Polymeric Heart Valve Leaflets Tested in an Accelerated Wear Tester Revealed a Stable Craze Microstructure

Area of Science:

• Materials Science
• Biomedical Engineering
• Polymer Science

Background:

• Aortic heart valve disease necessitates bioprosthetic valve replacement, with current glutaraldehyde-fixed bioprostheses suffering from limited durability due to calcification and tearing.
• Linear low-density polyethylene (LLDPE) offers potential as a durable material for heart valve leaflets due to its high tear strength and flexibility.
• Previous durability testing of LLDPE leaflets revealed premature failure, specifically wear and tear around commissure posts, necessitating investigation into the failure mechanisms.

Purpose of the Study:

• To investigate the microstructural changes and failure mechanisms of LLDPE leaflets used in bioprosthetic heart valves.
• To analyze the surface morphology and polymer structure of worn LLDPE leaflets using scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS).

Main Methods:

• Nine retrieved, worn LLDPE heart valve leaflets were chemically etched using a 2% w/v permanganate solution.
• SEM analysis was performed on both virgin and etched LLDPE samples to examine surface morphology and microstructure.
• SAXS was utilized to confirm findings related to polymer microstructure.

Main Results:

• SEM analysis of virgin LLDPE revealed spherulitic structures with well-organized lamellae.
• Etching of worn leaflets exposed surface wear and aligned polymeric material oriented perpendicular to the stress direction.
• A Kramer craze microstructure, characterized by 'cross-ties' of aligned and interlinked fibrils, was observed in one worn sample, providing direct evidence for a theorized structure.

Conclusions:

• The study provides the first direct evidence of the Kramer craze microstructure in a worn polymer from a medical device.
• Understanding these polymer failure mechanisms, including the observed cross-tie structure, is crucial for improving the durability of LLDPE-based heart valve leaflets.
• Further research into polymer microstructure and failure modes can guide the development of more robust and long-lasting bioprosthetic materials.