In our previous work, hydroxyapatite (HAP) was synthesized under two conditions: one in the presence of polyacrylic acid (in situ HAP) and the other in the absence of polyacrylic acid (ex situ HAP). Composites of both HAPs with polycaprolactone (PCL) were investigated for their applicability as scaffolds for bone tissue engineering. In the current work, bioactivity of these composites has been investigated by soaking them in simulated body fluid for different intervals of time. Nucleation and growth mechanism of apatite on these composites has also been investigated. Fourier transform infrared spectroscopy study suggests that although apatite growth starts with an intermediate phase, it completely transforms to HAP after 4 days of soaking. Nanoindentation results suggest that the apatite growing on in situ HAP/PCL composites has much higher hardness and elastic modulus as compared to the apatite growing on ex situ HAP/PCL composites. The apatite grown on the ex situ composites has a net-like interconnected structure. The observed differences in mechanical properties and morphology of apatite have been described on the basis of nucleation mechanisms. The nucleation of apatite on the in situ HAP/PCL composites proceeds through the formation of a complex between Ca2+ and COO- groups; on the other hand, nucleation occurs because of dissolution reaction of apatite in ex situ HAP/PCL composites.
2006 Wiley Periodicals, Inc. J Biomed Mater Res, 2006.