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. structural transformation | ductility | poly (L-lactide) | coronary heart disease | microdiffraction C ardiovascular disease (CVD) claims over 15 million lives per year-more lives than communicable, maternal, neonatal, and nutritional disorders combined and more than twice the number of deaths due to all cancers (1). Coronary heart disease (CHD), the narrowing of coronary arteries due to the deposition of plaque, accounts for nearly 50% of all CVD deaths (1). To restore blood flow, most patients receive minimally invasive balloon angioplasty followed by stent implantation (1 million in 2008 in the United States) (2). Stents are metal mesh tubes that are delivered to the target lesion while they are crimped onto a balloon. Once they are positioned at the lesion, inflation of the balloon compresses the plaque against the vessel wall and deploys the stent to provide support at the enlarged diameter after the balloon is deflated and withdrawn. Metal stents are permanent, and their stiffness prohibits vasomotion and dilation (3, 4). Further, they present a lifelong risk of late stent thrombosis (3-6). A new technology is poised to displace metal stents: bioresorbable vascular scaffolds (BVS), which have been deemed the "fourth revolution" in percutaneous coronary intervention (7,8).The goal of tissue scaffolds is to restore the healthy state of the tissue, rather than merely ameliorating the diseased state (9-11). Poly(L-lactide) (PLLA) was selected as the material for BVS because its semicrystalline structure gives it adequate radial strength [>300 mm Hg (12)], and it degrades into products that are metabolized by the human body (13-16). Clinically, bioresorption of PLLA vascular sca...

Coronary Heart Disease (CHD) is one of the leading causes of death worldwide, claiming over seven million lives each year. Permanent metal stents, the current standard of care for CHD, inhibit arterial vasomotion and induce serious complications such as late stent thrombosis. Bioresorbable vascular scaffolds (BVSs) made from poly L-lactide (PLLA) overcome these complications by supporting the occluded artery for 3-6 months and then being completely resorbed in 2-3 years, leaving behind a healthy artery. The BVS that recently received clinical approval is, however, relatively thick (~150 µm, approximately twice as thick as metal stents~80 µm). Thinner scaffolds would facilitate implantation and enable treatment of smaller arteries. The key to a thinner scaffold is careful control of the PLLA microstructure during processing to confer greater strength in a thinner profile. However, the rapid time scales of processing (~1 s) defy prediction due to a lack of structural information. Here, we present a custom-designed instrument that connects the strain-field imposed on PLLA during processing to in situ development of microstructure observed using synchrotron X-ray scattering. The connection between deformation, structure and strength enables processing-structure-property relationships to guide the design of thinner yet stronger BVSs.

Most visible-light photoinitiators are based on electron transfer processes and are comprised of two or more components. These initiators can lose effectiveness in viscous systems because the underlying reactions are diffusion controlled. In this contribution, the visible-light photoinitiator bis(cyclopentadienyl) bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium is characterized for polymerization of viscous systems and low light intensities. This compound absorbs visible light at wavelengths up to 550 nm, and does not rely on diffusion-controlled electron transfer reactions because it undergoes unimolecular decomposition. In contrast to trends observed for other photoinitiators, the effectiveness of the compound is found to increase markedly with the addition of protonic acids and with increasing system viscosity. For a given concentration of initiator and acid, a remarkably low optimal light intensity for effective polymerization is observed. The origins of these surprising results are discussed in terms of the mechanism of decomposition of the photoinitiator.

Tungsten disulfide nanotubes enhance flow-induced crystallization and radio-opacity of polylactide without adversely affecting in vitro toxicity, Acta Biomaterialia (2021), doi:

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 (). 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 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.