Hydroxyapatite Coating of Volcanic Glass Microballoons by a Homogeneous Precipitation Process

Abstract

Volcanic glass microballoons (so-called Shirasu balloons) are produced via the expansion of hot volcano ash sand. This work assessed the application of a biocompatible hydroxyapatite (HAp) coating to this material, which both floats in most solvents and has extremely thin outer shells. A two-step process consisting of homogeneous precipitation followed by conversion of the crystal phase using static liquid immersion was found to be effective for this purpose. A uniform HAp coating was obtained by converting the initial calcium phosphate layer to HAp by immersing the microballoons in an optimized solution under static conditions.