In fact, the SEM micrographs (Fig. 2) showed a good integration of the microparticles in the ceramic matrix, which was likely the STI 571 reason for the increased mechanical strength for one of the cements. It was also clear from the SEM micrographs that the polymer microparticles were much larger than the brushite and monetite crystallites, which could also have an effect on the resulting strength of the cement. Since the polymer microparticles were produced by mechanical crushing of a solid piece,19 smaller particles are hard to produce and the yield is quite low; however, smaller particles could possibly increase the strength further, and might be good to investigate in future studies. Figure 5. Conceptual drawing of the composite setting reaction.

(1) An exchange of glycerol to water starts when the cement is immersed in body fluids at 37 ��C. (2) The ceramic grains start to dissolve and since the temperature is around … From the XRD results it could be concluded that the ��-TCP content measured for all groups was slightly higher than the 10 mol% excess that was added to the mixtures. However, this was not surprising since the fast dissolving MCPA might diffuse out from the cement before the proper amount of ��-TCP has been dissolved and can react to form the end product. Since ��-TCP has a limited solubility at physiological pH��it needs a lower pH to dissolve��and MCPA decreases the pH in the vicinity after dissolution, the excess ��-TCP will not be dissolved after all MCPA is consumed.

It has previously been observed that the main product after reaction for premixed acidic calcium phosphate cements is dicalcium phosphate anhydrous, or monetite,16,20 and not brushite, which is seen when MCPM (or MCPA) and ��-TCP is mixed directly with water. Under physiological conditions monetite is the more stable phase; however, the nucleation and growth demands high energies, due to the high energies needed to dehydrate calcium, and nucleation and growth of brushite is thus favorable.23,24 In conditions where an insufficient amount of water is present two things can occur with the result of monetite being formed after setting. Either nucleation of brushite occurs, which is then decomposed to monetite to release water and continue the reaction,25 or if no water is present and the temperature is high enough to bridge the energy needed for monetite formation, it is likely that monetite is formed directly.

However, in this study a large variation of the monetite vs. brushite ratio was seen. This could be explained by the PEG enclosed inside the polymer microparticles. PEG is highly hydroscopic and due to its high molecular weight compared with glycerol it is retained within the material for a longer time. In the vicinity Brefeldin_A of PEG more water will be present than anywhere else in the material, thus the brushite will not be decomposed to monetite as easily as without the PEG.