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The aim of this study was to propose and validate a new unified method for testing dissolution rates of bioactive glasses and their variants, and the formation of calcium phosphate layer formation on their surface, which is an indicator of bioactivity. At present, comparison in the literature is difficult as many groups use different testing protocols. An ISO standard covers the use of simulated body fluid on

Glasses along four compositional tie lines, I: xAI(PO,),.(lx)NaPO, 11: xAIPO,.(l -x)NaPO,, 111: xAl,O,.(lx)NaPO,, and I V xNaAlO,.(l -x)NaPO,, have been prepared and a number of their properties evaluated. In general, AI,O, additions increase the glass transition temperature, decrease thermal expansion, and improve aqueous durability. However, the composition/property relationships are strongly influenced by relative cation ratios and thus the overall O/P ratio. The nature of these glasses undergoes significant transformations at both the pyrophosphate (O/P = 3.5) and the orthophosphate (O/P = 4.0) compositions, producing significant changes in the property behavior. Spectroscopic analyses of glass structure, described in part I1 of this report, reveal that changes in A1 coordination coincide with the breaks in the property behavior.

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We have used 27AI and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to describe quantitatively the relationships between the composition, structure, and properties of glasses in the Naz0~AI2O3-P,O, (NAP) system. In general, the glass properties (evaluated in part I) are most sensitive to changes in A1 coordination. "AI MAS NMR spectra reveal that octahedrally coordinated A1 is most abundant in glasses with O/P ratios less than 3.5, the pyrophosphate structural limit. Tetrahedrally coordinated A1 is most abundant in glasses with O/P greater than 3.5. Decreasing AI(OP),/AI(OP), ratio generally correlates with decreasing glass transition temperature and refractive index. The compositional dependence of glass structure and properties can be qualitatively understood using a crystal chemical model based on oxygen charge balancing by the different Al and P moieties.

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