Surface Treatments of Nearly Equiatomic NiTi Alloy (Nitinol) for Surgical Implants

Since the discovery of the shape memory effect in equiatomic NiTi alloy by Buechler in 1962 in the Naval Ordnance Laboratory [1], nitinol (Nickel-Titanium Naval Ordnance Laboratory) has attracted a great deal of commercial interest especially in medical applications [2, 3]. T. Duerig, A. Pelton, and D. Stockel wrote an excellence overview on nitinol medical applications in 1999 [3]. They pointed out that there were three reasons for the sudden explosive growth of Nitinol in the 1990’s. The most important was that the medical industry had been trying to pare costs and simplify medical procedures. Conventional materials like 316L stainless steel could not fulfill this new demand by medical devices. Furthermore, the availability of microtubing and ability to laser cut tubings with high precision favored new materials like Nitinol. Last but not least, sharing of technology developed by materials scientists and companies among product designers and doctors should not be underestimated. They specifically pointed out 11 specific reasons for the application of Nitinol to the medical industry [3, 4]: a. elastic deployment allowing an efficient deployment of a medical device; b. thermal deployment and by using the shape memory effect, the nitinol device can recover to its ‘pre-programmed’ shape by body temperature after the deployment; c. kink resistance which allow the medical device to pass through tortuous paths without stain localization and changing its shape; d. good biocompatibility which means that the foreign implants are well accepted by the body. Nitinol has been reported to have extremely good biocompatibility due to the formation of a passive titanium-oxide layer (TiO2) [3]. However, Ni is allergenic and toxic to humans and reports have shown that the Ni release from commercial ready-touse nitinol orthodinitc wires vary in a wide range from 0.2 to 7 μg cm-2 [5]. Therefore, Ni release from nitinol remains a serious health concern and surface modification of nitinol devices will be discussed later in this chapter; e. constant stress allowing the design of a medical device that applies a constant stress over a wide range of shapes;