Nitinol: Transforming Medical Technology
In this sponsored technical blog post, Alleima’s Dr. Bernd Vogel highlights the intricate balance required to preserve the alloy’s critical properties while achieving the precision necessary for high-performance medical applications. Dr. Bernd Vogel, serving as Global Technology and Innovation Manager at Alleima’s medical division, has dedicated his career to optimizing nitinol manufacturing workflows.
Introduction
Nitinol is a revolutionary smart material characterized by its unique shape-memory effect, a property that has reshaped the landscape of medical device innovation. This advanced nickel-titanium alloy exhibits remarkable biocompatibility and adaptability, making it indispensable in developing cutting-edge medical solutions. However, the exceptional properties of nitinol come with significant processing challenges that demand specialized expertise.
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Why did nitinol revolutionize medicine?
In 1962, William J. Buehler, a researcher at the Naval Ordnance Laboratory (NOL), discovered the extraordinary shape-memory properties of nickel-titanium (NiTi) alloys, leading to the creation of nitinol. In the medical device industry, many procedures involve the implantation of various metallic constructions. Traditional materials like stainless steel and titanium lack the flexibility and elasticity inherent in living tissues. This biomechanical incompatibility negatively affects adjacent tissue, leading to issues such as the loosening of bone implants. However, nitinol’s properties are much closer to living tissue than any other material. Consequently, nitinol has been continuously used as an implant material for bone fractures. Access to super-elastic nitinol tubes paved the way for vascular implants such as self-expanding stents, filters, and grafts. You can say that the discovery of nitinol revolutionized medicine.
Do standards exist for designers who lack in-depth knowledge of nitinol?
For R&D engineers navigating the complexities of nitinol, adherence to material standards is essential yet remains a challenge. While the American Society for Testing and Materials (ASTM) has established specifications for nitinol as medical device and implant material, these standards primarily address testing methods for wire, tube, and sheet forms. However, their broad definitions often fail to cater to the nuanced requirements of specialized applications. Engineers must contend with significant variations in material properties caused by differences in supplier standards, nickel content, and processing techniques. Even subtle changes in cold work or composition can drastically influence nitinol’s stress-strain behavior, super elastic range, and fatigue resistance. This variability underscores the need for collaborative supplier relationships and tailored R&D processes to ensure material consistency and optimal performance for advanced medical solutions.
Which processing methods can be applied to nitinol?
The fabrication of stand-alone nitinol components can be categorised into the following three groups:
- Structuring the wrought material: If the product is fabricated from tubes or sheets, the following methods are suitable: laser cutting, wire Electrical Discharge Machining (EDM), waterjet cutting, and chemical etching. On the other hand, if a wire, strip, or ribbon is needed, profiling would most likely involve abrasive techniques such as grinding. In some cases, eroding or chemical etching to be successful.
- Shape setting: The process required to set the shape is similar whether starting with wire, sheet, or tube material. The structured component from step one must be constrained on a fixture, followed by heat treatment. The heat treatment may involve a salt bath, sand bath, air or vacuum furnace, or various heating methods such as inductive heating.
- Surface finish: Depending on the heat treatment, the material will have a different surface condition (oxide layer). The resulting surface finish will affect the performance of the instrument. For implants, it will impact durability, corrosion resistance, and other properties.
Which joining methods do you need to consider for nitinol?
If the nitinol component is only one part of the assembly, designers must carefully consider the joining methods. Nitinol can be easily welded to itself using a laser or e-beam. However, one must consider that welds do not have super-elastic properties and should be placed in a section of the assembly where only slight deformation occurs. When joining nitinol to other materials like titanium or stainless steel, the process can be quite difficult. Research has shown successful joining using soft soldering with aggressive fluxes, while resistance or diffusion welding can also be applied. Generally, alternative mechanical options like crimping or shrink-fitting are preferred.
Why is it difficult to convert nitinol fabrication into industrial automation?
1. Deformation-resistant applications
Within every process step in processing components in the category of deformation-resistant applications, one must be aware of the effect it has on the properties of the material. The amount of cold work, the heat treatment, the rate of strain, and the number of cycles all impact material properties and therefore change the stress-strain behaviour. Therefore, the industry is having a hard time converting nitinol fabrication steps into industrial automation. Today, medical products made from nitinol are mainly processed through manual operations, handcrafted with a very low degree of automation. This makes the outcome dependent on the worker, and the process is very difficult to validate. That is also why there has been little demand for serial production from the industry. In the last few decades, the alloy has become popular in the treatment of peripheral vascular diseases. Stents were the first medical products with high demand.
Unfortunately, the variety of required versions of these implants is so high that the profitability of installing automated processes is questionable. Therefore, these implants are mainly processed with a low level of automation. There has however, been mass production of nitinol articles. The first article in this field was the super-elastic, kink-resistant nitinol guidewire used in surgical procedures. By using such products, the surgeon can reach a point of interest in the body in a minimally invasive or non-invasive way. Due to its elasticity, the ability of the wire to follow a tortuous path in the body and still rotate smoothly led to improved performance.
2. Shape-restoring applications
Moreover, the largest family in medical instruments is the group of shape-restoring applications. These products are temporarily deformed to be introduced into the body, and when removed, spring back to their original shape without the need to increase the temperature. They are used for various functions such as loops, snares, retractors, and baskets, as well as for removing foreign bodies and blood clots. In the urological field, there is already a worldwide yearly need for stone retrieval baskets. The same instrument can also be used to catch various stones that are anywhere from 1-12 millimeters in size. Therefore, the diversity of versions needed in this field is very low. This enables the possibility of installing a certain degree of automation in the manufacturing process. The most common version of stone retrieval baskets is manufactured from wires, which form a basket-like structure.
For which medical applications do you recommend using nitinol?
When looking into a factory of medical device manufacturers, the quality of the product is certainly the first thing we have to think about. Following this comes productivity and thereafter, profitability. These parameters do not have to depend on whether you have an all-hands-on operation or an automated process. You simply have to team up with a partner who is experienced in processing nitinol and knows every process step and what effect it has on the properties of the material. The amount of cold work, the heat treatment, the rate of strain, and the number of cycles all have an impact on material properties and therefore change the stress-strain behavior.
Alleima is one of those partners with the necessary experience and processing capabilities to bring life-changing medical solutions to life. In my role as Global Technology and Innovation manager, I am working with a team of experienced materials specialists, development and mechatronics engineers, quality managers, and skilled operators to support our customers in realizing their ideas and lead them through each stage of the challenging development process, all the way to the market approval. This really sets us apart from the competition.
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About the Supplier
With deep expertise in material science, a broad range of precision manufacturing capabilities, and a commitment to advancing together with its customers, Alleima continues to be a trusted leader in transforming complex medical visions into reality. From initial concept to commercialization, we are dedicated to delivering life-changing medical solutions that combine innovation, reliability, and precision. Explore Alleima’s in-stock components to move faster—or submit an RFQ for custom nitinol solutions built for your next innovation.