Reactions of Borate Glasses in Aqueous Solutions

Borate glasses are used for a variety of biomedical applications that depend on the controlled release of species from the glass to the surrounding physiological environment. As a consequence, a detailed understanding of their dissolution processes is required to design glasses for specific applications. Under neutral, dilute conditions, borate glasses dissolve congruently at rates that decrease for compositions with greater fractions of four-coordinated borate sites and modified with metal oxides that have greater cation field strengths. These dissolution rates then can be understood by considering the concomitant hydrolysis of the borate glass network and hydration of the metal ion sites. Ca ions released from borate glasses dissolved in phosphate-solutions, including physiological environments, react with those phosphate anions to form amorphous calcium phosphates that can transform to hydroxyapatite, and in so doing, promote tissue regeneration. The mesoscopic structures of these calcium phosphate phases depend on the glass composition and the dissolution conditions.