Effect of TiO2 addition on structure, solubility and crystallisation of phosphate invert glasses for biomedical applications

Phosphate invert glasses in the system P2O5–CaO–MgO–Na2O are completely soluble and exhibit a neutral pH in aqueous media, and they are therefore of interest for use as degradable implant materials. Their structure consists of small phosphate units such as pyrophosphate (P2O74−), and hence they are prone to crystallisation. Addition of TiO2 is known to improve processing of the melt and also to control glass solubility. The glass structure of phosphate glasses with 37 and 35 mol% P2O5 and addition of 1 to 10 mol% TiO2 was analysed using 31P MAS NMR, and the influence of structural changes on solubility, thermal properties, processing window and crystallisation behaviour was investigated. Glasses showed an increase in activation energy for crystallisation with increasing TiO2 content, resulting in an increased processing window, thereby allowing for fibre drawing and sintering of porous scaffolds. Deconvolution of 31P MAS NMR and calculation of network connectivity and average chain lengths suggest that Ti is acting as a network modifier with Ti4+ units acting as ionic crosslinks between phosphate units thereby impeding crystallisation as well as chain hydration and subsequent chain dissolution.