Biodegradable wire for medical devices

Abstract

A bioabsorbable material composition includes magnesium (Mg), lithium (Li) and calcium (Ca). Lithium is provided in a sufficient amount to enhance material ductility, while also being provided in a sufficiently low amount to maintain corrosion resistance at suitable levels. Calcium is provided in a sufficient amount to enhance mechanical strength and/or further influence the rate of corrosion, while also being provided in a sufficiently low amount to preserve material ductility. The resultant ductile base material may be cold-worked to enhance strength, such as for medical applications. In one application, the material may be drawn into a fine wire, which may be used to create resorbable structures for use in vivo such as stents.

What is claimed is: 
     
         1 . A magnesium-based alloy wire, comprising:
 between 3 wt. % lithium and 7 wt. % lithium;   between 0.1 wt. % calcium and 1 wt. % calcium; and   balance magnesium and trace impurities.   
     
     
         2 . The magnesium-based alloy wire of  claim 1 , wherein said wire comprises between 0.20 and 0.30 wt. % calcium. 
     
     
         3 . The magnesium-based alloy wire of  claim 2 , wherein the alloy exhibits sufficient ductility to be subjected to 98% cold work without fracture. 
     
     
         4 . The magnesium-based alloy wire of  claim 2 , wherein:
 the alloy is formed as a wire product having 98% retained cold work, the wire having a yield strength reaching 276 MPa.   
     
     
         5 . The magnesium-based alloy wire of  claim 2 , wherein:
 the alloy is formed as a wire product having 98% retained cold work, the wire having an ultimate tensile strength reaching 334 MPa.   
     
     
         6 . The magnesium-based alloy wire of  claim 1 , wherein said wire comprises between 0.9 wt. % and 1 wt. % calcium. 
     
     
         7 . The magnesium-based alloy wire of  claim 6 , wherein the alloy exhibits sufficient ductility to be subjected to 88% cold work without fracture. 
     
     
         8 . The magnesium-based alloy wire of  claim 6 , wherein:
 the alloy is formed as a wire product having 98% retained cold work, the wire having a yield strength reaching 240 MPa.   
     
     
         9 . The magnesium-based alloy wire of  claim 6 , wherein:
 the alloy is formed as a wire product having 98% retained cold work, the wire having an ultimate tensile strength reaching 271 MPa.   
     
     
         10 . The magnesium-based alloy wire of  claim 1 , further comprising between 0.9 wt. % and 5 wt. % aluminum. 
     
     
         11 . The magnesium-based alloy wire of  claim 1 , further comprising between 0.25 wt. % and 7 wt. % rare earth metal. 
     
     
         12 . The magnesium-based alloy wire of  claim 1 , further comprising between 0.10 wt. % and 6 wt. % zinc. 
     
     
         13 . The magnesium-based alloy wire of  claim 1 , further comprising between 0.10 wt. % and 1 wt. % manganese. 
     
     
         14 . The magnesium-based alloy wire of  claim 1 , further comprising between 0.10 wt. % and 1 wt. % zirconium. 
     
     
         15 . The magnesium-based alloy wire of  claim 1 , wherein the wire lacks any other element in addition to magnesium, lithium and calcium in an amount above 0.05 wt. %. 
     
     
         16 . The magnesium-based alloy wire of  claim 1 , wherein said wire has a diameter up to 2.5 mm. 
     
     
         17 . The magnesium-based alloy wire of  claim 1 , wherein said wire comprises a fine wire having a diameter between 20 μm and 1 mm. 
     
     
         18 . The magnesium-based alloy wire of  claim 1 , wherein said wire comprises one of a wire having a round cross section, a flat wire, a strand, a cable, a coil and tubing. 
     
     
         19 . A stent including the magnesium-based alloy wire of  claim 1 . 
     
     
         20 . A bimetal composite wire, comprising:
 an outer shell formed of a first biodegradable metallic material; and   an inner core formed of a second biodegradable metallic material,   said first and second biodegradable metallic materials being different from one another whereby said first and second biodegradable metallic materials have differing biodegradation rates, and   one of said first and second biodegradable materials comprising a magnesium-based alloy selected from the group consisting of:
 a Mg—Li—Ca alloy having between 3.0 wt. % and 7.0 wt. % Li and between 0.10 wt. % and 1.0 wt. % Ca; 
 a Mg—Li—Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li, between 0.10 wt. % and 1.0 wt. % Ca, and between 0.25 wt. % and 7.0 wt. % RE, wherein “RE” is at least one rare earth element; 
 a Mg—Li—Ca—Al alloy having between 3.0 wt. % and 7.0 wt. % Li and between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % Al; and 
 a Mg—Li—Al—Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li, between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and between 0.25 wt. % and 7.0 wt. % RE, wherein “RE” is at least one rare earth element. 
   
     
     
         21 . The bimetal composite wire of  claim 20 , wherein said magnesium-based alloy has an ultimate tensile strength reaching 334 MPa. 
     
     
         22 . The bimetal composite wire of  claim 20 , wherein the other of said first and second biodegradable materials is selected from the group consisting of pure metallic iron (Fe) and an iron-based alloy (Fe alloy). 
     
     
         23 . The bimetal composite wire of  claim 20 , wherein an outer diameter of said outer shell is less than 1 mm. 
     
     
         24 . The bimetal composite wire of  claim 20 , wherein the wire lacks any other element in addition to magnesium, lithium, calcium, aluminum and RE in an amount above 0.05 wt. %. 
     
     
         25 . A stent including of the bimetal composite wire of  claim 20 . 
     
     
         26 . A method of manufacturing a wire, comprising the steps of:
 providing an outer shell made of a first biodegradable material;   inserting a core into the outer shell to form a wire construct, the core formed of a second biodegradable material, the first and second biodegradable materials being different from one another, one of the first and second biodegradable materials comprising a magnesium-based alloy selected from the group consisting of:
 a Mg—Li—Ca alloy having between 3.0 wt. % and 7.0 wt. % Li and between 0.10 wt. % and 1.0 wt. % Ca; 
 a Mg—Li—Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li, between 0.10 wt. % and 1.0 wt. % Ca, and between 0.25 wt. % and 7.0 wt. % RE, wherein “RE” is at least one rare earth element; 
 a Mg—Li—Ca—Al alloy having between 3.0 wt. % and 7.0 wt. % Li and between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % Al; and 
 a Mg—Li—Al—Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li, between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and between 0.25 wt. % and 7.0 wt. % RE, wherein “RE” is at least one rare earth element; and 
   
     
     
         27 . The method of  claim 26 , further comprising imparting cold work at room temperature to the wire construct by drawing the wire construct from a first outer diameter to a second outer diameter less than the first outer diameter. 
     
     
         28 . The method of  claim 27 , further comprising, after said imparting step, the additional step of annealing the wire construct. 
     
     
         29 . The method of  claim 26 , further comprising forming the wire into a stent. 
     
     
         30 . The method of  claim 26 , wherein the other of said first and second biodegradable materials is selected from the group consisting of pure metallic iron (Fe) and an iron-based alloy (Fe alloy). 
     
     
         31 . The method of  claim 26 , wherein the wire lacks any other element in addition to magnesium, lithium, calcium, aluminum and RE in an amount above 0.05 wt. %.