Enhanced osteogenic activity of Ti alloy implants by modulating strontium configuration in their surface oxide layers

Abstract

To guarantee the long-term stability of an orthopaedic implant, non-degradable surface coatings with the ability to selectively release bioactive drugs or ions are especially desirable. In this study, SrO-TiO₂ composite coatings were deposited on the surface of Ti alloys, whose release behavior of bioactive Sr ions was modulated by the Sr configurations, either interstitial atoms in solid solution (Ti _y Sr_2-2y O₂) or strontium titanate (SrTiO₃). A perfect linear relationship between the amount of the released Sr ions and the Sr content in the coating was observed. Among the SrO-doped TiO₂ coatings, the 20% SrO-TiO₂ coating where Sr existed in both forms of Ti _y Sr_2-2y O₂ and SrTiO₃ not only promoted proliferation of bone cells but also enhanced their osteogenic differentiation, which was proved to be related to its Sr release behavior. However, overdosing with 30% SrO only resulted in one single Sr configuration (SrTiO₃) and an inferior osteogenic function. This study suggests that Sr configurations of both interstitial atoms of the solid solution and SrTiO₃ can realize the selective release of Sr, but they possibly have different effects on the biological functions and other properties including corrosion resistance.

Fig. 1. XRD patterns of the TiO₂ and SrO–TiO₂ coatings.

Fig. 2. SEM images of the TiO₂ (A), 10% SrO–TiO₂ (B), 20% SrO–TiO₂ (C) and 30% SrO–TiO₂ (D) coatings.

Fig. 3. Potentiodynamic polarization curves of TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ coatings.

Fig. 4. The surface morphology of the TiO₂ (A), 10% SrO–TiO₂ (B), 20% SrO–TiO₂ (C), 30% SrO–TiO₂ (D) coatings after immersion in 2× SBF for 14 days.

Fig. 5. XRD patterns of the TiO₂ coating and the SrO–TiO₂ coatings soaked in 2× SBF for 14 days.

Fig. 6. Concentrations of Sr ions released from TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ coatings after immersion in culture medium for 3, 6 and 9 days.

Fig. 7. Observation of cell initial attachment on the coatings, fluorescence microscope images (A) and SEM (B) of cells cultured on Ti6Al4V, TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ for 24 h. Scale bar, 20 μm.

Fig. 8. The proliferation and ALP activity and of rBMSCs cultured on the Ti6Al4V, TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ coatings at day 14.

Fig. 9. Quantitative PCR analysis of the cells cultured on TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ coatings for 7 and 14 days, house-keeping gene GAPDH was used as an internal control. *Statistically significant difference among different samples (p < 0.05).

Fig. 10. SEM and EDS of crystals precipitated on its surface after incubation with cells for 24 h (A), the ratio of anatase and rutile phase in the composite coatings (B), quantitative PCR analysis of the cells cultured on TiO₂, 10% SrO–TiO₂, 20% SrO–TiO₂, 30% SrO–TiO₂ coatings dissolution products (C).