Crystalline hydroxyapatite

Blood Calcium Ion Level. In normal adults, the blood Ca " level is estabhshed by an equiUbrium between blood Ca " and the more soluble intercrystalline calcium salts of the bone. Additionally, a subtle and intricate feedback mechanism responsive to the Ca " concentration of the blood that involves the less soluble crystalline hydroxyapatite comes into play. The thyroid and parathyroid glands, the fiver, kidney, and intestine also participate in Ca " control. The salient features of this mechanism are summarized in Figure 2 (29—31). 

The secret to our success with hydroxyapatite and other strong calcium phosphates is that we seek syntheses for these bioceramics at temperatures on the order of400-800°C, where if hydroxyapatite is used as a reactant it does not decompose. Previously, most attempts failed to produce calcium phosphate bioceramic materials that were even 20% as strong as crystalline hydroxyapatite. Most crumbled under even moderate crompession in vivo. 

In France, a number of researchers have explored the washing, phosphate stabilization, and sintering or calcination of residues (Derie 1996 Iretskya et al. 1999 Nzihou Sharrock 2002 Piantone et al. 2003). The formation of more crystalline hydroxyapatite reaction products helps reduce leachability of metals incor-... 

Eanes, E. D., Gillessen, I. H., Posner, A. S. Mechanism of conversion of non-crystalline calcium phosphate to crystalline hydroxyapatite. In Crystal Growth, pp. 373. Peiser, H. S. (ed.). Oxford Pergamon Press Inc. 1967... 

The presence of phosphate increases the ability of the mitochondria to accumulate Ca2+, partly because of the buffering effect of the phosphate on the pH of the matrix, and partly because of the precipitation of insoluble calcium phosphate within the matrix. The formation of insoluble calcium phosphate lowers the internal free [Ca2+] and favours further influx. The amorphous nature of the precipitate is of interest as the formation of crystalline hydroxyapatite would be expected. Mitochondria must contain a factor that inhibits this process. 

Calcium phosphate will ordinarily precipitate at concentrations typically exceeding 5 ppm PO4 or less, forming amorphous calcium orthophosphate (tricalcium phosphate) sludge, Ca3(P04>2, in the bulk water and crystalline hydroxyapatite, Caio(OH)2(P04)2, at heat-transfer surfaces. 

Holmes, J. M., and Beebe, R. A. Surface areas by gas adsorption on amorphous calcium phosphate and crystalline hydroxyapatite. Calc. Tiss. Res. 7, 163-174 (1971). 

Ninety-nine percent of the calcinm and 85% of the phosphoms in the body are fonnd in bones, mostly a poorly crystalline hydroxyapatite Caio(P04)6(OH)2. In addition, calcium is involved in blood coagulation and is an intermediary factor between impulses and muscle contraction. Calcium has an activating effect on enzymes such as phosphorylase kinase and pyruvate dehydrogenase (lipoamide)-phosphatase. 

Eanes, E.D., Gillessen, I.H., and Posner, A.S. Mechanism of conversion of non-crystalline calcium phosphates to crystalline hydroxyapatite, p 373-375, "Proc. Int. Cong, on Crystal Growth, Boston, 1966. Pergamon Press, Oxford, 1967. Termine, J.D., Peckauskas, R.A., and Posner, A.S. Calcium phosphate formation in vitro. II. Effect of environment on amorphous crystalline transformation. Arch. Biochem. Bio-phys. 140. 318-325 (1970). 

Hashimoto. Y. and Sato, T., Removal of aqueous lead by poorly-crystalline hydroxyapatites, Chemosphere, 69, 1775, 2007. 

Crystalline hydroxyapatites are the major mineral of bones and teeth in a matrix of the protein collagen. The hydroxyls reside in chaimels running along the crystal c-axis which provide easy access to the external environment. Hydroxyapatites in vivo are usually described as poorly crystalline, calcium deficient and containing carbonate substitutions. The carbonate substitutions can occur at both the hydroxyl and the phosphate... 

Fig. 10.33 Comparison of the infrared spectra of (a) highly crystalline hydroxyapatite (b) poorly crystalline hydroxyapatite and (c) defatted, deproteinated, dried bone showing the regions of the hydroxyl stretching vibration on the right and the hydroxyl libration on the left. Reproduced from [83] with permission from the PCCP Owner Societies.

Fig. 10.34 INS spectra of bone as the organic component is progressively removed [83]. (a) Dried, powdered bone, (b) after removal of fat and drying, (c) after removal of protein and drying and (d) highly crystalline hydroxyapatite.

Fig. 10.35 INS spectra of bone and model compounds in the O—H stretching region, recorded by two groups of workers. Left spectra recorded on LRMECS (IPNS) [84] (a) nanometre sized crystalline hydroxyapatite (b) micron sized crystalline hydroxyapatite, (c) bovine bone mineral, (d) bone mineral from rat. Right spectra recorded on HET (ISIS) [85] (e) highly crystalline hydroxyapatite, (f) ox femur bone mineral, and (g) brushite CaHP04.2H20. The dashed vertical lines show the position of the hydroxyapatite peaks.

Later work was centred on the chemical activation of titanium surfaces with alkalis that lead to precipitation of amorphous sodium titanate the Na+ ions of which will be exchanged against Ca2+ ions to form amorphous calcium titanate (Kokubo et al., 1996). This compound in turn will be transformed into ACP acting as a template for the subsequent precipitation of nano-crystalline hydroxyapatite (Kokubo, Kim and Kawashita, 2003). The sequence of events is shown schematically in Figure 5.2a-e. 

Dey, A. and Mukhopadhyay, A.K. (2014) Nanoindentation study of phase-pure highly crystalline hydroxyapatite coatings deposited by microplasma spraying. Open Bioeng. /, in press. 

L. T., Rossi, A.L., Ospina, R.O., Borghi, F.F., Silva Filho, J.G., and Rossi, A.M. (2013) Growth of crystalline hydroxyapatite thin films at room temperature by tuning the energy of the rf-magnetron sputtering plasma. ACS Appl. Mater. Interfaces, 5 (19), 9435-9445. 

Keller, L. and Dollase, W.A. (2000) X-ray determination of crystalline hydroxyapatite to amorphous calcium phosphate ratio in plasma sprayed coatings. /. Biomed. Mater. 

Consequently, the intensity of the v1 vibration, together with the relative intensities of the side peaks, will give information on the degree of ordering of the structure of crystalline hydroxyapatite. The decrease of the intensity of the OH stretching vibration at 3575 cm-1 from as-received HAp powder (a) to powder annealed at 1300 °C (b) to a plasma-sprayed coating (c) is indicative of the consecutive loss of OH ions with increased heat treatment (Figure 7.8). 

Figure 7.13 H-MAS NMR spectrum of as-sprayed hydroxyapatite (Hartmann ef a ., 2001). The inset shows the spectrum of a completely ordered, highly crystalline hydroxyapatite (Hartmann ef a/., 2000). L highly...

Crystalline hydroxyapatite is formed on very slow precipitation from extremely dilute solutions ... 

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