What is meant by Nitinol in terms of SMA?
The first observation of a shape memory effect was made by A. Ölander in the early 1930s on an AuCd alloy, which, however, could not be explained at that time. It was not until 1963 that intensive research on shape memory alloys was triggered by a chance discovery during research on heat shields. As a result of this discovery at the Naval Ordnance Laboratory (Maryland, USA), the shape memory alloys based on nickel and titanium with the highest technological significance to date now have
the trade name Nitinol. To date, the largest area of application for Nitinol has been in medical technology. A few years ago, about 90% of NiTi applications were found there. It was here that the cardiological stent was produced as the first mass-produced product. Due to its pseudoelastic material behavior, it can be passed through a microcatheter. When it emerges from the catheter in an artery, it resumes its original shape and can thus keep the vessel open and prevent it from
reoccluding. This treatment method is used, for example, in cases of arteriosclerosis, a hardening of the arteries. Nitinol is also used in the dental industry (e.g. root canal files), surgery (surgical tools) and orthopedics (orthoses).
Impurities in Nitinol production
The slightest impurities can have a negative effect on the production of Nitinol alloys. However, these inclusion levels can now be significantly reduced by special melting and processing methods. To assess the purity of nitinol in relation to industrial applied manufacturing processes, the 2015 technical scientific publication by Scott Robertson et al. is an important reference in the nitinol market (Scott W. Robertson, Maximilien Launey, Oren Shelley, Ich Ong, Lot Vien, Karthike Senthilnathan, Payman Saffari, Scott Schlegel, Alan R. Pelton,
JMBBM 2015 p119-131). Impressively, Robertson demonstrates the role of inclusions in assessing the functional fatigue of nitinol. Smaller inclusions, larger inclusions, and smaller numbers have a large statistical effect on fatigue behavior in superelastic wires and tubes made of nitinol. Other publications demonstrate the positive effect of smaller precipitates on corrosion behavior (Harshad M. Paranjape ∗, Bill Ng , Ich Ong , Lot Vien , Christopher Huntley ; Scripta Mat 2020 p442-446).
Their equally positive influence on fatigue behavior has also been scientifically documented (demonstrated by the example of in stents, Fan Sun, Laurence Jordan, Valérie Albin, Virginie Lair, Armelle Ringuedé, and Frédéric Prima, ASSODF 2020 p3073-3079). For exciting insights on the topic, check out our Medical Technology Insight Talk.