Investigation of the surface properties of shape memory nitinol materials in biomedical applications

Nitinol, categorized as a shape memory alloy, has gained widespread application in various engineering fields, particularly in biomedical applications, due to its remarkable properties such as shape memory effect, superelasticity, high mechanical strength, corrosion resistance, and excellent biocompatibility. The biocompatibility of Nitinol alloys is attributed to the protective passive layer formed on their surfaces, and the quality of this passive layer is directly influenced by the surface characteristics of the alloy. Electro-polishing (EP), one of the surface improvement techniques, has emerged as an effective method to enhance the passive layer by increasing its thickness and ensuring the formation of a homogeneous TiO₂ layer. In this study, the surface characteristics of Nitinol after electro-polishing (EP) and thermal passivation treatments were comprehensively investigated using three different electrolyte solutions (K₁, K₂, and K₃). The electrolyte solutions used in the study were K₁ (sulfuric acid, methanol, and glycol), K₂ (sulfuric acid, hydrochloric acid, and distilled water), and K₃ (hydrogen peroxide, hydrochloric acid, and distilled water). The effects of these electrolytes on electro-polishing were evaluated in terms of surface roughness, chemical composition, and biocompatibility. The experimental methods were optimized to achieve a smooth surface and minimize nickel ion release. Surface morphology analysis was performed using FESEM (Field Emission Scanning Electron Microscope), which revealed an increase in oxygen content indicative of TiO₂ layer formation after EP. Surface roughness measurements were carried out using AFM (Atomic Force Microscopy), with the K₃ electrolyte yielding a surface roughness value of 66 nm at an optimum voltage of 4.8 V. Additionally, nickel ion release tests conducted with ICP-MS (Inductively Coupled Plasma Mass Spectrometry) demonstrated that all electrolytes effectively prevented nickel release after EP and thermal passivation, with the K₃ electrolyte providing superior results compared to those reported in the literature. In conclusion, this study presents significant findings on the optimization of Nitinol surface treatments to improve biocompatibility and surface properties, contributing valuable insights and novel recommendations for the advancement of Nitinol in the field of biomaterials.