Dissolution of borate glasses and precipitation of phosphate compounds

Borate glasses have been developed for biomedical applications such as scaffolds for soft tissue and bone repair. The dissolution processes of borate glasses in phosphate-containing aqueous solutions were studied by µ-Raman spectroscopy which provided information about the types and concentrations of borate species released into the solution as a function of time and characterized the formation of calcium phosphate reaction products on the glass surface. Boric acid molecules (H3BO3) and borate anions (B(OH)4-) can be detected in solution and their relative concentrations depend on the solution pH. Static and dynamic single-pass flow-through experiments were employed to study the dissolution kinetics of a borate bioactive glass 13-93B3 in water, simulated body fluid (SBF), and other solutions. As the glasses react, B-, Ca-, Na-, K-, Mg-, and P-species were released from the glass and a magnesium-containing amorphous calcium phosphate (ACP) or hydroxyapatite (HAP) layer formed on the surface of the glass. The formation of crystalline hydroxyapatite was favored with faster flow rates, longer reaction times, and increased phosphate concentration in solution. Under static conditions, the dissolution rates are initially described by a reaction-controlled model (linear kinetics), but after the glass is ~25-30% reacted, a diffusion-controlled model (parabolic kinetics) better describes the dissolution rates. The change in reaction mechanism is attributed to the diffusion of species from the glass through the ACP layer. The activation energy for the reaction-controlled process is 41.1±0.6 kJ/mol, whereas the activation energy for the diffusion process is 32.3±0.1 kJ/mol. For the SPFT experiments, glasses dissolved faster under faster flow rates and smaller glass volumes. The ion release rate was calculated and found to range from 1.7x10-5 g/m2/s for slow flow rates to 1.1x10-3 g/m2/s for lower glass volumes. --Abstract, page iv