Single-step shape memory alloy expansion

Abstract

To provide a Nitinol medical equipment formation method capable of overcoming defects of the prior art.SOLUTION: In a state where no strain is imparted or minimum strain is imparted, Nitinol is subjected to a shape fixation temperature. Then the shape of Nitinol is substantially deformed during a time when it has a high temperature. After the deformation, Nitinol is kept at the high temperature for a prescribed time so that the shape of the material is fixed. Then Nitinol is returned approximately to a room temperature 20°C by, for example, water-quenching or air cooling.SELECTED DRAWING: Figure 2

Description

[Background technology]

[0001] It is well known to employ various percutaneously delivered intravascular endoprostheses to treat various body vascular conditions. These types of endoprostheses are commonly referred to as stents. Stents are generally tubular devices formed from biocompatible materials, such as nitinol. It is commonly known to fabricate stents from nitinol tubes cut by methods such as laser cutting, water jet cutting, electrical discharge machining, and chemical polishing. Nitinol is considered a shape memory alloy (SMA). Nitinol also has a shape-set temperature. The shape-set temperature is defined as any temperature within a temperature range at which an SMA article, when exposed to that temperature in a constrained shape for a predetermined period of time, will substantially retain that constrained shape when the article is subsequently released from the constrained state.

[0002] Nitinol tubing is expensive to manufacture. The larger the diameter of the Nitinol tubing, the more expensive it becomes. As a result of the cost constraints of large diameter Nitinol tubing, larger diameter Nitinol tubes (and / or Nitinol stents) are made by cutting patterns (e.g., stent patterns) into small diameter Nitinol tubing and then gradually expanding and shape-setting these tubes.

[0003] One common method for shape-setting Nitinol involves deforming and constraining Nitinol into a desired shape at or below room temperature (typically about 20°C). The Nitinol is then exposed to high temperatures (typically about 500°C), for example in a furnace, while being constrained in the desired shape for a predetermined time (typically about 5-20 minutes). The Nitinol is then cooled to room temperature by water quenching or allowing it to air cool. This shape-setting process imparts a new shape to the Nitinol. The new shape is the result of specifically pre-deforming and constraining the cut tube.

[0004] When expanding pre-cut nitinol tubing, a series of incremental expansion and shape-setting steps is typically used. A traditional nitinol stent device manufacturing method is described by Poncin et al. (SMST-2000 Conference Proceedings, pp 477-486), which states that "the device is expanded to its final size by a series of incremental shape-setting steps accompanied by heat treatment." The use of a series of incremental expansion steps reduces the occurrence of fracture or cracking of the pre-cut nitinol tubing during shape-setting.

[0005] In one example, a stent pattern can be laser cut into a nitinol tube with an outer diameter of approximately 4 mm. To expand this 4 mm pre-cut tube into a 24 mm pre-cut tube, a series of incremental expansion steps can be employed. For example, the pre-cut tube can be expanded from a 4 mm diameter to an 8 mm diameter and then shape-set. Next, the pre-cut tube can be expanded from an 8 mm diameter to a 12 mm diameter and then shape-set. This process continues until the desired 24 mm diameter pre-cut tube is obtained.

[0006] To avoid stent fracture during the shape-setting process, it is common to use a series of expansion steps when forming a stent. The example above used five expansion steps to achieve the desired 24 mm diameter stent. Omitting even one of these expansion steps, such as the expansion and shape-setting step from 4 mm to 12 mm, can result in the stent fracturing during shape-setting. This process of incrementally forming Nitinol through a series of shape-setting steps is costly and time-consuming.

[0007] It is also common practice in the art to cool Nitinol stents to form thermally induced martensite before expanding the Nitinol tube. Nitinol tubes, which are primarily austenitic at room temperature, are first cooled to form thermally induced martensite, making them easier to deform and expand diametrically. Because martensitic Nitinol is easier to deform than austenitic Nitinol, forming thermally induced martensite before expanding the Nitinol tube is believed to minimize crack formation within the stent. Despite this thermal induction of martensite before expanding the Nitinol tube, crack formation during expansion of the Nitinol tube remains a problem. Thermally inducing martensite before expanding the Nitinol tube does not eliminate the need for incremental expansion steps to diametrically expand the Nitinol tube and set its shape. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, there is a need to have a method for forming Nitinol medical devices that overcomes the shortcomings of the prior art. The present invention provides such a solution. [Means for solving the problem]

[0009] According to the present invention, a method of forming nitinol is provided. In one embodiment, nitinol is exposed to a shape-setting temperature of at least 300°C to about 650°C while in an unstrained or minimally strained state. The shape of the nitinol is then substantially deformed at this elevated temperature. After deformation, the nitinol is held at this elevated temperature while constrained in the desired shape for a predetermined time to shape-set the material. In another embodiment, the nitinol may be deformed at this elevated temperature one or more times. The nitinol is then returned to about room temperature (about 20°C) while still constrained, for example, by water quenching or air cooling.

