The present invention is related to magnesium-based alloys that may be used to manufacture bioabsorbable stents.
Magnesium alloys demonstrate excellent specific properties that make them potentially suitable candidates for replacing heavier materials in several commercial, defense, and medical applications. Magnesium is less toxic and an attractive alloy for biodegradable medical implant applications. Potential limitations of the current magnesium alloys include low ductility, low strength, limited high-temperature properties and poor corrosion resistance. Magnesium has a hexagonally close-packed (HCP) crystal structure resulting in relatively low ductility (compared to face centered cubic (FCC) and body-centered cubic (BCC) alloys). Several commercial magnesium-based alloys have been developed that include Mg—Al—Zn (AZ-type alloys), Mg—Zn—Mn (ZM-type alloys) and their variants containing additional elements such as rare earth (RE) elements to achieve improved strength, ductility and corrosion resistance. However, the corrosion resistance of the aforementioned alloys is limited due to presence of cathodic second phase particles (or precipitates) that promote galvanic coupling resulting in dissolution of the matrix. Second-phase particles can provide resistance to grain growth during annealing and other heat treatments.
It is desirable to achieve uniform single phase microstructure (without any second phases) to prevent internal galvanic coupling and thereby achieve superior corrosion resistance. In addition to galvanic coupling, impurities present in the Mg-based alloys may play a crucial role in driving corrosion. Elements such as Fe, Ni and Cu should be minimized in the system to improve corrosion. Hence, it is desirable to identify alloying elements that getters the aforementioned impurities. In the absence of any second phase particles providing precipitation strengthening, the single phase alloy will have to rely on other strengthening sources such as solid solution strengthening, grain refinement, and cold work (dislocation strengthening). It is noted that grain refinement improves corrosion resistance and ductility.
In a multiphase system, it is desirable to ensure that the discrete 2nd phase is relatively more electronegative compared to the matrix phase to slow corrosion of the matrix. Alloying elements that could (i) balance the difference in electrochemical potential between matrix and discrete 2nd phase particles and/or (ii) increase the electro positivity of the interconnected matrix relative to the discrete 2nd phase can be added. In addition to galvanic coupling, impurities present in the Mg-based alloys may play a crucial role in driving corrosion. Elements such as Fe, Ni and Cu should be minimized in the system to improve corrosion resistance.
In accordance with an aspect of embodiments of the present invention, there is provided a stent comprising a magnesium alloy. The magnesium alloy consists essentially of: 0-10 weight % rare earth element; 0-5 weight % Li; 0-1 weight % Mn; 0-1 weight % Zr; and Mg for the balance. The rare earth element may be selected from the group consisting of: Sc, Y, La, Gd, and Nd.
In an embodiment, a stent is formed from a magnesium alloy, the magnesium alloy consisting essentially of: about 1 weight % Sc; about 0.5 weight % Y; about 1 weight % Li; and balance Mg.
In an embodiment, a stent is formed from a magnesium alloy, the magnesium alloy consisting essentially of: about 1 weight % Sc; about 0.8 weight % Y; and balance Mg.
In an embodiment, a stent is formed from a magnesium alloy, the magnesium alloy consisting essentially of: about 1.5 weight % Sc; about 0.7 weight % Li; and balance Mg.
In an embodiment, a stent is formed from a magnesium alloy, the magnesium alloy consisting essentially of: about 1.5 weight % Y; about 0.7 weight % Li; and balance Mg.
In accordance with an aspect of embodiments of the present invention, there is provided a stent comprising a magnesium alloy. The magnesium alloy consists essentially of: 0-5 weight % rare earth element; 0-8 weight % Li; 0-1 weight % Mn; 0-1 weight % Sn; 0-3 weight Al; 0-4 weight % Zn; and Mg for the balance. The rare earth element may be selected from the group consisting of: Sc, Y, La, Gd, and Nd.
In an embodiment, a stent is formed from a magnesium alloy, the magnesium alloy consisting essentially of: about 3 weight % Li; about 1 weight % Al; and balance Mg.
