Enhanced Fatigue Performance of <scp>VAR</scp> / <scp>EBR</scp> Nitinol for Transcatheter Heart Valve Applications: Material Characterization and Design Implications

Transcatheter cardiovascular devices require high-purity Nitinol materials with exceptional fatigue resistance to meet stringent Class III regulatory durability requirements. Ultra-clean VAR/EBR (Vacuum Arc Remelt/Electron Beam Remelt) Nitinol represents a metallurgical advancement that achieves unprecedented control over inclusion size and distribution. This study characterizes the fatigue behavior of VAR/EBR Nitinol with nominal inclusion sizes below 10 μm under conditions representative of transcatheter mitral valve replacement (TMVR) applications, using the HighLife Mitral Valve Replacement system as a clinical case study. Diamond-shaped fatigue specimens were manufactured from ultra-clean VAR/EBR Nitinol tubing used in the HighLife Mitral Valve Replacement system and tested under physiologically relevant conditions to 10⁷, 10⁸, and 4 × 10⁸ cycles. Testing included multiple combinations of mean strains (1.5%-9%) and strain amplitudes (0.50%-2.50%) to simulate the multi-strain operating environments encountered in TMVR devices. VAR/EBR Nitinol demonstrated a conservative 130% improvement in 4 × 10⁸-cycle Fatigue Strain Limit (FSL) compared to conventional VAR Nitinol at mean strains between 3% and 5%. The FSL behavior revealed two distinct strain regimes correlating with stress-induced martensitic transformation. Fractographic analysis confirmed elimination of inclusion-initiated fatigue failure, with crack initiation occurring independently of microstructural defects. The ultra-clean microstructure of VAR/EBR Nitinol enables a fundamental shift from flaw-dominated to stress-dominated fatigue behavior, providing unprecedented safety margins for complex transcatheter devices operating across diverse mechanical conditions. This material advancement has broad implications for Class III cardiovascular device design, enabling devices like the HighLife system and other TMVR platforms to meet stringent regulatory durability requirements while maintaining safety across complex multi-strain operating environments.