Nanostructure evolution and calcium distribution in sol–gel derived bioactive glass

Abstract

Sol–gel derived bioactive glasses (70 mol% SiO₂ and 30 mol% CaO) have high potential as materials for bone regeneration and devices for sustained drug delivery. They bond to bone and have a controllable degradation rate. They have a unique tailorable nanoporosity, which enhances their surface area and exposes hydroxyl groups and affects protein adsorption and cellular response. This study aims to fully characterise the evolution of the nanoporous structure of sol–gel derived bioactive glass for the first time, to fully understand its nanostructure evolution and control, so that materials with specific nanoporous networks can be produced to further enhance effects on tissue regeneration. It was confirmed that nanopores of sol–gel derived bioactive glass are interstitial spaces between nanoparticles. Nanoparticles, approximately 5 nm in diameter that were produced early in the process, agglomerated into larger particles during the gelation process (10–30 nm in diameter) during stabilisation via heat treatment. Inductive coupled plasma (ICP) analysis of the pore liquor after ageing revealed that calcium nitrate (the calcium precursor) dissolves in pore liquor before drying. Thermal real time X-ray diffraction and MAS-NMR data revealed that calcium nitrate coated the silica nanoparticles during drying and calcium did not enter the silica network until the material was heated to 400 °C. This has implications for ensuring a homogeneous calcium distribution in bioactive glasses made by the sol–gel route.