Introduction: Inorganic bone scaffolds based on calcium phosphates have mineral compositions similar to human hard tissue. However, the scaffolds with high porosity and connectivity prepared by the commonly-used method show a low mechanical strength and undesired biological properties. Bivalent cations such as Sr, Zn, Mg and Cu, which are essential trace elements required for bone and connective tissues, have specific osteogenic effects on bone formation and selective inhibitory effect by stimulating bone cells revival, and collagen synthesis in osteoblastic cells in vitro and in vivo[1],[2]. Calcium phosphate scaffolds prepared with these ions doped HA whiskers would expect to present a cancellous bone-like scaffold structure, showing a good mechanical and biological performance. Although metal ions doped HA powders have been successfully developed[3]-[6], no more literatures were found about the structural characteristic and their biological effects of metal ions doped HA whiskers, therefore, the preparation of ions doped HA whiskers and study on their constitution, microstructure and antibacterial properties would be valuable to find a reasonable doping moral proportion of these ions in HA crystals.
Materials and Methods: Metal ions doped HA whiskers were prepared by a hydrothermal homogeneous precipitation method[7]. The aqueous solutions containing 2-40 mol% Sr, Zn, Mg or Cu ions were prepared by dissolving appropriate amounts of their nitrate, NH4H2PO4 and amine additive in deionized water. After the initial pH value being adjusted to 3.0, the mixture was placed into a high-temperature reactor and hydrothermally treated at 170~190 °C for 10h. The precipitates obtained were then filtered and washed using deionized water, finally dried at 70-80℃ for 1-2h in a vacuum oven for use. The structure characteristics and morphology of the products were characterized using XRD, FTIR and SEM. In vitro biocompatibility was evaluated by cytotoxicity through MTT assay. The antibacterial activity of the whiskers was examined by bacterial counting method using Escherichia coli.
Result and Discussion: The bivalent cations (Mg2+, Zn2+, Sr2+ or Cu2+) doped HA whiskers had a length of 60 - 110mm and aspect ratio 30-80, depending on the ions species and their concentration. To obtain a single phase composition, the doping concentration should be < 15 mol% for Mg and Zn, and < 8% for Cu. Further increasing the doping proportion, the second phase would accompany with the whiskers. Whereas, for Sr doped HA, its doping proportion was up to 25-40 mol%. The morphology of HA whiskers was obviously affected by the species of doping ions and their content in the solution. Low doping ions concentration favored the precipitation of long whiskers with high aspect ratio.
The presence of bivalent cation in HA crystals not only affected their crystallization behavior, solubility and surface chemical properties, but also exerted a different effect on the antibacterial properties and cytotoxicity. Mg2+ and Sr2+ doping showed good mineralization ability in SBF solution and no obvious effects on the antibacterial and cytotoxicity were found. For Zn2+ and Cu2+ doped HA whiskers, both antibacterial properties and cytotoxicity were observed. The effects of these ions greatly depended on their doping concentration.
Conclusion: Sr, Zn, Mg or Cu doped HA whiskers were successfully synthesized in a moderate acid solution. Ca2+ was partially substituted by these ions, resulting in lattice distortion and lower crystallinity. To obtain uniform whiskers with single phase, doping proportion of Sr, Zn, Mg or Cu ion should be <25mol%, 10 mol%, 10mol% and 8 mol%, respectively. The presence of bivalent cation in HA crystals not only affected their crystallization behavior and solubility, but also exerted a different effect on the antibacterial properties and cytotoxicity.
This work was supported by National Nature Science Foundation of China (No.51372182)
References:
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