Hydroxyapatite morphology

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]. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

As summarized in Table 14.1, teeth, bones, shells, etc. are indispensable components, consisting of inorganic mineral crystals and protein film, with sizes, morphologies, and textures suitable to fulfil the function of the particular organs involved. In this section we will look at hydroxyapatite, aragonite and calcite (two polymorphs of CaCO ), and magnetite in greater detail. 

Hydroxyapatite (HA) NP-5-I-NP-9 and cyclohexane To compare differences in the sizes, morphologies and specific surface areas of HA powder prepared by W/0 microemulsion, bicontinuous microemulsion and the emulsion method [162]... 

Deslandes, Y., Morphology of hydroxyapatite as suspension stabilizer in the polymerization of poly(styrene-co-butadiene), J. Appl. Polym. Sci. 34 (1987) 2249. 

Wang, M., Joseph, R., and Bonfield, W., Hydroxyapatite-polyethylene composites for bone substitution effects of ceramic particle size and morphology. Biomaterials, 19, 2357, 1998. 

H]o.8[(OCH2CH2)20Me]i.2 n were allowed to react with Si(OEt)4 under hydrolytic conditions.The rate of hydroxyapatite formation from CaHP04.2H20 and Ca4(P04)20 in aqueous solution is influenced by the presence of NP[OC6H4(C02Na)-4]2)n- The same holds for the morphology of the hydoxy-apatite formed. 

Recently, PCL containing bovine bone hydroxyapatite (HA) and hydroxyapa-tite/Ag (HA-Ag) composite nanoflbers were prepared via an electrospinning process [43]. The morphology, structure and thermal properties of the PCL, PCL/HA, and PCL/HA-Ag composite nanoflbers before and after immersion in SBF were characterized. SEM images revealed that the nanoflbers were well-oriented and incorporated the HA-Ag nanoparticles well. Mechanical study revealed that the yield stress of PCL/HA-Ag composite nanoflbers showed a higher value than that of PCL/HA composite, possibly due to the addition of metallic Ag nanoparticles [43]. 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

However, not only the kinetics but also the morphology of precipitated HAp nanocrystals will be modified by structure-mediated (epitaxial) adsorption of organic constituents such as poly(amino acids) at prominent lattice planes of HAp. For example, adsorption of poly(l-lysine) on (0 0 1) planes causes formation of polycrystalline nanocrystals of HAp whereas adsorption of poly(l-glutamic acid) leads to precipitation of large flat micron-sized single crystals of HAp (Stupp and Braun, 1997). Similar relations have been found in experiments involving adsorption of recombinant human-like collagen (Zhai and Cui, 2006) and bovine serum albumin (BSA) (Liu et al., 2003) on hydroxyapatite surfaces. 

Figure 5.15 Morphological changes of carbonated hydroxyapatite deposited by the thermal substrate technique from a solution containing 0.7 mmol CaCI2 and 0.3 mmol Ca(H2P04)2 with varying amounts of NaHCOj (a and b) as well as collagen I added (c and d). (a) Carbonated HAp with type B substitution (+0.5 mmol NaHCOj, 140 °C, 15 min).

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