Construction of surface HA/TiO2 double bioactive coating on porous titanium and evaluation for its biological performance

摘要

Introduction: With superior mechanical property, corrosion resistance and good biocompatibility, Titanium and its alloys have been widely used for load-bearing bone defect repair. To reduce the elastic modulus and improve the bioactivity of them, porous scaffolds or coatings were often introduced to the metal implants, such as the titanium beads or fibres sintered to the surface of hip joints. However, the formed porous coatings lack of enough micropores, which has been confirmed to play a key role in the osteogenesis of osteoinductive biomaterials. Besides, the traditional plasma spray hydroxyapatite (HA) coating cannot be made in the inner macropores of a porous scaffold or coating. In our previous study, we developed a new method fabricating porous titanium with stack sintering of a centrifugally granulated microporous titanium spheres.In this work, a simple approach to fabricating HA/TiO2 bioactive coating was developed, based on the combination of chemical treatment and electrochemically deposited HA methods. The biological performance of this double bioactive coating was evaluated by in vitro cell experiments. Materials and Methods: Porous titanium scaffolds with uniform macropores and abundant micropores were fabricated according to our previous study. The porous titanium samples were in turn pretreated by petroleum ether, acetone, alcohol and deionized water to clear away the surface contamination. The samples were then subjected to acid and alkali treatments based on the previous study to form the surface TiO2 layer. After that, the treated samples were electrochemically treated to deposit the outer HA layer on the outside TiO2 layer. The effect of the electrochemical parameters on the morphology, thickness and chemical composition was investigated by SEM, XRD and TEM. To evaluate the bioactivity of this coating, MC3T3-E1 osteoblasts were cultured on the samples, and the proliferation, growth and differentiation of the cells were tested. Results and Discussion: After acid & alkali treatment, the microporous network formed on the surface of the porous titanium. The further pulse electrochemical treatment led to the uniform sediment occurred on the microporous network. However, under direct current deposition mode, the sediment only loosely distributed on the surface and could not form a uniform deposition layer. Under the condition of pulse current mode and 10 mA/cm2 of the high current density, the thickness of the sediment increased with the increase in the deposition cycles. At 90 cycles, the sediment covered the whole surface completely. The morphology of the sediment was affected by the high current density, sheet structure formed at 5 mA/cm2, and nano rod-like structure occurred at 10 mA/cm2, but no any sediment could be observed when the high current density increased to 20 mA/cm2. The mass transfer process influenced the homogeneity of the sediment evidently. Under stirring or ultrasonic condition, the fast mass transfer led to formation of a uniform deposition layer on the surface. On the contrary, the scattered sediments were observed under static condition. By SEM observation of the fraction surface of the treated samples, it could be found that there was a double layer structure on the substrate surface, as could be ascribed to the acid & alkali treatment and electrochemical deposition, respectively. The thicknesses of them were about 500 nm and 1 mm, respectively. The XRD analysis for the coating confirmed the existence of HA and TiO2 phases. The TEM analysis for the particles detached from the coating by ultrasonic vibration, the crystal structure of HA was further identified. The results of protein adsorption experiments showed that compared to the single TiO2 surface, the HATiO2 surface allowed more adsorption of serum proteins and better adhesion and growth of MC3T3-E1 osteoblasts. Besides, the HA/TiO2 surface could further enhance the activity of ALP of the osteoblasts. Conclusion: By acid & alkali and subsequent electrochemical deposition treatments, a HA/TiO2 double bioactive surface could form on porous titanium and endow the material with excellent bioactivity. The porous titanium with a HA/TiO2 double bioactive surface could be an ideal scaffold or coating biomaterial for load-bearing bone defect repair.