Method of Increasing Stent Retention of Bioabsorbable Scaffolding with a Sheath

Abstract

A medical device includes a polymer stent crimped to a catheter having an expansion balloon. The stent is crimped to the balloon by a process that includes heating the stent to a temperature below the polymer's glass transition temperature to improve stent retention without adversely affecting the mechanical characteristics of the stent when later deployed to support a body lumen. A variable diameter sheath with a central portion that prevents expansion of the stent when the balloon is pressurized and larger diameter ends is disposed over the crimped stent-balloon assembly. The balloon is pressurized and the larger diameter ends of the sheath allow the balloon beyond the ends of the stent to expand. The balloon is then depressurized.

1 . A method of making a stent delivery system, comprising the steps of:
 providing a polymeric scaffolding crimped tightly over a delivery balloon;   providing a tubular sheath comprising a middle portion and two end portions, wherein the middle portion of the sheath have an inside diameter equal to or 1-2% larger than the outer diameter of the crimped scaffolding and the end portions of the sheath have a diameter 3-100% greater than the diameter of the crimped scaffolding;   disposing the polymeric scaffolding and balloon within the tubular sheath, wherein the middle portion of the sheath is disposed over the scaffolding and the end portions of the sheath extend beyond the ends of the scaffolding over end portions of the balloon;   pressurizing the balloon to cause the end portions of the balloon to inflate beyond the outer diameter of the crimped scaffolding; and   depressurizing the balloon.   
     
     
         2 . The method of  claim 1 , wherein a length of the scaffolding is 12 mm or less. 
     
     
         3 . The method of  claim 1 , wherein the retention force of the scaffolding after depressurizing the balloon is between 0.7 to 2 lb. 
     
     
         4 . The method of  claim 1 , wherein ends of the middle portion of the sheath coincide with the ends of the scaffolding. 
     
     
         5 . The method of  claim 1 , further comprising heating the end portions of the balloon prior to pressurizing the balloon. 
     
     
         6 . The method of  claim 1 , wherein the middle portion of the sheath prevents an increase in a diameter of the scaffolding. 
     
     
         7 . The method of  claim 1 , wherein the end portions of the pressurized balloon abut against the sidewalls of struts at the ends of the scaffolding. 
     
     
         8 . The method of  claim 1 , wherein the end portions of the depressurized balloon abut against the sidewalls of struts at the ends of the scaffolding. 
     
     
         9 . The method of  claim 1 , wherein a diameter of the end portions of the depressurized balloon is greater than an outer diameter of the scaffolding. 
     
     
         10 . A method of making a stent delivery system, comprising the steps of:
 providing a polymeric scaffolding;   crimping the scaffolding to a final crimped diameter over a balloon to form a crimped stent-balloon assembly, wherein the crimping includes at least one crimping step in which the scaffolding is crimped to a first diameter greater than the final diameter and holding the scaffolding at the first diameter while the balloon is inflated to a pressure against the scaffolding;   providing a tubular sheath comprising a central portion;   disposing the polymeric stent and balloon within the tubular sheath, wherein the central portion prevents expansion of the stent when the balloon is pressurized;   pressurizing the balloon to cause end portions of the balloon proximal and distal to ends of the stent to expand; and   depressurizing the balloon.   
     
     
         11 . The method of  claim 10 , wherein a length of the scaffolding is 12 mm or less. 
     
     
         12 . The method of  claim 10 , wherein the sheath further comprises end portions having diameters greater than the outside diameter of the crimped stent, wherein the end portions of the sheath control the expansion of the balloon ends. 
     
     
         13 . The method of  claim 10 , wherein the sheath further comprises end portions having diameters equal to or 1-2% larger than the outside diameter of the crimped stent. 
     
     
         14 . The method of  claim 10 , wherein balloon material penetrates through gaps in the scaffolding surface of the crimped scaffolding. 
     
     
         15 . The method of  claim 10 , wherein the scaffolding is crimped to the balloon while the scaffolding has a temperature of between about 45° C. and 55° C. 
     
     
         16 . The method of  claim 10 , wherein the end portions of the pressurized balloon abut against the sidewalls of struts at the ends of the scaffolding. 
     
     
         17 . The method of  claim 10 , wherein the end portions of the depressurized balloon abut against the sidewalls of struts at the ends of the scaffolding. 
     
     
         18 . The method of  claim 10 , wherein a diameter of the end portions of the depressurized balloon is greater than an outer diameter of the scaffolding. 
     
     
         19 . A stent delivery system, comprising:
 a tubular sheath comprising a middle portion and two end portions;   a polymeric scaffolding crimped tightly over a delivery balloon, wherein the polymeric scaffolding and balloon are disposed within the tubular sheath;   wherein the middle portion of the sheath has an inside diameter equal to or 1-2% larger than the outer diameter of the crimped scaffolding and the end portions of the sheath have a diameter greater than the diameter of the crimped scaffolding;   wherein the middle portion of the sheath is disposed over the scaffolding and the end portions of the sheath extend beyond the ends of the scaffolding over end portions of the balloon.   
     
     
         20 . The system of  claim 19 , wherein a length of the scaffolding is 12mm or less. 
     
     
         21 . The system of  claim 19 , wherein ends of the middle portion of the sheath coincide with the ends of the scaffolding. 
     
     
         22 . The system of  claim 19 , wherein the end portions of the balloon abut against the sidewalls of struts at the ends of the scaffolding. 
     
     
         23 . The system of  claim 19 , wherein a diameter of the end portions of the balloon is greater than an outer diameter of the scaffolding.