The influence of cobalt incorporation and cobalt precursor selection on the structure and bioactivity of sol–gel-derived bioactive glass

Cobalt (Co) is a potential therapeutic ion used to enhance angiogenesis through a stabilizing effect on hypoxia-inducible factor 1 alpha (HIF-1α), and its incorporation into the structure of bioactive glass is a promising strategy to enable sustained local delivery of Co to a wound site or bone defect. Here Co-releasing bioactive glasses were obtained through the sol–gel method, comparing cobalt nitrate and cobalt chloride as precursors. The effect of using different Co precursors on the sol–gel synthesis and in the obtained bioactive glass structure, chemical composition, morphology, dissolution behaviour, hydroxycarbonate apatite (HCA) layer formation was investigated. When the chloride salt was used as Co precursor, evidence of crystalline cobalt (II, III) oxide (Co3O4) phase formation was found, along with the presence of Co3+ species as evaluated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), whereas an amorphous glass containing mainly Co2+ species was obtained when the nitrate salt was the Co source. The presence of a crystalline phase decreased the surface area and pore volume of the final glass, consequently reducing the Co-release rate. Evidence of HCA layer formation after immersion in simulated body fluid (SBF) was still found when different precursors were used, although the rate of formation was reduced by the presence of Co. Therefore, this study showed that Co incorporation and the proper selection of the precursor could affect the final material structure, and properties, and should be considered when designing new bioactive glass compositions for tissue engineering applications.