He Tian

The influence of surface oxides on the distribution and release of nickel from Nitinol wires.

The patterns of Ni release from Nitinol vary depending on the type of material (Ni-Ti alloys with low or no processing versus commercial wires or sheets). A thick TiO(2) layer generated on the wire surface during processing is often considered as a reliable barrier against Ni release. The present study of Nitinol wires with surface oxides resulting from production was conducted to identify the sources of Ni release and its distribution in the surface sublayers. The chemistry and topography of the surfaces of Nitinol wires drawn using different techniques were studied with XPS and SEM. The distribution of Ni into surface depth and the surface oxide thickness were evaluated using Auger spectroscopy, TEM with FIB and ELNES. Ni release was estimated using either ICPA or AAS. Potentiodynamic potential polarization of selected wires was performed in as-received state with no strain and in treated strained samples. Wire samples in the as-received state showed low breakdown potentials (200 mV); the improved corrosion resistance of these wires after treatment was not affected by strain. It is shown how processing techniques affect surface topography, chemistry and also Ni release. Nitinol wires with the thickest surface oxide TiO(2) (up to 720 nM) showed the highest Ni release, attributed to the presence of particles of essentially pure Ni whose number and size increased while approaching the interface between the surface and the bulk. The biological implications of high and lasting Ni release are also discussed.

Stability of Ni in nitinol oxide surfaces

The stability of Ni in titanium oxide surface layers on nitinol wires known to release certain amounts of Ni was investigated by first principles density functional theory and transmission electron microscopy. The oxides were identified as a combination of TiO and TiO(2) depending on the thickness of the layer. The calculations indicate that free Ni atoms can exist in TiO at ambient temperature while Ni particles form in TiO(2), which was confirmed by the transmission electron microscopy observations. The results are discussed with respect to surface stability and Ni release due to free Ni atoms and Ni particles.

TEM study of the Ni-Ti shape memory micro-wire

Today, due to its unique mechanical properties, durability and biocompatibility, applications of Nitinol in a wide variety of medical implants are progressively increasing [1–3]. However, as Nitinol consists of about 50 at. % Ni, certain applications are still hindered by the concern of free Ni release in the surrounding tissue. It is known that the biocompatibility of the implants made from TiNi alloys depends on a corrosion resistant titanium oxide layer avoiding the severe toxicological and allergic responses of Ni. The latter is controlled by the structure of (near-) surface layers which can strongly be affected by various surface treatments [3,4]. In this present work, the oxide surface of an as-received Nitinol micro-wire is investigated by different TEM techniques. FIB was used to prepare near surface cross-section sample.

TEM investigations on novel shape memory systems with Ni-depletions

Many of today’s shape memory systems contain a substantial amount of Ni. The most typical system is of course Nitinol® based on the binary Ni-Ti alloy used near to its equiatomic composition. Another well known system is Ni-Al, which has been very thoroughly studied more for its fundamental physical characteristics rather than its practical applications. Recently, however, new ternary systems have attracted a lot of attention attempting to, e.g., introduce magnetic transitions and driving forces, such as in Ni-Mn-Ga or Co-Ni-Al or lower the hysteresis and increase the transformation temperatures in Ni-Ti-(Au,Pd,Pt), replacing mainly Ni with the ternary compound. At the same time, new studies have revealed unwanted Ni release in commercial wires for medical use. Focusing on Ni thus remains important for understanding the behaviour of martensitic transformations and shape memory applications.