| 其他摘要 | Magnesium alloys have shown potential applications as bone implants due to their good biocompatibility, biodegradability and close modulus to nature bones. In-vitro and in-vivo degradation properties of two new magnesium alloys, Mg-Mn and Mg-Mn-Zn, and bone response to the alloys were investigated in this dissertation. A phosphate conversion coating system was developed and optimized to improve the corrosion resistance and the surface bioactivity of the extruded Mg-Mn-Zn alloy. Meanwhile, formation mechanism of the phosphate conversion coating was discussed. Finally, the bioactivity of the phosphated Mg-Mn-Zn alloy was evaluated both in-vitro and in-vivo. The main results are summarized as followings:
A long passivation stage and a noble breakdown potential were observed in the polarization tests of Mg-Mn and Mg-Mn-Zn alloys, indicating that a passive layer was rapidly formed on the surfaces of the magnesium alloys after immersion in a phosphate buffered SBF solution. Immersion test showed that the corrosion rate was different between Mg-Mn and Mg-Mn-Zn alloys, but these two alloys demonstrated a similar corrosion tendency in the solution. Magnesium alloys were corroded rapidly at the initial stage of immersion, and then were covered by a Mg-, Na- and Ca-containing phosphate reaction layer after 24h immersion. This phosphate layer protected the magnesium alloys from fast corrosion in the further immersion. Then, the magnesium alloy kept a stable corrosion rate with immersion time.
The in vivo tests indicated that a degradation layer consisted of Mg-containing calcium phosphate was rapidly formed on the surface of the magnesium alloy implants after implantation. Mg, Mn and Zn released from the corrosion of magnesium implants were absorbed by the surrounding bone tissue. The blood normal examination and blood biochemical examination up to 26 weeks post-implantation showed that in vivo degradation of the magnesium alloy implants did not cause significant change in blood composition, disorders in functions of liver and kidney, and increase in inorganic ions concentrations. Based on the histological analysis, the bone response to the magnesium alloy implants can be summarized as following: the formation of a blood blot and hematoma around the implant; the formation of a crystalline calcium magnesium phosphate layer on the surface of the implant which can protect the implant from fast corrosion/degradation and improved the surface osteoconductivity of the implant; the attachment and adhesion of osteoblast to the phosphate layer and then proliferation, differentiation and calcification; and finally, the formation of new bone on the surface of the implant.
A calcium phosphate coating on the surface of Mg-Mn-Zn alloy was optimized by an orthogonal regression method to improve the surface bioactivity of the magnesium alloy. The optimized coating parameters are: Ca/Zn>1.50, pH>3.5, T>44℃, t>5min. The phosphate conversion coating was mainly composed of dicalcium phosphate-dehydrate (DCPD). It was confirmed that the corrosion resistance of Mg-Mn-Zn alloy was improved significantly by the phosphate conversion coating. In addition, the formation mechanism of phosphate conversion coating was discussed. It was believed that the formation process of the phosphate coating can be divided into four stages: the initial corrosion stage of Mg-Mn-Zn alloy, the formation stage of a passive film, the formation stage of DCPD layer and the stable growth stage of the DCPD layer. At the initial corrosion stage, the Mg alloy was corroded rapidly and the open circuit potential (OCP) dropped fast down to the lowest level. Then a passive film containing Mg, Ca, O and P was formed on the surface of the Mg alloy at the formation stage of passive film, and the OCP increased linearly with an increase of immersion time. At the formation stage of DCPD layer, the OCP increased slowly with increase of the immersion time and DCPD crystal nucleus began to deposite, grow up and cover the surface of the passive film. At the stable growth stage of the DCPD layer, the formation and the dissolution of the DCPD layer happened simultaneously. The formation process of the DCPD layer did not finish until the formation and the dissolution of the layer reached a dynamic balance, and the OCP did not change with time anymore.
The noble Ecorr and the high Rp obtained from the electrochemical test of the phosphated samples showed that the phosphating treatment greatly improved the corrosion resistance of the Mg alloy. X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) results identified that the DCPD layer was transformed into hydroxyapatite (HA) after immersion in SBF solution.
The phosphated Mg alloy exhibited lower cell toxicity than the un-treated alloy. The in vitro cell tests also showed that the cells L-929 exhibited much good adherence and very high growth rate and proliferation characteristic（p<0.05）on the surface of the phosphated Mg alloy, demonstrating that the phosphating treatment can significantly improve the cell biocompatibility and surface bioactivity of the magnesium alloy.
No lymphocytic infiltration was observed by pathological examination in the in vivo studies, indicating that the phosphated Mg alloy exhibited better in vivo compatibility than the un-treated Mg alloy. The immunohistochemical analysis demonstrated that the phosphated Mg alloy exhibited significantly good osteoconductivity and osteogenesis at the early first 4 weeks post-operation (p<0.05), which would benefit the early healing process of bone tissue. |
修改评论