Autologous bone or allogenic implants (i.e., from a donor) are still used in current clinical practice. However, synthetic materials offer undoubted advantages like their lower cost and availability, less pain for the patients and above all reduced risk of infections or adverse effects. Bioactive glasses have been one of the most intensively studied synthetic biomaterials. Their composition is based on a mixture of oxides from the SiO₂-CaO-MgO-Na₂O-K₂O-P₂O₅ system, and represent one of the most suitable materials for bone repair owing to their high bioactivity resulting in a special ability to attach to the bone tissue through the formation of a mineral hydroxyapatite layer when soaked in a biological solution. These materials are mainly synthesised either by the melting and crushing method or by the sol-gel method. The latter has multiple advantages over the former, such as perfect control of the chemistry, giving rise to high purity feedstocks and a simple way to modify the composition, as well as low processing temperatures. However, the sol-gel method requires long material processing times.
 
The main objective of this work was to synthesise bioactive glasses by a hydrothermal chemical route assisted by microwaves. Microwave processing could be of great interest because of its potential applications in the ceramic industry. The use of microwaves results in rapid and uniform heating (no selective heating of the surface), energy saving process (low power consumption in order to reach low to moderate working temperatures), higher yield and shorter preparation time, lower processing cost, small narrow particle size distribution and high purity.
 
The bioactive glass 58S (58% SiO₂, 33% CaO and 9% P₂O₅ in wt%) has been chosen as the working composition. Aqueous-based solutions were prepared using tetraethyl orthosilicate, triethyl phosphate and calcium nitrate as precursors of SiO₂, P₂O₅ and CaO respectively. In addition, nitric acid was also used as catalyst for the hydrolysis and condensation of tetraethyl orthosilicate. The sols were introduced into a microwave digestion system (MW Ethos One, Milestone) and several residence times, different temperatures (in the range of 100 – 250 ºC) and pressures (up to 35 bar) were tested. The resulting aged gel was crushed in a mortar, dried at 80 ºC and stabilised by a thermal treatment using a lab-made microwave sintering furnace. In all cases, several temperatures were tested based on a thermal analysis. For the sake of comparison, the same glass was synthesised by the sol-gel method and stabilised using a convencional furnace.
 
The resulting glasses were characterised in terms of microstructure and phase nature by scanning electron microscopy and X-ray diffraction respectively. Moreover, the elemental composition was assessed by energy dispersive X-ray spectroscopy and the different chemical bonding groups were analysed by Fourier-transform infrared spectroscopy. Furthermore, all glasses were immersed in simulated body fluid (SBF) for different periods of time following a standard protocol.
 
 
Acknowledgment
The authors thank the Spanish Ministry of Universities the support provided by the Margarita Salas postdoctoral contract (MGS/2022/20) financed by the European Union - NextGenerationEU