Surface modification of hydroxyapatite with polyhedral oligomeric silsesquioxane

The remineralizing potential of hydroxyapatite (HA) prompted its use as a filler in dental composites. However, the compatibility between HA and organic compounds remains still insufficient, resulting in low mechanical properties. Moreover, HA is highly hydrophilic, leading to low mass stability of the composite. Polyhedral oligomeric silsesquioxanes are promising additives to dental composites; therefore, the compound from this group was used to modify the HA surface. Methacrylate-bearing silsesquioxane was synthesized and used as a modifier. Its structure was confirmed by Fourier-Transform Infrared Spectroscopy (FT-IR), Nuclear Magnetic Resonance Spectroscopy (NMR), and Mass Spectrometry (MS). HA modifications were conducted in several ways which efficiency was evaluated with FT-IR, Energy Dispersive Spectrometry (EDS), Thermogravimetric Analysis (TGA), Elemental analysis, and Scanning Electron Microscopy (SEM). Contact angle measurements also proved the wettability changes of the HA surface. All modification procedures were effective. The highest efficiency (78.6%) was obtained in the presence of the polymerization initiator and cross-linking agent at room temperature. The modified HA changed from highly hydrophilic to hydrophobic, which can be used in applications where its lower water sensitivity is required, i.e. in fillers for dental composites. The organic layered HA will bond stronger with the organic components of the composite matrix, improving therefore its mechanical properties.