Hydroxyapatite growth

Aryal, S., Bahadour, K.C.R., Bhattarai, J.R., Prabu, P. and Kim, H.Y. (2006) Immobilization of collagen on gold nanoparticles, preparation, characterization and hydroxyapatite growth, foumal of Materials Chemistry, 16, 4642-4648. 

Lindberg, L, Heinrich, J., Ericsson, F., Thomas, P., and Engqvist, H. (2008) Hydroxyapatite growth on singlecrystal rutile substrates. Biomaterials,... 

In the previous chapter, two examples of multifunetional drag delivery systems applying PSi luminescent and photonic properties were described, but there are many other possibilities to include additional functionalities to the delivery systems. One of the most studied applications is PSi-poljuner composite structures in which PSi have a dual role as a hydroxyapatite growth activator and drag carrier (Mukheijee et al. 2006 Fan et al. 2009, 2011). The composite structures are described in detail in chapter Poljnner-Porous Silicon Composites. ... 

Hydroxyapaite, the mineral constituent of bone, is appHed to the surfaces of many dental implants for the purpose of increasing initial bone growth. Some iavestigators beHeve that an added benefit is that the hydroxyapatite shields the bone from the metal. However, titanium and its aHoy, Ti-6A1-4V, are biocompatible and have anchored successfuHy as dental implants without the hydroxyapatite coating. 

The cell growth was much faster on the chitosan-hydroxyapatite scaffolds with the glass than on the chitosan-hydroxyapatite scaffold without the glass. The total protein content of cells increased over time on both composites. The cells on the chitosan-hydroxyapatite-glass also expressed significantly higher amount of alkaline phosphatase at days 7 and 11 and osteocalcin at day 7 than those on chitosan-hydroxyapatite [165]. 

Sodium carboxymethyl chitin and phosphoryl chitin had most evident influences on the crystallization of calcium phosphate from supersaturated solutions. They potently inhibited the growth of hydroxyapatite and retarded the rate of spontaneous calcium phosphate precipitation. These chitin derivatives were incorporated into the precipitate and influenced both the phase and morphology of the calcium phosphate formed (flaky precipitate resembling octacalcium phosphate instead of spherical clusters in the absence of polysaccharide) [175]. 

Eanes, E.D. and Posner, A.S. 1970 A note on the crystal growth of hydroxyapatite precipitated from aqueous solutions. Materials Research Bulletin 5 377-384. 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Humic substances. Analogous to the reactions described above, humic substances (the polymeric pigments from soil (humus) and marine sediments) can be formed by both enzymatic and non-enzymatic browning. High concentrations of free calcium and phosphate ions and supersaturation with respect to hydroxyapatite can sustain in soil, because adsorption of humic acids to mineral surfaces inhibits crystal growth (Inskeep and Silvertooth, 1988). A similar adsorption to tooth mineral in a caries lesion can be anticipated for polycarboxylic polymers from either the Maillard reaction or enzymatic browning. 

M.R. Christoffersen, J. Christoffersen, J. Arends, Kinetics of dissolution of calcium hydroxyapatite. VII. The effect of fluoride, J. Cryst. Growth 67 (1984) 107-114. 

Lower et al. (1998b) can shed more light on this phenomenon. They again used AFM, SEM, TEM, SEM-EDS, electron diffraction, and XRD to study the reactions between 0.5 and 500 mg/L of Pb with hydroxyapatite at pH 6 and a reaction temperature of 22 °C. A commercial hydroxyapatite was used at sorbent concentrations of 0.5 g/L. Reactions were observed over a 2 h period. At high initial Pb concentrations, Pb solution concentrations dropped from 500 mg/L to <100 mg/L. At concentrations of 0.5-100 mg Pb/L, after reaction, Pb levels dropped to less than 15 pg/L. In both cases, hydroxyapatite dissolved and hydroxypyromorphite formed. The authors applied some nucleation and crystal growth theory developed... 

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