Methods for increasing a retention force between a polymeric scaffold and a delivery balloon

Abstract

A medical device includes a scaffold crimped to a catheter having an expansion balloon. The scaffold is crimped to the balloon by a process that includes inflating the delivery balloon during a diameter reduction to improve scaffold retention and maintaining an inflated balloon during the diameter reduction and prior and subsequent dwell periods.

What is claimed is: 
     
         1 . A method for crimping a balloon-expanded scaffold to a balloon, comprising the steps of:
 providing a tube comprising PLLA characterized by a glass transition temperature range having a lower limit of about TG-low;   radially expanding the tube to increase its radial stiffness;   forming the scaffold from the radially-expanded tube; and   crimping the scaffold to the balloon while the scaffold has a crimping temperature of between about 5 to 15 degrees below TG-low including maintaining a pressure in the balloon as the scaffold diameter is being reduced in size.   
     
     
         2 . The method of  claim 1 , wherein the crimping step includes a first crimping step, a final alignment step, then a final crimping step, wherein the balloon pressure is maintained only during the final crimping step. 
     
     
         3 . The method of  claim 2 , wherein the scaffold diameter is reduced in diameter by about 50% for the first crimping step and by about 30% for the final crimping step. 
     
     
         4 . The method of  claim 3 , wherein the crimping temperature is about 48° C. 
     
     
         5 . THe method of  claim 2 , wherein following the first crimping step the balloon is inflated and the scaffold maintained at a first diameter for a first dwell period, followed by a third crimping step, wherein the balloon is inflated during the first dwell period to a pressure of about twice the balloon pressure for the final crimping step. 
     
     
         6 . The method of  claim 1 , wherein a final crimped diameter for the scaffold has a retention force that is greater than 1.0 lbs, the scaffold has a length of no more than 18 mm and the final crimped diameter is at least 2.5 times less than the scaffold diameter before crimping. 
     
     
         7 . A method for securing a scaffold to a balloon, comprising the steps of:
 crimping the scaffold to the balloon while the scaffold has a temperature of between about TG and 15 degrees below TG, the scaffold being made from a polymer comprising PLLA or PLGA, the crimping step including the steps of
 (a) reducing the scaffold diameter from a starting size to a first size, 
 (b) maintaining the first size for a first dwell period to allow for stress relaxation in the polymer, 
 (c) reducing the scaffold diameter from the first size to a second size, 
 (d) maintaining the second size for a second dwell period to allow for stress relaxation in the polymer while the balloon has a first pressure, and 
 (e) reducing the scaffold diameter from the second size to a third size, 
 wherein the balloon has a second pressure while the scaffold diameter is being reduced from the second size to the third size. 
   
     
     
         8 . The method of  claim 7 , wherein the polymer comprises PLLA or PLGA. 
     
     
         9 . The method of  claim 8 , wherein the polymer comprises PLGA, the temperature range is about 43 to 54 degrees centigrade and the GA content of PLGA is between about 15% to 0%. 
     
     
         10 . The method of  claim 7 , wherein the scaffold has a circumferential series of closed cells having a W-shape and linear link struts connecting the W-shape cells. 
     
     
         11 . The method of  claim 7 , wherein the scaffold diameter is reduced in size by about 30% in step (e). 
     
     
         12 . The method of  claim 7 , wherein the scaffold diameter is reduced in size by about 50% from steps (a) to (d). 
     
     
         13 . The method of  claim 7 , wherein the scaffold is crimped in a crimper mechanism, further including removing the scaffold from the crimper mechanism after the scaffold diameter is reduced to the second size,
 wherein the scaffold is removed from the crimper after its diameter is reduced by 50%, and   wherein the scaffold diameter is reduced in diameter by an additional about 30% when it attains the third size.   
     
     
         14 . The method of  claim 13 , wherein the first pressure is about twice the second pressure. 
     
     
         15 . The method of  claim 1 , wherein the scaffold has a starting diameter,
 intermediate diameter, and final diameter and the crimping step uses a crimping mechanism, the crimping step further including:
 reducing the scaffold diameter to the intermediate diameter, and 
 removing the scaffold from the crimping mechanism to adjust, if necessary, the position of the scaffold on the balloon before reducing the scaffold to the final diameter, 
 placing the scaffold back into the crimping mechanism, and 
 reducing the scaffold to the final diameter; 
 wherein the ratio of the final diameter to the intermediate diameter is greater than the ratio of the intermediate diameter to the final diameter.