  Phosphate glasses for bioresorbable implants display dissolution rates that
vary significantly with composition, however currently their mechanisms of
dissolution are not well understood. Based on this systematic study we present
new insights into these mechanisms. Two-stage dissolution was observed, with
time dependence initially parabolic and later linear, and a two-stage model was
developed to describe this behaviour. Dissolution was accelerated by lower Ca
concentration in the glass, and lower pH in the dissolution medium. A new
dissolution mechanism is proposed, involving an initial stage where
diffusion-controlled formation of a conversion layer occurs. Once the
conversion layer is stabilised, layer dissolution reactions become
rate-limiting. Under this mechanism the transition time is sensitive to the
nature of the conversion layer and solution conditions. These results reveal
the dependence of P$_{2}$O$_{5}$-CaO-Na$_{2}$O glass dissolution on solution
pH, and provide new insight into the dissolution mechanisms, particularly
regarding the transition between the two dissolution stages.
