Behavior of rat bone marrow stem cells on titanium surfaces modified by laser-beam and deposition of calcium phosphate

Objectives The aim of this study was to evaluate the behavior of rat bone marrow stem cells seeded on a Ti-15Mo alloy surface modified by laser-beam irradiation followed by calcium phosphate deposition. Materials and methods A total of four groups were evaluated: polished commercially pure titanium (cpTi): Ti-P; laser irradiation + calcium phosphate deposition on cpTi: Ti-LCP; polished Ti-15Mo alloy: Ti15Mo-P; and laser irradiation + calcium phosphate deposition on Ti-15Mo alloy: Ti15Mo-LCP. Before and after laser irradiation and calcium phosphate deposition on the surfaces, physicochemical and morphological analyses were performed: Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDX). The wettability of the samples was evaluated by contact angle measurement. In addition, the behavior of osteoblast-like cells to these surfaces was evaluated for cell morphology, adhesion, proliferation and viability, evaluation of alkaline phosphatase formation and gene expression of osteogenesis markers. Results Surfaces wet-abrade with grit paper (P) showed oriented groves, while the laser irradiation and calcium phosphate deposition (LCP) produced porosity on both cpTi and Ti15Mo alloy groups with deposits of hydroxyapatite (HA) crystals (SEM). EDX showed no contamination after surface modification in both metal samples. A complete wetting was observed for both LCP groups, whereas P surfaces exhibited high degree of hydrophobicity. There was a statistical difference in the intragroup comparison of proliferation and viability (p < 0.05). The ALP activity showed higher values in the Ti15Mo alloy at 10 days of culture. The gene expression of bone related molecules did not present significant differences at 7 and 14 days among different metals and surface treatments. Conclusion Ti15-Mo seems to be an alternative alloy to cpTi for dental implants. Surface treatment by laser irradiation followed by phosphate deposition seems to positively interact with bone cells. Clinical relevance Ti-15Mo alloy surface modified by laser-beam irradiation followed by calcium phosphate deposition may improve and accelerate the osseointegration process of dental implants.

Formation Mechanism of Pores Inside Structure Fabricated by Metal-Based Additive Manufacturing

The powder bed fusion (PBF) technique is a metal-based additive manufacturing (AM) method in which metal powder is deposited on a substrate and melted by selective laser-beam irradiation. Given that the process and parameters of metal-based AM are complicated, there are various problems in high-precision fabrication. One of these is that although metal-based AM can be used for fabrication of high-density parts, pores can easily form inside the fabricated structure owing to process instabilities. Pore formation degrades the mechanical strength of the fabricated structure. Therefore, this study investigated the pore formation mechanism inside a structure fabricated by PBF. Pore suppression by controlling the substrate temperature was also evaluated. Small- and large-sized pores with diameters of 10 μm and more than 50 μm, respectively, were found. Furthermore, differences in pore formation in the cross-section of the fabricated structure were observed owing to a variation in the volume-specific energy density and substrate temperature. At a substrate temperature of 25°C, the number of pores decreased more at the upper position than at the lower position owing to repeated melting and solidification under the laser-beam irradiation. At a substrate temperature of 200°C, the number of pores decreased significantly more than at 25°C. Furthermore, as the substrate temperature increased, the wettability of the molten metal improved, resulting in smaller contact angles of the fabricated structure in the single-line track. In PBF, multiple lines are fabricated in each layer. At low substrate temperatures, interstices were formed between the lines owing to the low wettability of the molten metal. These interstices acted as the origins of pores when the next layer was fabricated. Heating the substrate made the surface of the structure smooth owing to the high wettability of the molten metal and a reduction in the number of pores. Therefore, the formation of large pores could be reduced by controlling the substrate temperature.

Laser Polishing of Additive Manufactured 316L Stainless Steel Synthesized by Selective Laser Melting

One of the established limitations of metal additive manufacturing (AM) methods, such as selective laser melting (SLM), is the resulting rough surface finish. Laser polishing is one method that can be used to achieve an improved surface finish on AM printed parts. This study is focused on the laser surface polishing of AM parts using CO2 laser beam irradiation. Despite the fact that several researchers have investigated the traditional abrasive polishing method, there is still a lack of information reporting on the laser surface polishing of metal parts. In this study, AM 316L stainless steel cylindrical samples were polished using CO2 laser beam irradiation in continuous wave (CW) working mode. Two design of experiment models were developed for the optimization of the input processing parameters by statistical analysis of their effect on the resulting roughness. The processing parameters investigated were the laser beam power, the rotational speed of the sample, the number of laser scan passes, the laser beam focal position, and the percentage overlap of the laser tracks between consecutive passes. The characterization of the measured roughness and the modified layer microstructure was carried out using 3D optical and scanning electron microscopy (SEM). A maximum reduction of the roughness from 10.4 to 2.7 µm was achieved and no significant change in the microstructure phase type and micro-hardness was observed.