A comparative investigation on the mechanical properties and cytotoxicity of Cubic, Octet, and TPMS gyroid structures fabricated by selective laser melting of stainless steel 316L.

Metallic lattice structures can be fabricated by selective laser melting (SLM) with purposefully designed pores and controlled pore sizes that can bio mimic the natural bone, providing adequate mechanical and biological support for the patients. Strut-based structures, like Cubic, Octet; and sheet-based structures, like triply periodic minimal surface (TPMS) gyroid, have been studied extensively in the past. However, it lacks enough comparative study on the mechanical properties and cytotoxicity among these structures. Therefore, Cubic, Octet, and TPMS gyroid of Stainless steel 316 L (SS316L) are designed, manufactured, and characterized at 40/50/60% relative densities in this study. Moreover, the flowability, density characteristics, and cytotoxicity of SS316L powder are validated to ascertain its suitability for 3D printing and implant application. Based on refining the Gibson-Ashby model, it is possible to predict or design the mechanical properties via adjusting the relative densities. The results indicate these structures demonstrated appropriate Young's modulus and outstanding biocompatibility.

A biomechanical evaluation on Cubic, Octet, and TPMS gyroid Ti6Al4V lattice structures fabricated by selective laser melting and the effects of their debris on human osteoblast-like cells.

Lattice structures are widely used in orthopedic implants due to their unique features, such as high strength-to-weight ratios and adjustable biomechanical properties. Based on the type of unit cell geometry, lattice structures may be classified into two types: strut-based structures and sheet-based structures. In this study, strut-based structures (Cubic & Octet) and sheet-based structure (triply periodic minimal surface (TPMS) gyroid) were investigated. The biomechanical properties of the three different Ti6Al4V lattice structures fabricated by selective laser melting (SLM) were investigated using room temperature compression testing. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to check the 3D printing quality with regards to defects and quantitative compositional information of 3D printed parts. Experimental results indicated that TPMS gyroid has superior biomechanical properties when compared to Cubic and Octet. Also, TPMS gyroid was found to be less affected by the variations in relative density. The biocompatibility of Ti6Al4V lattice structures was validated through the cytotoxicity test with human osteoblast-like SAOS2 cells. The debris generated during the degradation process in the form of particles and ions is among the primary causes of implant failure over time. In this study, Ti6Al4V particles with spherical and irregular shapes having average particle sizes of 36.5 µm and 28.8 µm, respectively, were used to mimic the actual Ti6Al4V particles to understand their harmful effects better. Also, the effects and amount of Ti6Al4V ions released after immersion within the cell culture media were investigated using the indirect cytotoxicity test and ion release test.

A study of Titanium and Magnesium particle-induced oxidative stress and toxicity to human osteoblasts.

Hybrid implants combine both Titanium (Ti) and Magnesium (Mg) are prevalent nowadays. The long-term implications of Ti and Mg implants within the human body are not yet fully understood. Many implant failure cases due to inflammation, allergic responses, and aspect loosening have been reported frequently. Particles generated through daily wear and tear of implants may worsen the situation by causing acute complications. An in-depth understanding of the behavior of metal particles with human osteoblasts is necessary. In this study, a novel and systematic attempt was made to understand the effects of different concentrations of Ti and Mg particles to the osteoblastic SAOS2 cell: toxicity, alterations to mitochondria, and changes to the specific gene and protein expression. Ti particles were found toxic to SAOS2 cells at different dosages, while Mg particles at lower concentrations could improve cell viability. To understand this phenomenon better, we have measured cellular reactive oxygen species (ROS) production and cell apoptosis & necrosis percentage. We also have checked the mitochondrial structure with transmission electron microscope (TEM), and mitochondrial function using Tetramethyl rhodamine, ethyl ester staining (TMRE). NDUFB6, SDHC, and ATP5F1 were the essential mitochondrial genes involved in the ROS production and ATP production. Immunocytochemistry (ICC) and real-time polymerase chain reaction (qPCR) were implemented to check the regulations of these related genes.

Binder Jetting Additive Manufacturing of High Porosity 316L Stainless Steel Metal Foams.

High porosity (40% to 60%) 316L stainless steel containing well-interconnected open-cell porous structures with pore openness index of 0.87 to 1 were successfully fabricated by binder jetting and subsequent sintering processes coupled with a powder space holder technique. Mono-sized (30 µm) and 30% (by volume) spherically shaped poly(methyl methacrylate) (PMMA) powder was used as the space holder material. The effects of processing conditions such as: (1) binder saturation rates (55%, 100% and 150%), and (2) isothermal sintering temperatures (1000 ○C to 1200 ○C) on the porosity of 316L stainless steel parts were studied. By varying the processing conditions, porosity of 40% to 45% were achieved. To further increase the porosity values of 316L stainless steel parts, 30 vol. % (or 6 wt. %) of PMMA space holder particles were added to the 3D printing feedstock and porosity values of 57% to 61% were achieved. Mercury porosimetry results indicated pore sizes less than 40 µm for all the binder jetting processed 316L stainless steel parts. Anisotropy in linear shrinkage after the sintering process was observed for the SS316L parts with the largest linear shrinkage in the Z direction. The Young's modulus and compression properties of 316L stainless steel parts decreased with increasing porosity and low Young's modulus values in the range of 2 GPa to 29 GPa were able to be achieved. The parts fabricated by using pure 316L stainless steel feedstock sintered at 1200 ○C with porosity of ~40% exhibited the maximum overall compressive properties with 0.2% compressive yield strength of 52.7 MPa, ultimate compressive strength of 520 MPa, fracture strain of 36.4%, and energy absorption of 116.7 MJ/m3, respectively. The Young's modulus and compression properties of the binder jetting processed 316L stainless steel parts were found to be on par with that of the conventionally processed porous 316L stainless steel parts and even surpassed those having similar porosities, and matched to that of the cancellous bone types.

