Fort Wayne Metals - Technical Blog

Magnetic susceptibility of medical metals: New insights for MR conditional devices

Tue, 16 May 2023 15:13:00 Z

A recent study, coauthored by Fort Wayne Metals and MED Institute researchers, provides valuable insights into the magnetic susceptibility of 45 metallic materials commonly used in medical devices. This research offers critical data for selecting materials in applications where MR safety is a concern.

Why magnetic susceptibility matters in medical devices

Magnetic susceptibility refers to a material’s degree of magnetization in response to an external magnetic field. In the medical field, this property is essential for devices that may be used in magnetic resonance (MR) environments. Materials with high magnetic susceptibility can:

Cause image distortion in MRI scans
Generate dangerous forces and torques within strong magnetic fields
Impact the safety of implanted or external medical devices

To achieve an MR Conditional label, a device must be designed with materials that minimize these effects.

Key findings

The study measured the magnetic susceptibility of a wide range of metals and alloys, presenting results in ascending order. Key takeaways include:

Titanium and Nitinol alloys: Extremely low magnetic susceptibility, making them ideal for MR-compatible implants
Cobalt-chromium (CoCr) alloys: Moderate susceptibility but often acceptable for certain applications
Stainless steel: Vary widely depending on composition and cold working, with some grades exhibiting significantly higher susceptibility
Nickel-based alloys: Generally higher susceptibility, which can limit MR compatibility

Material selection for MR Conditional devices

For medical device manufacturers, this data supports informed material selection

Best choices for MR compatibility: Titanium and Nitinol alloys
Moderate risk materials: Certain CoCr alloys and austenitic stainless steels
High susceptibility materials to avoid: Cold-worked stainless steels and some nickel-based alloys

Conclusion

This study provides a comprehensive reference for selecting medical metals based on magnetic susceptibility. By considering this factor early in the design process, manufacturers can enhance MR safety, improve imaging quality, and ensure compliance with MR Conditional labeling standards.

Advancing neurovascular treatment: FeMnN—Mo composite wires for absorbable flow Diverters

Mon, 06 Feb 2023 14:35:00 Z

The growing demand for innovative solutions to treat intracranial aneurysms has driven advancements in medical materials. Researchers at Fort Wayne Metals have developed FeMnN-Mo composite wires as a foundation for absorbable flow diverters. These devices, designed to treat aneurysms by redirecting blood flow and promoting clot formation, are expected to dissolve after achieving their purpose, reducing long-term complications.

Challenges with existing materialsExisting flow diverters employ permanent metals such as 35N LT® or Nitinol. These devices function well but are unnecessary after aneurysm occlusion and can impede secondary procedures. Absorbable polymers like polyglycolic acid (PGA) and poly-l-lactic acid (PLLA) have been investigated as temporary options but require larger struts for sufficient strength, compromising device profile and flexibility. Absorbable Mg- and Fe-based devices have also been investigated but suffer from rapid degradation and premature fracture.

Innovative FeMnN-Mo composite wiresThe study introduces composite DFT® wires made of:

FeMnN shell: Provides strength, elasticity, and a cell-friendly surface.
Molybdenum (Mo) core: Offers enhanced radiopacity and staged corrosion protection.

These wires, available in diameters as fine as 25 µm, mimic the dimensions of traditional metallic flow diverters while addressing their limitations.

Key findings

Mechanical performance

The composite wires achieved mechanical properties comparable to non-absorbable counterparts, with customizable strength and elasticity by varying Mo content.
Braided prototypes demonstrated crush resistance similar to commercial devices, making them suitable for neurovascular applications.

Enhanced radiopacity

The Mo core significantly improved visibility under fluoroscopic guidance, essential for precise device placement.
Radiopacity increased proportionally with Mo content, ensuring adequate imaging performance without permanent markers.

Corrosion Behavior

In vitro and in vivo tests showed progressive and controlled degradation of the FeMnN shell, while the Mo core remained intact for at least six months.
This staged degradation minimizes the risk of premature fragmentation.

Biocompatibility

Cytotoxicity testing confirmed minimal impact on cellular health, supporting the material's safety for clinical use.

Advantages for Neurovascular Devices

Minimized profile: Comparable to traditional flow diverters, enabling easier navigation in small vessels.
Reduced long-term risks: Absorbability eliminates concerns like chronic inflammation, side branch blockage, and imaging artifacts from permanent implants.
Improved healing: Supports endothelial tissue regeneration over aneurysm necks for effective occlusion.

Future Directions
While these findings highlight the potential of FeMnN-Mo DFT® composite wires, further research is needed to optimize their degradation timeline and assess long-term clinical performance. Exploring additional configurations and alloy combinations could further enhance their functionality.

Conclusion
FeMnN-Mo DFT® composite wires represent a promising step forward in the development of absorbable flow diverters. By addressing critical challenges in material performance and compatibility, this innovation paves the way for safer and more effective neurovascular treatments.

