Unanticipated stabilization of zinc-silicate glasses by addition of lanthanum: Implications for therapeutic inorganic ion delivery systems

ABSTRACT Two series of lanthanum containing zinc-silicate glasses were developed (A: 0.51SiO2–0.29Na2O–(0.20-X)ZnO–XLa2O3, 0 ≤ X ≤ 0.09; B: 0.51SiO2–0.35ZnO–(0.14-Y)Na2O–YLa2O3, 0 ≤ Y ≤ 0.14). Glasses were characterized in order to examine composition–structure–property–function relationships with a view to their potential use as therapeutic inorganic ion delivery systems. Glasses were melt synthesized and the role of lanthanum was investigated with respect to (i) glass structure based on density, glass transition temperature (Tg) and 29Si MAS-NMR, (ii) the effect on radiopacity (computed tomography) as a function of composition and (iii) the ion release characteristics under simulated physiological conditions. A linear increase in density and Tg was observed with the addition of La2O3 to each glass series. Generally, glasses demonstrated a constant molar volume, while density was observed to increase linearly with the addition of La2O3. The peak maxima of the 29Si MAS NMR spectra remained relatively unchanged by the addition of La2O3 in both glass series, but the line shapes and widths reveal modifications in the local silica structure. Radiopacity increased with La2O3: series A ranged between 1770 ± 33 and 6590 ± 89 HU and B Series B ranged between 2340 ± 54 and 9120 ± 290 HU. Series A glasses were observed to release only Na, it was also noted that increased La2O3 caused a reduction in Na+ release. Series B glasses released all constituent glass elements. Interestingly, in series B, La3 + concentration decreased over time indicating possible reprecipitation. Based on the structural data, lanthanum does not behave as a traditional network modifier and may in fact act to stabilize the network against degradation by the formation of mixed cation clusters.