Evaluation of the effect of Two-Step Acid Etching on the Surface Treatment and Improved Bioactivity of Nitinol

Surface properties, including morphology, submicron and chemical morphology, are important factors affecting the quality and biological responses at the contact location of tissue with the implant material. This could significantly affect the healing process of bones.
In this laboratory study, morphology and biocompatibility of polished mechanical nitinol (NiTi) shape memory alloys surfaces treated with mixed solution of three acids (HCl-HF-H3PO4) were evaluated. In the next stage, further chemical treatment was conducted in solution of HNO3 and HCl with ratio of 1:1 Vol.% at room temperature and the results were assessed. Seventy-five samples were selected for chemical modification of the surface, biological evaluation and surface roughness; furthermore, nine samples were selected as control. Scanning the Electron Microscope (SEM), atomic force microscope (AFM) and inspection of Nitinol surface roughness (NiTi) were carried out to evaluate the surface. In addition, MG-63 cell was cultured on different nitinol surfaces to assess cell adhesion, growth and proliferation. Data analysis was performed using t-test and one-way ANOVA.
The obtained results indicated that HNO3 and HCl treatment produced significantly higher roughness compared to (HCl-HF-H3PO4) treatment and untreated surfaces. Moreover, evaluation of cell culture revealed significantly higher amounts of cell adhesion, growth and proliferation in two-stage modified nitinol surfaces, compared to the groups modified with triple acid solution and unmodified surfaces.
According to the results, not only chemical surface modification of nitinol changed the surface morphology and created submicron roughness, but it also led to altered chemistry of the surface. This could increase bone-bonding ability in implant and decrease the toxic effects of nickel in the body.