Answer
MP35N (a Cobalt-Nickel-Chrome-Molybdenum alloy) is selected for wave springs in medical implants, such as heart valves or vascular stents, due to its unmatched biocompatibility and extreme fatigue life. MP35N offers a tensile strength exceeding $250$ ksi and is virtually immune to all forms of corrosion in physiological fluids (saline). The material's high modulus of elasticity ($E \approx 234$ GPa) allows for very thin, low-profile wave springs that can still provide high clamping forces. Furthermore, it is non-magnetic, making it MRI-safe. The processing of MP35N requires cold working followed by aging at $1000^{∘}F$ ($538^{∘}C$) to achieve the required spring properties, making it one of the most high-performance and expensive materials in the industry.
