Phosphate Glasses - Bio-Glasses: An Introduction - page 55

composition is heat-treated, a crystalline surface layer appears while the
centre of the monolith is free of crystals (see inset in Figure 4.12).

4.5 PHOSPHATE GLASS DISSOLUTION

The main reason why phosphate glasses are of interest for use as
biomaterials is their ability to dissolve completely in aqueous solutions
into safe non-toxic dissolution products.
The dissolution rate of phosphate glasses is very sensitive to glass
composition [8]. For dissolution of vitreous P 2 O 5 and ultraphosphate
glasses, cleavage of P-O-P bonds is necessary. As mentioned above,
hydrolysis of vitreous P 2 O 5 occurs readily, but also ultraphosphate
glasses are prone to hydrolysis, which results in degradation in the
presence of atmospheric humidity.
By contrast, for dissolution of meta- and polyphosphate glasses,
no P-O-P bond cleavage is necessary. Comparison of phosphate chain
lengths in the glass and those found in solution shows that the phosphate
chains stay intact during glass dissolution [8]. These glasses dissolve by
hydration of entire phosphate chains and subsequent chain disentangle-
ment and dissolution. Hydrolysis of polymeric phosphate chains occurs
in solution after the initial hydration and dissolution, but at a much
slower rate.
In general, solubility decreases with decreasing P 2 O 5 content.
Figure 4.13 shows glass dissolution (presented as dissolved phosphate
relative to total phosphate in the glass) as a function of phosphate
content in the glass, and a general trend confirms an increase in solubility
with increasing phosphate content, in the order invert glasses →

polyphosphate glasses metaphosphate glasses.

When phosphate glasses dissolve, they affect the pH of the surrounding
solution differently to silicate glasses (Chapter 2). Metaphosphate glasses
are known to give an acidic pH, while decreasing phosphate content
increases the pH change towards neutral (Figure 4.14). The resulting
pH, however, depends on several factors, such as the ratio of glass to
liquid, fluid flow/exchange or the buffering capacity of the solution.
The pH of the surrounding solution also affects glass dissolution. In
an acidic environment, phosphate glass dissolution increases dramati-
cally [8], while a basic pH also increased glass dissolution (compared to
pH 7) although at a slower rate. This shows that maintaining the pH in
a neutral range is important if glass degradation needs to be controlled
(e.g. for degradable implant materials). If a phosphate glass, such as a
metaphosphate glass, lowers the local pH upon initial degradation, it

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