. The coil sensor pull ring10includes components which are primarily electrical and/or magnetic of a wire coil16and electrical circuit wires18for the wire coil16.

The pull ring12is placed in the distal end of the catheter adjacent the catheter tip, and is used to bend the distal end of the catheter during navigation through human tissue (such as an artery or vein) so the catheter can be advanced to the desired catheter deployment site. The pull wires14must have sufficient flexibility to curve through the catheter path within the human tissue, while being able to support the pull force needed to deflect the angle of the pull ring12for navigation as the catheter is advanced into the human body. In the first example shown, the pull wires14have a rectangular cross-section, oriented to match the primary navigational direction intended, i.e., as shown in the orientation ofFIG. 1, the pull wires14are used to deflect the catheter tip in the left-to-right direction, and the pull wires14have a greater height than width so they are more flexible in the left-to-right direction than the up-and-down direction. The pull wire height should be less than 50% of the pull ring outer diameter, withFIG. 1showing a pull wire14with a height which is about 8% of the pull ring outer diameter. Other embodiments utilize pull wires of different cross-sections for part or all of their length, such as the circular cross-sectioned pull wire20shown inFIG. 5.

In the preferred embodiments, the pull wires14are attached to the pull ring12by cutting a longitudinally-extending pull ring slot22in the proximal end of the pull ring12for each pull wire14, and then laser welding the pull wire14to the pull ring12within the slot22. The preferred laser welding positions the pull wire14within its slot22so the pull wire14does not stand proud or extend beyond either the inner diameter or the outer diameter of the tubular pull ring shape, which facilitates both assembly with other catheter structures and functionality of the pull ring12with minimal friction/opportunity for snagging during assembly and use of the catheter.

The pull ring12has an outer diameter which is a majority (i.e., at least 50%) of the outer diameter of the distal end of the catheter in which it is used, and more preferably from 80 to 100% of the outer diameter of the distal end of the catheter in which it is used. In most catheter assemblies, the pull ring axis24is coincident with the catheter axis (not separately shown). Catheter diameters depend heavily upon the particular intended deployment site, but pull ring outer diameters are typically in the range of 3 to 34 French (0.039-0.445″, or 1-11.3 mm).

The wall thickness of the pull ring12should be as thin as possible to provide as much interior space as possible, while still supporting the rigidity of the pull ring12in use. For pull rings formed of metal, the wall thickness will typically be 2-20% of the outer diameter, with the example shown having a wall thickness of about 5% of the outer diameter.

The pull ring12of the present invention has a length in the same order of magnitude as its outer diameter, such as within a range of 30-300% of the outer diameter, which will typically make the tubular pull ring12between 0.05″ and 0.5″ in length. The particular embodiments shown inFIGS. 1, 2 and 5have a length which is about 5% longer than its diameter, butFIGS. 3 and 4show both shorter and longer pull rings. The pull ring12is most preferably cylindrical, but could alternatively have a more square, rectangular or other polygon or oval shape.

The material chosen for the pull ring12is selected for sterility and as being biologically compatible for the likelihood of contact with body tissues for the length of time the catheter is within the body, and also for having both high magnetic permeability and high strength. The requirements for high magnetic permeability and high strength are important due to the interaction between the pull ring12and the primary electrical/magnetic components of the coil sensor pull ring10. To maximize magnetic permeability, the material for the pull ring12should be a ferrite-based or soft magnetic material, with one preferred material being mu-metal. More preferably, a ferritic stainless steel in the full-hard condition is used for the pull ring material to better satisfy physical requirements. The preferred material choice is a 400-series ferritic stainless steel, which maximizes EM sensor sensitivity and is chemically suited for medical purposes. Most preferred materials include SS410 and SS 416, with a potential to use SS444 if extra corrosion-resistance is preferred. Catheter applications that are tolerant of a softer pull ring should utilize SS430, which has a higher permeability. Where maximization of sensitivity is not critical (as is the case for larger coils with diameters exceeding 0.150″) or where MRI-compatibility is necessary, an austenitic stainless steel such as SS304 or SS316 may be used for the pull ring body12. Cobalt or ferrite nanoparticles (not separately shown) can be added or coated onto the pull ring body12to increase the magnetic permeability and/or saturation-flux-density. High-strength magnetically active alloys such as Permendur, Vadnadium Permendur, and HiperCo50, which have sufficient hardness for the physical pull ring requirements, can also be used.

