Patent Application
20070131318
The material and methods disclosed herein relate to medical alloys having two or more alloying elements, and including a dispersion of discrete particles substantially free of the two or more alloying elements. In a specific embodiment, the materials and methods disclosed herein relate to a nickel-titanium alloy, and a nickel-titanium alloy with a dispersion of discrete particles substantially free of nickel and titanium.
Nickel-titanium alloy, more specifically nitinol, is valued in a number of industries because of its unique properties of superelasticity and shape memory. For example, nitinol is used in a number of components in minimally invasive surgery, including but not limited to catheters and stents. Other medical alloys are used for a wide range of implantable applications.
While nitinol has its benefits, it also has its drawbacks. Particularly, nitinol is subject to brittle fracture. Once a fracture or crack is initiated, it is subject to propagation under fatigue conditions. This propagation could have serious consequences, especially if the nitinol is being used in a medical device placed in a patient. Another disadvantage of nitinol is that nitinol has low radiopacity, meaning that the nitinol has limited visibility when viewed by x-ray based imaging systems. Low radiopacity and low resistance to fatigue crack propagation limit the effectiveness of nitinol in the medical device industry. Other medical alloys with better radiopacity than nitinol may be used in applications such as arterial stents where the device section is small and visibility of the device is limited under x-ray visualizations.
Development efforts have focused on creating a ternary or higher order alloy from the base binary nitinol or adding dense metallic additions to other non-nitinol medical alloys. The efforts with nitinol have achieved moderate improvement in radiopacity, but the other drawbacks noted for binary nitinol still exist. The efforts with the dense metallic additions to other medical alloys creates a new alloy, i.e., one metal dissolved in another to create a new composition. Those new compositions face many regulatory hurdles, and therefore are problematic.
One embodiment of the invention is an article having a medical alloy and a uniform dispersion of discrete particles in the article. The medical alloy includes two or more alloying elements. The discrete particles are substantially free of the two or more alloying elements.
In another embodiment, the article is made by melting two or more alloying elements to form a medical alloy and dispersing at least one type of discrete particle free of the two or more alloying elements in the medical alloy to form an ingot. The melting and dispersing are performed at a temperature above the alloying temperature of the composition and below the melting temperature of the discrete particles. The ingot is hot worked to form a processed ingot. The processed ingot is cold worked and annealed to form the article.
In a further embodiment, an article having a medical alloy including two or more alloying elements and a uniform dispersion of discrete particles substantially free of the two or more alloying elements is made by vacuum induction melting a composition including the two or more alloying elements to form a cast ingot. The melting is performed at or above an alloying temperature for the composition. During the vacuum induction melting, the discrete particles are pour stream injected into the cast ingot at a temperature below the melting point of the discrete particles. The cast ingot is hot worked to form a processed ingot. The processed ingot is cold worked and annealed to form the article.
In yet another embodiment, an article having a medical alloy including two or more alloying elements and a uniform dispersion of discrete particles substantially free of the two or more alloying elements is made by preparing an electrode having a hollow center. Dispersion particles are introduced into the hollow center. The electrode and particles are vacuum arc melted at a temperature above the alloying temperature of the composition and below the melting point of the particles to form a cast ingot. The cast ingot is hot worked to form a processed ingot. The processed ingot is cold worked and annealed to form the article.
For the purpose of illustrating the invention there are shown in the drawing a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.
The single drawing shows an article according to one embodiment of the invention disclosed herein.
With reference to the drawing, where like numerals identify like elements, there is shown an article 10 in accordance with the materials and methods disclosed herein. As illustrated, the article 10 is a stent. While the article is shown as a stent, the article is not so limited. The article can be all or just a portion of any structure having the elements described herein. The article can be a product or component of a product including for example, but not limited to, actuators, hydraulic line couplings, electrical connectors, fishing lures, eyeglass frames, and golf clubs. The invention is particularly suited for medical devices or components including, but not limited to, catheters, biopsy sectioning and retrieval equipment, vena cava filters, and stents.
