Bioactive glasses (BAG), first melt-derived in the late 1960s by Larry Hench, obtained good clinical results in dentistry, due to their properties of good bioactivity, when used to treat bone defects [1]. The composition of this bioactive glass was 45 wt% SiO 2 , 24.5 wt% Na 2 O, 24.5 wt% CaO, and 6 wt% P 2 O 5 , which was later termed as 45S5 or Bioglass®. Recently, various researchers have incorporated BAG into experimental[2-5] and commercial dental resin composite materials [6]. The release of calcium and phosphate ions was used as a means to assist with prevention of demineralization of dentine from an initial caries attack. Furthermore, BAG-containing resin compos-ites can reduce bacterial penetration into marginal gaps due to their ability to increase local pH, precipitate apatite on the surface, or in this case within the gap [7]. In addition, a novel design of resin composite-based implant containing bioactive glass has successfully been used for many years [8,9]. The fiber-reinforced composite implants loaded with bioactive glass were supported to enhance biological bone repair and the formation of vascularized structures, in addition to providing improved antimicrobial properties for implants. Chemically speaking, this type of "bioactive" action is a mineralization reaction. At the beginning, a silica-rich layer with Si-OH groups forms on the surface by the exchange of Na + and Ca 2+ ions from the glass with surrounding H + ions, which increases surrounding pH. Then, Ca 2+ and PO 3 4− from surrounding solution forms amorphous calcium phosphate (ACP, Ca x (PO 4 ) y •nH 2 O)[10] on the surface, which is transformed into octacalcium phosphate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O) [10] and finally evolves into nanocrystalline carbonated hydroxyapatite (CHA, Ca 10−x (PO 4 ) 6−x (CO 3 ) x (OH) 2−x−2y (CO 3 ) y ) not hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) in human body as bone or tooth enamel [11,12].
Hydroxyapatite (HA)-coated TiO2 nanotubes (TNTs) have been reported to enhance osteogenesis. However, the nanoscale topography of TNTs usually vanishes due to the uncontrollable mineralization on the surface. In this study, TNTs with different diameters(small, 25 nm; medium, 55 nm; and large, 85 nm) were fabricated by anodization in 3 different voltages. Enzyme-directed biomineralization was adopted to deposit calcium phosphate on the above TNTs. The surface structures and properties of the coatings were characterized by scanning electron microscopy, dispersive X-ray spectrometry, X-ray diffraction, and Fourier-transform infrared spectroscopy. The osteogenesis effect of the hybrid TNT/HA and the original TNTs were evaluated. The results showed that hydroxyapatite deposited homogeneously along the TiO2 nanotubes while preserving the intrinsic nanotopography. Mechanically, alkaline phosphatase(ALP) played a critical role in the mineralization and large nanotube size is more favorable for the mineralizing process because of more ALP absorption. Besides, the hybrid nanosurface TNT/HA coating was found to improve the adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 cells compared to pure TNTs. Our study suggests that the hybrid TNT/HA coating constructed by enzyme-directed biomineralization on TiO2 nanotubes is a promising modification strategy for titanium implants.
Numerous studies have shown that there is an amorphous calcium phosphate (ACP) phase preceding the precipitation of crystalline hydroxyapatite (HA) in calcium phosphate solutions. It has also been shown that the addition of magnesium to the solutions has a stabilizing effect by inhibiting the transformation of ACP to HA. The stabilizing effect of Mg2+ is attributed to the stronger bonds between water molecules and the magnesium ions adsorbed on the surface of the ACP particles, making it harder for them to dehydrate. However, the kinetics of the reactions between calcium and phosphate ions to form ACP and then HA crystals, and the effects of varying concentrations of Mg on the kinetics have not been studied theoretically in detail. In this study, we develop and validate a kinetic model for analyzing such reactions. The pertinent rate constants are derived by calibrating the model against temporal changes in Ca2+ concentration reported by others. The predicted onset and growth of HA crystallization for solutions with different Mg concentrations are consistent with those measured. As it is capable of predicting the production of ACP and the subsequent transformation to HA under different assumed conditions, the kinetic model developed can help further our understanding of the mechanism of mineralization of calcium phosphate solutions.
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