Wollastonite-derived 316L-CaTiSiO5 core-shell scaffolds for bone substitutes: mechanical properties and in-vitro study

In this work, co-extrusion Direct Ink Writing 3D printing technology was used to produce 316L-sphene (CaTiSiO 5 ) core-shell scaffolds, with the main objective of developing a new generation of bone substitutes capable of mimicking the mechanical behavior of natural bone under compressive load. The presence of a ductile core in the struts of bioceramics scaffolds increases the strain energy density, preventing brittle fracture and enabling a more graceful failure. The proposed CO₂-free synthesis route for sphene, without decomposition byproducts, preserves the intrinsic ductility of the stainless-steel core while maintaining a level of porosity suitable for promoting the bioactive properties of the bioceramic shell. Compression tests on the 3D-printed bio-scaffolds demonstrated that the incorporation of a ductile metallic core alters the fracture behavior under compressive loading, resulting in a fourfold increase in strain energy density compared with fully ceramic scaffolds, while preserving a high porosity (~70%). The in-vitro studies confirmed the bioactive behavior of both sphene and 316L–sphene scaffolds, as evidenced by the formation of calcium phosphate phases and apatite-like precipitates after immersion in SBF. ICP-MS analyses revealed very low metal ion release (Fe < 0.3 ppm; Ti < 20 ppb; Cr < 10 ppb; Ni < 5 ppb; Mn < 100 ppb). Cytotoxicity tests showed cell viability consistently above the 70% threshold defined by ISO 10993-5, confirming the absence of cytotoxic effects. These results represent a first step toward the development of a new generation of composite bio-scaffolds, in which the combination of materials with both structural and functional properties may enable the fabrication of patient-specific implants. Further studies will require the optimization of the core-shell interface strength, the study of their mechanical behavior under dynamic loading conditions, and the assessment of the effect of scaffold architecture on their bioactive response.