The article 10 comprises an alloy component 12. The alloy component can comprise any medical alloy. As used herein, medical alloys are alloys generally used in the medical device industry in the manufacture of both implantable medical devices and non-implantable medical devices. Implantable medical devices are devices that can be placed within a person. Implantable medical devices include stents, valves, prostheses, trauma fixation plates, screws and nails, CRM devices, leads and connectors, shunts, filters and a number of other devices. Medical alloys for implantable medical devices generally include stainless steel, cobalt alloys, and titanium alloys.
More specifically, medical alloys for implantable medical devices and components can be 316L per ASTM F138, nitrogen strengthened stainless steel per ASTM F1314 and F2229, Ti6A1 4V per ASTM F136 and F1472, Ti6A1 7Nb per ASTM F1295, Cobalt Chrome Moly Tungsten per ASTM F90, Cobalt Chrome Moly per ASTM F75, F799 and F1537, Cobalt Nickel Chrome Moly per ASTM F562, and Nickel Titanium per ASTM 2063.
Non-implantable medical devices are devices used in medicine, but that are not intended to be placed within a person. Non-implantable medical devices and components of devices include but are not limited to: device delivery systems such as catheters, trials for orthopedic prosthesis sizing, tools such as reamers, drills, and screw drivers, guides for locating drilled holes, retracters, trocars, staplers, and endoscopes. Medical alloys for non-implantable medical devices can be austenitic, ferritic, precipition hardenable, and martensitic stainless steels. Medical alloys used in non-implantable medical devices are generally listed in ASTM F899.
The selection of a suitable medical alloy for a particular application is based on the functionality of the medical alloy as it relates to that particular application. For example, for implantable medical devices, the major functional requirement is biocompatibility. Consequently, a suitable medical alloy for use in an implantable medical device is preferably biocompatible.
A medical alloy, as with any alloy, includes two or more alloying elements. Alloying elements are pure or substantially pure metals that can be combined to form an alloy. For example, where the medical alloy is a nickel-titanium alloy, nickel metal and titanium metal are alloying elements.
Nickel-titanium is a preferred medical alloy. The nickel-titanium alloy can be a binary or higher alloy. Preferably, the nickel-titanium alloy is a binary “nitinol.” Nitinol includes a family of nickel-titanium alloys having a substantially equiatomic composition of nickel and titanium. The equiatomic composition results in an ordered crystalline structure with the unique property of deformation with a high degree of recoverable (or pseudo-elastic) strain, which allows the composition to be returned to its original shape after deformation.
The unique properties of nitinol are referred to as superelasticity and shape memory. Superelasticity refers to the unusual ability of certain metals/alloys to undergo large elastic deformation. When mechanically loaded, a superelastic nitinol article undergoes a recoverable deformation up to very high strains (e.g., up to 8%). The load creates a stress-induced martensitic transformation in the article. Upon unloading, a spontaneous reversal of the transformation occurs, causing the article to return to its original shape. No change in temperature is needed for the alloy to recover its initial shape.
In contrast to superelasticity, shape memory describes the characteristic that allows a plastically deformed article to be restored to its original shape by heating it. The article is heated above the austenite finish temperature of the article, causing crystalline transformation and returning the article to its original shape. Shape memory is important to many nitinol-based products. For example, orthopedic staples can be inserted into holes drilled into the bone on each side of a fracture. When the staple is heated it attempts to return to its original shape, applying a closure force to the fracture. The shape memory property is beneficial for reusable medical instruments. Medical personnel can shape the instrument to fit the desired need (e.g., fit the patient's physiology). After use, the instrument can be heat sterilized, which results in the instrument returning to its original shape for future use.
