With its extreme flexibility and resistance to damage, the metal alloy, nitinol, has become an essential resource in medical devices manufacturing and other leading-edge industries.
However, until recently, manufacturers of nitinol products have faced challenges as the alloy can be difficult to machine and product design has been limited to simple structures. Pierre Forêt, Associate Director, Additive Manufacturing at Linde Gases, looks at how the latest AM technologies are enabling greater design freedom and improved productivity.
Discovered over 50 years ago, nitinol (NiTi) is an especially valuable metal alloy that has revolutionised numerous industries. Made of 50% atomic nickel and 50% atomic titanium, it has unique properties, allowing for its superelasticity and “shape memory effect” – meaning it can change shape depending on temperature. This special behaviour is now seeing nitinol increasingly used in the medical and dental industries and in aerospace for solar panels.
Nitinol is an excellent material to use when creating components for minimally invasive medical devices, such as guidewires, catheters, and stents. When medical professionals need to navigate in particularly tight areas, nitinol has both the flexibility to change shape as needed and the durability to endure high amounts of strain.
Biocompatibility in practice
Stents that are used to keep arteries open are perhaps the clearest example of why the superelasticity and the shape memory effect of nitinol are so beneficial. When stents are inserted into the body, they can be compressed down to tiny size to be used in minimally invasive procedures. When they are placed at the right point in the artery, however, they expand to fill the necessary space and brace the inside arterial wall, a procedure that could not be done using stainless steel.
In orthopaedic procedures, surgeons need components that will help patients regain flexibility and range of motion, as well as easily adapting to an individual patient’s tissue. Nitinol is the preferred material for these components, as it mimics bone mechanical behaviour. Orthodontists also need wires and brackets that hold braces together and perform the function of moving teeth, so nitinol’s shape memory is particularly useful with archwire applications.
Challenges in production
Despite all its advantages, developing components made from nitinol can have its challenges. The alloy can be difficult to machine, so product design has typically been limited to simple structures. Such difficulties include high toughness, high ductility and work hardening in cutting processes. So traditional machining results in excessive tool wear, high cutting forces and surface degradation, often ending in low workpiece quality with inferior chip breaking and burrs formation.
And while laser bed powder fusion (LBPF) additive manufacturing can offer greater production efficiency greater and design freedom, there are still issues to be overcome when printing with nitinol.
Vaporisation of the nickel during the laser enabled process can lead to a decrease in the nickel / titanium ratio, thereby increasing the transformation temperature. Additionally, oxygen pick-up inside the material can also affect the transformation temperature, negatively impacting on shape memory and affecting the overall performance of the intended application. Surface oxidation can also be an issue, meaning the part needs significant post production cleaning. To avoid both issues, it is vital to reduce the amount of oxygen in the print chamber.
Maintaining the right atmosphere
While additive manufacturing can optimise production of medical devices, ensuring high-quality repeatability of the process and requiring less post-print finishing, the atmosphere in the printing chamber needs to be optimal and reproducible.
Although the atmosphere in the chamber is purged with high-purity argon to rid it of oxygen, impurities still remain present due to incomplete purging and small leakages. Even extremely small variations in oxygen content can impair the mechanical or chemical properties of metals and alloys sensitive to oxygen – including nitinol – and can affect the composition of the end product resulting in negative physical characteristics such as discolouration and even poor fatigue resistance. Typically, after purging, the level of residual oxygen is around 1,000ppm – far off the ideal of less than 10ppm.
Linde has dedicated the past few years to developing pioneering technology to overcome these atmospheric impurities in order to give manufacturers optimal printing conditions. The result – ADDvance® O2 precision – provides continuous analysis of the gas atmosphere, detecting oxygen levels with high precision without cross-sensitivity. Recognising O2 concentrations as low as 10 parts per million (ppm), the unit automatically initiates a purging process to maintain the atmosphere as pure as needed.
The technology is already in use at medical device manufacturing companies at the vanguard of additive manufacturing, including 3D Medlab, a division of the Marle Group, in France. It was with the collaboration of 3D Medlab that Linde has developed a solution to deliver stringent control of oxygen levels for the production of nitinol components using ADDvance O2 precision oxygen monitoring system and ADDvance® Laser230 process gas mixture.
ADDvance Laser230 is a bespoke gas mixture developed specifically to optimise printing outcomes in LPBF processes. Combining argon with helium, the process gas mixture will reduce particle redeposition by up to 30% and powder loss by up to 20%. It also saves on maintenance time, with fewer changes of filters required. In addition, it mitigates fume formation and accelerates printing times, making the printing process safer and lowering the cost per part. It is alloy agnostic and ideal for additive manufacturing of lattice structures.
With such advancements in additive manufacturing technologies, nitinol will not only enable the future of medical procedures to become less invasive, it will also open up possibilities to manufacture leading-edge products and components that have previously been out of reach.