The pull ring12may be coated with a parylene layer (not separately shown) or other chemically inert dielectric substance to provide electrical insulation and render the pull ring12compatible with medical applications. The parylene layer is particularly important in a groove or connection recess36where the electrical connections are made to the coil wire leads26. Alternatively the electrical connections can be made to the coil wire leads26using the interconnect ring disclosed in U.S. patent application Ser. No. 16/040,052, incorporated by reference. The parylene or similar layer is also particularly important if nanoparticles or a high-strength magnetically active alloy is used, to reduce the potential for chemical activity.

The coil wire16is wound around the pull ring12, including a plurality of turns so as to be able to sense and/or create an electric, magnetic or electromagnetic field through body (human) tissue as is common in medical imaging. For instance, the coil16may be wound with about 100-10000 turns or more around the coil area28of the pull ring12, providing an inductance in the microhenry-millihenry range. The electromagnetic field detector (not shown) is used to sense the position and/or the orientation of the catheter according to the electromagnetic field generated in the vicinity of the catheter. Alternative embodiments use the coil16for other purposes, such as for sensing temperature or pressure.

The coil wire16is quite thin, typically having a size smaller than 40 AWG, such as within the range of 40-60 AWG. In the preferred examples shown, the coil wire16is an insulated 58 AWG copper wire, meaning the copper wire is a tiny thread of about 0.0004 inches in diameter. For comparison, the thickness of a human hair is about 0.002-0.004 inches in diameter, i.e., about five to ten times thicker than the copper conductor of the coil wire16. Being so very thin, the flexible coil wire16is also quite fragile. The coil wire16can be closely wound in a single layer around the pull ring12, but more preferably is closely wound in numerous layers (such as 5 to 20 layers) around the outwardly facing surface of the pull ring12. Two leads26for the coil16extend beyond the coil area28.

A pair of magnet circuit wires18are electrically connected to the coil wire leads26to carry the signal longitudinally out of the proximal end (not shown) of the catheter. In the preferred embodiment, the circuit wires18are substantially larger in diameter than the coil wire, such as with a range from 32 to 46 AWG, provided as a twisted pair within a sheath30(drawn somewhat translucent and shorter than its actual length inFIGS. 1 and 5, to better show the twisted pair circuit wires18) for protective shielding. At this larger diameter, the circuit wires18can withstand the twisted pair bending twist as well as the bending of the catheter without breaking, whereas the coil wire, including both the coil16and its leads26, is intended to be entirely stationary relative to the pull ring12throughout use of the catheter.

The pull ring12itself is preferably formed with one or more geometric features to accommodate the coil16and the electrical connections for the coil16. To accommodate the coil16without having the coil16stand proud of the outer diameter of the pull ring12, the coil area28of the pull ring12is machined to a smaller wall thickness, such as removing 5 to 70% of the full wall thickness. The term “stand proud”, as used herein and relative to full wall thickness, refers to a physical geometry extending outside the shape defined by the full wall thickness if the entire tubular structure of the pull ring had a uniform wall thickness. Thus, since pull ring12is cylindrical, the coil16does not “stand proud” of the pull ring by having the largest coil turn with an outer diameter which is no greater than the maximum outer diameter of the pull ring12. For instance, if seven layers of turns of 58 AWG wire are used for the coil16, the machining can remove about 0.0028″ of material (or slightly less, depending upon how the different coil turn layers are nested into each other) from the outer diameter of the cylindrical tubular pull ring12. In the example depicted inFIG. 1, this is about 50% of the wall thickness of the pull ring12.

