The Effect of Two-Stage Acid Treatment on Surface Behavior and Improvement of Bioactivity of Nitinol Alloy

Surface properties, including morphology, submicron morphology, and surface chemistry, are essential factors that affect the quality and manner of biological responses at the site of tissue contact with the implant, affecting the bone healing process. In this in vitro study, morphology and biocompatibility of nitinol (NiTi) memory alloy surfaces mechanically polished and modified with a chemical solution consisting of three types of acid (HCl-HF-H3PO4) and then chemical operations in solution (HNO3 and HCl) with a Volumetric scale of 1:1 and examined at ambient temperature. 75 samples were used for surface chemical modification, biological evaluations, and surface roughness, and also 9 samples as control. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and nitinol alloy (NiTi) surface roughness measurements were performed to analyze the surfaces. Besides, MG-63 cells were cultured on different nitinol alloy levels to evaluate adhesion and cell growth and proliferation. Data were analyzed using t-test and one-way analysis of variance. The results show that the chemical surface modification operation with two-stage acid solution had a higher roughness compared to the unmodified surfaces and the surface chemical modification operation with the acidic solution with an only solution consisting of (HCl-HF-H3PO4). Cell culture evaluations also showed that the two-stage modified nitinol levels showed significant cell adhesion and significant growth and proliferation compared to the tertiary acid-modified and unmodified levels. The surface chemical modification method for nitinol alloy can change the surface chemistry and change the surface morphology and create sub-micron scale roughness. This can increase the connectivity of the implant tissue and reduce the toxic effect of nickel.

Machinability Analysis and Optimization in Wire EDM of Medical Grade NiTiNOL Memory Alloy

NiTiNOL (Nickel–Titanium) shape memory alloys (SMAs) are ideal replacements for titanium alloys used in bio-medical applications because of their superior properties like shape memory and super elasticity. The machining of NiTiNOL alloy is challenging, as it is a difficult to cut material. Hence, in the current research the experimental studies on machinability aspects of medical grade NiTiNOL SMA during wire electric discharge machining (WEDM) using zinc coated brass wire as electrode material have been carried out. Pulse time (Ton), pause time (Toff), wire feed (WF), and servo voltage (SV) are chosen as varying input process variables and the effects of their combinational values on output responses such as surface roughness (SR), material removal rate (MRR), and tool wear rate (TWR) are studied through response surface methodology (RSM) based developed models. Modified differential evolution (MDE) optimization technique has been developed and the convergence curve of the same has been compared with the results of differential evolution (DE) technique. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrography (EDS) analysis are carried out to study the surface morphology of the machined alloy. SV is found to be more influential process parameter for achieving better MRR with minimal SR and TWR, followed by Ton, Toff, and WF. The WF has good impact on reduced SR and TWR responses and found to be least significant in maximizing MRR.