Stents with high radial strength and methods of manufacturing same

Abstract

Polymeric stents having fracture toughness and resistance to recoil after deployment are disclosed along with methods of manufacturing such stents. Improvements to mechanical characteristics and other improvements may be achieved by having polymer chains within individual stent struts oriented in a direction that is closer to or in line with the axis of the individual stent struts. The desired orientation of polymer molecules may be achieved by one or any combination of extruding polymers into hollow tubes to induce axially oriented polymer chains, applying a tensile load to polymer tubes to induce axially oriented polymer chains, and radially expanding polymer tubes to induce circumferentially oriented polymer chains. Stent patterns include struts defining diamond shaped cells and/or W-shaped cells.

1 . A method of making an endoprosthesis, comprising:
 radially deforming a polymer tube to increase radial strength of the polymer tube; and   forming linear ring struts, link struts, and curved hinge elements from the deformed polymer tube, the ring struts connected to each other by the hinge elements, the ring struts and the hinge elements defining a plurality of rings, the rings connected to each other by the link struts, the hinge elements adapted to allow the rings to move from a non-deformed configuration to a deformed configuration, the non-deformed configuration corresponding to the state of the rings before crimping, the deformed configuration corresponding to the state of the rings after crimping,   wherein the ring struts, hinge elements, and link struts define a plurality of W-shaped closed cells, each W-shaped cell touching six other W-shaped cells, each W-shaped cell having a perimeter that includes eight of the ring struts and two of the link struts, and when the rings are in the non-deformed configuration, the ring struts are oriented relative to each other at an interior angle greater than 100 degrees.   
     
     
         2 . The method of  claim 1 , wherein when the rings are in the non-deformed configuration, the interior angle is between about 124 degrees and about 130 degrees. 
     
     
         3 . The method of  claim 1 , wherein an outer diameter of the polymer tube after the radial deformation is greater than or equivalent to about five times an inner diameter of the polymer tube prior to the radial deformation. 
     
     
         4 . The method of  claim 1 , wherein an outer diameter of the polymer tube after the radial deformation is greater than or equivalent to about six times an inner diameter of the polymer tube prior to the radial deformation. 
     
     
         5 . The method of  claim 4 , wherein when the rings are in the non-deformed configuration, each ring strut is oriented at a non-zero angle less than about 40 degrees relative to the direction in which the polymer tube was radially deformed. 
     
     
         6 . The method of  claim 1 , wherein each ring has a center point, at least two of the center points defining a central axis, and the link struts are oriented parallel or substantially parallel to the central axis when the rings are in the non-deformed configuration, and wherein the perimeter of each W-shaped cell includes ten of the hinge elements. 
     
     
         7 . The method of  claim 1 , further comprising, after the forming step, crimping the rings onto a delivery device. 
     
     
         8 . The method of  claim 1 , wherein the polymer tube to be radially deformed contains poly(L-lactide). 
     
     
         9 . The method of  claim 9 , further comprising extruding poly(L-lactide) to form the polymer tube to be radially deformed. 
     
     
         10 . A method of making an endoprosthesis, comprising:
 radially expanding a polymer tube so that an outer diameter of the polymer tube after expansion is greater than about four times an inner diameter of the polymer tube prior to expansion; and   forming ring struts and link struts from the radially expanded polymer tube, the ring struts defining a plurality of rings capable of moving from a non-deformed configuration to a deformed configuration, each ring having a center point, at least two of the center points defining a central axis, the link struts oriented parallel or substantially parallel to the central axis and connecting the rings together, the link struts and the ring struts defining W-shaped closed cells, each W-shaped cell abutting six other W-shaped cells, each W-shaped cell having a perimeter that includes eight of the ring struts and two of the link struts,   wherein when the rings are in the non-deformed configuration, the ring struts are oriented relative to each other at an interior angle greater than 100 degrees.   
     
     
         11 . The method of  claim 10 , wherein the interior angle is between about 124 degrees and about 130 degrees when the rings are in the non-deformed configuration. 
     
     
         12 . The method of  claim 10 , wherein the outer diameter of the polymer tube after expansion is equivalent to or greater than about six times the inner diameter of the polymer tube prior to expansion. 
     
     
         13 . The method of  claim 12 , wherein when the rings are in the non-deformed configuration, each ring strut is oriented at an angle between about 25 degrees and about 28 degrees relative to the direction of the radial expansion. 
     
     
         14 . The method of  claim 10 , wherein the ring struts on each ring define a series of crests and troughs that alternate with each other, each crest on each ring connected by one of the link struts to another crest on another ring. 
     
     
         15 . The method of  claim 10 , further comprising, after the forming step, crimping the rings onto a delivery device. 
     
     
         16 . The method of  claim 10 , wherein the non-deformed configuration corresponds to the state of the rings before crimping, and the deformed configuration corresponds to the state of the rings after crimping. 
     
     
         17 . The method of  claim 10 , wherein the polymer tube to be radially deformed contains poly(L-lactide). 
     
     
         18 . The method of  claim 10 , further comprising extruding poly(L-lactide) to form the polymer tube to be radially deformed.