[0010] Accordingly, one embodiment of the present invention includes a method of forming a shape memory alloy (SMA) article, the method comprising providing an SMA article having an initial shape, wherein the SMA has a shape-setting temperature; heating the SMA article to about the shape-setting temperature; deforming the SMA article to a final shape while the SMA article is at about the shape-setting temperature; and cooling the SMA article under restraint, thereby substantially retaining the final shape. In one embodiment, after deforming the SMA article while the SMA article is at about its shape-setting temperature, the deformed SMA article is allowed to remain at about its shape-setting temperature. In another embodiment, the shape-setting temperature is from about 300°C to about 650°C. In another embodiment, the SMA is Nitinol. In another embodiment, deforming the SMA article to the final shape is achieved by applying an internal force. In another embodiment, deforming the SMA article to the final shape is achieved by applying an external force. In another embodiment, transforming the SMA article into the final shape is accomplished by using a tapered mandrel. In another embodiment, the SMA article is formed into a medical device. In another embodiment, the medical device is an implantable medical device. In another embodiment, the implantable medical device is selected from the group consisting of a stent, a cardiac occluder, a valve, and an intraluminal filter. In another embodiment, the SMA initial shape is formed by machining. In another embodiment, the machining includes laser cutting, water jet cutting, electrical discharge machining, and / or chemical etching.

[0011] Another embodiment of the present invention includes a method of forming a stent, comprising providing a machined shape memory alloy (SMA) tube, the machined SMA tube having a stent pattern, a first (smaller) diameter, and a shape-setting temperature; heating the machined SMA tube to about the shape-setting temperature; deforming the machined SMA tube to a second (larger) diameter while the machined SMA tube is at about its shape-setting temperature; and cooling the SMA article under restraint, thereby substantially retaining the second diameter. In one embodiment, the machining comprises laser cutting, water-jet cutting, electrical discharge machining, and / or chemical etching. In another embodiment, the stent pattern comprises a sinusoidal, diamond, U-, V-, or ovaloid shape. In another embodiment, the SMA tube has a circular cross-section. In another embodiment, after deforming the SMA tube while the SMA tube is at about the shape-setting temperature, the deformed SMA tube is maintained at about the shape-setting temperature. In another embodiment, the shape-setting temperature is about 300°C to about 650°C. In another embodiment, deforming the SMA tube into the second shape is achieved by applying an internal force. In another embodiment, deforming the SMA tube into the second shape is achieved by applying an external force. In another embodiment, deforming the SMA tube into the second shape is achieved by using a tapered mandrel. In another embodiment, the ratio of the second (large) diameter shape to the first (small) diameter shape is greater than about 1.25:1. In another embodiment, the ratio of the second (large) diameter shape to the first (small) diameter shape is greater than about 1.5:1. In another embodiment, the ratio of the second (large) diameter shape to the first (small) diameter shape is greater than about 2:1. In another embodiment, the ratio of the second (large) diameter shape to the first (small) diameter shape is greater than about 3:1.In another embodiment, the ratio of said second (larger) diameter shape to said first (smaller) diameter shape is greater than about 4:1.

[0012] In another embodiment, the present invention includes a medical device including a shape memory alloy (SMA) article adapted to transition between a first state, a second state, and a third state, wherein the SMA has a shape-set temperature, the article has a first circumferential perimeter in the first state, the article has a plurality of annular perimeters in the second state, the article has a third annular perimeter in the third state, each annular perimeter in the second state being larger than the annular perimeter in the first state and smaller than the annular perimeter in the third state, and the shape memory alloy (SMA) article is maintained at the shape-set temperature while transitioning between the first state, the second state, and the third state.

[0013] Another embodiment of the present invention is an apparatus for deforming an SMA article, the apparatus comprising an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer circumference, (ii) the tube having a length, a lumen therethrough, and a wall, (iii) the lumen defining a first inner circumference, (iv) the tube having at least two slots therethrough, (v) the slots being oriented essentially parallel to the longitudinal axis of the tube, and (vi) the slots extending partially along the length of the tube; and an expandable manifold. The apparatus includes a mandrel, (i) the mandrel includes a first portion having an essentially constant first circumference, (ii) the mandrel includes a second tapered portion, (iii) the second tapered portion has a varying circumference that transitions from the first circumference of the mandrel to a larger second circumference, and (iv) the circumference of the first portion of the mandrel is sized to be inserted within a first inner circumference of the lumen of the slotted elongate tube, and a shape memory alloy article surrounding at least a portion of the slotted elongate tube. In one embodiment, the SMA is Nitinol. In another embodiment, the SMA article is a medical device. In another embodiment, the medical device is selected from the group consisting of a stent, a cardiac occluder, a valve, and an intraluminal filter.

