Structural and physico-chemical analysis of calcium/strontium substituted, near-invert phosphate based glasses for biomedical applications

Neutron diffraction, Na-23 and P-31 NMR, and FTIR spectroscopy have been used to investigate the structural effects of substituting CaO with SrO in a 40P(2)O(5)center dot(16-x)CaO center dot 20Na(2)O center dot 24MgO center dot xSrO glass, where x is 0, 4, 8, 12 and 16 mol%. The P-31 solid-state NMR results showed similar amounts of Q(1) and Q(2) units for all of the multicomponent glasses investigated, showing that the substitution of Sr for Ca has no effect on the phosphate network. The M-O coordinations (M = Mg, Ca, Sr, Na) were determined for binary alkali and alkaline earth metaphosphates using neutron diffraction and broad asymmetric distributions of bond length were observed, with coordination numbers that were smaller and bond lengths that were shorter than in corresponding crystals. The Mg-O coordination number was determined most reliably as 5.0(2). The neutron diffraction results for the multicomponent glasses are consistent with a structural model in which the coordination of Ca, Sr and Na is the same as in the binary metaphosphate glass, whereas there is a definite shift of Mg-O bonds to longer distance. There is also a small but consistent increase in the Mg-O coordination number and the width of the distribution of Mg-O bond lengths, as Sr substitutes for Ca. Functional properties, including glass transition temperatures, thermal processing windows, dissolution rates and ion release profiles were also investigated. Dissolution studies showed a decrease in dissolution rate with initial addition of 4 mol% SrO, but further addition of SrO showed little change. The ion release profiles followed a similar trend to the observed dissolution rates. The limited changes in structure and dissolution rates observed for substitution of Ca with Sr in these fixed 40 mol% P2O5 glasses were attributed to their similarities in terms of ionic size and charge. Statement of Significance Phosphate based glasses are extremely well suited for the delivery of therapeutic ions in biomedical applications, and in particular strontium plays an important role in the treatment of osteoporosis. We show firstly that the substitution of strontium for calcium in bioactive phosphate glasses can be used to control the dissolution rate of the glass, and hence the rate at which therapeutic ions are delivered. We then go on to examine in detail the influence of Sr/Ca substitution on the atomic sites in the glass, using advanced structural probes, especially neutron diffraction. The environments of most cations in the glass are unaffected by the substitution, with the exception of Mg, which becomes more disordered. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.