Role of annealing-induced residual stress in the osteogenic performance of TiO2 nanotube arrays

Titanium dioxide nanotubes (TNTs) layers are widely used to improve the biological performance of titanium implants. However, the influence of annealing on TNTs properties and their regulation of cellular behavior remains controversial. In this study, TNTs arrays were fabricated via anodization and annealed at 200, 300, 450 and 600 °C. Physicochemical characterizations show that increasing annealing temperature decreases the thickness of the nanotubes, thickens tube walls and drives phase transformation from amorphous to anatase/rutile, accompanied by increased hardness and Young's modulus. The retained compressive residual stress shows a non-monotonic variation, with the highest value observed at 200 °C and the lowest at 450 °C. Correspondingly, the 200 °C-annealed surface exhibits the greatest enhancement in proliferation and osteogenic differentiation of MC3T3-E1 cells, whereas 450 °C-annealed surface shows the weakest. Further analyses demonstrated increased insulin-like growth factor-1 (IGF-1) secretion together with enhanced activation of ERK1/2 and p38 MAPK signaling pathways on the 200 °C-annealed surface, and supplementation with recombinant IGF-1 largely restored the osteogenic capacity of the less osteoinductive groups. Overall, this study indicates that retained compressive residual stress is closely associated with the osteogenic performance of annealed TNTs and may serve as a useful engineering descriptor of the annealing-induced surface state.