[0014] Another embodiment of the present invention is an apparatus for deforming an SMA article, the apparatus comprising: an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer circumference, (ii) the tube having a length, a lumen therethrough, and a wall, (iii) the lumen defining a first inner circumference, (iv) the tube having at least two slots therethrough, (v) the slots being oriented essentially parallel to the longitudinal axis of the tube, and (vi) the slots extending partially along the length of the tube; and the apparatus comprising: an expansion mandrel, (i) the mandrel being The device includes a mandrel, (ii) the mandrel includes a first portion having an essentially constant first circumference, (iii) the second tapered portion has a varying circumference that transitions from the first circumference of the mandrel to a larger second circumference, and (iv) the circumference of the first portion of the mandrel is sized to be inserted within a first inner circumference of the lumen of the slotted elongate tube, wherein the slotted elongate tube surrounds at least a portion of the expansion mandrel, and the device includes a shape memory alloy article surrounding at least a portion of the slotted elongate tube. In one embodiment, the slotted elongate tube surrounds at least a portion of the first portion of the expansion mandrel. In another embodiment, the slotted elongate tube surrounds at least a portion of the second tapered portion of the expansion mandrel. In another embodiment, the expansion mandrel further includes a third portion having an essentially constant second circumference. In another embodiment, the slotted elongate tube surrounds at least a portion of the third portion of the expansion mandrel.

[0015] Another embodiment of the present invention is an apparatus for deforming a shape memory alloy (SMA) article, comprising an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer periphery, (ii) the tube having a length, a lumen therethrough, and a wall, (iii) the lumen defining a first inner circumference, (iv) the tube having at least two slots therethrough, (v) the slots being oriented essentially parallel to the longitudinal axis of the tube, (vi) the slots extending partially along the length of the tube, (vii) the tube having a first portion with an essentially constant first circumference, and (viii) the tube having a first portion having a second ... the tube having a second tapered portion, (ix) the second tapered portion having a varying circumference that transitions from a first circumference of the tube to a second, larger circumference; a translation instrument, (i) the translation instrument including a rod sized to extend and slide through a through lumen of the slotted elongate tube, and (ii) the rod having at least two fins sized to extend and slide through the slots through a wall of the slotted elongate tube; and a shape memory alloy article surrounding at least a portion of the slotted elongate tube. [Effects of the Invention]

[0016] Thus, the method of the present invention allows for the expansion of a nitinol tube (eg, a stent) to a significantly larger diameter (eg, six times or more) in a single processing step.

[0017] Exemplary embodiments of the present invention will now be described in connection with the accompanying drawings, in which, where appropriate, like numerals indicate like elements and are offset by 100. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a time / temperature graph illustrating a multi-step thermoforming process currently known in the art.

[0019] [Figure 2] FIG. 2 is a time / temperature graph illustrating a single step thermoforming process according to the present invention.

[0020] [Figure 3] 3A and 3B are perspective views of the cut patterned tube before and after expansion.

[0021] [Figure 4] FIG. 4 is a perspective view of the expansion fixture of the present invention showing the slotted mandrel, expansion die, and pulling rod.

[0022] [Figure 5A] FIG. 5A is a side view of a stent expansion mandrel assembly of the present invention showing a slotted mandrel, expansion die, pull rod, and stent.

[0023] [Figure 5B] FIG. 5B is a side view of the stent expansion mandrel assembly of the present invention showing the slotted mandrel, expansion die, pull rod and stent.

[0024] [Figure 5C] FIG. 5C is a side view of a stent expansion mandrel assembly of the present invention showing the slotted mandrel, expansion die, pull rod and partially expanded stent.

[0025] [Figure 5D] FIG. 5D is a side view of a stent expansion mandrel assembly of the present invention showing the slotted mandrel, expansion die, pull rod and fully expanded stent.

[0026] [Figure 5E] FIG. 5E is a side view of the stent expansion mandrel assembly of the present invention showing the slotted mandrel and fully expanded stent.

[0027] [Figure 6A] FIG. 6A is a perspective view of a slotted tube and unexpanded stent of the present invention.

[0028] [Figure 6B] FIG. 6B is a perspective view of a tapered mandrel of the present invention.

[0029] [Figure 6C] FIG. 6C is a perspective view of the stent expansion mandrel assembly of the present invention showing the slotted tube, tapered mandrel and unexpanded stent.

[0030] [Figure 6D] FIG. 6D is a perspective view of the stent expansion mandrel assembly of the present invention showing the slotted tube, tapered mandrel and expanded stent. DETAILED DESCRIPTION OF THE INVENTION

[0031] As currently known in the art, nitinol tubing of various diameters and wall thicknesses can be cut to form desired patterns, such as stent patterns. The cut tubing can be placed on an expansion fixture and expanded approximately 20% while at ambient temperature. The cut tubing and expansion fixture can then be heated to an elevated temperature, and after a suitable dwell time, the cut tubing and fixture can be quenched to return the cut tubing to ambient temperature. This process can be repeated, with each cycle expanding the tubing an additional approximately 20% resulting in the desired diameter (i.e., 100% expansion).

