Effects of pH and temperature on microstructure and morphology of hydroxyapatite/collagen composites synthesized in vitro

Hydroxyapatite/collagen composites (HAp/Col) were synthesized, in vitro, using the self-organization mechanism, by the co-precipitation of collagen, extracted from pork skin, and aqueous H3PO4 and Ca(OH)2. The effects of pH and temperature, on the microstructure and morphology of HAp/Col composites, were extensively studied.It was shown that the yield of the composite is closely related to the pH value, during preparation. At high pH, the white precipitate of the composites was formed in a large quantity. In contrast, the yield was quite low, at low pH, owing to the dissolution of the composite precipitates. The electrophoretogram of the as-prepared composite shows that the α1(I) chain was dominant and that there was no protein residue in the supernatant of the reaction solution. Accordingly, it is reasonable to assume that the collagen molecules had completely reacted with hydroxyapatite.A closer look at the structural evolution of HAp/Col composites revealed that, at pH = 5, there was no sign of interfacial interaction between hydroxyapatite and collagen, during the first 36 h. In contrast, under alkaline conditions, the amount of α(I) chains was indeterminate, at the start of precipitation, while the concentration of β-chains decreased gradually. The chemical structure of HAp/Col, as determined by FTIR spectra, revealed that the bending vibration of phosphate contours can be attributed to the dissolution of HAp, at low pH. The crystalline of HAp was readily discerned, for all samples, as revealed by the XRD patterns. The morphology of the HAp/Col, prepared under neutral to alkaline conditions (7 ≤ pH ≤ 9), exhibited a compact, coral reef-like structure. In summary, HAp/Col composites can be synthesized, in vitro, under conditions native to animal physiology. The desirable conditions for synthesis are a higher pH (8 to 9) and a temperature of 40 °C.