Study of Titanium Dioxide Nanotube (TNT) Growth on Various Surface Morphologies Manufactured Using DMLS 3D Printing

The development of titanium dioxide nanotubes (TNT) has greatly improved the field of biomedical implants, particularly in the context of Direct Metal Laser Sintering (DMLS) printed titanium implants. To achieve optimal results in terms of microstructure and surface roughness, it is essential to carefully select and control the input process parameters in DMLS. These parameters include laser power, scan velocity, and hatch distance. Laser power typically expressed in watts is the the energy input into the material, scan velocity expressed in mm/s controls the time the laser interacts with the powder, and the hatch distance is the lateral distance between adjacent scan tracks.<br/>In this study, Ti6Al4V ELI samples were prepared using nine different combinations of DMLS input process parameters. Ti6Al4V ELI is a grade 23 titanium alloy used in implant materials due to its excellent mechanical strength, corrosion resistance, and biocompatibility. Variations in laser power from 180 to 270 watts, scan velocity from 720 mm/s to 1000 mm/s and layer thickness from 40 to 60 microns were used. These range of values have been optimized from earlier literature studies which show promising mechanical strength of implants. Consistency in surface roughness (Ra) measurements was confirmed by polishing all samples to a mirror-finished surface. The surface roughness within a consistent range of results, specifically between 2 to 6 nanometers (nm) was measured. Surface roughness was assessed using a white light interferometer (WLI). Electron Backscatter Diffraction (EBSD) characterization was used to study grain orientations and microstructural characteristics of the samples. Alpha martensite grain dimension measurements were the main area of interest. This focus was based on the fact that the length of the alpha martensite needles significantly influences the dissolution behavior of titanium samples.<br/>Titanium Nanotubes (TNT) growth on the DMLS printed titanium samples was carried out using an electrochemical anodization (EA) process. When a valve metal such as titanium is subjected to this method, it develops a passive layer on the surface which protects the layer beneath the surface thus improving the corrosion resistance of the surface. EA is based on the mechanism of controlled oxidation and dissolution of the titanium surface. It creates a layer of aligned self-organised nanotubes. The electrolyte chosen for this process was ethylene glycol, water (2.5% v/v H2O), and ammonium fluoride (0.5% w/w NH4F). The choice of electrolyte is a key component and third-generation organic electrolyte was chosen as it directly affects the formation and properties of the nanotubes. Before the growth of TNTs, a dissolution behavior of the DMLS Ti6Al4V samples was undertaken in the same electrolyte to investigate the underlying mechanism. Field Emission Scanning Electron Microscopy (FESEM) was utilized for structural characterization of the samples after anodization. FESEM images were analyzed for measurement of TNT diameter and lengths.<br/>A key finding of this study is that TNT dimensions are dependent on the lengths of alpha martensite needles. While the TNT diameters are a function of anodization voltages, the TNT lengths were greatly affected by changes in martensite needle lengths. The relationship between TNT growth and trends in microstructural changes with varying DMLS parameters was examined to gain a better knowledge of TNT growth on various surface morphologies. Our findings demonstrate a correlation between DMLS processing parameters, resulting microstructures, and nanotube growth characteristics. Future research directions may include, investigating cellular responses to various nanotubular configurations, and exploring the potential for incorporating bioactive agents within the nanotubes for localized drug delivery. This work represents a significant step towards integrating advanced manufacturing techniques with nanotechnology in the biomedical field.