Effect of immersion in SBF on porous bioactive bodies made by sintering bioactive glass microspheres

Abstract

Since the mid 90s it has been possible to draw fibers and manufacture microspheres of novel bioactive glasses. Thus, by sintering bioactive glass microspheres it is now possible to form porous textures, in which the bioactive surface area is increased manifold compared with non-porous bodies. Four different types of porous bodies were made by sintering glass microspheres of diameter 250–300 μm. Two of the body types contained only one kind of spheres; either highly bioactive glass spheres or spheres made of glass having a low bioactivity (biocompatible glass). Two additional types of test bodies were obtained by sintering mixtures of bioactive and biocompatible spheres (composites). The dissolution of silica and calcium into simulated body fluid (SBF) was determined at different time intervals using a direct current plasma atomic emission spectrometer (DCPAES). The influence of immersion on the mechanical strength of the porous structures was studied by means of a compression test. Further, the thickness of the silica-rich gel formation on the surface of bioactive glass spheres was measured at each time interval using back-scattered electron imaging of scanning electron microscopy (BEI-SEM). A non-porous glass rod made from the same bioactive glass was used as the control. The results showed that dissolution of silica and calcium into SBF from the porous glass texture was inversely related to the silica content of the glass. The rate of silica gel formation on the sintered bioactive microspheres was significantly higher than on a rod made from the same glass. The initial mechanical strength of porous bodies consisting of only one kind of glass was 17–20 MPa. However, these bodies lost their mechanical strength at an early stage of the immersion showing compression strength of only 7–8 MPa at 14 days of immersion. The initial strength of composite glass bodies (7–11 MPa) was lower compared with bodies containing only one kind of glass but the bodies showed no notable mechanical weakening during the test. Softening of the surface of smooth bioactive glass plates correlated well with the formation of the silica-rich layer on the plate. Interestingly, the study also showed that in porous glass structures containing both bioactive and biocompatible glass the biocompatible glass can act as a site for calcium phosphate precipitation.