Templated Bioactive Glasses
Bioactive glasses are materials that form a biologically compatible hydroxycarbonate apatite (HCA) layer at the interface with bone in vivo. This layer allows the glass to establish a continuous bond with bone. An HCA layer can be formed on the surface of bioactive materials when they are soaked in simulated body fluid (SBF), an aqueous solution that has approximately the same ion concentration and pH as human blood plasma. Traditionally, bioactive glasses have been prepared from a melt of glass components or from sol-gel precursors. We have investigated the use of colloidal crystal templating to produce threedimensionally ordered macroporous (3DOM) bioactive glasses (mixed SiO2/CaO compositions). In vitro hydroxycarbonate apatite growth is studied by soaking the bioactive glass in SBF at body temperature. The formation of a hydroxycarbonate apatite layer on the glass can be followed by diffraction, microscopic, spectrocopic and chemical analysis. We have demonstrated that the 3DOM bioactive glass exhibited faster growth of apatite than a non-templated control sample, probably due to the more accessible surface in the 3DOM material. 3DOM glasses possess several features relevant for bioactive materials that result in their faster dissolution and more rapid growth of HCA in SBF. These features include (1) the size of the overall 3DOM particle (up to mm-size), which should prevent rejection of the particle by the body defense system; (2) the large pores which permit facile transport of SBF and probably also natural body fluids; (3) the thin walls which are generated by filling interstices between close-packed templating spheres; and (4) the relatively large accessible surface. In a related project, we have synthesized 3DOM hydroxyapatite materials directly by colloidal crystal templating and investigated the product as a host for uptake and controlled release of drug components.
Fig. 1. Schematic diagram of bioglass conversion in simulated body fluid. It is believed that apatite forms from the dissolved precursor species on top of a silica nucleation layer.
Fig. 3. SEM images of 3DOM hydroxyapatite/tricalcium phosphate.
Fig. 2. SEM images of 3DOM CaO/SiO2 before exposure to SBF (top), after immersion in SBF for 3 h (middle), and after immersion in SBF for 4 days.
Fig. 4. Release of norfloxacin from charged 3DOM hydroxyapatite/tricalcium phosphate in SBF at 37°C.