Immunological response triggered by metallic 3D printing powders

Powder bed fusion-based Additive Manufacturing (3D printing) methods, particularly Selective Laser Melting, are increasingly being employed for customized medical implants. However, partially-melted residual metal powder is difficult to fully remove from the as-printed materials, and the existence of loose powder is another potential defect in the as-printed implant lattice. Understanding what effect loose powder and partially remelted powders have on biocompatibility requires a detailed study of the immunological response associated with the presence of metal powder particles, and the present study is designed to address this critically important concern. Typical powders for implant materials, including 316 L stainless steel, Ti, and Ti-6Al-4 V, with a range of particle sizes (15–100 μm) have been investigated in this study. Macrophage cells were selected to determine the immune response to metallic powder exposure and the cell responses were evaluated using confocal laser scanning microscopy and enzyme-linked immunosorbent assay analysis. Real-time monitoring of the cell reaction to the metal powders, cell viability, and secretion behavior of the inflammatory cytokines was employed for the different metal powders with various particle sizes. The main findings from the study are: (a) low concentrations (e.g. 1 × 10⁴ particles/mL) of metal powders were not found to affect the cell viability, (b) in high particle concentrations (e.g. 1 × 10⁶ particles/mL), the particle size was observed to be a significant factor in determining the cell reaction, and (c) the 316 L powder triggerd macrophages to secret the most pro-inflammatory cytokines and this tends to induce higher inflammatory reaction than the Ti and Ti-6Al-4 V powders. The underlying reasons are discussed from the perspectives of powder surface chemistry and powder morphology, which were investigated by detailed X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy studies. A comparison between the metal powder for 3D printing and conventional wear debris has also been briefly addressed. The results answer some critical questions relating to additively manufactured medical materials and lay a good foundation for better and more systematic assessment of the biological response of metal implants containing residual powders for potential orthopedic applications.