Surface, Interface, and Bulk Structure of Borate Containing Apatitic Biomaterials

Nuclear magnetic resonance (NMR) has been used for a detailed investigation of the borate incorporation in apatitic biomaterials prepared by high-temperature solid-state reaction sintering. The NMR data clearly show that crystalline hydroxyapatite (HAp) does exist, but it contains only about 30% of the entire phosphate content of the sample. The main phosphate content of about 70% forms a disordered calcium phosphate phase (BCaP) that accommodates the borate units in two structurally different trigonal BO3 3− groups besides some minor linear BO2 − units. The average chemical composition of BCaP was estimated from the NMR spectra. Furthermore, a structural model of these particles is proposed, where HAp forms the crystalline core of these crystals covered by the disordered BCaP, suggesting that the BCaP phase is responsible for the adhesion properties of organic molecules like proteins and not HAp that is the only significant crystalline phase (XRD). Furthermore, the presence of an interface between HAp and BCaP is discussed based on various NMR experiments, including a triple-resonant 11B−31P cross-polarization edited 31P NMR spectrum with subsequent 31P{1H} REDOR (Rotational Echo DOuble Resonance) dephasing.