The coil area28is longitudinally in a middle portion of the pull ring12, between a proximal section32of full wall thickness and a distal section34of full wall thickness. The two sections32,34of full wall thickness greatly help to maintain the overall shape and rigidity of the pull ring12, particularly important to avoid damage to the shape during handling of the coil sensor pull ring10prior to assembly into a catheter. After assembly into the catheter, the material thickness of the coil area28must still withstand the compression and twisting forces seen during catheter deployment and provide sufficient hoop strength to withstand any residual tension in the coil wire16. (During the winding operation of the coil16onto the pull ring12, the pull ring12is supported throughout its inner diameter, so the tension seen during winding does not have to be withstood by the coil area thickness.)

A longitudinally extending connection recess36is machined or otherwise formed into the pull ring12proximally outside the coil area28. The connection recess36is preferably deep enough such that the circuit wires18can be received within the connection recess36without standing proud of the outer cylindrical diameter of the pull ring12.

The circuit wires18are electrically connected to the winding wire leads26to lead out the coil16to a proximal connector (not shown). The electrical connection can be achieved by resistance welding or soldering, with the leads26then positioned within the connection recess36of the pull ring12. The termination locations can be protected with heat shrink material (not shown) and/or then potted with adhesive (not shown) to provide a more durable dielectric barrier between the wires18,26and pull ring12. Such potting provides improved strength to ensure the wires18,26and termination site remain intact during assembly and operation of the catheter.

The preferred connection recess36has a planar bottom surface38. The planar bottom surface38of the connection recess36provides a flat platform that is better suited for adhering the bond sites via a cyanoacrylate or similar adhesive (not shown). The relatively large surface area of the bottom surface38of the connection recess36allows a very durable bond. Alternatively, the flat base38of the connection recess36could provide a platform for adhering bonding-pads or micro printed circuit boards (“PCBs”) (not shown). The flat bonding platform38would make such bonding operations more efficient. The preferred machining operation to achieve the flat bottom surface38is through milling with an end mill (not shown).

The connection recess36improves the quality of the electrical connection and the strength of mechanical connection for the wires18,26. The pull strength, particularly on the coil lead wires26, is improved, resulting in fewer failures. With a better electrical connection, the electrical response of the coil16is more accurately transmitted to the circuit wires18for reading with appropriate electrical equipment. Manufacturability is improved and made easier, and the resulting EM sensor is more reliable.

FIG. 3shows a second embodiment of a pull ring40. In this embodiment, the side walls42defining the coil area44define planes which are not perpendicular to the pull ring axis24, but rather are offset or skewed relative to the normal plane of pull ring axis24by an offset angle θ. Both side walls42define planes which are parallel to each other. Preferred embodiments use an offset angle θ within a range of 1 to 10°, with the most preferred embodiment using an offset angle θ of about 4°. When the coil wire is wound about the pull ring40, the turns of the coil wire are offset by the same offset angle θ, such as by moving the pull ring40longitudinally back and forth (or pivoting the pull ring40back and forth) relative to the coil wire source (or vice versa) during each rotation of the pull ring40while winding. With the windings of the coil laid off-axis from the pull ring axis24, the coil can provide compact 6-Degree-of-Freedom tracking capabilities.

FIG. 4shows a third embodiment of a pull ring46. This embodiment is similar toFIG. 3, but then adds a second coil area48on the pull ring46. The second coil area48is offset relative to the first coil area44, such as using an offset angle θ2of about −4° relative to the normal plane of pull ring axis24. Separate coils (not shown) are wound in the two distinct coil areas44,48, each attached to their own separate circuit wires (not shown) such as within their own separate connection recess36,50. This configuration allows more robust 6-Degree-of-Freedom tracking capabilities. Crosstalk between the two coils can be minimized by use of an austenitic stainless material for the pull ring46, such as SS304 or SS316.

