Glass - Back to the Future!

Presenting Author:
Delia S. Brauer

article posted 21 March 2016

Delia S. Brauer is a junior professor at the Otto Schott Institute, University of Jena, Germany. After finishing her studies in environmental chemistry she completed her PhD on phosphate glasses at the University of Jena. She worked as a postdoctoral researcher at the University of California, San Francisco, Nagoya Institute of Technology, Japan, and Imperial College and Queen Mary University of London, UK, before returning to Jena in 2012. Her research focuses on the structure–property relationship in glasses, with a focus on degradable and highly disrupted glass systems including phosphate glasses and phospho-silicate glasses. She is a member of Technical Committee 04 (Bioglasses) of the International Commission on Glass, member of the Basic Sciences and Technology Committee of the Society of Glass Technology, Associate Editor of a new journal Biomedical Glasses and winner of the Gottardi Prize of the ICG in 2015.

Ion release from mixed alkali phospho-silicate glasses

R. Brückner1, M. Tylkowski1, L. Hupa2 & D.S. Brauer1,*
1 Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Germany
2 Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Turku, Finland

Bioactive phospho-silicate glasses are used clinically to regenerate bone, but their pronounced crystallisation tendency limits high temperature processing. Mixed alkali compositions are therefore of interest owing to their reduced crystallisation tendency. Here, we are investigating the influence of alkali substitution on ion release and dissolution.
In melt-derived Bioglass 45S5 (46.1 SiO2, 2.6 P2O5, 26.9 CaO, 24.4 Na2O; mol%) Na2O was replaced by either Li2O or K2O (0, 25, 50, 75 or 100%). Static (6 hours to 3 days) and dynamic (continuous-flow; up to 2500 s) dissolution experiments were performed on glass powders in tris(hydroxymethyl)aminomethane (Tris) buffer at pH 7.4, followed by ICP-OES analysis. Dissolution experiments showed slower pH change and ion release with increasing Li for Na substitution (Figure 1a) and faster pH change ion release with increasing K for Na substitution (Figure 1b). This suggests that in these glasses, ion release is controlled by the packing density of the glass: substitution of K for Na (ionic radii: K+ 138 pm, versus Na+ 102 pm) expands the network and thus facilitated ion exchange with the surrounding solution, while the opposite effect was observed for Li substitution (Li+ 76 pm). Changes with composition did not show any minima or maxima, indicating that the mixed alkali effect did not play any pronounced role during the time periods studied. This is in contrast with thermal properties of the same glasses, which previously showed the typical mixed alkali effect,(1) and with ion release from more crosslinked, conventional mixed alkali silicate glasses.(2)

Figure 1: Ions in solution (shown as percentage of ions present in the untreated glass) for (a) Li and (b) K substituted glasses at 6 hours in Tris buffer during static dissolution experiments. SEM images showing surface corrosion features at 7 days in ambient atmosphere for (c) 100% Li substituted and (d) 100% K substituted composition.

Freshly fractured, uncoated glass monoliths were investigated in an SEM (Jeol JSM 7001F; 2 kV) directly after fracturing and again after being in contact with ambient air for various time periods. SEM images showed circular sub-micrometre features (Figure 1c,d), which increased in size and changed in general appearance when in contact with atmospheric humidity. This surface corrosion was more pronounced with in K substituted glasses than in Li substituted glasses, in agreement with results from dissolution experiments.
Taken together, these results suggest that the ion release from highly disrupted phospho-silicate glasses is not reduced for mixed alkali compositions but that variations in modifier ionic radius affect ion release via variations in glass molar volume.

1. M. Tylkowski & D.S. Brauer, Mixed alkali effects in Bioglass® 45S5, J Non-Cryst Solids, 376 (2013) 175-181.
2. M.F. Dilmore et al., Chemical durability of Na2O-K2O-CaO-SiO2 glasses, J Amer Cer Soc, 61 (1978) 439-443.