Fabricating an implantable medical device from an amorphous or very low crystallinity polymer construct
Methods of fabricating a polymeric implantable device with improved fracture toughness through annealing, nucleating agents, or both are disclosed herein. A polymeric construct that is completely amorphous or that has a very low crystallinity is annealed with no or substantially no crystal growth to increase nucleation density. Alternatively, the polymer construct includes nucleating agent. The crystallinity of the polymer construct is increased with a high nucleation density through an increase in temperature, deformation, or both. An implantable medical device, such as a stent, can be fabricated from the polymer construct after the increase in crystallinity.
1. A method of making a stent comprising:
forming a polymeric tube comprising nucleating agent particles dispersed within the polymer, wherein the nucleating agent particles comprise insoluble particles which provide nuclei for heterogeneous crystallization to increase nucleation density, wherein crystal growth is minimized or prevented during formation of the tube so that the polymer of the formed tube has a crystallinity of less than 5%;
annealing the polymeric tube at a temperature in a temperature range of above Tg and below Tm with no crystal growth during a selected annealing time of greater than 1 hr;
after annealing, inducing and allowing crystal growth around the nucleating agent particles dispersed within the polymeric tube to obtain a desired crystallinity,
wherein the crystal growth is induced by radially deforming the polymer tube to provide circumferential orientation in the polymeric tube; and
fabricating a stent from the tube after the crystal growth.
2. The method of claim 1 , wherein the polymer of the polymeric tube is selected from the group consisting of PLLA and PGA.
3. The method of claim 1 , wherein the polymer of the polymeric tube is PLLA.
4. The method of claim 1 , wherein the polymeric tube is completely or near completely amorphous prior to inducing crystal growth.
5. The method of claim 1 , wherein forming the polymeric tube comprises fabricating the polymeric tube using extrusion which includes quenching the polymeric tube as it exits an extruder to a temperature below Tg of the polymeric tube to prevent recrystallization.
6. The method of claim 1 , wherein the polymer of the polymeric tube is bioabsorbable.
7. The method of claim 1 , wherein the annealing of the polymeric tube is at a temperature in a temperature range of Tg to Tg+5° C.
8. A method of making a stent comprising:
forming a polymeric tube comprising nucleating agent particles dispersed within the polymer, wherein the nucleating agent particles comprise insoluble particles which provide nuclei for heterogeneous crystallization to increase nucleation density, wherein crystal growth is minimized or prevented during formation of the tube so that the polymer of the formed tube has a crystallinity of less than 5%;
annealing the polymeric tube at a temperature in a temperature range of above Tg and below Tm with no crystal growth during a selected annealing time of greater than 1 hr;
after annealing, inducing and allowing crystal growth around the nucleating agent particles dispersed within the polymeric tube to obtain a desired crystallinity,
wherein the crystal growth is induced by increasing the temperature of the polymeric tube; and
fabricating a stent from the tube after the crystal growth.
9. The method of claim 8 , wherein the polymer of the polymeric tube is PLLA.