You can see: https://www.hindawi.com/journals/jm/2011/501483/

Medical applications of shape memory alloys. L.G. Machado and M.A. Savi.

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-879X2003000600001

http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=2500499

https://www.google.com/patents/US6379393

The Nitinol has become the material of choice for minimally invasive surgical techniques. Devices and instruments, such as orthodontic arch wires, stents dental, guidewires, catheter tubes, ASD patches, among others, make use of the unique properties provided by this material. Indeed, shape memory is no longer looking for problems, has found a comfortable home in the medical industry

Look at the work that follows:

Nickel-titanium alloy:

Application in orthodonts

http://sonda.sfg.hr/wp-content uploads/2015/04/Me%C5%A1trovi%C4%87-S.-et-al.-%2%80%

Dear Yasaman,

Very good question. I agree, they have told us about the advantages but, as in any other business I guess, they have not told us about the disadvantages.

Besides production costs, which you pointed, I would add something: we are inserting in our bodies Nickel, which is dangerous. Actually it is not Nickel alone but alloyed with Titanium and thats why it's safe. I don't know the state of the art at this point. I know Nickel-Titanium can be produced with a thin layer of TiO2 which protects the inside. Depending on the thickness and stress this layer can be cracked. I also know there are several papers about cracks in NiTi alloys, but I haven't read about the possibility of escaping Nickel atoms to the body.

They also use many other coatings. I've found this one attached which is interesting as they take care of this by adding a ceramic coating.

So, can any NiTi device producer 100% assure that no Ni ions will be released to the body during the life of the patient?

Article Hydroxyapatite coating on NiTi shape memory alloy by electro...