In an article recently published in the open access journal Materialsthe researchers discussed the sintering of the new bioactive glass enriched with magnesium and strontium oxide at low temperatures.
Study: Low temperature sintering of a new bioactive glass enriched with magnesium oxide and strontium oxide. Image Credit: nevodka/Shutterstock.com
The discovery of bioactivity in the late 1970s marked a turning point in the study of biomaterials. Since then, orthopedics and dentistry have widely used the first bioactive (BG) glass, 45S5 Bioglass® (45S5), for biomedical use. Unfortunately, BGs have inherited several disadvantages of ordinary glasses, including fragility and, in particular, the propensity to crystallize needed to sinter powders or deposit bioactive coatings on metal substrates.
According to reports, crystallization reduces ionic release from glass and inhibits their bioactivity, which limits the responsiveness of these new materials to biological processes. Unlike ordinary silicate glasses, 45S5 and BGs generally contain significantly more modifying oxides, which disrupt the silicate network.
The composition of 45S5 has been changed several times. Although the resulting glasses showed a reduced tendency to crystallize, the impact of these ions on the reactivity of the glass is not yet fully understood. A new class of BG has recently been created with high crystallization temperature and notable biological reaction. The high biological activity of these materials is explained by the presence of magnesium and strontium, whose therapeutic value has been supported by several investigations.
HM analysis of BGMS_LS pressed powders. Image Credit: Bellucci, D et al., Materials
About the study
In this study, the authors discussed the development of a new bioactive glass (BGMS LS) that could solve the main drawbacks of commercial BGs. The developed material could be sintered at extremely low temperatures without crystallizing, preserving all of its biological potential. It also contained strontium and magnesium, two elements whose medicinal importance is well known.
This work improved the composition of the glass by lowering the silicon content and increasing the sodium content, resulting in an even more reactive glass known as BGMS LS. BGMS LS had one of the lowest sintering temperatures of around 686°C due to its extraordinarily low crystallization compared to conventional BGs.
The team suggested the possibility of sintering the new BGMS LS while leaving it entirely amorphous, which would preserve the in vivo dissolution rate, biological activity and ionic release of the finished product. This had advantages from an economic point of view. Fusion-quenching was used to create BGMS LS.
To determine the typical temperature of the material, the glass powders were tested by heating microscopy (HM) and differential thermal analysis (DTA). The glass particles were compressed onto discs and heated at 686°C for three hours. An in vitro test in SBF, where it was possible to see both the significant precipitation of apatite and the creation of a layer of silica gel (sg), which preceded and followed the precipitation of the apatite itself , was used to confirm the considerable bioactivity of BGMS LS.
The researchers used the volume shrinkage D% of the glass discs after heat treatment and the sintering parameter to evaluate the sintering attitude of the BGMS LS. By X-ray diffraction (XRD), crystallization in sintered samples was determined. The hardness and Young’s modulus of the sintered samples were evaluated using a micro-indentation technique and an open-platform apparatus.
By soaking the samples for 14 and 7 days in a simulated body fluid (SBF) solution, which had an ion concentration close to that of human plasma, the in vitro bioactivity of BGMS LS discs was studied. Micro-Raman spectroscopy and X-ray energy dispersive spectroscopy (EDS) were used to study the potential precipitation of apatite. A Raman microscope spectrometer was used to achieve this goal.
(a,b) The formation of HA on BGMS_LS samples after soaking in SBF; (vs) Typical Raman spectra acquired on HA precipitates. Arrows indicate major HA peaks. Image Credit: Bellucci, D et al., Materials
At 810°C, the BGMS LS began to devitrify and peak crystallization occurred soon after at an incredibly high temperature of approximately 860°C. The processing window of BGMS LS at 200 °C was larger than that of 45S5 and its Sc sintering parameter was positive and high, indicating independent sintering and crystallization kinetics that occurred prior to devitrification. The EDS results showed that except for small fluctuations, the Ca/P ratio in the precipitate approximated that of apatite, providing evidence for the presence of apatite.
The symmetric v1 stretching vibration of phosphate anions, which was the dominant peak in the Raman spectra at 960 cm-1, dominates the spectrum. At 430cm-1 and 590cm-1, two additional large peaks were observed that gradually emerged from the background. The notable bioactivity of BGMS LS and its low temperature sintering could be attributed to several competing elements from the composition of the glass.
The glass lattice was weaker due to the reduction of SiO2 content compared to 45S5 which favors the reactivity of the glass. The greater thermal stability was the result of the reduction of Na2Level O. The positive impact of MgO was enhanced by glass sintering due to its stronger field than calcium. Strontium made the new multi-component BGMS LS more viscous and increased the entropy of mixing, which promoted the persistence of a disordered amorphous state and prevented crystallization. These factors made the notable reduction of alkali oxides and the inclusion of strontium and magnesium, whose biological importance was established, an attractive solution to circumvent the well-known drawbacks of conventional bioactive glasses.
BGMS_LS samples after immersion in SBF for 14 days: (a) cross section and (b–D) EDS spectra. Image Credit: Bellucci, D et al., Materials
In conclusion, this study revealed that the new BGMS LS had exceptional thermal stability due to its composition, which allowed the glass to sinter at a significantly lower temperature than standard 45S5. The fritted glass was entirely amorphous despite the heat treatment, allowing the creation of highly bioactive scaffolds and coatings.
The authors mentioned that BGMS LS is extremely promising, especially for the creation of goods that need to undergo a thermal process, such as scaffolds for bone tissue regeneration. They also said the biological reactivity of magnesium and strontium could make the new glass even more promising in light of a clinical application for BGMS LS.
Bellucci, D., Cannillo, V., Low temperature sintering of a new bioactive glass enriched with magnesium oxide and strontium oxide. Materials, 15(18), 6263 (2022). https://www.mdpi.com/1996-1944/15/18/6263