Preparation And Formation Mechanism Of Mesoporous Apatite Microspheres And Coatings

Apatite is a major inorganic component of the hard tissues of human being, and the corresponding synthetic apatite particles and coatings have been used respectively as bone cavity filling materials and artificial bone graft substitutes because of their biocompatibility, bioactivity, osteoconductivity, nontoxicity, and noninflammatory. Recently, the studies have shown that the bone-forming bioactivity of biomaterials is associated not only with their chemical composition, but also with their microstructures, such as pore size, pore volume and pore structure. Mesoporous structure of biomaterials can promote cell adhesion, adsorption of biologic metabolites, and resorbability at controlled rates to match that of tissue repair. In this work, we proposed chemical immersion method, emulsion-like method, and electrophoretic depositon-chemical immersion method, which were used to prepare mesoporous apatite microspheres and coatings. The morphologies, phases, mesoporous structure and formation mechanism of meosporous apatite microspheres and coatings were studied by means of XRD, FTIR, SEM, TEM, XPS, BET, and TG-DSCThermodynamic calculation has shown that nacre powders and calcium carbonate powders can be converted to calcium phosphate phases at low temperatures after soaking in phosphate buffer solutions (PBS). Although apatite crystals are stabler thermodynamically than other calcium phosphate phases, the conversion products are determined by the pH values of PBS. If the pH value of PBS is kept at 6.0 or 6.4, nacre powders or calcium carbonate powders are converted mainly to octacalcium phosphate (OCP) or dicalcium phosphate dehydrate (DCPD). If the pH value of PBS is kept at 7.4 or 8.0, the main products are apatite. The formation mechanism of apatite is dissolution-precipitaion reaction. After soaking nacre powders and calcium carbonate powders in PBS, calcium ions are dissolved firstly from the smaller particles, react with PO43? ions to form apatite crystals, and precipitate them on the large particle surfaces. Decreasing particle size, prolonging reaction time, and increasing the concentrations of PBS can improve the conversion percentages of apatite.Calcium carbonate microspheres were prepared by mixing Na2CO3 solution and CaCl₂ solution with nacre organic materials. Both chemical immersion method and emulsion-like method were used to convert cacium carbonate microspheres to mesoporous apatite microspheres with low crystalinity. The PO₄^3- ions in apatite lattice are substituted partially by CO₃^2- and HPO42- ions. The mesoporous apatite microspheres obtained by chemical immersion method have irregular shape, and the nitrogen adsorption-desorption isotherms are identified as type IV isotherms with type H₃ hysteresis loops. The mesoporous structure is unimodal with the pore size of 3.9 nm. However, the mesoporous apatite microspheres converted from calcium carbonate microspheres in a cetyltrimethylammonium bromide (CTAB)/Na2HPO4 solution/cyclohexane/n- butanol emulsion system are monodispersed with the particle size of ⁵μm. The mesoprous structure is bimodal with the pore size of 3.9 nm and 9.0 nm. With increasing the reaction time and improving the temperature, the bigger mesopores begin to disappear. The formation mechanism of mesopores with the pore size of 3.9 nm is attributed to the aggregation of nanoparticles, and that of 9.0 nm is attributed to the CTAB micelles served as templates.Mesoporous apatite coatings were fabricated by electrophoretic depositon- chemical immersion method. This method consists of a two-stage application route: the deposition of nacre powders or CaCO3 powders on Ti6Al4V substrates by electrophoresis, and the conversion of nacre coatings or CaCO3 coatings to apatite coating by treatment with PBS. After soaking nacre coatings or CaCO3 coatings in PBS for 1 day, plate-like apatite coatings with mesoporous structure are formed. The pore sizes are distributed around 3.9 nm. After soaking for 9 days, the plate-like structure is turned into a sponge-like structure, and the mesopores partially disappear. A TiO_x layer and PO₄^3- ions appear on the Ti6Al4V substrate surfaces by pretreatment with a H₃PO4/HF solution. The TiO_x and PO₄^3- ions can induce the formation of apatite crystals, resulting in a composition gradient in the TiO_x layer. Simulated body fluid (SBF) immersion tests reveal that the calcium deficiencies in apatite lattice, the mesoporous structure, and nacre organic materials can improve the in vitro apatite forming ability of the mesoporous apatite coatings.Magnetic mesoporous apatite coatings were fabricated by electrophoretic deposition of CaCO₃/Fe₃O₄ particles on Ti6Al4V substrates followed by treatment with PBS at 37°C. After soaking CaCO₃/Fe₃O₄ coatings in PBS, apatite nucleates heterogeneously on the surfaces of CaCO₃/Fe₃O₄ particles and forms a plate-like structure. Fe₃O₄ increases the velocity of nucleus formation of apatite. After soaking for 1 day, the percentage of unreacted calcium carbonate is 9.1%, lower than the 41.0% for apatite coatings without magnetism. The pore size of mesopores is distributed around 3.9 nm, and the mesopores do not disappear after treatment with PBS for 9 days. SBF immersion tests reveal that Fe₃O₄ improves the in vitro apatite forming ability of biocoatings.