Embodiments of the present invention are directed to a novel Mg-based composition combined with a special recipe for processing to achieve single microstructure with superior corrosion resistance. Additionally, preliminary evaluations reveal good strength and ductility when compared against pure Mg. The alloying elements include:
The RE elements include Sc, Y, La, Gd, Nd. Mg—Li-RE with Mn and/or Zr is a novel composition wherein the alloying elements are specifically chosen to achieve single phase corrosion-resistant microstructure with minimal impurities. The Mg—Li-RE-Mn—Zr based alloy with Li demonstrates improved ductility as Li is known to activate several additional slip systems in HCP-Mg during deformation. Additionally, Li contributes to solid solution strengthening. RE elements such as Sc and Y promote grain refinement, and due to their high solubility in HCP-Mg, will also improve solid solution strengthening. It was also observed that Sc could getter impurities such as Fe during melting to form second phase (Fe2Sc) that could sediment to the bottom of the melt. Subsequently, a small portion at the bottom of melt-pool in the crucible may be avoided when pouring to the mold to ensure that gettered Fe-rich phase is removed. This is one of the unique melting strategies that may be followed to improve corrosion resistance.
Mn and Zr in the system may assist in gettering impurities. Additionally, Mn and Zr may promote grain refinement. Initial evaluations using thermodynamic and property models indicate the alloy composition to demonstrate enhanced corrosion resistance, strength and ductility compared to incumbent Mg-based alloys. Possible alloy compositions (all compositions in weight %) are listed in Table I:
Hence, it is desirable to identify alloying elements that getters the aforementioned impurities. In the multiphase system, precipitates (or discrete particles) can contribute to strengthening by acting as shearable or Orowan obstacles. Additional sources of strengthening include solid solution strengthening, grain refinement, and cold work (dislocation strengthening). It is noted that grain refinement improves corrosion resistance and ductility.
Example 14, which is an alloy having 2 wt. % Y, 0.3 wt. % Zr, and Mg as the balance, is designed to explore the effect of Zr, which could be an efficient grain refiner (innoculant) and has been reported to enhance corrosion resistance. Zr is also reported to getter impurities such as Fe in Mg-based alloys. The expected eutectic temperature is greater than 550° C., and the expected solvus temperature is expected to be 180° C. (based on Mg24Y5). After melting, homogenization may be completed at 500° C. for 10 hours, then the melt may be extruded at 300° C. and the extrusion ratio may be greater than 45, then a tube may be drawn at 300° C. (Zr innoculant may be present as second phase in the system), and if needed, SHT/annealing may be completed at 350° for 4 hours.
Embodiments of the present invention are also directed to novel Mg-based compositions with a novel recipe for processing to achieve two phase microstructure with a more anodic discrete phase (distributed) relative to a more cathodic matrix (or interconnected) phase. The anodic discrete phase in this case may impart corrosion resistance to the continuous interconnected Mg-rich matrix by behaving as a sacrificial anode, and may dissolve (corrode) preferentially to the matrix phase. The alloying elements include:
The RE elements include Sc, Y, La, Gd, Nd. Mg—Li-RE-Al—Zn—Mn is a novel composition space wherein the alloying elements and respective weight percentage are specifically chosen to achieve two phase corrosion-resistant microstructure with the discrete phase being more anodic. The compositions should ensure formation of interconnected HCP Mg—Li phase with varying second phase particles. The addition of Li may contribute to solid solution strengthening and enhanced ductility. RE elements such as Sc and Y may also improve solid solution strengthening.
