Super-elastic alloy braid structure

This invention relates to a tubular structure comprising a plurality of super-elastic alloy constituent rectangular cross-section ribbons interwoven into a braid, wherein the super-elastic alloy comprises a nickel-titanium alloy containing at least about 1.5% (wt) of at least one alloying member selected from the group consisting of vanadium, chromium, manganese, iron and cobalt.

There are a number of uses for superelastic alloy braids, particularly those which have small diameters of 6.35 mm (1/4") or less, which are also both flexible and consistent along their length in size and physical characteristics. I have found that braids made using nickel-titanium alloys, generally known as "nitinol", are not consistent in their physical properties or diameter. For instance, a braid made of a nickel-titanium alloy containing no more than trivial portions of iron or chromium does not retain its shape during subsequent handling or assembly steps even when heat-treated according to my invention. The diameter of such a braid varies as much as 10-20% causing the local stiffness to vary as well. I have surprisingly found that the use of a nickel-titanium alloy containing at least 1.5% (wt) of at least one alloying member selected from the group consisting of vanadium, chromium, manganese, iron and cobalt solves these problems.

These braids are of special use in medical devices particularly when they are of smaller diameter. The invention is not so limited, however.

Other small diameter braids are known in the medical device area. For instance, there are a number of medical catheters discussed in the literature which have catheter bodies having multiply-wrapped reinforcing material. These catheters may include structures having braided bands or ones in which the spirally wound material is simply wound in one direction and the following layer or layers are wound in the other.

Crippendorf, U.S. Patent No. 2,437,542, describes a "catheter-type instrument" which is typically used as a ureteral or urethral catheter. The physical design is said to be one having a distal section of greater flexibility and a proximal section of less flexibility. The device is made of intertwined threads of silk, cotton, or some synthetic fiber. It is made by impregnating a fabric-based tube with a stiffening medium which renders the tube stiff yet flexible. The thus-plasticized tubing is then dipped in some other medium to allow the formation of a flexible varnish-like layer. This latter material may be a tung oil base or a phenolic resin and a suitable plasticizer.

Similarly, U.S. Patent No. 3,416,531, to Edwards, shows a catheter having braiding-edge walls. The device further has additional layers of other polymers such as TEFLON and the like. The strands found in the braiding in the walls appear to be threads having circular cross-sections.

U.S. Patent No. 3,924,632, to Cook, shows a catheter body utilizing fiberglass bands wrapped spirally for the length of the catheter. As is shown in Figure 2 and the explanation of the Figure at column 3, lines 12 and following, the catheter uses fiberglass bands which are braided, that is to say, bands which are spiralled in one direction cross over and under bands which are spiraled in the opposite direction. Additionally, it should be observed that Figure 3 depicts a catheter shaft having both an inner lining or core 30 and an outer tube 35.

U.S. Patent No. 4,425,919, to Alston, Jr. et al., shows a multilayered catheter assembly using multi-stranded flat wire braid. The braid 14 in Figure 3 further covers an interior tubing or substrate 12.

U.S. Patent No. 4,484,586 shows a method for the production of a hollow, conductive medical tubing. The conductive wires are placed in the walls of hollow tubing specifically for implantation in the human body, particularly for pacemaker leads. The tubing is preferably made of an annealed copper wire which has been coated with a body-compatible polymer such as a polyurethane or a silicone. After coating, the copper wire is wound into a tube. The wound substrate is then coated with still another polymer to produce a tubing having spiral conducting wires in its wall.

A document showing the use of a helically wound ribbon of flexible material in a catheter is U.S. Patent 4,516,972, to Samson. This device is a guiding catheter and it may be produced from one or more wound ribbons. The preferred ribbon is an aramid material known as Kevlar 49. Again, this device is a device which must be fairly stiff. It is a device which is designed to take a "set" and remain in a particular configuration as another catheter is passed through it. It must be soft enough so as not to cause substantial trauma, but it is certainly not for use with a guidewire.