[0032] FIG. 1 shows a time / temperature graph illustrating a typical expansion process commonly known in the art. In this example, a five-step expansion process is shown, with each expansion occurring at approximately ambient temperature (approximately 20°C). Each of the five expansion steps expands the cut tube approximately 20% of its expanded diameter. After each expansion, the cut tube and expansion fixture are heated to approximately 500°C, and after a suitable dwell, the cut tube and expansion fixture are water quenched and returned to ambient temperature. As shown, the process is repeated four additional times, resulting in the desired diameter (approximately 100% total expansion).

[0033] FIG. 2 is a graph (with the same axes as FIG. 1) illustrating the process of expanding a pre-cut tube to a desired diameter (i.e., approximately 100% expansion) in a single expansion step. As shown in FIG. 2, the pre-cut tube is placed on an expansion fixture. The pre-cut tube and expansion fixture can then be heated to an elevated temperature, and at this elevated temperature, the expansion fixture can be activated to expand the pre-cut tube approximately 100% in a single expansion step. In another embodiment, the expansion fixture can be activated to expand the pre-cut tube approximately 200%, approximately 300%, approximately 400%, and / or approximately 500%. By comparing FIG. 1 with FIG. 2, it is clear that the method taught herein reduces the number of steps, and therefore the time, required to expand a Nitinol tube.

[0034] FIG. 3A is a partial perspective view of an exemplary pre-cut tube 300a having an initial small diameter 302. The pre-cut tube 300a has a wavy shape including peaks 304 and valleys 306, typical of an implantable stent. FIG. 3B is a partial perspective view of an expanded pre-cut tube 300b after it has been expanded to a large diameter 308. The large diameter 308 is approximately 100% larger than the initial small diameter 302. The tube 300a can be cut to have any desired pattern. For example, the tube 300a can be cut to form individual rings, interconnected rings, open and / or closed cells, or shapes such as sinusoidal, diamond, U-, V-, or oval shapes, or any other pattern tailored to a given application. The tube 300a can comprise Nitinol or any other similar metal having a shape-locking temperature range. Nitinol refers to a family of alloys including binary nickel-titanium binary shape memory alloys and nickel-titanium based alloys with ternary and quaternary additions of alloying elements (e.g., but not limited to, iron, niobium, chromium, copper, cobalt, vanadium, platinum, and hafnium). Shape memory alloys include Nitinol alloys as well as other alloys capable of reversible crystalline phase changes (e.g., but not limited to, AgCd, AuCd, CuAlZn, CuAlNi, CuAlBe, CuSn, NiAl, FePt, FePd, MnCu, and FeMnSi alloy systems).

[0035] The tube 300a can have a diameter ranging from about 0.5 mm to about 100 mm, with a preferred range being about 2 mm to about 40 mm. The tube 300a can have a wall thickness ranging from about 0.05 mm to about 10 mm, with a preferred range being about 0.1 mm to about 0.5 mm. The length of the tube 300a can range from about 1 mm to about 250 mm. The length of the tube 300a can be configured according to any particular application.

[0036] FIG. 4 shows a perspective view of at least one expansion fixture 400. In this example, the expansion fixture 400 includes a tapered, slotted tubular mandrel 402 fabricated from a high-temperature metal, such as Inconel, stainless steel, or other suitable material. The slotted mandrel 402 has a large diameter portion 404, an intermediate tapered portion 406, a small diameter portion 408, and a series of longitudinal slots 410. The longitudinal slots 410 are cut through the mandrel wall and extend through the small diameter and tapered portions (408 and 406) of the slotted mandrel. The longitudinal slots 410 are cut through the mandrel wall and extend only partially along the large diameter portion 404, as shown in FIG. 4. In an optional configuration, the longitudinal slots can form a spiral. The intermediate tapered portion can have a varying taper angle or a sharp portion instead of a constant taper angle, if desired.

[0037] The expansion fixture 400 further includes an expansion die 412 having a series of fins 414 as shown in Figure 4. The fins 414 of the expansion die 412 engage with the slots 410 of the slotted mandrel 402, allowing the fins 414 of the expansion die to slide through the longitudinal slots 410 of the slotted mandrel 402 along a longitudinal axis indicated by directional arrow 416.

[0038] 4, the expansion die 412 is coupled to a pull rod 418. The pull rod 418 extends through a central bore of the tubular slotted mandrel 402 and out from a collar portion 422 of the slotted mandrel 402. When the pull rod 418 is pulled along the longitudinal axis indicated by directional arrows 416 and 420, the expansion die 412 is forced to slide over the small diameter portion 408, intermediate tapered portion 406, and large diameter portion 404 of the slotted mandrel 402.

[0039] The collar portion 422 of the slotted mandrel 402 is configured to secure the slotted mandrel to a heat source (not shown). The slotted mandrel 402 and expansion die 412 are positioned within the heat source. The heat source is connected to a tension rod 418 End 424 (opposite side of the expansion die) is configured to allow it to protrude from the heat source.

