Experimental Investigation of the Biofunctional Properties of Nickel–Titanium Alloys Depending on the Type of Production

Abstract

Nickel–titanium alloys used in dentistry have a variety of mechanical, chemical, and biofunctional properties that are dependent on the manufacturing process. The aim of this study was to compare the mechanical and biofunctional performances of a nickel–titanium alloy produced by the continuous casting method (NiTi-2) with commercial nitinol (NiTi-1) manufactured by the classical process, i.e., from remelting in a vacuum furnace with electro-resistive heating and final casting into ingots. The chemical composition of the tested samples was analyzed using an energy dispersive X-ray analysis (EDX) and X-ray fluorescence (XRF). Electron backscatter diffraction (EBSD) quantitative microstructural analysis was performed to determine phase distribution in the samples. As part of the mechanical properties, the hardness on the surface of samples was measured with the static Vickers method. The release of metal ions (Ni, Ti) in artificial saliva (pH 6.5) and lactic acid (pH 2.3) was measured using a static immersion test. Finally, the resulting corrosion layer was revealed by means of a scanning electron microscope (SEM), which allows the detection and direct measurement of the formatted oxide layer thickness. To assess the biocompatibility of the tested nickel–titanium alloy samples, an MTT test of fibroblast cellular proliferation on direct contact with the samples was performed. The obtained data were analyzed with the IBM SPSS Statistics v22 software. EDX and XRF analyses showed a higher presence of Ni in the NiTi-2 sample. The EBSD analysis detected an additional NiTi2-cubic phase in the NiTi-2 microstructure. Additionally, in the NiTi-2 higher hardness was measured. An immersion test performed in artificial saliva after 7 days did not induce significant ion release in either group of samples (NiTi-1 and NiTi-2). The acidic environment significantly increased the release of toxic ions in both types of samples. However, Ni ion release was two times lower, and Ti ion release was three times lower from NiTi-2 than from NiTi-1. Comparison of the cells’ mitochondrial activity between the NiTi-1 and NiTi-2 groups did not show a statistically significant difference. In conclusion, we obtained an alloy of small diameter with an appropriate microstructure and better response compared to classic NiTi material. Thus, it appears from the present study that the continuous cast technology offers new possibilities for the production of NiTi material for usage in dentistry.

Corrosion Behavior of Nickel–Titanium Continuous-Casted Alloys

“…The grain size of each metal material depends on the nucleation centers and the crystal's growth during the solidification of the melt. The higher nickel content in the NiTi cc alloy affected the increase in the number of initial grains during the nucleation process [23,27,28]. The grain size reduction can improve the alloy's superelastic properties, as it allows the desired orientation and stability of formatted grains due to the possibility of producing a larger number of martensite plates [29].…”

“…The scientific literature is poor, with evidence of indirect biocompatibility of the nitinol alloy. On the contrary, several studies indicate good biocompatibility of nickel-titanium in direct tests, showing that different cell lines can proliferate on alloys' surfaces [23,40]. From the perspective of passive alloys, direct contact means cells adhered to Ti and Ni oxides, depending on the weight percentage surface ratio.…”

“…NiTi as cast and NiTi cc discs were placed in a tube containing 10 mL of complete medium (DMEM/F12 with 10% FBS and 1% ABAM (all from Gibco, Thermo Fisher, MA, USA), incubated at 37 • C for 2 days, and removed, while the remaining supernatant was used in further experiments. Gingival fibroblast cells were used, as in our previous study [23]. The cells were seeded in a 96-well plate (5.000 cells/well), and the next day 100 µL of supernatant was added to corresponding wells and incubated at 37 • C in a humidified 5% CO 2 atmosphere for up to 7 days.…”