(28g) Multiplicity of Morphologies in Poly(L-lactide) Bioresorbable Vascular Scaffolds Provide Ductility, Strength and Tailored Hydrolysis Profile

Poly(L-lactide) (PLLA) is the structural material of the first clinically-approved bioresorbable vascular scaffold (BVS), a potential alternative to permanent metal stents for treatment of coronary heart disease. BVSs are transient implants that support the occluded artery for 6 mo and are completely resorbed in 2 years. Early clinical trials of BVS’s reported restoration of arterial vasomotion and elimination of serious complications of metal stents that occur 5 to 7 yr after implantation. Further improvements are needed, however, to overcome complications that occur during the first two years after implantation. Understanding the manufacturing process and the semicrystalline structure it creates may hold the key to the next generation of BVS. It is remarkable that a scaffold made from PLLA, known as a brittle polymer, does not fracture when crimped onto a balloon catheter or during deployment in the artery. We used X-ray microdiffraction to discover how PLLA acquired ductile character and found that the crimping process creates localized regions of extreme anisotropy. Dramatic gradients in the degree and direction of orientation and crystallinity are observed on micron-scale distances. The distinct morphologies in the crimped scaffold work in tandem to enable a low-stress response during deployment, which avoids fracture and provides the strength needed to support the artery. After deployment, the highly oriented morphology created at points of stress localization during crimping confer resistance to hydrolysis precisely where it is needed for the scaffold to retain strength even after 9 mo of hydrolysis. Thus, the ability to use processing to access exceptional semicrystalline microstructures in PLLA are essential to the clinically-approved BVS and open the way to thinner resorbable scaffolds in the future.

This work was conducted in collaboration with Abbott Vascular, including Mary-Beth Kossuth and James Oberhauser, and Caltech researchers Artemis Ailianou, Karthik Ramachandran and Tiziana Di Luccio. This research used resources of the Advanced Photon Source (APS), a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. We thank Dr. Zhonghou Cai at APS for his assistance in collecting x-ray microdiffraction data. Funding was provided by Abbott Vascular.