[0040] Book Another embodiment of the invention is an apparatus for deforming a shape memory alloy (SMA) article, the apparatus comprising: an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer circumference; (ii) the tube having a length, a lumen therethrough, and a wall; (iii) the lumen defining a first inner circumference; (iv) the tube having at least two slots therethrough; (v) the slots being oriented essentially parallel to the longitudinal axis of the tube; and (vi) the slots extending partially along the length of the tube; The apparatus includes an expansion mandrel, (i) the mandrel includes a first portion having an essentially constant first circumference, (ii) the mandrel includes a second tapered portion, (iii) the second tapered portion has a varying circumference that transitions from the first circumference of the mandrel to a larger second circumference, and (iv) the circumference of the first portion of the mandrel is sized to be inserted within a first inner circumference of the lumen of the slotted elongate tube, and a shape memory alloy article surrounding at least a portion of the slotted elongate tube. In one embodiment, the SMA is Nitinol. In another embodiment, the SMA article is a medical device. In another embodiment, the medical device is selected from the group consisting of a stent, a cardiac occluder, and an intraluminal filter.

[0041] Another embodiment of the present invention is an apparatus for deforming a shape memory alloy (SMA) article, the apparatus comprising: an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer circumference, (ii) the tube having a length, a lumen therethrough, and a wall, (iii) the lumen defining a first inner circumference, (iv) the tube having at least two slots therethrough, (v) the slots being oriented essentially parallel to the longitudinal axis of the tube, and (vi) the slots extending partially along the length of the tube; the apparatus comprising: an expansion mandrel; The device includes a mandrel including: (i) a first portion having an essentially constant first circumference; (ii) the mandrel including a second tapered portion; (iii) the second tapered portion having a varying circumference that transitions from the first circumference of the mandrel to a larger second circumference; and (iv) the circumference of the first portion of the mandrel is sized to be inserted within a first inner circumference of the lumen of the slotted elongate tube, wherein the slotted elongate tube surrounds at least a portion of the expansion mandrel; and the device includes a shape memory alloy article surrounding at least a portion of the slotted elongate tube. In one embodiment, the slotted elongate tube surrounds at least a portion of the first portion of the expansion mandrel. In another embodiment, the slotted elongate tube surrounds at least a portion of the second tapered portion of the expansion mandrel. In another embodiment, the expansion mandrel further includes a third portion having an essentially constant second circumference. In another embodiment, the slotted elongate tube surrounds at least a portion of the third portion of the expansion mandrel.

[0042] Another embodiment of the present invention is an apparatus for deforming a shape memory alloy (SMA) article, the apparatus comprising an elongated slotted tube, (i) the tube having a longitudinal axis and a first outer periphery, (ii) the tube having a length, a lumen therethrough, and a wall, (iii) the lumen defining a first inner circumference, (iv) the tube having at least two slots therethrough, (v) the slots being oriented essentially parallel to the longitudinal axis of the tube, (vi) the slots extending partially along the length of the tube, (vii) the tube having a first portion having an essentially constant first circumference, and (viii) the first portion having a first periphery. The device includes the tube, wherein (ix) the tube has a second tapered portion, the second tapered portion having a varying circumference that transitions from a first circumference of the tube to a larger second circumference; a translation device, wherein (i) the translation device includes a rod sized to extend and slide through a through lumen of the slotted elongate tube, and (ii) the rod has at least two fins sized to extend and slide through the slots through a wall of the slotted elongate tube; and an SMA article surrounding at least a portion of the slotted elongate tube.

[0043] Any suitable heat source can be used to heat the extension fixture 400, including a flowing bath, a salt bath, a hot liquid, a hot gas, radiant heating, induction heating, convection heating, electrical resistance heating, radio frequency heating, conduction heating, or a combination of different energy sources.

[0044] Accordingly, one embodiment of the present invention is a method for expanding pre-cut tubes, comprising the steps of: cutting a metal tube to form a desired cut pattern; placing the pre-cut metal tube over a reduced diameter portion of a slotted tapered mandrel; inserting a pulling rod having an expansion die attached thereto through a central bore of the slotted tapered mandrel; engaging a series of fins (integral with the expansion die) within the slots of the slotted tapered mandrel; placing the pre-cut tubes, the slotted tapered mandrel, and the expansion die into a heat source so that the ends of the pulling rod extend outside of the heat source; heating the slotted tapered mandrel and the expansion die to an elevated temperature (shape-setting temperature); and translating the pulling rod (while maintaining the cut tube, the slotted tapered mandrel, and the expansion die at the shape-setting temperature) to forcibly slide the expansion die over the smaller, tapered, and larger diameter sections of the slotted tapered mandrel, wherein as the pulling rod is translated, fins on the expansion die engage the cut SMA tube and forcibly slide the cut SMA tube over the smaller, tapered, and larger diameter sections of the slotted tapered mandrel.

