Biocompatibility and Corrosion Response of Laser Joined NiTi to Stainless Steel Wires

The biocompatibility of nickel titanium (NiTi) wires joined to stainless steel (SS) wires via laser autogenous brazing has been evaluated. The laser joining process is designed to limit the amount of mixing of the materials, thus preventing the formation of brittle intermetallic phases. This process has the potential for manufacturing implantable medical devices; therefore, the biocompatibility must be determined. Laser joined samples underwent nickel release rate, polarization, hemolysis, and cytotoxicity testing. Competing effects regarding grain refinement and galvanic effects were found to influence the corrosion response. After 15 days of exposure to a simulated body fluid, the total nickel released is less than 2 ug/cm2. Numerical modeling of the corrosion currents along the wires, by making use of polarization data, helped to explain these results. Microbiological testing found a maximum hemolytic index of 1.8, while cytotoxicity tests found a zero toxicity grade. All of these results indicate that the autogenous laser brazing process results in joints with good biocompatibility.

Nickel Release Rate of Type 303 Free Cutting Austenitic Stainless Steel

Type 303 austenitic stainless steel has been applied in jewelry production, and its nickel release rate has become a concerned issue in the jewelry industry. In this paper, the commercial type 303 stainless steel was used as the test material, while 304 stainless steel as the contrast material; their nickel release rates and corrosion behaviors in artificial sweat were studied. The results show that the actually measured nickel release rate of 303 stainless steel reaches 2.06μg/cm2/week, nearly 25 times higher than that of 304 stainless steel, which exceeds the threshold specified in nickel release standard EN1811:2011 for jewelries coming into direct and prolonged contact with the skin (0.88μg/cm2/week), and its adjusted value also exceeds the threshold specified in Nickel Directive 2004/96/EC for puncture accessories (0.35μg/cm2/week). The high nickel release rate for 303 stainless steel is mainly caused by its high sulfur content and the inevitable formation of manganese sulfide inclusions, which will cause the pitting and exacerbate the material corrosion. Considering the risk of nickel sensitization of 303 stainless steel, it is not suggested to use 303 stainless steel as the jewelry material, especially for piercing jewelry.