Ali Nemati; Abdorreza Sheikh Mehdi Mesgar; Fathollah Moztarzadeh
Volume 3, Issue 2 , June 2009, , Pages 135-149
Abstract
In this paper, dissolution kinetics of Amorphous Calcium Phosphate as well as cements in the Simulated Osteoclastic Medium (SOM) was evaluated based on the Shrinking Core models considering the liquid-solid reactions. Based on this model, three steps may be considered as controlling steps in the system: ...
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In this paper, dissolution kinetics of Amorphous Calcium Phosphate as well as cements in the Simulated Osteoclastic Medium (SOM) was evaluated based on the Shrinking Core models considering the liquid-solid reactions. Based on this model, three steps may be considered as controlling steps in the system: diffusion of component A through the surrounding films, reaction of component A with solid on the surface and diffusion through the interface. Two cases were considered here: 1. Shrinking Core model with formation of the intermediate phase 2. Shrinking Core model without formation of the intermediate phase Then, experimental data were used for the evaluation of the controlling steps and its mechanism (s). The results showed that enough amounts of calcium were entered into the solution in the initial stage of the process. This in turn causes to form a film on the particles, and the potential of calcium carbonate complex, resulted in the reduction of calcium saturation in the system. The amounts of entered calcium into the solution were higher in the amorphous system. In other words, a longer time is required in the crystalline system for more entrance of calcium into the solution (as in the sample H1T). Based on these observations, it was concluded that the approximately crystalline cements with carbonate falls between the crystalline cements without carbonate and amorphous system (The amounts of entered calcium into the solution). Dissolution rate of ACCPs in the Simulated Osteoclastic Medium (SOM) was dependent on the contents of carbonate and remaining water. Dissolution behavior in the SOM showed that the behavior of ACCP (high carbonate)–DCPD–PHA–Gelatin system was comparable to the ACCP (low carbonate)-DCPD. The presence of PHA and gelatin in cement system decreased the dissolution rate. The dissolution kinetics of the cements and ACCPs in the SOM was likely controlled by the formation of an acid-resisting ACP and/or DCPD as product layer.
Biomechanics of Bone / Bone Biomechanics
Abdorreza Sheikh Mehdi Mesgar; Zahra Mohammadi; Fathollah Moztarzadeh; Mahtab Ashrafi Khouzani; Zeinab Sadat Mohammadi
Volume 1, Issue 1 , June 2007, , Pages 39-51
Abstract
Amorphous carbonated calcium phosphates (ACCPs) with different carbonate contents and Ca/P ratios were reproducible synthesized by the reaction parameters as low temperature, high pH value, using initial solutions of calcium and phosphate at low concentrations, and various amounts of carbonate, as well ...
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Amorphous carbonated calcium phosphates (ACCPs) with different carbonate contents and Ca/P ratios were reproducible synthesized by the reaction parameters as low temperature, high pH value, using initial solutions of calcium and phosphate at low concentrations, and various amounts of carbonate, as well as freeze drying of the precipitates. The addition of carbonate to the solutions led to form precipitates with higher Ca/P ratios with respect to the initial solutions. Heat treatment of freezedried ACCPs at 500 °C had no influence on their amorphous structure. The results of elemental carbon and thermal analysis showed that the carbonate may be eliminated in a wide range of temperature (500−1150oC). Dissolution rate of ACCPs in the simulated bone resorption medium was dependent to the contents of carbonate and remaining water. Dissolution rate of the specimens with higher carbonate contents was controlled by the carbonate content, but the amount of remaining water had major influence on the dissolution rate of the precipitates with lower carbonate contents. The dissolution kinetics was found to follow a shrinking-core model, with product layer as the ratedetermining step. Formation of an amorphous calcium phosphate and/or thermodynamically desirable dicalcium phosphate dihydrate as possible product layer prevents complete resorption of ACCPs under bone resorption conditions, and promotes osteoblastic activation process through nucleation and growth of biological apatite.
Abdorreza Sheikh Mehdi Mesgar; Zahra Mohammadi
Volume -1, Issue 2 , June 2005, , Pages 143-151
Abstract
Crystallization behavior and in vitro bioactivity of the bioactive glasses in the system MgO-CaO-P2O5-SiO2 were studied. Crystallization of bulk glasses led to the formation of large cracks in crystallized product that was attributed to the precipitation of fibrous b-wollastonite crystals growing ...
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Crystallization behavior and in vitro bioactivity of the bioactive glasses in the system MgO-CaO-P2O5-SiO2 were studied. Crystallization of bulk glasses led to the formation of large cracks in crystallized product that was attributed to the precipitation of fibrous b-wollastonite crystals growing perpendicular to the outer surface of the glasses. Crack-free dense crystallized products were formed by crystallization of the same glasses in a powder compact. By substituting SiO2 for P2O5, there was no change in the kind of formed crystalline phases but the apatite contents decreased and wollastonite contents increased. The whitlockite phase was formed when glass powder compacts were heated above wollastonite crystallization temperature. The in vitro bioactivity of the glasses and glass-ceramics was evaluated by examining apatite layer formation on their surfaces in the simulated body fluid (SBF) with SEM/EDXA. All samples showed an apatite layer on their surfaces after immersion in SBF.
Tissue Engineering
Abdorreza Sheikh Mehdi Mesgar; Zahra Mohammadi
Volume -1, Issue 1 , June 2004, , Pages 47-55
Abstract
The conditions for synthesis of amorphous calcium phosphates (ACPs) according to the crystallization principles were described. By selecting reaction parameters correctly (low temperature, high pH, immediate removing of water and using solutions containing calcium cations and phosphate anions at low ...
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The conditions for synthesis of amorphous calcium phosphates (ACPs) according to the crystallization principles were described. By selecting reaction parameters correctly (low temperature, high pH, immediate removing of water and using solutions containing calcium cations and phosphate anions at low concentrations), full ACPs can be reproducibly synthesized. The X-ray diffraction patterns of synthesized amorphous specimens were showed a characteristic broad peak. The Ca/P ratio of the resulting precipitates was weakly dependent on the initial Ca/P ratio. Solubility of synthesized specimens in simulated osteoclastic activity conditions was showed that the rate of dissolution would be decreased with increasing crystallinity level. The synthesized amorphous calcium phosphates showed the highest dissolution rate in the simulated solution, and can be a suitable candidate for using in the field of hard tissue engineering applications.
