Elucidating the microstructural basis for the lasting radial strength of poly (L-lactide) bioresorbable vascular scaffolds during hydrolysis*

Drug-eluting metal stents (DES) are the current standard-of-care for restoring blood flow through an occluded artery. However, DES are made from metal alloys that are non-biodegradable and consequently, inhibit arterial vasomotion and pose a risk of thrombosis, a dreaded complication. Bioresorbabale vascular scaffolds (BVS) made from poly L-lactide (PLLA) are emerging as a promising alternative to permanent metal stents. The clinically-approved BVS (FDA-approval in 2016) supports the occluded artery for the requisite 3-6 months but is completely resorbed in 2-3 years. As a result, the BVS restores arterial vasomotion and can eliminate the late onset of thrombosis. The clinically-approved BVS presents a paradox as it hydrolyzes in the body – it suffers a ~40% decrease in molecular weight (M_n) but shows no decrease in radial strength. Using X-ray microdiffraction, we discovered that the BVS develops a unique microstructure in localized regions that make up <3% of the scaffold. These regions resist hydrolysis and reinforce struts in the BVS that are the most vulnerable to fracture. Thus, the global measure of degradation does not capture the presence of chains with M_n higher than average in regions that have a disproportionate impact on strength.