Laser powder bed fusion of Tixcu alloys for biomedical applications

Abstract

Laser powder bed fusion (LPBF) technology is an additive manufacturing (AM) technique used to build objects layer-by-layer from three-dimensional (3D) computer-aided design (CAD) models. This type of technology provides almost unchallenged design freedom and enables the manufacture of patient-specific components while maintaining short turnaround times. Titanium (Ti) and its alloys (particularly Ti6Al4V) are the preferred materials for bioengineering implants in the orthopaedic and dental fields due to their outstanding specific strength, light weight, biocompatibility, and corrosion resistance. However, Ti is bioinert, which limits its biocompatibility. Modifying the surfaces of Ti implants by coating them with polymer nanofibres (NFs) may promote their bioactivity. Electrospinning is the preferred method for coating surfaces with ultrafine polymer fibres that mimic the extracellular matrix (ECM). Metals and polymers do not inherently possess antibacterial properties and may attract bacterial attachment due to their biocompatibility. Copper (Cu) has broad-spectrum antibacterial activity and, unlike most metal-based antibacterial agents, is an essential element for human health and exhibits low cytotoxicity at controlled concentrations. Thus, adding Cu to Ti-based implants and their polymer coatings may impart bactericidal properties to the biomaterials. The current project aimed to employ LPBF to manufacture Ti6Al4V and commercially pure Ti (cpTi) implant materials in situ alloyed with Cu, and to coat the Ti-based materials with electrospun polymeric NFs and NFs imbued with Cu particles. The mechanical properties of the alloys were then determined, and the functionality of the alloys and coated material was investigated for antibacterial effect and biocompatibility with human cells. Several process parameters control the characteristics of the final part fabricated using the LPBF procedure; hence, crucial parameters that contribute to obtaining samples with the highest density (namely, layer thickness, laser power, scanning speed and hatching distance were optimised by conducting the single track experiment. The experiment entailed producing single tracks at various scanning speeds and laser powers, manufacturing double layers at selected speeds and powers while varying the hatch distance, analysing the microstructures and porosity of three-dimensional objects produced at selected speeds, powers, and hatch distances, and finally selecting the optimum parameters. Optimised parameters were applied to manufacture samples from powders of Ti6Al4V-xCu, cpTi and Ti-xCu (x = 3, 6 wt.%). Previously determined parameters were used for the Ti6Al4V ELI powder.vVarious polymer combinations were explored to determine the most suitable polymer blends for coating Ti6Al4V ELI and cpTi using the electrospinning technique. Suitability was primarily based on adhesion strength at the metal-polymer interface and the potential antibacterial effect. The polymer blends selected were polycaprolactone-polyurethane (PCL/PU) and its copper oxide nanoparticle (CuO NPs)- infused counterpart (PCL/PU/CuO). The PCL/PU and PCL/PU/CuO NPs solutions were used to coat Ti6Al4V ELI and cpTi test specimens successfully by electrospinning. The coatings were characterised. Their adhesion strengths were determined using the direct pull-off adhesion test and were found to be comparable to those reported in the literature. Mechanical properties (that is, yield strength, ultimate tensile strength, modulus of elasticity and ductility quantifiers) of LPBF-manufactured Ti6Al4V-xCu, cpTi and Ti-xCu (x = 3, 6 wt.%) specimens were determined using the standard tensile test. The results showed that strength increased and ductility decreased with increasing Cu content. Fractography was conducted to study the fracture behaviour of the tensile-tested surfaces. The effect of Cu was not evident during fracture analysis, and the Ti6Al4V-Cu alloys showed lower ductility than the cpTi alloys with Cu. The Ti6Al4V-xCu and Ti-xCu (x = 3, 6 wt.%) and NF-coated Ti6Al4V ELI and cpTi samples were investigated for antibacterial effectiveness and their ability to integrate with human cells without cytotoxicity. The antimicrobial efficacy of the samples was assessed using the agar diffusion assay and plate count method. Using the plate count method, antibacterial activity was evident on NF-coated surfaces and was associated with Cu/CuO-containing samples. C2C12 and MDA-MB-231 cell lines were used to study biocompatibility. Cell toxicity was observed from Ti6Al4V ELI, Ti6Al4V-3Cu, Ti6Al4V-6Cu and Ti-6Cu toward cell lines and was attributed to the Cu content. The NF-coated specimens exhibited the least cytotoxicity and the most cell growth promotion. The investigations were instrumental in achieving highly dense Ti6Al4V ELI and cpTi LPBF parts and modifying them with Cu alloying and surface modification through coating with PCL/PU and PCL/PU/CuO NFs using electrospinning. The addition of Cu did not adversely affect the renowned mechanical properties of Ti-based material. Moreover, Cu/CuO particles exhibited antibacterial properties. The NFs enhanced the antibacterial properties and biocompatibility of Ti surfaces with human cells.