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

Abstract

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the vessel wall and deploys the stent to provide support at the enlarged diameter after the balloon is deflated and withdrawn.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...

Novel materials

“…The first article, by Ailianou et al (1), describes fundamental studies of biodegradable polymers used in advanced vascular scaffolds designed for the treatment of coronary heart disease. These devices, known as stents, deploy into blood vessels where they provide structural reinforcement following procedures to remove obstructive plaques.…”

Modeling of PLLA near glass transition temperatures

“…The amorphous region of PLLA provides toughness. It has been recently observed that PLLA subjected to thermal annealing can result in the promotion of amorphous region thereby thinning the material for medical applications and improving the toughness [1]. In order to understand the response of PLLA subjected to thermal annealing, it is important to understand the evolution of its microstructure at various strain rate and temperature ranges.…”

“…Coronary Heart Diseases (CHD) is responsible for nearly 50% of the death of all Cardiovascular diseases [1]. CHD involves narrowing of an artery in the heart of a patient due to the formation of plaque with the passage of time, thereby, restricting the blood flow.…”

“…Bioresorbable Vascular Scaffolds (BVS) on the other hand supports the artery for 6-9 months and then dissolves completely in 2 years since they are metabolic in nature thus leaving behind a healthy artery. The base material for such scaffolds, which has been recently clinically approved [1], is Poly l-lactide Acid. However, the problem with PLLA is its inferior strength compared to metals making it necessary to use higher thicknesses and thus not making it ideal for implantation.…”

Physical gelation and macromolecular mobility of sustainable polylactide during isothermal crystallization

“…Polylactide (PLA), as a biodegradable semicrystalline polymer produced from renewable resources, has received increasing attention in recent years because of the environmental and economic concerns . Due to its balanced cost and properties, PLA has been viewed as the most promising sustainable substitution to some petroleum‐based polymers.…”