Bioactivity of Sodium Free Fluoride Containing Glasses and Glass-Ceramics

Abstract

The bioactivity of a series of fluoride-containing sodium-free calcium and strontium phosphosilicate glasses has been tested in vitro. Glasses with high fluoride content were partially crystallised to apatite and other fluoride-containing phases. The bioactivity study was carried out in Tris and SBF buffers, and apatite formation was monitored by XRD, FTIR and solid state NMR. Ion release in solutions has been measured using ICP-OES and fluoride-ion selective electrode. The results show that glasses with low amounts of fluoride that were initially amorphous degraded rapidly in Tris buffer and formed apatite as early as 3 h after immersion. The apatite was identified as fluorapatite by ¹⁹F MAS-NMR after 6 h of immersion. Glass degradation and apatite formation was significantly slower in SBF solution compared to Tris. On immersion of the partially crystallised glasses, the fraction of apatite increased at 3 h compared to the amount of apatite prior to the treatment. Thus, partial crystallisation of the glasses has not affected bioactivity significantly. Fast dissolution of the amorphous phase was also indicated. There was no difference in kinetics between Tris and SBF studies when the glass was partially crystallised to apatite before immersion. Two different mechanisms of apatite formation for amorphous or partially crystallised glasses are discussed.

Keywords: bioactive glass-ceramics; fluorapatite; fluoride; sodium-/alkali-free bioactive glass.

Figure 1

XRD patterns of glasses after immersion in buffer solutions: (a) GF 3.0 in Tris; (b) GF 3.0 in SBF; (c) GF 9.3 in Tris; (d) GF 9.3 in SBF; (e) SF 3.0 in Tris; (f) SF 3.0 in SBF; (g) SF 9.3 in Tris; (h) SF 9.3 in SBF. Label Ap in CaF₂ series is fluorapatite Ca₅(PO₄)₃F (00-034-0011), and strontium fluorapatite Sr₅(PO₄)₃F in SrF₂ series (00-050-1744).

Figure 2

FTIR spectra of glasses after immersion in buffer solutions: (a) GF 3.0 in Tris; (b) GF 3.0 in SBF; (c) GF 9.3 in Tris; (d) GF 9.3 in SBF; (e) SF 3.0 in Tris; (f) SF 3.0 in SBF; (g) SF 9.3 in Tris; (h) SF 9.3 in SBF.

Figure 3

³¹P (a) and ¹⁹F (b) MAS-NMR spectra of the samples collected after immersion in Tris buffer solution for 6 h.

Figure 4

pH measured at the end of the immersion time in Tris buffer. Open symbols are for the CaF₂ series (GF) and closed symbols are for the SrF₂ series (SF) with the same nominal fluoride content.

Figure 5

(a) The percentage of phosphate presented as concentration of elemental phosphorus in Tris for CaF₂-containing glasses; (b) The percentage of cations (Sr²⁺ or Ca²⁺) concentration in Tris; (c) The percentage of fluoride concentration measured at 3 and 6 h in Tris for CaF₂-containing glasses.