Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2002-05-24
2004-12-14
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S402000, C148S670000, C148S676000, C623S001180
Reexamination Certificate
active
06830638
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to application of nickel-titanium alloys to form medical devices. More precisely, the present invention is directed to cold worked nickel-titanium alloys and nickel-titanium clad alloys that have been processed by a deep drawing operation to produce a material exhibiting linear pseudoelastic behavior without a phase transformation or onset of stress-induced martensite that can be manufactured into a medical device.
A focus of developmental work in the treatment of heart disease is an endoprosthetic device referred to as a stent. A stent is a generally cylindrically shaped intravascular device that is implanted in a diseased artery to hold it open. The device is used to maintain the patency of a blood vessel immediately after intravascular treatments, and further reduces the likelihood of restenosis. In some circumstances, a stent can be used as the primary treatment device where it is expanded to dilate a stenosis and then left in place.
A limitation of some prior art stents, especially those of the balloon expandable type, is that they are stiff and inflexible. Often, the expandable type stents are formed from stainless steel alloys and are constructed so that they are expanded beyond their elastic limit. Such stents are permanently deformed beyond their elastic limits in order to hold open a body lumen and to maintain the patency of the body lumen. By the same token, since the material is stressed beyond its elastic limit into the plastic region, the material becomes stiff and inflexible.
There are several commercially available stents that are widely used and generally implanted in the coronary arteries after a PTCA (Percutaneous Transluminal Coronary Angioplasty) procedure, described above. Stents also can be implanted in vessels that are closer to the surface of the body, such as the carotid arteries in the neck or the peripheral arteries and veins in the leg. Because these stents are implanted so close to the surface of the body, they are particularly vulnerable to impact forces that can partially or completely collapse the stent and possibly block fluid flow in the vessel. Under certain conditions, muscle contractions may even cause the stent to partially or totally collapse. Since balloon expandable stents are plastically deformed, once collapsed or crushed, they remain so, possibly blocking or occluding the vessel. These balloon expandable stents, therefore, could possibly pose an undesirable condition to the patient.
Such important applications as mentioned above have prompted stent designers to use superelastic or shape memory alloys in their stents to exploit the self-expanding and elastic properties of these materials. Typically, the superelastic or shape memory alloy of choice is nickel-titanium. Nickel-titanium alloy, commonly referred to as Nitinol, an acronym for Nickel-Titanium Naval Ordinance Laboratory, where it was initially developed, is frequently chosen for use in self-expanding stents and other medical devices due to its highly elastic behavior and resiliency. As a result, a nickel-titanium stent does not deform plastically when deployed, and remains highly resilient inside the body lumen. Because of this resilience, the self-expanding nickel-titanium stent can encounter a deforming impact, yet will return to its initial shape. Therefore, the chance of a permanent collapse of the self-expanding stent due to an impact force is minimized. An example of such shape memory alloy stents is disclosed in, for example, European Patent Application Publication No. EP0873734A2, entitled “Shape Memory Alloy Stent.”
Near equi-atomic binary nickel-titanium alloys are known to exhibit “pseudoelastic” behavior when given certain cold working processes or cold working and heat treatment processes following hot working. Generally speaking, “pseudoelasticity” is the capacity of the nickel-titanium alloy to undergo large elastic strains on the order of 8 percent or more when loaded and to substantially fully recover all strain upon removal of the load. Substantially full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.
Pseudoelasticity can be further divided into two subcategories: “linear” pseudoelasticity and “non-linear” pseudoelasticity. “Non-linear” pseudoelasticity is sometimes used by those in the industry synonymously with “superelasticity.”
Linear pseudoelasticity results from cold working only. Non-linear pseudoelasticity results from cold working and subsequent heat treatment. Non-linear pseudoelasticity, in its idealized state, exhibits a relatively flat loading plateau in which a large amount of recoverable strain is possible with very little increase in stress. This flat plateau can be seen in the stress-strain hysteresis curve of the alloy. Linear pseudoelasticity exhibits no such flat plateau. Non-linear pseudoelasticity is known to occur due to a reversible phase transformation from austenite to martensite, the latter more precisely called “stress-induced martensite” (SIM). Linear pseudoelasticity has no such phase transformation associated with it. Further discussions of linear pseudoelasticity can be found in, for example, T. W. Duerig, et al., “Linear Superelasticity in Cold-Worked Ni—Ti,”
Engineering Aspects of Shape Memory Alloys
, pp. 414-19 (1990).
Because of the useful nature of the nickel-titanium alloy, some have attempted to change its properties to solve different design needs. For example, U.S. Pat. No. 6,106,642 to DiCarlo et al. discloses annealing Nitinol to achieve improved ductility and other mechanical properties. U.S. Pat. No. 5,876,434 to Flomenblit et al. teaches annealing and deforming Nitinol alloy to obtain different stress-strain relationships.
Binary nickel-titanium alloys have been used in the medical field. Some medical device related applications exploit the non-linear pseudoelastic capabilities of Nitinol. Examples include: U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 to Jervis; and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746; 5,720,754; and 6,004,629 to Middleman, et al.
Yet another application of nickel-titanium alloys is in an embolic protection or filtering device. Such embolic filtering devices and systems are particularly useful when performing balloon angioplasty, stenting procedures, laser angioplasty, or atherectomy in critical vessels, particularly in vessels such as the carotid arteries, where the release of embolic debris into the bloodstream can occlude the flow of oxygenated blood to the brain or other vital organs. Such an occlusion can cause devastating consequences to the patient. While the embolic protection devices and systems are particularly useful in carotid procedures, they are equally useful in conjunction with any vascular interventional procedure in which there is an embolic risk.
What has been needed and heretofore unavailable in the prior art is a medical device that exploits the benefits of linear pseudoelastic Nitinol. With the use of linear pseudoelastic Nitinol, the mechanical strength of the device is substantially greater per unit strain than a comparable device made of superelastic Nitinol. Furthermore, smaller component parts such as struts can be used because of the greater storage of energy available in a linear pseudoelastic Nitinol device.
SUMMARY OF THE INVENTION
The present invention is generally directed to cold worked nickel-titanium alloys and nickel-titanium clad alloys (nickel-titanium alloys clad with a layer of another metal) that have been deep drawn in a cold working process that produces linear pseudoelastic behavior in the alloy. The processed material may exhibit pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device.
In one aspect, the present invention is directed to a medical device for use in a body lumen comprising a structural element made from a cold formed nickel-titanium alloy which has been processed by plastically deforming a sheet-type
Boylan John F.
Boyle William J.
Huter Scott J.
Magrini Kevin M.
Advanced Cardiovascular Systems Inc.
Fulwider Patton Lee & Utecht LLP
Wyszomierski George
LandOfFree
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