Superelastic conductor materials: Enhancing implantable lead durability

Mon, 22 Aug 2022 12:00:00 Z

Implantable biostimulation leads are essential components in cardiostimulation and neurostimulation devices. These highly engineered wire constructs must endure millions of flexural cycles over decades of use. Fort Wayne Metals has introduced a new wire construct concept that significantly improves fatigue resistance, potentially enhancing the longevity and reliability of implantable medical devices.

Breakthrough in conductor materials

Traditional biostimulation leads have relied on materials like 35N LT® (CoNiCrMo), which offer good fatigue resistance but still present limitations in extreme flexural conditions. The introduction of Nitinol-based conductors presents a major advancement. When substituted for 35N LT®, Nitinol demonstrates a 50% to 100% improvement in cyclic strain-loading fatigue performance, making it a promising alternative for long-term implantable applications.

The new composite wire, NiTi-DFT®-30Ag, features a high-conductivity pure silver core encased in a Nitinol outer sheath. While its ultimate strength of 1000 MPa is lower than the 1600 MPa of traditional 35N LT®-DFT®-28Ag, it requires significantly more energy to fracture—65.9 mJ/mm³ compared to 23.7 mJ/mm³. This enhanced toughness suggests improved resilience under continuous flexing conditions.

Advantages for medical device applications

Nitinol’s unique superelastic properties allow it to elastically recover from strains exceeding 10%, making it an excellent candidate for applications requiring high fatigue resistance. This property has already led to its widespread use in guidewires and stents, and its introduction into implantable leads could revolutionize the field.

A key challenge in integrating Nitinol into lead designs has been its elasticity, which complicates coil formation and shape retention. However, Fort Wayne Metals has successfully demonstrated that polyimide coatings can provide the necessary electrical insulation while withstanding high-temperature shape-setting processes. This breakthrough enables the use of Nitinol-based bifilar coils, which maintain electrical isolation even after exposure to 450-550°C secondary shape-setting.

Potential future applications

The ability to shape set Nitinol-based conductors at high temperatures opens up new design possibilities for implantable leads. For example, future pacing and defibrillation systems may incorporate hybrid designs with transmyocardial leads that require exceptional flexibility and durability. Additionally, Nitinol-based leads could be programmed with deployable shapes that enhance passive fixation, reducing the risk of dislodgement or migration.

Beyond cardiac applications, neurostimulation devices could benefit from Nitinol’s compliance and resistance to mechanical fatigue. As neurostimulation leads are often subjected to constant movement within the body, improved fatigue resistance could lead to longer-lasting and more reliable therapies for conditions such as Parkinson’s disease and chronic pain.

Conclusion

The introduction of Nitinol-based conductors represents a significant advancement in implantable lead technology. With superior fatigue resistance, enhanced durability, and the ability to maintain electrical integrity under extreme conditions, these materials have the potential to redefine the future of biostimulation devices. As research and development continue, Nitinol-based leads may soon become the new standard for next-generation implantable medical devices.

CoNiCr-Nitinol composite wires for guidewire applications

Mon, 17 Feb 2020 12:00:00 Z

Fort Wayne Metals developed a CoNiCr-Nitinol composite wire targeted for use in advanced guidewire applications. This novel material combines the strength and stiffness of CoNiCr alloys with the superelasticity of Nitinol, offering a seamless transition from proximal stiffness to distal flexibility. These properties may enhance physician control, improve tip performance, and facilitate the crossing of chronic total occlusions (CTOs) without requiring a jointed core wire.

Material composition and manufacturingThe composite wire consists of:

Outer Shell: 35N LT® (CoNiCrMo alloy) for high strength and stiffness
Core: Ni50.8Ti49.2 Nitinol for superelasticity and kink resistance

Manufactured using DFT® technology, the wire undergoes co-processing, cold reduction, and heat treatments. This process combines two dissimilar materials into a single wire system, achieving optimal strength, flexibility, and superelastic performance.

Key performance characteristics

Mechanical strength and flexibility

The CoNiCr shell provides an initial elastic modulus of 197 GPa and an ultimate tensile strength of 2600 MPa.
The Nitinol core maintains an elastic modulus of 48 GPa with an ultimate tensile strength of 1170 MPa.
The wire demonstrates a seamless transition from stiffness to flexibility, allowing for precise control.

Torque control and one-to-one response

Rotational testing confirmed minimal lag between proximal and distal ends, providing sufficient torque transmission.
Optical tracking in a whip test showed that the wire exceeded ASTM F2819 standards for straightness.

Bending and kink resistance

The composite wire maintains higher bending stiffness in the proximal section for pushability.
The Nitinol core offers lower bending resistance in the distal end, improving navigation and kink resistance.

Joint-free design

Unlike traditional guidewires that require soldering or adhesives to join different materials, this composite wire integrates high strength and superelasticity into a single, continuous structure.
This composite wire product has the potential to reduce failure points and enhance reliability.