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

Abstract

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.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…Borate substitution with the HA structure formed at high temperature shows that the bulk of the borate units are incorporated in a disordered calcium phosphate phase as a surface layer that [88].…”

“…The interface between the HA core and the amorphous phosphate surface layer was probed by a combination of triple resonance 1 H-31 P-11 B CP and 31 P{ 1 H} REDOR [88]. In mineralised cartilage the inorganic surface species that interact with the organic surroundings have been probed by 1D 1 H and 31 P, 2D 13 C{ 31 P} heteronuclear correlation and 13 C{ 31 P} REDOR experiments [89].…”

Effect of Disordered Structure of Boron-Containing Calcium Phosphates on their In Vitro Biodegradability

“…Right: Slice taken at 0 ppm in the 1 H dimension. This 31 P line shape is representative of crystalline HAp 23 . Note that it is much narrower than the 31 P MAS spectrum shown in Fig.…”

“…5 in correlation with the 1 H– 31 P heteronuclear interactions. An additional feature in the profiles was a broad and weak resonance at −0.6 ppm, which comes from a disordered phase (HAp, cHAp) and the BCaP phase described by Bahreine et al 23 It grew for BHAp so as to broaden the OH − peak at 0 ppm.…”

“…Recently, Barheine et al 16,23 proposed a new structural model for boron‐containing hydroxyapatite (BHAp) particles, presented in Fig. 1, where core HAp is covered by a disordered calcium borophosphate (BCaP) layer which is hydrated to some extent.…”