FIG. 5shows another embodiment of an EM coil sensor pull ring52. This embodiment52shares many of the features of the EM coil sensor pull ring10ofFIG. 1and adds a second coil54similar to the second coil ofFIG. 4, but locates the second coil54around one of the pull wires20. To facilitate better wrapping of the coil54around the pull wire20, at least the portion of the pull wire20inside the coil54preferably has a circular or ovular or rounded corner cross-sectional shape. For instance, the distal end of a circular cross-sectioned pull wire20can be stamped into a rectangular cross-section, to better mate for the laser welding operation into its slot22in the pull ring12. If desired, multiple separate coils (not shown) can be longitudinally spaced along a pull-wire20, and be used to can provide visualization of the deflection in the catheter shaft. As in the case of the pull ring designs (12as compared to40,46), the windings may be made with an axis parallel to the axis of the underlying pull wire20or off-axis to allow 6-DOF localization. However, off-axis winding is much more difficult without machining a side wall (not shown, but similar to side wall42) into the pull-wire20. The pull-wire20should ideally be spring-tempered and non-magnetic for optimal physical properties and sensor performance. Most preferred material choices for the pull-wire20inside the coil54include SS304, SS316, and/or nitinol.

The pull-wire20should have a cross-sectional area exceeding ≈0.003 in2if a direct-winding approach is to be used. The length of the pull-wire20proximal to the coil54is generally irrelevant; typical lengths range from 4 to 72″. The winding wire54will typically lie between 50 and 58 AWG and be wound over a length of 0.3 to 0.5″. With about eight to twenty layers of windings, the windings54add approximately 0.004-0.010″ to the thickness of the pull-wire20due to the necessary number of winds.

Other embodiments utilize more than one coil around one pull wire20, or even use separate coils around each of the pull wires14,20, but omit the coil16around the pull ring12. Such embodiments reduce the cost and length of the pull ring12, potentially decreasing the rigidity of the catheter distal end. For all embodiments which utilize a coil54around the pull wire20while making connections on a recessed flat50of the pull ring, a downside is that the coil54can move relative to its leads26during flexing of the pull wire20, which increases the chance of breakage or damage to the thin coil wire54and/or its leads26.

In any of these embodiments, one or more strips (not shown) of higher magnetically permeability material can be added inside the coil wire. The slimness of the strip should not appreciably constrict the working channel of the catheter. Practical magnetic-strip dimensions are as little as 0.001″ to 0.012″ thick and 0.02″ to 0.08″ wide, with a length matching the length of the coil. The sensor coil16and/or54is wound directly over the magnetic strip as well as around the pull ring or pull wire to which the magnetic strip is attached.

The alloy of the strip is chosen to best fit the application. For many applications where such strip(s) is/are added, the strip(s) should be formed of a traditional high-permeability alloy such as permalloy. For the case of a coil54placed over a particularly narrow pull wire (≥0.020″), a high saturation-flux-density alloy such as HiperCo 50 or MetGlass 2605 may be necessary to avoid saturation. For applications where the coil is applied over flexing locations, MetGlass 2714, MetGlass 2605 and similar magnetic glasses have a smaller bending radius than most magnetic alloys, making them ideal for adding the higher permeability strip(s) while maintaining flexibility.

The present invention has at least several primary advantages over prior art solutions. The invention minimizes the intrusion of the EM sensor windings into the working volume of the catheter. Comparable EM sensors in the industry are not wound directly over existing catheter components and require an additional “core” to provide the EM sensor form. The coil sensor pull ring10can easily be incorporated into the catheter shaft as a pre-assembled assembly. The invention reduces EM sensor location offset as the coil16is automatically ‘centered’ around an existing structure in the catheter, typically having the coil axis24coincident with the catheter axis. Winding around a hollow feature, such as a pull ring12,40,46, maintains an ‘open ID’ (open inside diameter—thus having applications for both steerable closed shaft catheters and for steerable introducers used to deliver catheters and medical devices through a central lumen).