Surface Severe Plastic Deformation of an Orthopedic Ti-Nb-Sn Alloy Induces Unusual Precipitate Remodeling and Supports Stem Cell Osteogenesis through Akt Signaling

This work presents a strategy to augment the bioactivity of a new-generation metastable beta-Ti-Nb-Sn alloy through surface severe plastic deformation. Foremost, the alloy was strengthened by precipitation of alpha phase using a well-designed thermo-mechanical processing route. Subsequently, the surface of the aged alloy was subjected to severe plastic deformation via surface mechanical attrition treatment (SMAT). Upon SMAT, a unique phenomenon of strain induced precipitate coarsening was observed. A possible mechanism is proposed wherein the precipitates first dissolve due to significant slip transfer across the alpha/beta-interface followed by reprecipitation along the other precipitates thereby leading to coarsening. Coarsening of the precipitates abrogated the strengthening caused by plastic deformation as a result of which the hardness did not increase significantly after SMAT in sharp contrast to other alloys. SMAT led to a decrease in the attachment of human mesenchymal stem cells because of an increase in the roughness-mediated surface hydrophobicity. On the other hand, an increase in the roughness led to the formation of more number of focal adhesions. This in turn enhanced the proliferation rate and more importantly, osteogenic differentiation of stem cells. Detailed investigation into the underlying mechanism revealed that an increase in focal adhesions activated the Akt-mediated mechano-transduction signaling pathway that enhanced the osteogenic differentiation. In summary, the potential of surface severe plastic deformation to impart bioactivity to the next-generation of orthopedic beta-Ti alloys is underscored in this work.