More specifically, a method is disclosed in which the helically coiled wire can be continuously formed from a special nickel-titanium wire and spooled for convenient storage.

When the wire is later unspooled, it will snap back into the desired helical coil form.

BACKGROUND OF THE INVENTION Helically coiled wire has many uses, the most common of which is as an everyday spring. In the medical field, helically coiled wire is used in such applications as a medical stent to hold blood vessels open or as reinforcement for medical tubing such as blood catheters, drainage catheters or endotracheal tubes.

A particularly advantageous material to use in forming helically coiled wire for medical applications is a nickel-titanium based alloy, such as"Nitinol."It has been found that Nitinol can be formed into a very thin wire while retaining great strength and integrity of shape. These properties are very important when helically coiled wire must be inserted into very small blood vessels in the human body. For these medical applications, a wire is needed that will not kink, collapse or break. For obvious reasons, a wire that kinks, collapses or breaks inside the human body will often be difficult and messy to retrieve."Heat/cool"and "superelastic"forms of Nitinol are now currently available in the marketplace."Heat/cool" Nitinol is characterized by its ability to be readily deformed below certain temperatures and its tendency to expand back to its original shape when heated. By contrast, helical coils made

of"superelastic"Nitinol are not significantly affected by heat or cold and yet have high expansion ratios. A highly elastic, adjustable helical coil stent formed of superelastic Nitinol alloy is described in the inventor's U. S. Patent No. 6,027,516.

Up to now, forming helical coils for medical devices has often been done one device at a time or in small batches. In other words, the wire will be separately formed around a mandrel for each medical device or for a small group of such medical devices. The inventor is unaware of any prior art processes in which the helical coil wire can be continuously formed, stretched to assume a straight shape and wrapped around a spool, economically stored or shipped and then, when unspooled, will quickly snap back into the desired helical coil form. For example, while Buehler's U. S. Patent No. 3,985,177 discloses a method for continuously forming Nitinol wire from a furnace, there is no teaching that the wire can be helically coiled, stored on a spool and then unspooled into a helical coil shape.

Similarly, while Kenmore's U. S. Patent No. 3,399,702 discloses a pickling process in which a wire is unspooled, coiled, pickled and then respooled, there is no disclosure in the Kenmore patent that the wire will automatically reform into a desired helical coil shape when it is unspooled again.

SUMMARY OF THE INVENTION The present invention provides a method of and apparatus for continuously forming wire into a helical coil, straightening the formed wire, spooling that straightened wire for convenient transportation and/or storage and then having that wire automatically snap back into the desired helical coil shape when the wire is unspooled. Through use of this method, large amounts of spooled wire with a"memorized"helical coil shaped can be stored and then used as needed to make helical coil devices. In the preferred embodiment, Nitinol wire is passed through a spring forming unit to curve the wire, wrapped around a cylindrical

mandrel, passed through a high temperature oven and cooled so that the helical coil shape will be memorized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic view of the helical coiling method and apparatus of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS FIG. 1 shows a preferred method of and apparatus for continuously creating helically coiled memory wire. The method starts with raw wire 10 which often arrives on a spool 12. It is important to select a wire that has the inherent ability to memorize a helical coil shape upon heat treatment so that after it has been heat treated, straightened, respooled and then unspooled, it will automatically reform into the desired helical coil shape. It has been found that nickel-titanium based alloy, such as"Nitinol,"has these desired memory properties. Nitinol, both in its"heat/cool"and"superelastic"forms is commercially available from such suppliers as Fort Wayne Metals of Fort Wayne, Indiana.

In its preferred form, the process begins by unspooling the raw wire 10 and passing that wire 10 through a Teflon tape unit 14 which, under pressure from weight 16, provides a low friction path for the wire 10. After the Teflon tape unit 14, the wire is then passed through a spring forming unit 20 to impart a desired curvature to the wire. In the preferred embodiment, this spring forming unit 20 consists of highly polished rectangular or square slabs 22,24,26 of carbide or polycrystalline diamond coated steel and alignment roller 29. The edge 28 of slab 26 over which the wire is curved should be formed to a small radius. This radius should be chosen to make the helical coil tightly wound in an appropriate diameter. For example, the radius of the edge 28 can be as small as 0.001 inches to make a

very tightly would helical coil spring for medical applications or 0.020 inches or larger to make a larger diameter helical coil spring. To provide versatility, the edge 28 can be tapered along its length to a variety of radiuses so that many different types of helical coil springs can be made using the same apparatus. The forming of this edge 28 can be accomplished by first grinding with a diamond wheel and then polishing. The space between the slabs 22,24,26 in the spring forming unit 20 should be kept very narrow, ideally of approximately the same dimensions as the wire 10. Teflon shims (not shown) can advantageously be placed along the surfaces of the slabs 22,24,26 which are contacted by the wire to reduce friction and assist with alignment.

