| Apatite biomaterials, especially hydroxyapatite (Ca10(PO4)6(OH)2, HA) andβ-tricalcium phosphate (β-Ca3(PO4)2,β-TCP) have biocompatiblity and bioactivity in human body. However, due to their brittleness and lack of strength, they are not suitable to be directly used in load-bearing applications. On the other hand, metallic biomaterials can fulfill the requirements of load-bearing because of their good mechanical properties. However, metallic biomaterials do not develop a chemical bond with bone, and wear and corrosion of the metallic implant overtime can lead to the release of metallic ions, causing different pathologies that could finally end in the removal of the implant.When coating apatite on metallic substrate, the perfect cooperation between the superior mechanical properties of the metal (or alloy) and the unique biocompatibility of apatite coating makes it an ideal choice for most biomedical applications.However, there are still many concerns about the practical application of apatite coatings. Research topics related to these coatings include their high dissolution rate, bioactivity and adhesion strength, which affect the long-term stability.Recent studies have shown that bioactivity and physical and chemical properties of apatite can be improved through the substitution of Ca2+ in apatite with cations in natural bone such as K+, Na+, Zn2+, Mn2+ and Mg2+ and their roles in bone metabolism, promoting catalytic reaction and controlling biological functions.Magnesium is one of the most important bivalent ions in biological bone. It is always associated with the mineralization of calcified tissues, mainly in bones and teeth. Magnesium indirectly influences mineral metabolism or even controls the crystallization processes (crystal size, lattice parameters) of mineral substances as well as the pattern of mineral formation. Therefor, Mg-containing apatite attracts much attention in recent years. This project focuses on synthesizing magnesium apatite coatings on Ti6A14V substrate by sol-gel dip-coating method. Influence of magnesium on mechanical properties and biological properties are studied in detail. The properties of coatings are optimized to achieve high bioactivity and reduced solubility, acceptable morphology and excellent adhesion strength. Major conclusions are achieved through the study as follows: 1. A series of magnesium apatite coatings with different magnesium content of 0= x= 2 in (Ca10-xMgx)(PO4)6(OH)2 is synthesized on Ti6A14V substrate by a sol-gel dip-coating method and sintering at 600℃. The coatings are dense, uniform, about 2μm in thickness, no cracks and particles appear in the coatings.2. The synthesization of magnesium into apatite structure has significant influences on the crystal phase. When the content of magnesium in the coating is x=1, both HA andβ-TCP phase coexist in the coating. While x>1, onlyβ-TCP is observed. Replace of Ca ion in HA by Mg ion can cause crystal mismatch. This crystal mismatch inhibits the forming of HA from amorphous apatite but enhances the formingβ-TCP. Mg also changes the elements composition in the coating. When x=1, Mg concentration in the coatings almost match the designed concentration in the sol. As x>1, Mg in the coating becomes more than that designed in the sols due to part of the magnesium in the coating aggravating as MgO.3. The synthesization of magnesium into apatite coating has influences on the mechanical properties. Magnesium in the coating decreases the hardness and Young's modulus of the coating because it enhances the forming ofβ-TCP. Therefor, it is necessary for the coating to combine with metal substrate in load bearing circumstance. The adhesion strength can be improved by the synthesization of magnesium to the apatite coating. The superior adhesion strength is attributed to form more Mg-Ti-O chemical bonds in the transitional region due to the faster diffusion ability of Mg ions than Ca ions.4. Magnesium in the coating has significant influence on the dissolution behaviors in physiological saline solution. When x= 1, Mg in the coatings decreases the dissolution rate of the coating. While x>1, Mg in the coatings can increase the dissolution rates. The dissolution of magnesium apatite coatings can provide Mg2+ to the surrounding solutions. Due to the key role of magnesium in the cell growth and metabolism process, Mg in the coatings will benefit for the implantation in Mg-deficient circumstance.5. Magnesium apatite coatings with different magnesium content show different bioactivities in simulated body fluid (SBF). As x=1, magnesium in the coating increases the nucleation rate of apatite in SBF on the coating surface, which in favor of deposition of new apatite on the coating surface from SBF. While x>1, magnesium increases the dissolution rate of the deposited layer, which inhibits the further deposition of new apatite on the coating surface from SBF. Glucose in SBF (1g/L) has negligible influence on the deposition of new apatite on the coating surface. It co-deposits with apatite to form new apatite layer on primary apatite coatings. In contrast, the addition of BSA in SBF (1g/L) has significant influence on the deposition of new apatite. A protein containing apatite layer forms on the coating surface, which inhibits the further deposition of new apatite.6. All the magnesium apatite coatings (0=x=2) have positive effect on osteoblastic cell (MG63) attachment and proliferation on the coating surface. However, due to the enrichment of Mg2+ in the culture medium, no significant difference is observed between HA and magnesium apatite coatings.7. Mg, F, Zn ions in the coating have different influence on the physical-chemical properties of coating. F and Zn can be synthesized into the crystal structure of HA to stabilize the crystal structure. While Mg in the coating can destabilize the HA structure and promote the forming ofβ-TCP phase. The synthesization of F into magnesium apatite coating can inhibit the transformation of HA toβ-TCP and stabilize the HA structure.8. Mg, F, Zn in the coating has influence on the mechanical properties of coating. Cations such as Mg2+ and Zn2+ can replace Ca2+ in HA structure and decrease the hardness and Young's modulus of the coatings, while anions like F- replace OH- group in HA structure and increase the hardness and Young's modulus of the coating. All of the added elements can increase the the adhesion strength between coating and metallic substrate due to forming more chemical bonding in the transitional region. Among all elements, F shows the most significant influence on the increasing of adhesion strength.9. Based on the performance of magnesium apatite coating in physical-chemical, mechanical and bioresponse, it is recommended that the optimum magnesium content in the coating should be 0.5=x=1. The magnesium apatite coating with the optimum magnesium content has the potential for the future medical applications.
|
PDF Full Text Request