Evaluation of Biocompatibility and Corrosion Resistance of Shape Memory NiTi Alloy in Simulated Physiological Fluids

Nickel–titanium (NiTi) shape memory alloys have gained considerable attention in the field of biomedical engineering because of their exceptional mechanical and biological properties. These alloys exhibit unique characteristics such as excellent corrosion resistance, high biocompatibility, superelasticity, and a remarkable shape memory effect. Because of these properties, NiTi alloys are widely considered suitable materials for various medical devices and implants, including stents, orthodontic wires, and bone fixation systems. From a physiological perspective, materials used in the human body must be able to function safely within complex biological environments that contain electrolytes, proteins, and cells. Therefore, the interaction between biomaterials and body fluids is an important factor in determining their long-term stability and safety. One of the important features of NiTi alloys is the formation of a protective titanium dioxide (TiO₂) passive layer on their surface. This oxide layer acts as a barrier that helps prevent the release of nickel ions into surrounding tissues and body fluids. Since excessive nickel release can cause allergic reactions or cytotoxic effects in biological systems, the stability and protective ability of this passive layer are critical. However, despite the presence of this protective layer, the relatively high nickel content in NiTi alloys and the long-term durability of the passive film in physiological environments remain subjects of ongoing scientific debate and investigation. In the present study, a NiTi shape memory alloy with a nominal composition of 50.7 atomic percent nickel was investigated to evaluate its corrosion behavior and biological compatibility. Electrochemical corrosion tests were conducted in two simulated physiological environments: Ringer solution and 0.9% sodium chloride (NaCl) solution. These solutions are commonly used to simulate the ionic composition of body fluids because they contain electrolytes similar to those found in extracellular fluid in the human body. The electrochemical results indicated that the breakdown potential of the NiTi alloy was higher in the 0.9% NaCl solution than in the Ringer solution, suggesting variations in corrosion resistance under different physiological conditions. Surface morphology and corrosion characteristics were further examined using Scanning Electron Microscopy (SEM). The observations revealed that relatively low pitting corrosion occurred in the Ringer solution compared with the NaCl solution during potentiostatic testing. Following the electrochemical experiments, an increase in the pH value of both solutions was observed. This change in pH may be associated with electrochemical reactions occurring at the alloy surface during corrosion processes. In addition, X-ray diffraction (XRD) analysis identified the presence of hydride phases, which indicates a reduction in hydrogen ion concentration within the solution. From a physiological standpoint, such changes in ionic concentration and pH can influence cellular responses and tissue compatibility.