Despite its beneficial characteristics, nitinol has low radiopacity and high susceptibility to fatigue crack propagation. Both the low radiopacity and high susceptibility to fatigue crack propagation have limited the use of nitinol in products such as implantable medical devices.
To increase radiopacity and decrease susceptibility for fatigue crack propagation, the article 10 includes a uniform dispersion of discrete particles 14. The discrete particles are substantially free of the two or more alloying elements of the alloy component 12. As used herein, “substantially free of” means that the discrete particles have, at most, only a trace amount, but preferably none, of the two or more alloying elements of the alloy component 12. Also as used herein, “substantially free of” means that the discrete particles do not form an alloy with the two or more alloying elements of the alloy component 12, or only form an alloy with, at most, a trace amount of the two or more alloying elements. In short, “substantially free of” means that some diffusion from the alloy component may occur, but that the diffusion is minimal. For example, for articles where nitinol is the alloy component, the discrete particles are any material other than nickel and titanium. Melting and dispersion are performed such that the discrete particle material does not form an alloy with the nickel and/or titanium. The discrete particles can be one or more elements including, but not limited to, iridium, platinum, gold, rhenium, tungsten, palladium, rhodium, tantalum, silver, ruthenium, and hafnium. The discrete particles can be one or more alloys containing one or more elements including, but not limited to, iridium, platinum, gold, rhenium, tungsten, palladium, rhodium, tantalum, silver, ruthenium, and hafnium.
As shown in the figure, the discrete particles 14 form islands in the alloy component. The islands are substantially free of the two or more alloying elements (e.g., nickel and titanium for articles having nitinol as the alloy component) and do not form alloys with the two or more alloying elements (e.g., nickel and titanium for articles having nitinol as the alloy component) of the alloy component. The islands can be formed in any geometric or non-geometric shape. Preferably, the islands are spherical in shape. Preferably, if the islands are elongated, they are oriented such that the major axis of the discrete particles in the island are aligned perpendicular to the likely direction of fatigue crack propagation in the article. The islands can be located anywhere in the article.
The nitinol materials described herein provide several benefits over prior alloys used in medical devices such as binary nitinol, and nitinol alloyed with other elements to form a conventional ternary or high order alloy. For example, the discrete particles will have significantly lower impact upon the phase transformational changes of the nitinol that result in superelastic and/or shape memory effects.
A benefit of the materials described generally herein is increased resistance to crack propagation. If a crack is formed in an article without particles, the crack may continue unabated thereby damaging and/or destroying the article. However, if a crack front encounters a discrete particle, further propagation will be impeded, thereby preserving the integrity of the article.
If the particle is chosen from the aforementioned list of elements or alloys, then there is an additional benefit of increasing the radiopacity of the resulting system over the radiopacity of many of the medical alloys described herein when those medical alloys are used alone (i.e., without the particles). Increased radiopacity will allow physicians or other medical personnel to view medical devices containing the material with greater clarity using x-ray based imaging techniques.
Several methods are contemplated for making the article 10. The methods described herein are directed to an article having a nickel-titanium alloy, but it is within the scope of this invention that articles with other medical alloys can be made following the general parameters of the same methods. The methods produce an article having a medical alloy including two or more alloying elements and a dispersion of discrete particles in the articles wherein the discrete particles are substantially free of the two or more alloying elements and do not form an alloy with any of the two or more alloying elements. The methods preferably produce an article having a nickel-titanium alloy and dispersion of discrete particles in the article, wherein the discrete particles are substantially free of nickel and titanium and do not form an alloy with nickel and/or titanium of the nickel-titanium alloy.
Generally, the methods include the steps of melting a composition having two or more alloying elements to form an alloy and dispersing at least one type of discrete particle in the alloy to form an ingot. Preferably, the composition is a substantially equiatamic composition of nickel and titanium. The melting and dispersing are performed at a temperature above the alloying temperature of the composition and below the melting temperature of the at least one type of discrete particle.