[0045] One expansion process for pre-cut SMA tubing according to the present invention is illustrated in Figures 5A-5E. Figure 5A shows an expansion fixture 500. Expansion fixture 500 includes a tapered, slotted tubular mandrel 502. Slotted mandrel 502 has a large diameter section 504, an intermediate tapered section 506, a small diameter section 508, and a series of longitudinal slots 510. Longitudinal slots 510 are cut through the mandrel wall and extend through the small diameter and tapered sections (508 and 506) of the slotted mandrel. Longitudinal slots 510 are cut through the mandrel wall and extend only partially along large diameter section 504, as shown in Figure 5A.

[0046] A pre-cut tube 524 a having an initial small diameter is placed over the small diameter portion 508 of the slotted mandrel 502 .

[0047] An expansion die 512 having a series of fins (414 in FIG. 4) formed to engage slots 510 in slotted mandrel 502 is coupled to a pull rod 518. Pull rod 518 extends through a central bore of tubular slotted mandrel 502 and out the end of the slotted mandrel opposite the expansion die.

[0048] As shown in FIG. 5B, the pull rod 518 is translated in the direction indicated by arrow 520 to advance the expansion die 512 and allow the fins (414 in FIG. 4) of the expansion die to engage with the slots 510 of the slotted mandrel 502.

[0049] The cut tubing extension fixture is then placed over the heating chamber so that the collar portion 522 and protruding pull rod 518 are outside the heated chamber (direction arrow 526 ), while the remainder of the slotted mandrel 502, expansion die 512, and cut tube 524a are positioned in the heated region of the heating chamber (indicated by directional arrow 525 The temperature of the heating chamber is then raised to the desired temperature. If a salt bath or similar heat transfer medium is used, the medium can be preheated or fully heated to the desired elevated temperature.

[0050] As shown in FIG. 5C, after proper residence within the heated chamber, the pull rod 518 is further advanced along direction 520, causing the expansion die 512 to force the cut tube 524b over the tapered portion 506 of the slotted mandrel 502.

[0051] 5D, the pulling rod 518 is further advanced along direction 520 such that the expansion die 512 forces the cut tube 524c over the large diameter portion 504 of the slotted mandrel 502. The translational motion of the pulling rod 518 can include continuous, intermittent, or variable speed motion.

[0052] The expansion fixture 500 with the fully expanded pre-cut tube 524c is then removed from the heating chamber. The pull rod 518 and expansion die 512 are then attached to the slotted mandrel. 502 The slotted mandrel 502 and fully expanded pre-cut tubes 524c are then quenched in a several temperature water bath. After reaching ambient temperature, the fully expanded pre-cut tubes 524c can be removed from the slotted mandrel 502.

[0053] 5A-5E depict a short length of tubing, any length of tubing can be expanded using the above process. The large diameter portions 404, 504 of the slotted mandrels 402, 502 can be any size to accommodate tubing of any length.

[0054] 5A-5E is one method of using an internal force (a force inside the tube to be expanded) to expand the SMA tube, but other methods can be used. These include an expansion mandrel that expands the tube over the mandrel.

[0055] In another embodiment, expansion of the pre-cut SMA tubing is achieved by applying an external force that pulls the tubing apart. Hooks or clamps that grip specific areas of the tubing can pull the tubing apart, thereby expanding the tubing.

[0056] While particular embodiments of the present invention have been shown and described herein, the present invention should not be limited to such illustrations and descriptions. Changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims. The following examples are provided to further illustrate the present invention.

[0057] example Example 1: Loading and expanding pre-cut Nitinol tubing on a slotted mandrel The nitinol stent ring 524a shown in Figures 5A and 5B was obtained. The stent ring 524a was laser cut from a nitinol tube with an inner diameter (ID) of approximately 4 mm and a wall thickness of approximately 0.5 mm. The length of the stent ring 524a was approximately 10 mm.

[0058] A tapered slotted mandrel 402 was custom fabricated from a suitable high temperature steel as shown in Figure 4. Slotted Mandrel 402 Large diameter portion 404 was about 26 mm. The minor diameter of slotted mandrel 402 was about 8 mm. The length of slotted mandrel 402 was about 11 cm. An expansion die 412 was custom fabricated from a suitable high temperature steel.

[0059] The expansion die 412 was designed so that the fins 414 of the die engaged the slots 410 of the slotted mandrel 402 , allowing the expansion die 412 to slide through the slotted mandrel 402 .

[0060] The expansion die 142 is attached, for example, by laser welding, to a pulling rod 418. The pulling rod 418 is approximately 2 mm in diameter, approximately 60 cm in length, and is fabricated from a suitable high-temperature steel. A fluidized bath (Techne Fluidized Bath Model FB-08) was obtained to be used for the heat treatment section.

