Surface Modification of Additively Manufactured Nitinol by Wet Chemical Etching

Three-dimensional printed nitinol (NiTi) alloys have broad prospects for application in medicine due to their unique mechanical properties (shape memory effect and superplasticity) and the possibilities of additive technologies. However, in addition to mechanical properties, specific physicochemical characteristics of the surface are necessary for successful medical applications. In this work, a comparative study of additively manufactured (AM) NiTi samples etched in H₂SO₄/H₂O₂, HCl/H₂SO_4, and NH₄OH/H₂O₂ mixtures was performed. The morphology, topography, wettability, free surface energy, and chemical composition of the surface were studied in detail. It was found that etching in H₂SO₄/H₂O₂ practically does not change the surface morphology, while HCl/H₂SO₄ treatment leads to the formation of a developed morphology and topography. In addition, exposure of nitinol to H₂SO₄/H₂O₂ and HCl/H₂SO₄ contaminated its surface with sulfur and made the surface wettability unstable in air. Etching in NH₄OH/H₂O₂ results in surface cracking and formation of flat plates (10-20 microns) due to the dissolution of titanium, but clearly increases the hydrophilicity of the surface (values of water contact angles are 32-58°). The etch duration (30 min or 120 min) significantly affects the morphology, topography, wettability and free surface energy for the HCl/H₂SO₄ and NH₄OH/H₂O₂ etched samples, but has almost no effect on surface composition.