Calcium-containing inorganic polymers as potential bioactive materials

In vitro studies are reported of the behaviour of potassium aluminosilicate inorganic polymers containing 10 wt% Ca(OH)2, nanostructured calcium silicate and Ca3(PO4)2 exposed to simulated body fluid (SBF). Heating to 600 °C lowers the alkalinity of Ca3(PO4)2-containing samples, but their X-ray powder diffraction characteristics, 27Al, 29Si and 43Ca MAS NMR spectra are unchanged by heating. Exposure of the heated compounds to SBF usually results in the formation of the crystalline biomineral phases hydroxylapatite and hydroxycarbonate apatite in the samples containing Ca(OH)2 and Ca3(PO4)2, but scanning electron microscopy/energy dispersive spectroscopy suggests that all the samples in the present study form calcium phosphates on exposure to SBF. This conclusion is also consistent with the removal of P from the SBF by all the samples. The concentrations of Al leached from the samples containing nanostructured calcium silicate and Ca3(PO4)2 (0.05 and 0.47 ppm, respectively) are acceptable for biomaterials use, but apart from the Ca3(PO4)2-containing sample, which takes up Ca from the SBF, the levels of Ca released into the SBF by the other samples are well in excess of the published optimum amount for stimulation of new bone growth by gene transcription in osteoblasts. Only the calcium silicate-containing samples release Si into the SBF, but in a concentration that falls short of the optimum amount. The strength of all the present compounds after heating is probably adequate for applications as biomaterials, but the Ca3(PO4)2-containing compound shows slightly greater strength. Thus, on balance, the Ca3(PO4)2-containing compound appears to be the most promising as a bioactive material.