[0061] As shown in FIG. 5A, the stent ring 524a is inserted into the small diameter slot of the slotted mandrel 502. portion508. To load the stent ring 524a, having an ID of approximately 4 mm, onto the approximately 8 mm smaller diameter end 508 of the slotted mandrel 502, a tapered mandrel having a diameter of approximately 4 mm at one end and an diameter of approximately 8 mm at the opposite end was used to first expand the stent ring 524a to approximately 8 mm (at room temperature). At this point, the stent is minimally constrained (or substantially unconstrained). The approximately 8 mm end of the tapered mandrel was then abutted against the approximately 8 mm smaller diameter end 508 of the slotted mandrel 502, and the stent ring 524a was transferred from the tapered mandrel to the slotted mandrel 502 at room temperature. As shown in FIG. 5A, a pulling rod 518 with an attached expansion die 512 was inserted into the slotted mandrel. The fins 414 (FIG. 4) of the expansion die 512 were engaged with the slots 510 of the slotted mandrel 502, as shown in FIG. 5B.

[0062] The assembly consisting of slotted mandrel 502, stent ring 524a, expansion die 512, and pull rod 518 was then immersed in a fluidized bath preheated to a temperature of approximately 550°C and allowed to dwell for approximately 3 minutes. After approximately 3 minutes, pull rod 518 was withdrawn from the position shown in FIG. 5B to the position shown in FIG. 5D. It took approximately 2 seconds to withdraw pull rod 518 from the position shown in FIG. 5B to the position shown in FIG. 5D. As an upward force is applied to pull rod 518, fins 414 (FIG. 4) of attached expansion die 512 exert a force on stent ring 524b, pulling it along slotted mandrel 502 as shown in FIG. 5C. The orientation of slots 510 and fins 414 (FIG. 4) also helps maintain uniform diametric expansion of stent ring 524c as shown in FIG. 5D. Then, after a residence time of approximately 15 minutes in the preheated fluidized bath, the assembly consisting of slotted mandrel 502, expanded stent ring 524c, expansion die 512, and pulling rod 518 was removed from the fluidized bath and water quenched. Pulling rod 518 and attached expansion die 512 were then removed from slotted mandrel 502. The expanded nitinol stent ring 524c and slotted mandrel 502 following heat treatment and shape setting in the fluidized bath are shown in FIG. 5E. The resulting nitinol stent ring 524c was expanded and shape set to a diameter of approximately 26 mm.

[0063] 4, it will be apparent to one skilled in the art that additional fixtures can be used to interface the slotted mandrel 402, which expands the stent, with the fluidized bath. To accommodate such fixtures, a collar 422 can be cut into the slotted mandrel 402. The collar 422 can be used to attach additional fixtures that allow the mandrel to be safely immersed in the heated medium of the fluidized bath.

[0064] As will be apparent to those skilled in the art, various modifications can be made to the present invention. For example, the slotted mandrel 402 as shown in FIG. 4 could have four slots 410 instead of eight slots 410. Additionally, the expansion die 412 could have four fins 414 instead of eight fins 414. Additionally, the length of the slotted mandrel and the resulting taper angle could be modified. For example, the length of the slotted mandrel 402 could be increased to 20 cm instead of approximately 11 cm. This may reduce the force required during stent expansion.

[0065] Example 2: Expansion of pre-cut Nitinol tubing without heating 5A-5E, a nitinol stent ring 524a was loaded onto a slotted mandrel 502 using the method and materials of Example 1. The stent ring 524a was then expanded at about room temperature (about 20°C) by raising the pull rod 518 from the position shown in Figure 5B to the position shown in Figure 5D. The assembly of the slotted mandrel 502, stent ring 524c, expansion die 512, and pull rod 518, as shown in Figure 5D, was then immersed in a fluidized bath preheated to a temperature of about 550°C and allowed to dwell for about 15 minutes.

[0066] The assembly consisting of slotted mandrel 502, stent ring 524c, expansion die 512, and pulling rod 518 was then removed from the fluidized bed and water quenched. The resulting nitinol stent ring was fractured and had intact discontinuities within the stent ring.

[0067] Example 3: Expanding pre-cut Nitinol tubing using an expandable mandrel Another expansion fixture is shown in Figures 6A-6D. The slotted tube 610 shown in Figures 6A and 6C was formed from a suitable high-temperature steel and was approximately 15 cm in length. The slotted tube had an inner diameter of 4.2 mm and a wall thickness of approximately 0.25 mm. The slots 604 cut in the tube and the resulting tube segments 606 were each approximately 12 cm in length.

[0068] As shown in Figure 6B, a tapered mandrel 618 was formed from a suitable high-temperature steel and was approximately 40 cm long. The large diameter section 612 was approximately 8 mm in diameter and approximately 8 cm in length. The small diameter section 616 was approximately 4 mm in diameter and approximately 28 cm in length. The tapered section 614 of the tapered mandrel 618 transitioned from approximately 8 mm in diameter to approximately 4 mm in diameter and was approximately 4 cm long.

[0069] The nitinol stent 624 shown in Figure 6A was obtained. The stent rings 624 were laser cut from nitinol tubing with an inner diameter (ID) of approximately 4.1 mm and a wall thickness of approximately 0.25 mm. The stent length was approximately 60 mm. The stent 624 was loaded onto the slotted tube 610, close to the slotted end 602 of the slotted tube 610. The smaller diameter end 616 of the tapered mandrel 618 was then inserted into the slotted end 602 of the slotted tube 610.

