Microstructure, phase compositions and in vitro evaluation of freeze casting hydroxyapatite-silica scaffolds

Hydroxyapatite-silica (HA-SiO2) scaffolds with different SiO2 content (0, 2, 5 and 10 wt% SiO2) were fabricated by freeze casting. After sintering, the scaffolds maintained the interconnected unidirectional pore channels by removing the frozen ice crystals via sublimation. X-ray diffraction (XRD) analysis indicated that SiO2 promoted the decomposition of HA to tricalcium phosphate (TCP), comprised of α-TCP and β-TCP, which became more apparent with the increase of SiO2 content. The microstructure observation of scanning electron microscope (SEM) showed that the scaffolds surface feature had great changes in terms of grain size and grain boundary with the addition of SiO2. Moreover, the addition of SiO2 could increase the porosity and pore size of the scaffolds, even allowing it to reach a maximum as the SiO2 content increased to 5 wt%. Compression tests investigated the variation in the compressive strength of the scaffolds with the increase in the SiO2 content, which showed first decreasing and then increasing behavior. In vitro evaluation results in simulated body fluid (1.5×SBF) revealed that the introduction of SiO2 enhanced the growth rate of bone-like layer, especially the scaffold with 5 wt% SiO2, which exhibited faster growth rate of bone-like layer than the other scaffolds. The XRD and fourier transformed infrared spectroscopy (FT-IR) characterization confirmed that the bone-like layer formed on the scaffold surface was a carbonate-containing hydroxyapatite bone-like layer.