Crimping-induced structural gradients explain the lasting strength of poly l-lactide bioresorbable vascular scaffolds during hydrolysis.

Biodegradable polymers open the way to treatment of heart disease using transient implants (bioresorbable vascular scaffolds, BVSs) that overcome the most serious complication associated with permanent metal stents-late stent thrombosis. Here, we address the long-standing paradox that the clinically approved BVS maintains its radial strength even after 9 mo of hydrolysis, which induces a ∼40% decrease in the poly l-lactide molecular weight (Mn). X-ray microdiffraction evidence of nonuniform hydrolysis in the scaffold reveals that regions subjected to tensile stress during crimping develop a microstructure that provides strength and resists hydrolysis. These beneficial morphological changes occur where they are needed most-where stress is localized when a radial load is placed on the scaffold. We hypothesize that the observed decrease in Mn reflects the majority of the material, which is undeformed during crimping. Thus, the global measures of degradation may be decoupled from the localized, degradation-resistant regions that confer the ability to support the artery for the first several months after implantation.

Multiplicity of morphologies in poly (l-lactide) bioresorbable vascular scaffolds.

Poly(l-lactide) (PLLA) is the structural material of the first clinically approved bioresorbable vascular scaffold (BVS), a promising 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 y. Clinical trials of BVSs report restoration of arterial vasomotion and elimination of serious complications such as late stent thrombosis. 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; PLLA chains in the scaffold change orientation from the hoop direction to the radial direction on micrometer-scale distances. This multiplicity of morphologies in the crimped scaffold works in tandem to enable a low-stress response during deployment, which avoids fracture of the PLLA hoops and leaves them with the strength needed to support the artery. Thus, the transformations of the semicrystalline PLLA microstructure during crimping explain the unexpected strength and ductility of the current BVS and point the way to thinner resorbable scaffolds in the future.