[0070] A fluidized bath (Techne Fluidized Bath Model FB-08) was obtained for use in heat treating sections.

[0071] FIG. 6C shows an extended fixture 600. The assembly of slotted tube 610, stent 624, and tapered mandrel 618, as shown in Figure 6C, was then immersed in a fluidized bath heated to a temperature of about 550°C and allowed to dwell for about 3 minutes. After about this 3 minutes, tapered mandrel 618 was pulled in the direction 620 shown in Figure 6C to the position shown in Figure 6D. It took about 3 seconds to pull tapered mandrel 618 from the position shown in Figure 6C to the position shown in Figure 6D.

[0072] After approximately 15 minutes in the preheated fluidized bath, the assembly consisting of expanded slotted tube 628, expanded stent 626, and tapered mandrel 618, shown in FIG. 6D, was then removed from the fluidized bath and water quenched. Following heat treatment and shape setting, expanded stent 626 was then removed from expanded slotted tube 628. The resulting nitinol stent was expanded and shape set to a diameter of approximately 8.5 mm.

[0073] It will be apparent to one skilled in the art that additional fixtures can be used to interface the stent expansion hardware shown in Figures 6A and 6B with the fluid bath. Additionally, it will be apparent to one skilled in the art that the dimensions of the hardware shown in Figures 6A and 6B can be modified to improve the interface between the stent expansion hardware and the fluid bath. For example, the length of the small diameter end 616 of the tapered mandrel 618 can be further extended, if necessary, so that it extends sufficiently above the level of the heated medium in the fluid bath. Additionally, the length of the unslit end 608 of the slotted tube 610 can be further extended so that it extends sufficiently above the level of the heated medium in the fluid bath.

[0074] As will be apparent to those skilled in the art, various modifications can be made to the present invention. For example, the slotted tube 610 shown in Figure 6A can have eight slots 604 instead of four. Additionally, the tapered mandrel 618 can have longitudinal grooves that allow it to interlock with the segments 606 of the slotted tube 610. These grooves allow the segments to move along the tapered portion of the tapered mandrel 618. classification As it moves to 614, it will control the expansion of segment 606.

[0075] Example 4: Expansion of pre-cut Nitinol tubing using an expandable mandrel without heat treatment Using the method and materials of Example 3, a nitinol stent 624 was loaded onto a slotted mandrel 610. The stent 624 was then expanded at approximately room temperature (approximately 20°C) by pulling a tapered mandrel 618 in the direction shown in FIG. 6C to the position shown in FIG. 6D. The assembly of the slotted tube 610, stent 624, and tapered mandrel 618, as shown in FIG. 6C, was then immersed in a fluidized bed preheated to a temperature of approximately 550°C and allowed to dwell for approximately 15 minutes. The assembly of the expanded slotted tube 628, expanded stent 626, and tapered mandrel 618 was then removed from the fluidized bed and water quenched. The resulting nitinol stent 626 had numerous fractures.

[0076] In addition to the embodiments described above and the claims that follow, the present invention also relates to embodiments having different combinations of the above patents and the claims that follow, and as such, the present invention also relates to other embodiments having any other possible combination of the independent features recited in the claims that follow.

[0077] Numerous features and advantages of the present invention are set forth in the foregoing description, including preferred and alternative embodiments, along with details of the structure and function of the invention. The present disclosure is intended to be illustrative only and, as such, is not intended to be comprehensive. As will be apparent to those skilled in the art, various modifications may be made within the principles of the present disclosure, particularly with respect to structure, materials, elements, components, shapes, sizes, and arrangements of parts, to the full extent indicated by the broad and general meaning of the terms in which the appended claims are expressed. These various modifications are intended to be included in the present disclosure to the extent that they do not depart from the spirit and scope of the appended claims.

Claims

1. 1. A method of forming a shape memory alloy (SMA) article, comprising: Providing an SMA article having an initial shape; heating the SMA article to a shape-setting temperature in the range of 300°C to 650°C; using an expansion fixture to deform the SMA article to its final shape in a single expansion step to 100% expansion within 3 seconds while maintaining the SMA article within the range of 300°C to 650°C; and cooling the SMA article under restraint, thereby substantially retaining said final shape; The method comprising:

2. 10. The method of claim 1, wherein after the SMA article is deformed, the deformed SMA article is allowed to rest at about the shape-setting temperature.

3. 3. The method of claim 1 or 2, wherein deforming the SMA article into a final shape is achieved by applying an internal force.

4. 3. The method of claim 1 or 2, wherein deforming the SMA article into a final shape is achieved by applying an external force.

5. 5. The method of any one of claims 1 and 2 to 4, wherein the initial shape has an initial size and the SMA article is deformed into a final shape such that the final shape is at least twice the initial size.

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