Effect of MgO addition on the sinterability, mechanical properties and biological cell activities of sintered silicon-substituted hydroxyapatite

The effect of 1 wt% magnesium oxide (MgO) as sintering additive on the sintering behaviour, microstructure, mechanical properties and cell activities of 0.4 and 1.6 wt% silicon-substituted hydroxyapatite (MgO-0.4Si-HA and MgO-1.6Si-HA) was investigated. The powders were synthesized by the wet-chemical neutralization method and the compacts were pressureless sintered in air between 1150 and 1250 degrees C for 2 h. The results showed that MgO-0.4Si-HA promoted the formation of the secondary phase of beta-TCP when sintered at 1250 degrees C, whilst the stability of the hydroxyapatite phase was not disrupted for the MgO-1.6Si-HA. The presence of MgO was found to be beneficial in suppressing grain growth of Si-HA when sintered at 1250 degrees C; however, it did not promote densification at lower temperatures (1150 and 1200 degrees C). It was observed that despite the lower densification, Vickers hardness and compressive strength of the MgO-Si-HA were higher than those of the undoped Si-HA samples. The beneficial effect of MgO in enhancing the biocompatibility of Si-HA has been revealed. It was found that the MgO-doped samples dramatically enhanced the osteogenic differentiation of MC3T3-E1 cells as shown from the higher cell proliferation, alkaline phosphatase activity and bone-like nodule formation when compared to pristine HA.