The addition of Nd to HCP Mg—Li may promote formation of a BCC Mg—Li phase, which is expected to be relatively electronegative (i.e. anodic) compared to an HCP matrix. Certain compositions containing Zn may require a novel process path in which the alloy will be homogenized at low temperature and after thermomechanical processing such as extrusion, should be tempered/aged at higher temperature (relative to homogenization temperature) to promote formation of anodic second phases. Al-containing alloys promote precipitation of Al—Li phase in HCP Mg—Li matrix. Al—Li phase is expected to be anodic relative to the matrix thereby reducing the corrosion rate of the alloy. The phase fraction of the anodic second phase could be up to 30%. Additionally, Al may contribute to solid solution strengthening. Sn is known to be highly cathodic and presence of Sn in the HCP Mg matrix solid solution is expected to make the matrix cathodic relative to the precipitate phase. Certain compositions are designed to ensure the precipitate phase is present during thermomechanical processing at intermediate temperature to assist in grain pinning Mn is expected to assist in gettering impurities during melting. Potential compositions (all compositions in weight %) are listed in Table II below:
Testing of Alloys
Five of the examples above were selected for melting, extrusion, and testing. Specifically, single-phase alloy Examples 9, 10, 12, and 15 and two-phase alloy Example 24 were extruded at 350° C. at an extrusion ratio of 20.25 to form extruded rods at least three feet in length. The extrusion dies were lubricated with moly-disulfide and the melts were cooled by being quenched with liquid nitrogen. The actual composition of each sample was measured using inductively coupled plasma mass spectrometry (ICP-MS), and Table III lists the measured compositions (with balance Mg) of the extruded samples, in ppm (except where otherwise noted).
The microstructure of a sample extruded from the Example 10 alloy was also characterized and the composition of the sample was measured by energy-dispersive X-ray spectroscopy. Table IV lists the measured compositions, in weight %, of the matrix and of second phases in the alloy. The second phases present in the alloy are Mg-oxides with limited RE solubility. The oxides are expected to be relatively inert and not significantly affect the corrosion resistance of the alloy.
One of the extruded rods having the composition of the Example 10 alloy was also further extruded into a wire having a diameter of about 0.15 mm. The extruded wire was tested for mechanical properties, including ultimate tensile strength, yield strength, and elongation, and the results of the mechanical testing, as well as mechanical testing of three commercial Mg alloys are listed in Table V. As shown, the wire made from the Example 10 alloy had improved combined mechanical properties (strength and elongation) as compared to three commercial Mg alloys, in which AE42 is an alloy of Mg with 4 wt % Al and 2 wt % RE; ZM21 is an alloy of Mg with 2 wt % Zn and 1 wt % Mn, and AZ31 is an alloy of Mg with 3 wt % Al and 1 wt % Zn.
Corrosion of a stent that was manufactured from the Example 10 alloy showed improved uniformity, which is desirable for a stent application. The stent made from the Example 10 alloy corroded in a uniform pattern from the outside to the center of the wire, while the commercial alloys listed above had non-uniform corrosion with a high level of pitting. It is estimated that a stent made from the Example 10 alloy will keep its integrity until the end of corrosion and stents made from the commercial alloys listed above will break apart during the corrosion due to pitting and/or localized corrosion.
The microstructure of a sample of the Example 15 alloy was also characterized and the composition of the sample was measured by energy-dispersive X-ray spectroscopy. Table VI lists the measured compositions, in weight %, of the matrix and of second phases in the alloy. The second phases present in the alloy are Mg-oxides and RE-oxides. Two shapes of oxide particles were observed, including circular MgO and elongated MgO+Y2O3.
The microstructure of a sample of the Example 9 alloy was also characterized. The matrix composition was measured to be close to the nominal composition, and the observed second phases were oxides, including Mg-oxides and RE-oxides. Barring the oxides, the microstructure is single phase (in accordance with predictions).
The microstructure of a sample of the Example 24 (two-phase) alloy was also characterized. The matrix composition was close to the nominal composition, and the observed oxides were Mg-oxides and Al-oxides.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, Tables I and II include additional Examples that were not described in detail, but still fall within the scope of the present invention and are claimed below. In addition, Table VII lists additional compositions (all elements in weight %) that may be used to enhance corrosion resistance of stents comprising Mg, and fall within the scope of the present invention. The additional compositions may include 0.0 to 3.5 wt % Li; 0.0 to 9 wt % Sc; 0.0 to 5 wt % Y; 0.0 to 1 wt % Mn; 0.0 to 1 wt % Zr; and balance Mg.
The descriptions above are intended to be illustrative, not limiting. For example, although the alloys are described as being used to make a stent, it should be appreciated that other medical devices may also be fabricated with such alloys in accordance with embodiments of the invention. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/544,373, filed Oct. 7, 2011, the entire content of which is incorporated herein by reference.
Number | Date | Country | |
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61544373 | Oct 2011 | US |