When the titanium concentration is close to 50 weight percent, molten nitinol is highly reactive and must be melted in a vacuum. It is advantageous to melt other medical alloys in a vacuum as well. Vacuum induction melting (VIM) and vacuum consumable arc melting (VAR) are preferred ways to vacuum melt the materials. Alternate vacuum melting methods can also be used.
VIM involves melting a metal composition under vacuum conditions by inducing alternating electrical eddy currents in the metal. The composition is placed in an electrically conductive crucible, preferably graphite or calcia, or in a induction melter without a crucible. The composition and crucible are placed in a vacuum chamber. The composition is heated by eddy currents causing the composition to melt.
The vacuum chamber can be a furnace having an air-tight, water-cooled steel jacket that is capable of withstanding the vacuum required for melting. The inside of the furnace is typically lined with refractory materials.
VIM or vacuum skull melting (VSM) can be used in an embodiment of the presently disclosed method. In that embodiment, a substantially equiatomic composition of nickel and titanium is vacuum induction melted to form a cast ingot. The melting is performed at or above an alloying temperature for the composition. Following the vacuum induction melting, particles are pour stream injected during the casting of the ingot. The injection is performed at a temperature below the melting point of the particles. If more than one type of particle is injected, then the temperature is below the lowest melting point of all of the particles.
Alternatively, VAR can be used in place of VIM or VSM. In such an embodiment, an electrode is prepared from a substantially equiatomic composition of nickel and titanium. The electrode is prepared having a hollow center into which particles are introduced. The particles can be introduced by injecting the particles into the hollow center, by packing a powder form of the particles into the hollow center, or any other known manner. After the particles are introduced, the electrode is vacuum arc melted to form a cast ingot. The vacuum arc melting is performed at a temperature above the alloying temperature of the composition and below the melting point of the particles.
In a further alternative, VAR can be used in conjunction with VIM or VSM. In such an alternative, VIM or VSM melting is typically performed first, followed by VAR melting. With that order, the VIM melting creates an ingot. The ingot is then used as a consumable electrode in the VAR melting. The VIM/VAR combination combines the benefits of VIM (e.g., thorough mixing) with the benefits of VAR (e.g., high purity of the resultant alloy).
Alternate methods can be used to introduce the uniform dispersion into the cast ingot.
After the cast ingot is formed by any of the methods described herein or by any other similar method, the ingot is typically refined by additional deformation processes in order to optimize the beneficial properties, such as shape memory, superelasticity, or resistance to fracture. The additional deformation is typically done by first hot working the ingot to form a processed ingot with a useful shape, while at the same time changing the microstructure in the processed ingot into one that has optimized beneficial properties. Hot working can include, for example, press forging, rotary forging, extrusion, swaging, bar rolling, rod rolling, or sheet rolling.
To further optimize the beneficial properties, the processed ingot can undergo a series of cold working steps. The cold working steps provide the final shape, surface finish, refined microstructure, and mechanical properties of the article. Preferably, cold drawing or cold rolling are used to cold work the processed ingot. Typically a cold working step is followed immediately by an annealing step. Cold working and subsequent annealing can be repeated multiple times, if necessary.
Generally, the result of each of the methods disclosed herein is an article composed of a medical alloy having two or more alloying elements and a uniform dispersion of discrete particles substantially free of the two or more alloying elements. In a preferred embodiment, the result of each of the methods disclosed herein is an article composed of a nickel-titanium alloy and a uniform dispersion of discrete particles substantially free of nickel and titanium.
It will be appreciated by those skilled in the art that the present invention may be practiced in various alternate forms and configurations. The previously detailed description of the disclosed methods is presented for clarity of understanding only, and no unnecessary limitations should be implied therefrom.
This is a Continuation in Part application of patent application Ser. No. 11/299,476, filed Dec. 12, 2005, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11299476 | Dec 2005 | US |
| Child | 11595034 | Nov 2006 | US |
