Low Ion Release Research Articles

TiN coating improves the corrosion behavior of superelastic NiTi surgical alloy

The key to the biocompatibility of NiTi surgical devices resides in the improvement of the material's corrosion resistance. To protect the surface of NiTi from corrosion, an original PIRAC nitriding method was proposed. In the present work, the corrosion and electrochemical properties of NiTi samples annealed under a low pressure of highly reactive nitrogen at 900 and 1000°C were studied. The microstructure of PIRAC coatings was characterized employing XRD and SEM/EPMA and was found to consist of a hard TiN outer layer followed by a Ti 2Ni layer. Electrochemical tests were performed in Ringer's solution, both upon immersion in deaerated solution and after long term exposure (400 h) under aeration at open circuit potential (OCP). PIRAC nitriding treatment was found to significantly improve the corrosion resistance of NiTi alloy. Samples nitrided at 900°C, 6 h and at 1000°C, 0.5–5 h were passive in a wide range of potentials with no signs of activation up to 1.1 V. A very low anodic current in the passive region and an exceedingly low metal ion release rate were measured on PIRAC coated samples. The relationship between the nitriding parameters and protective properties of PIRAC coatings has been established and the role of the outer and inner surface layers in the improvement of NiTi corrosion resistance is discussed.

Corrosion behavior of titanium nitride coated Ni–Ti shape memory surgical alloy

Nickel–titanium (NiTi, nitinol) shape memory alloy was nitrided using an original powder immersion reaction assisted coating (PIRAC) method in order to modify its surface properties. PIRAC nitriding method is based on annealing the samples in the atmosphere of highly reactive nitrogen supplied by decomposition of unstable nitride powders or, alternatively, by selective diffusion of the atmospheric nitrogen to the sample surface. Being a non-line-of-sight process, PIRAC nitriding allows uniform treatment of complex shape surgical implants. Hard two-layer titanium nitride (TiN)/Ti 2Ni coatings were obtained on NiTi surface after PIRAC anneals at 900 and 1000°C. PIRAC coating procedure was found to considerably improve the corrosion behavior of NiTi alloy in Ringer's solution. In contrast to untreated nitinol, no pitting was observed in the samples PIRAC nitrided at 1000°C, 1 h up to 1.1 V. The coated samples were also characterized by very low anodic currents in the passive region and by an exceedingly low metal ion release rate. The research results suggest that PIRAC nitriding procedure could improve the in vivo performance of NiTi alloys implanted into the human body.

Long-term evaluation of titania-based ceramics compared with commercially pure titanium in vivo

Fifty-four cylinders (2.8 mm in diameter) machined from hot isostatically pressed titania (TI) and titania-hydroxyapatite (TI/HA-15 vol%) sintered at 925°C, as well as commercially pure titanium (c.p. Ti), were implanted in the fermoral cortical bone of New Zealand white rabbits for 1, 3 and 12 months. The shear strength between bone and implant was measured by a push-out test. The TI/HA composite showed a significantly higher bonding strength to bone compared to c.p. Ti at all times, while no differences were observed between TI and c. p. Ti at 1 and 3 months after implantation. Titania-based materials had a significantly higher bonding strength than that of c.p. Ti one year after implantation. The results indicate that bioactivity of HA in TI/HA composite contributes to the early bone apposition reflected by high bonding strength, while the stability of the oxide, determines the development of long-term bonding strength. Both effects may be explained by the level and type of ions released from the ceramic implant. HA has a positive conduction to bone ingrowth while TI has a limited interaction to the bone apposition due to the extraordinary low ion release in vivo. Under light microscopy, similar patterns of bone-implant interfaces were seen from titania-based materials and c.p. Ti in 3-month samples, indicating high biocompatibility of these materials. However, histological evaluation by light microscope cannot identify the differences between physical contact and chemical bonding of implant-bone interface, and thus does not give information on bonding mechanism and the level of shear stresses developed.