U.S. Patent No. 4,806,182, to Rydell et al, shows a device using a stainless steel braid imbedded in its wall and having an inner layer of a polyfluorocarbon. The process also described therein is a way to laminate the polyfluorocarbon to a polyurethane inner layer so as to prevent delamination.

U.S. Patent No. 4,832,681, to Lenck, shows a method and apparatus useful for artificial fertilization. The device itself is a long portion of tubing which, depending upon its specific materials of construction, may be made somewhat stiffer by the addition of a spiral reinforcement comprising stainless steel wire.

U.S. Patent No. 4,981,478, to Evard et al., discloses a multi-sectioned or composite vascular catheter. The interior section of the catheter appears to have three sections making up the shaft. The most interior (and distal) section, 47, appears to be a pair of coils 13 and 24 having a polymeric tubing member 21 placed within it. The next, more proximal, section is 41, and Figure 4 shows it to be "wrapped or braided" about the next inner layer discussed just above. The drawing does not show it to be braided but, instead, a series of spirally wrapped individual strands. Finally, the outermost tubular section of this catheter core is another fiber layer 49, of similar construction to the middle section 26 discussed just above.

Another catheter showing the use of braided wire is shown in U.S. Patent No. 5,037,404, to Gold et al. Mention is made in Gold et al of the concept of varying the pitch angle between wound strands so to result in a device having differing flexibilities at differing portions of the device. The differing flexibilities are caused by the difference in pitch angle.

U.S. Patent No. 5,057,092, to Webster, Jr., shows a catheter device used to monitor cardiovascular electrical activity or to electrically stimulate the heart. The catheter uses braided helical members having a high modulus of elasticity, e.g., stainless steel. The braid is a fairly complicated, multi-component pattern shown very well in Figure 2.

U.S. Patent 5,176,660 shows the production of catheters having reinforcing strands in their sheath wall. The metallic strands are wound throughout the tubular sheath in a helical crossing pattern so to produce a substantially stronger sheath. The reinforcing filaments are used to increase the longitudinal stiffness of the catheter for good "pushability". The device appears to be quite strong and is wound at a tension of about 1723 MPa (250,000 lb./in.²) or more. The flat strands themselves are said to have a width of between 0.152 and 0.51 mm (0.006 and 0.020 inches) and a thickness of 0.038 and 0.1 mm (0.0015 and 0.004 inches).

Another variation which utilizes a catheter wall having helically placed liquid crystal fibrils is found in U.S. Patent No. 5,248,305, to Zdrahala. The catheter body is extruded through an annular die, having relatively rotating inner and outer mandrel dies. In this way, the tube containing the liquid crystal polymer plastic-containing material exhibits a bit of circumferential orientation due to the rotating die parts. At column 2, line 40 and following, the patent suggests that the rotation rate of the inner and outer walls of the die may be varied as the tube is extruded, with the result that various sections of the extruded tube exhibit differing stiffnesses.

Japanese Kokai 05-220,225, owned by the Terumo Corporation, describes a catheter in which the torsional rigidity of the main body is varied by incorporating onto an inner tubular section 33, a wire layer which is tightly knitted at the proximal section of the catheter and more loosely knitted at a midsection.

U.S. Patent No. 5,501,694 discloses a medical device in which an expandable material removal element is made up of a plurality of braided wires. The wires preferably have a substantially round cross-section defining an outer diameter of about 0.05-0.15 mm (0.002"-0.006"), although it does mention that wires having flat, square or triangular cross sections can also be used. In an exemplary embodiment of the removal element the wires comprise nitinol superelastic wire, chromium-doped as drawn. Exemplary dimensions for the contracted and expanded diameters of the material removal element are 1-1.125 mm and 4 mm respectively.

U.S. Patent No. 5,437,282 discloses a drive shaft for acoustic imaging catheters and flexible catheters and comments generally that nitinol alloys having a third element, for example, chromium, vernadium or iron, are generally stiffer and stronger than a pure nickel-titanium alloy.

