Zinc  Substituted Bioactive Glasses as Remiineralizing Additives for Toothpaste

Method:  ZnO (2 mole%) was substituted in a BG for SiO₂ along with additional charge balancing cations in order to investigate the role of ZnO and not change the network connectivity associated with zinc substitution for CaO.

Glass dissolution and apatite formation was followed by XRD, FTIR spectroscopy and ³¹P and ¹⁹F MAS-NMR spectroscopy. The ions in solution following immersion in Tris buffer were determined by ICP-OES and by a fluoride ion selective electrode.

Result: The zinc free glass formed a fluorapatite (FAP) like phase in under two hours that was evident in the XRD patterns and FTIR spectra. The zinc containing glass did not show sharp XRD diffraction peaks consistent with the formation of an apatite phase until 24 hours immersion and did not show the characteristic vPO₄ peaks of a crystalline orthophosphate in the FTIR spectra. ³¹P and ¹⁹F MAS-NMR spectra showed that the zinc free glass formed FAP in under two hours and the dissolution of the glass was complete in <6 hours. The zinc containing glass also formed a FAP, but the apatite formed was in smaller amounts and the peaks broader indicating a more disordered apatite. The dissolution of the glass in terms of its fluorine and phosphate content was not complete in 24 Hrs.

Conclusion: small substitutions of zinc oxide in bioactive glasses retard dissolution and apatite formation and this effect is not solely due to changes in network connectivity or due zinc ions in solution inhibiting apatite nucleation and growth.