Preparation Process, Related Mechanism And Properties Of HA Nanofibers Synthesized In Simulated Body Fluid

Hydroxyapatite (HA) is the major inorganic in human bones. The distinct features of HA in bones are calcium-deficient, small size (nanometer sized), low crystallinity and presence of carbonate ions in the crystal lattice. Human bones always cure themselves in near neutral or slightly acidic environments. Therefore, chemical synthesis of HA nano-powders in near neutral or slightly acidic environments is very significant for probing the experimental condition similar to growth environment of human apatite. In this paper, HA nano-fibers were synthesized in near neutral or slightly acidic SBF solution, using CaCl₂ and K₂HPO₄ · 3H₂O as reagents. The microstructures of synthesized HA products were investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Thermogravimetric (TG) and differential thermal analyses (DTA), Transmission electron microscopy (TEM), High resolution transmission electron microscopy (HRTEM). Based on the study upon the effect of different processing parameters on the microstructures of HA nano-fibers, the synthetic processing parameters were optimized. The synthetic mechanism of HA nano-fibers was discussed and chemical reaction kinetic model was established. The growth directions of HA nano-fibers synthesized in simulated body fluid (SBF) were determined, some biology properties of HA nano- fibers were also studied.HA nanofibers were first synthesized in SBF solution at 37℃, without the need of high-temperature sintering and adjusting pH value. The range of pH value (8.00 to 4.94) during the whole experiment was very close to that of pH value (7.2 to 5.4) human bone curing. HA fibers are of nanometer sized in three dimensions, low crystallinity and presence of carbonate ions, therefore their composition and structure is probably close to human apatite. This work should provide the basis to enlarge the biological applying of HA. Reagent concentration, reaction time and reaction temperature are key factors affectingthe HA synthesizing process.During the synthetic process, the effect of reagent concentration on the phase composition of HA products was studied by XRD and FT-IR. The results showed that reagent concentration was one key factor and must be strictly controlled. When the reagent concentration was high, the result HA powders contained dicalcium phosphate dihydrate (DCPD) and dicalcium phosphate anhydrous (DCPA). DCPD and DCPA in HA products decreased as the reagent concentration being reduced. When the reagent concentration was reduced to 15SBF and the reaction time was 24 hours, the pure HA nano-fibers were prepared which Ca/P molar ratio is 1.65, that is, calcium-deficient HA. The heat treatment temperature of HA nano-fibers was low.The effect of reaction time on the shape and crystallization level of HA nano-powders was studied. Under the condition of 15SBF and 37°C and during the range of 0.5²4h, the XRD results showed that the amount of impurities in HA products decreased gradually as the reaction time prolonged. Pure HA powders were synthesized when the reaction time was above lOh with all the peaks of impurities disappearing. HA powders synthesized for lOh exhibited fibered and spherical shapes. As the reaction time extended, spherical HA powders dropped off and disappeared basically until 24h. The results showed that fiber-shape HA nano-powders with uniform shape and low crystallinity were synthesized for 24h.The reaction temperature is another key factor influencing the crystallinity and the size of HA powders. Under the condition of 15SBFand 24h, at the range from 37 to 80°C, the crystallinity and the size of HA powders increased as the reaction temperature increased. Washing manner had also distinct effect on the formation of HA fibers. HA fibers washed by de-ionized water exhibited serious agglomerate phenomenon and HA fibers washed by ethanol dispersed well.The pH values of the experimental solution were mensurated under the condition of 15SBF and at the range of 37 to 70°C. PH values changed from8.00 to 4.94 during the whole experiments. As the pH reduced gradually, acidic material should created, which acidic material presented was analyzed, and chemical reaction equations probably taken place were determined. Second-order kinetic model was established. According to reaction active energy, the process of HA synthesis was determined to be reaction control Therefore, the measure to enhance solution mixing level generally has no effect on reaction rate. The reaction rate and the reaction translation ratio increased as the temperature increased. During the experimental temperature range, pH values basically changed before 6.5h. The result showed that the chemical reaction almost finished when the reaction time was 6.5h and the rest time was used for improving crystallinity and the growth of HA crystals.High resolution electron micrology may study perfect crystals, artificial crystals, amorphous materials, may also study vacancies, dislocations, stacking faults, etc. HRTEM studies provided further insight into microstructures of the HA fibers. Under HRTEM observation, it was found that the synthesized HA fibers were very unstable under strong electron beam irradiation. After about 5-8s of electron irradiation, the edges of the fibers became irregular or even disappear. The growth directions of HA fibers were calculated according to HRTEM lattice images. Characterization by HRTEM shows that the synthesized HA fibers grow along [00l]or [010], and there exist structural defects in the synthesized HA nano-fibers, such as stacking faults and edge dislocations.Bactenostatic effects of HA nano-fibers on streptococcus mutans and coliforms, as well as adsorptive effects of HA nano-powders on the above two bacteria were studied. Two routes were applied for the bacteriostatic experiment, that is, bacterium count and bacteriostatic ring. The bacteriostatic experiment result showed that HA nano-fibers had no effect on the two bacteria. But adsorptive experiment result showed that HA nano-fibers had distinct adsorption effect on the two bacteria. During the experimental concentration range, HAnano-fibers had the similar adsorption rule on the two bacteria. The adsorption rate increased as HA concentration increased during the low concentration range, the adsorption rate was highest at 0.5%, and the reverse trend presented during the high cincentration range . The highest adsorption rate was 45.9% and 39.5% respectively, when HA concentration was 0.5%.