Fabrication of Ti + Mg composites by three-dimensional printing of porous Ti and subsequent pressureless infiltration of biodegradable Mg.

A semi-degradable Ti + Mg composite with superior compression and cytotoxicity properties have been successfully fabricated using ink jet 3D printing followed by capillary mediated pressureless infiltration technique targeting orthopaedic implant applications. The composite exhibited low modulus (~5.2 GPa) and high ultimate compressive strength (~418 MPa) properties matching that of the human cortical bone. Ti + Mg composites with stronger 3D interconnected open-porous Ti networks are possible to be fabricated via 3D printing. Corrosion rate of samples measured through immersion testing using 0.9%NaCl solution at 37 °C indicate almost negligible corrosion rate for porous Ti (~1.14 µm/year) and <1 mm/year for Ti + Mg composites for 5 days of immersion, respectively. The composite significantly increased the SAOS-2 osteoblastic bone cell proliferation rate when compared to the 3D printed porous Ti samples and the increase is attributed to the exogenous Mg2+ ions originating from the Ti + Mg samples. The cell viability results indicated absent to mild cytotoxicity. An attempt is made to discuss the key considerations for net-shape fabrication of Ti + Mg implants using ink jet 3D printing followed by infiltration approach.

Insight into cytotoxicity of Mg nanocomposites using MTT assay technique.

In the present study, Young's modulus measurements and indirect cytotoxicity test were performed on Mg (0.97, 1.98, 2.5) vol% TiO2, Mg (0.58, 0.97, 1.98) vol% TiC, and Mg (0.58, 0.97, 1.98) vol% TiN, synthesized using the Disintegrated Melt Deposition technique to determine the cytotoxicity of low volume nano-particulate reinforcement on magnesium. The results of the indirect MTT assay on Day 3 and Day 5 indicate that 2.5vol% TiO2, TiC and TiN has little effect on the cytotoxicity when added as low volume fraction reinforcement to magnesium. While (0.97, 1.98) vol% TiO2 negatively affected the cytotoxicity when added to Mg. The Young's modulus of the materials was found to remain close to that of cortical bone which would suggest that the stress shielding effect would be reduced as the increase in Young's modulus was mitigated by the low volume addition of reinforcements.

Reinforcing Low-Volume Fraction Nano-TiN Particulates to Monolithical, Pure Mg for Enhanced Tensile and Compressive Response.

Novel Mg (0.58, 0.97, 1.98 and 2.5) vol. % TiN nanocomposites containing titanium nitride (TiN) nanoparticulates of ~20 nm size are successfully synthesized by a disintegrated melt deposition technique followed by hot extrusion. Microstructural characterization of Mg-TiN nanocomposites indicate significant grain refinement with Mg 2.5 vol. % TiN exhibiting a minimum grain size of ~11 µm. X-ray diffraction studies of Mg-TiN nanocomposites indicate that addition of up to 1.98 vol. % TiN nanoparticulates aids in modifying the strong basal texture of pure Mg. An attempt is made to study the effects of the type of titanium (metal or ceramic), size, and volume fraction addition of nanoparticulates on the microstructural and mechanical properties of pure magnesium. Among the major strengthening mechanisms contributing to the strength of Mg-Ti-based nanocomposites, Hall-Petch strengthening was found to play a vital role. The synthesized Mg-TiN nanocomposites exhibited superior tensile and compression properties indicating significant improvement in the fracture strain values of pure magnesium under loading. Under tensile and compression loading the presence of titanium (metal or ductile phase) nanoparticulates were found to contribute more towards the strengthening, whereas ceramics of titanium (brittle phases) contribute more towards the ductility of pure magnesium.

Effects of Primary Processing Techniques and Significance of Hall-Petch Strengthening on the Mechanical Response of Magnesium Matrix Composites Containing TiO2 Nanoparticulates.

In the present study, Mg (1.98 and 2.5) vol % TiO2 nanocomposites are primarily synthesized utilizing solid-phase blend-press-sinter powder metallurgy (PM) technique and liquid-phase disintegrated melt deposition technique (DMD) followed by hot extrusion. Microstructural characterization of the synthesized Mg-TiO2 nanocomposites indicated significant grain refinement with DMD synthesized Mg nanocomposites exhibiting as high as ~47% for 2.5 vol % TiO2 NPs addition. X-ray diffraction studies indicated that texture randomization of pure Mg depends not only on the critical amount of TiO2 NPs added to the Mg matrix but also on the adopted synthesis methodology. Irrespective of the processing technique, theoretically predicted tensile yield strength of Mg-TiO2 nanocomposites was found to be primarily governed by Hall-Petch mechanism. Among the synthesized Mg materials, solid-phase synthesized Mg 1.98 vol % TiO2 nanocomposite exhibited a maximum tensile fracture strain of ~14.5%. Further, the liquid-phase synthesized Mg-TiO2 nanocomposites exhibited higher tensile and compression properties than those primarily processed by solid-phase synthesis. The tensile-compression asymmetry values of the synthesized Mg-TiO2 nanocomposite was found to be lower than that of pure Mg with solid-phase synthesized Mg 1.98 vol % TiO2 nanocomposite exhibiting as low as 1.06.