According to the present invention there is provided a tubular structure comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a plurality of superelastic alloy constituent rectangular cross-section ribbons interwoven into a braid and forming an inner passageway,</li><li>wherein the tubular structure has been heat-treated after braiding to impart a consistent diameter to said structure,</li><li>wherein said superelastic alloy constituent ribbons have a major axis between 0.0635 and 0.3048 mm (2.5 and 12 mils) and a minor axis between 0.00635 and 0.0889 mm (0.25 and 3 mils), and</li><li>wherein said superelastic alloy comprises a nickel-titanium alloy containing at least about 1.5% (wt) of at least one alloying member selected from the group consisting of vanadium, chromium, manganese, iron and cobalt.</li></ul>

This invention is especially suitable for use in medical such as catheter bodies and the like. In general, however, the inventive braids may be used to provide specific physical strengths of various types, e.g. torsional rigidity, stiffness, kink resistance, composite elasticity, etc. to a large number of different devices.

The invention may provide a medical device, e.g. a catheter comprising such a braided tubular structure.

The invention will be further described by way of example, with reference to the accompanying drawings, in which:- <ul id="ul0002" list-style="none" compact="compact"><li>Figures 1 and 2 show a side view of sections of the inventive braid.</li><li>Figure 3 shows a side view of a section of non-inventive braid having multiple members in the braiding fabric.</li><li>Figures 4A-4D show various cross-sectional views of a number of structures not in accordance with the invention.</li></ul>

This invention is a braided tubular structure made up of a plurality of interwoven fibrous members, a majority of which comprise one or more superelastic and (at least) ternary alloys of nickel, titanium, and at least about 1.5% (wt) of one or more alloying members selected from the group consisting of vanadium, chromium, manganese, iron and cobalt. The braided structure may contain a minority of fibrous members of radio-opaque materials, polymeric materials, other metals or alloys, and highly conductive materials. Highly conductive materials are considered to be those having a specific resistance less than about 3.28 ohms/cm (100 ohms per foot), preferably less than 1.64 ohms/cm (50 ohms per foot), and most preferably less than about 0.33 ohms/cm (10 ohms per foot). Once the braid is woven, it is heat treated to "set" the woven structure in its tubular form. The braid structure of this invention is particularly useful in applications where consistency of size (e.g., diameter) and physical properties (e.g., flexibility) is desirable. This invention is especially suitable for use in medical devices such as catheter bodies and the like. Because of the suppleness of the smaller sizes of the inventive braid, it may be used as electromagnetic shielding during various diagnostic procedures. In general, these inventive braids may be used to provide specific physical strengths of various types, e.g., torsional rigidity, stiffness, kink resistance, composite elasticity, etc. to a large number of different devices.

Figure 1 shows one variation of the inventive metallic braid (100) made up of a number of metallic ribbons (102). A majority of the metallic ribbons (102) in braid (100) are superelastic alloys. In this variation, there is a significant amount of spacing between adjacent turns of the braid ribbons (102).

A technical basis for super-elastic alloys is found in the class of titanium/nickel materials known as nitinol -- alloys discovered by the U.S. Navy Ordnance Laboratory. These materials are discussed at length in U.S. Patent Nos. 3,174,851 to Buehler et al., 3,351,463 to Rozner et al., and 3,753,700 to Harrison et al. Alloys suitable for use in my invention are those which also containing at least 1.5% (wt) and up to about 8% (wt) or more, of one or more alloying members selected from the group consisting of vanadium, chromium, manganese, iron, and cobalt.

When using such superelastic alloys in my braids, an additional step is employed to help to preserve the shape of the braid. For instance, with a Cr-containing Ni/Ti superelastic alloy which has been rolled into a 0.0254 × 0.1016 mm (1 x 4 mil) ribbon and formed into a 16-member braid, some heat treatment is desirable. The braid may be placed onto a mandrel, usually metallic, of an appropriate size. The braid is then heated to a temperature of 343.3-398.8°C (650°-750°F) for a few minutes, possibly (but not necessarily) annealing the constituent ribbon. After heat treatment, the braid retains its shape and the alloy retains its superelastic properties.

