Structural characterization of boron-containing glassy and semi-crystalline Biosilicate® by multinuclear NMR

Abstract

In vitro, in vivo, and clinical studies - including histopathological, cytotoxicity, and genotoxicity analyses - over the past 25 years have proven the tremendous potential of Biosilicate® in appliance as osteoinductive powders, scaffolds and even monolithic pieces. Within continuing efforts to improve the properties of this bioactive material, in this article we explore the structural consequences of incorporating boron in small amounts and its effects on selected properties, as it is known to influence vital processes such as embryogenesis, bone growth, and psychomotor skills, among others. Solid state 11B MAS NMR studies indicate that in the glassy precursor boron is likely present in the form of three-coordinate pyroborate units, with only minor fractions of four-coordinate species present. In contrast, 31P MAS-NMR spectra reveal that phosphorus is almost exclusively present in the form of orthophosphate. The demand of the anionic borate network former for cationic charge compensation leads to an increase in average connectivity of the silicate network, as evident from 29Si MAS NMR. 31P/11B dipolar recoupling experiments indicate negligible amounts of borate–phosphate linkages in these glasses, presumably because of the low concentrations of each element. Crystallization of the glassy Biosilicate® precursor produces crystalline NaCaPO4 and Na2CaSi2O6 as well as a residual amorphous material for which the fraction of four-coordinate boron is significantly increased and no B-O-P linkages can be detected. Surprisingly, we find that in this particular glass boron reduces the glass stability against crystallization and has only a weak influence upon the TRIS-buffer solution dissolution kinetics.