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At least 775 records · Page 43Linked to original sources
PubMed1995

Apatite formation on (2-x)CaO.x/3 M2O3 x 2SiO2 glasses (M = La, Y; 0 < or = x < or = 0.6) in a simulated body fluid.

Glasses were prepared by substituting La2O3 or Y2O3 for CaO in glassy wollastonite composition (CaO.SiO2). Their behaviour when they are soaked in a simulated body fluid (SBF) was studied by means of an electron microscope equipped with an energy dispersive system for elemental analysis, and by means of IR spectroscopy. Electron microscopy and energy dispersive microanalysis were performed on samples soaked as polished bulk samples, and IR analysis was on samples soaked as fine powders. A carbonate-containing hydroxyapatite layer is formed when glasses of low La2O3 content are soaked in SBF. When Y2O3-containing glasses are considered, even in the case of small substitution for CaO, the same layer forms only on fracture surfaces. The experimental results agree with the mechanism reported in the literature.

Apatites↗
PubMed2004

Influence of synthesis and sintering parameters on the characteristics of carbonate apatite.

A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO(2) flux, as a source of carbonate, in the HA synthesis process based on the neutralisation reaction. The reactants are abundant and inexpensive. The yield is good compared to other CHA powder synthesis. The reaction may be performed at low temperature and without pH control and does not produce any by-products. The influence of the synthesis parameters (temperature, H(3)PO(4) solution dropping rate, i.e. reaction time, CO(2) flux, ageing time) has been tested to optimise the process conditions in order to obtain the highest carbonation degree and favour the B-type CHA precipitation with respect to A-type one. The prepared powder (5.8wt% of total carbonate with an A/B ratio of 0.78) was thermally treated at various temperatures in the range 500-1400 degrees C in different atmospheres (air, wet and dry carbon dioxide). The thermal treatments were performed with a double aim, to eliminate selectively the carbonate groups in A-position maintaining the B-type substitution, and to evaluate the thermal stability of the CHA and the total loss of carbonate as a function of temperature. The thermal treatment at 900 degrees C in wet CO(2) gave the best result in terms of a high carbonate residue and a low A/B ratio. We also investigate the use of different techniques (inductively coupled plasma, TGA, Fourier transformed infrared spectroscopy, X-ray diffraction) for characterising CHA and calculating sensitivity and accuracy in the quantification of carbonate ions for each molecular site.

Apatites↗
PubMed2005

Cyclic silicate active site and stereochemical match for apatite nucleation on pseudowollastonite bioceramic-bone interfaces.

Hydroxyapatite (Ca5(PO4)3(OH)) forms on pseudowollastonite (psW) (alpha-CaSiO3) in vitro in simulated body fluid, human parotid saliva and cell-culture medium, and in vivo in implanted rat tibias. We used crystallographic constraints with ab initio molecular orbital calculations to identify the active site and reaction mechanism for heterogeneous nucleation of the earliest calcium phosphate oligomer/phase. The active site is the planar, cyclic, silicate trimer (Si3O9) on the (001) face of psW. The trimer has three silanol groups (>SiOH) arranged at 60 degrees from each other, providing a stereochemical match for O atoms bonded to Ca2+ on the (001) face of hydroxyapatite. Calcium phosphate nucleation is modeled in steps as hydrolysis of surface Ca-O bonds with leaching of Ca2+ into solution, protonation of the surface Si-O groups to form silanols, calcium sorption as an inner-sphere surface complex and, attachment of HPO4(2-). Our model explains the experimental solution and high resolution transmission electron microscopy data for epitaxial hydroxyapatite growth on psW in vitro and in vivo. We propose that the cyclic silicate trimer is the universal active site for heterogeneous, stereochemically promoted nucleation on silicate-based bioactive ceramics. A critical active site-density and a point of zero charge of the bioceramic less than physiological pH are required for bioactivity.

Aluminum↗