To pull the wire 10 through the spring forming unit 20, a motor drive unit 30 is used in the preferred embodiment. The rollers 32,34 for this motor drive unit 30 may advantageously be formed of hardened steel. While the combination of a spring forming unit 20 and motor drive unit 30 have been described as the preferred means of curving the wire, those in the art will recognize that there are other suitable techniques for forming a helical coil including conventional coil forming machines.

After the wire 10 passes through the motor drive unit 30, it preferably enters a flattened Teflon tubing to temporarily change it from a curved to a flat wire, or it can be directly applied to the mandrel 40. The wire 10 is then wrapped in a helical coil shape 42 around a mandrel 40, which can be formed, for example, from a stainless steel rod or tube.

This wrapping around the mandrel 40 should be a relatively passive process, with all forming work being previously done in the spring forming unit 20. As more and more wire 10 is wrapped around the mandrel 40, it moves toward the high temperature oven 50. At this point, if the helically coiled wire is to be used immediately, it can be removed from the mandrel by suitably cutting the wire. By skipping the heating and cooling steps, though, the

helically coiled Nitinol wire 42 will gradually lose some"memory"and thereby adversely affect the quality of the finished product.

Heating of the helical coiled wire to solidify its"memory"is accomplished using a high temperature oven 50. In one preferred embodiment, this high temperature oven 50 takes the form of a 0.5 inch internal diameter Pyrex glass tube 52 which is 3 inches in length. A hot gas 54, such as air, argon or another inert gas, is heated to about 800° F to 1200° F and passed by the helically coiled wire 42 at an appropriate flow rate, such as 10 liters per minute. The temperature of this hot gas 54 can be maintained at an appropriate level by known methods, such as by use of a thermocouple temperature sensor (not shown).

This hot gas 54 raises the temperature of the helical coil sufficiently to set the wire 42 in its helically coiled form. The high temperature oven 50 is suitably insulated to prevent gas leaks and other temperature losses. Those of skill in the art will, of course, recognize that other forms of high temperature ovens can be used to impart memory to the wire 42 such as an oven using a heating laser, molten salt or electrical current. After the helical coiled wire 42 exits the high temperature oven 50, it is allowed to cool to room temperature.

The process of forming helically coiled memory wire is completed by removing the previously heated helically coiled wire 56 from the mandrel 40 in straightened form, passing the wire through a second Teflon tape unit 60, passing the wire through a second motor drive unit 70, passing the wire over a motor driven sled 80 for proper alignment and finally spooling the wire on second spool 90. The difference between the wire on the original spool 12 and the wire on the second spool 90 is that the wire on the second spool has a memorized helical coil shape even though it is also wound onto a spool in straightened form. When the wire is removed from the second spool 90, it will automatically reform into the same desired helical coil shape it had when it was moving along mandrel 40.

The speed of the two motor drive units 30,70 are carefully coordinated so that the length of wire delivered to the mandrel 40 by motor drive unit 30 exactly matches the length of wire taken up by motor drive unit 70. If motor drive unit 70 is transiently speeded up for a few seconds, the resulting coil will become heat set in a smaller diameter.

Conversely, if motor drive unit 70 is transiently slowed down, the resulting coil will be heat set in a slightly larger diameter. Hence, it becomes possible to, within a narrow range, slightly enlarge or reduce the resulting diameter of the formed coils.

To use the finished wire created by the process of the present invention, one need only unwind an appropriate length of wire from the second spool 90. Because of its "memory"properties, this finished wire will automatically reform into the desired helical coil shape after being unspooled. The reformed helical coil wire can then be used for any number of purposes. For example, to make a wire reinforced catheter tube, the helically coiled wire can be placed around an inner plastic tube and then coated with plastic polymer.

Alternatively, the tube can be co-extruded around the reformed wire. As another application, the reformed wire can be made into a helical coil stent of the type described in the inventor's U. S. Patent No. 6,027,516, which is incorporated by reference, by coating the wire with an elastomer and affixing the coated wire to a control tube in the manner shown. Other applications for the reformed helical coil wire of the present invention are well known to those in the art.

In the foregoing specification, the invention has been described with reference to specific preferred embodiments and methods. It will, however, be evident to those of skill in the art that various modifications and changes may be made without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, the wires formed from the methods of the present invention may be used not only in medical applications but also in virtually any application involving helically wound springs. The specification and drawings are, accordingly, to be regarded in an illustrative, rather than restrictive, sense; the invention being limited only by the appended claims.