Etching of Failed Polymeric Heart Valve Leaflets Reveals Cross-Tie Craze Microstructure

Rheumatic and calcified aortic heart valve disease presents a global health concern, impacting millions of individuals across various age groups. The gold standard medical treatments recommend replacing the sick heart valves with xenograft-based bioprosthetic valves that are chemically fixed using glutaraldehyde, commonly sourced from bovine or porcine. Clinical investigations over more than two decades have revealed fixed tissues are prone to premature calcification and tearing, thereby limiting their durability. Moreover, challenges related to supply chains, inconsistent mechanical characteristics, and high costs have been constant issues for tissue valves.<br/><br/>An innovative alternative approach involves enhancing polyethylene-based materials with hyaluronic acid (HA), a glycosaminoglycan naturally occurring in native heart valve leaflets. By incorporating small amounts of HA into a low-density linear polyethylene (LLDPE) thin film at the molecular level, a robust and hydrophilic biomaterial of <i>in vivo</i> anti-calcific and anti-thrombotic properties is achieved. The LLDPE thin film has high tear strength and excellent flexibility, making it an appealing choice for developing heart valves. Nonetheless, during durability testing according to ISO 5840-2005 standards, these valves exhibited premature failure. To uncover the fundamental mechanisms behind these failures, this study conducted a failure analysis of semicrystalline blown LLDPE thin film-based heart valves.<br/>The valves consistently tear and wear around highly stressed (from finite element analysis) commissure posts. Six of these worn posts were retrieved from failed valves and chemically etched. Low-voltage scanning electron microscopy (SEM) imaging was taken before and after chemical etching. The semicrystalline LLDPE polymer, with a crystallinity of 36% as determined by DSC, underwent chemical etching using a standard 2% w/v permanganate etching solution, followed by multistep washing. The SEM analysis of pristine LLDPE unveiled distinctive spherulitic structures consisting of well-organized lamellae with diameters of approximately 3 µm and lamellae thickness of 40-80 nm. The etching process effectively eliminated low-energy amorphous regions, revealing the spherulites. A similar study was carried out on the worn and torn LLDPE valves.<br/><br/>The SEM images of the worn surfaces displayed signs of surface wear and aligned fibrils oriented perpendicular to the principal stress direction, akin to a phenomenon known as crazing. Following etching, some fibrils remained partially intact, while others exposed the crystals beneath them. These crystals within the fibrils lost their spherulitic superstructure, exhibiting thickened and fractured lamellae. The presence of these fibrils after etching suggested a state of hyper-crystallinity, a characteristic often observed in polyethylene-drawn filaments. In contrast, the worn surfaces that had tears displayed multiple irregularities at the torn surface. These included bumps, holes, a visually rough texture, and a lack of exposed crystals. Crystal disruptions were notably concentrated within 200-300 μm areas near the tear and wear, while the rest of the samples retained spherulitic morphology.<br/><br/>Remarkably, one of the worn samples unveiled the Kramer craze microstructure "cross-tie," composed of aligned and interlinked lamellae. The spacing between cross-tie lamella ranges between 100-200 nm, and the thickness remained 40-80 nm. To the best of the author’s knowledge, the cross-tie structure has been only theorized with indirect evidence collected from lab-grown crazes. Direct evidence of cross-tie structure indicates crazing initiates the tears. Stress disentangles polymer strands, causing lamella fibrillation, but some do not disentangle. Instead, they pile up to bridge between fibrils. This study holds the potential to shed light on preventing polymer disentanglement, thereby mitigating crazing and improving heart valve durability.