Biomimetic Precipitation

From a thermodynamical point of view, only two calcium phosphate phases are stable at ambient temperature and pressure in SBF, monetite (DCPA, dicalcium phosphate anhydrate, CaHP04) (Nancollas, 1977 Jinawath et al., 2001) 

0 Dicalcium phosphate dihydrate (brushite) CaHP04-2H20 6.59 

these calcium phosphate phases are members of the same family and their expression in aqueous precipitates is strictly governed by extraneous factors such as supersaturation, temperature and pH level. It is well known that OCP is a transitional state during the formation of HAp in vitro (Brown and Chow, 1986) and that its nucleation rate is substantially higher than that of HAp (Lu and Leng, 2005 Ito et al., 2014). In vivo proliferation and differentiation studies on murine bone marrow stromal ST-2 and primary calvarial osteoblastic cells support the conclusion that OCP promotes appositional bone formation more significantly than hydrolysed Ca-deficient hydroxyapatite, at least in its early stages up to 12 weeks (Suzuki et al., 2006). 

The complex chemical pathway of the individual reactions is thought to be as 

The main idea of this sequence of chemical events was confirmed in principle by Webster et al. (2003) who showed that osteoblasts preferentially adhered to nanometre-rough calcium titanate formed as a consequence of the reaction between HAp and titanium during coating annealing. As bioadhesion is a crucial prerequisite for mineralisation of osteoblasts towards bony tissue, 

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It might be argued that the results by Jager et al. (2006) discussed and interpreted earlier pertain only to nanocrystalline hydroxyapatite synthesised from an aqueous solution. However, they are clearly relevant to calcium phosphate biomimetically precipitated from SBFs used to validate the degree of osseoconductivity of bioceramic coatings. 

Hence in coatings incubated for only up to 28 days the newly formed layer of secondary hydroxyapatite is decoupled from the stress state of the original layer showing essentially zero stress. Increasing the incubation time may also increase the compressive stress state again caused by reorganisation, consolidation and densification of the biomimetically precipitated Ca-deficient bone-like apatite layer (Gotze etal., 2001). 

To mimic the constituents of natural bone extracellular matrix, the crosstinked fibers were coated with carbonated hydroxyapatite through a biomimetic precipitation, resulting in an attractive bio-... 

Finally, control of fluorapatite particle size (from 15 nm to 200 pm) and morphology can be achieved by varying precipitation conditions using aqueous precipitation from a micro-emulsion or by biomimetic synthesis in a gelatin matrix [129],... 

Furthermore, diatoms and radiolaria exert strict species-specific control over the patterns and pore sizes of the silica, which is precipitated at ambient conditions. Controlled industrial synthesis of silica, on the other hand, requires extreme pHs and/or temperatures. Biomimetic silica synthesis has, therefore, attracted much interest among materials scientists and phycologists for potential industrial applications, such as components of industrial separators, catalysts, and electronic devices.64-67... 

Proteins such as rhBMP can be incorporated into calcium phosphate layers when biomimetically co-precipitated with the inorganic components (Liu et al., 2004). For example, rhBMP-2 (0.1-10pgml-1) retains its osseoinductive potential when incorporated into the crystal lattice of hydroxyapatite in a dose-dependent manner as ascertained by protein blot staining and enzyme-linked immunosorbent assay (ELISA). The positive osseogenicity of rhBMP-2 was assessed quantitatively by monitoring ALP activity (see Section 7.10.3.1) on rat BMSCs grown directly on the calcium phosphate coatings. 

Knecht, M. R. Wright, D. W., Functional analysis of the biomimetic silica precipitating activity of the R5 peptide from Cylindrotheca fiisiformis. Chem. Commun. 2003, (24), 3038-3039. 

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