Metallic ribbons (102) that are suitable for use in this invention are of rectangular cross-section and between 0.00635 and 0.0889 mm (0.25 mil and 3.5 mil) in thickness and 0.0635 and 0.3048 mm (2.5 mil and 12.0 mil) in width. They should have an aspect ratio of at least 0.5 (thickness/width).

The ribbons (102) making up the braid (100) shown in Figure 1 may also contain a minor amount of non-superelastic materials. Although metallic ribbons may be preferred as the ancillary materials because of their strength-to-weight ratios, fibrous materials (both synthetic and natural) may also be used. Preferred, because of cost, strength, and ready availability are stainless steels (SS304, SS306, SS308, SS316, SS318, etc.) and tungsten alloys. In certain applications, where radioopacity is desired, radiopaque metals and alloys, e.g., gold, platinum, palladium, rhodium, rhenium, tungsten, their alloys and mixtures, etc. may be used. A platinum alloy with a few percent of tungsten is preferred partially because of its radiopacity.

Suitable non-metallic ribbons include high performance materials such as those made of polyaramids (e.g., KEVLAR) and carbon fibers.

The braids of this invention may be made using commercially available tubular braiders. The term "braid" is meant to include tubular constructions in which the fibrous materials making up the construction are woven in an in-and-out fashion as they cross to form a tubular member defining a single passageway. The braids may be made up of a suitable number of ribbons, typically six or more. Ease of production on a commercial braider typically results in braids having eight or sixteen ribbons.

The braided structure shown in Figure 1 has a nominal pitch angle of 45°. Clearly the invention is not so limited. Other braid angles from 20° to 60° are also suitable. An important variation of this invention is the ability to vary controllably the pitch angle of the braid either at the time the braid is woven or at the time the braid is assembled into another device.

Although the braid (100) shown in Figure 1 and the other Figures has a single size of ribbon, the braid need not be so limited; multiple sizes of ribbon may be used as desired. The major limitations are simply the size, e.g., diameter, of the overall braid as finally constructed and the desired added stiffness to be added to the braid structure.

The braids typically useful in this invention comprise an even number of ribbons: one half of the ribbons wound one way, i.e., clockwise, and the remainder are wound the other way. A typical braid will be of eight to 16 ribbons. The braid may have a single pitch, an angle of a constituent ribbon measured against the axis of the braid, or it may have a pitch which varies along the axis of the braid.

The braid structure (100) shown in Figure 1 has a relatively constant diameter. Although the heat treatment step noted above in conjunction with the specified alloys results in a tubular structure having a shape corresponding to the particular mandrel chosen for the heat treating step, the shape of the mandrel and hence the shape of the tubular structure may have a varying, e.g., an increasing or decreasing diameter.

The braid structure (100) may be rough to the touch if not covered or further processed. Procedures such as rolling, sanding, or grinding may be used to smooth the surface of the braid structure if so desired. Removal of any produced particulates is, of course, desirable.

Figure 2 shows a variation of the inventive braid structure (106) in which the spacing between the adjacent ribbons (108) is minimal. That is to say that each ribbon (108) is adjacent the next. This tight structure is typically stiffer than more loosely woven braids.

Figure 3 shows a close-up of a device not in accordance with the present invention in which the non-rectangular filamentary members (112) are not a single weave as is shown in the Figures above. The filamentary members (112) weave around the tubular structure (110) in a band of (for instance) four to five filaments much in the same way that the single ribbon is woven around the Figure 1 and 2 devices. I refer to this variation as a "multiple member braid structure."

Finally, this invention is not limited to a tubular structure which is simply round (140) as is shown in Figure 4A. Other cross-sections contemplated in this invention include triangular (e.g. radiused triangular) (142) as is shown in Figure 4B and radiused square or rectangular (144) as shown in Figure 4C. An oval shape (146) is shown in Figure 4D. It should be noted that none of the tubular structures illustrated in Figures 4A-4D is in accordance with the present invention due to the cross-section of the ribbons being oval, not rectangular.