Bioactivity in silica/poly(γ-glutamic acid) sol–gel hybrids through calcium chelation

Bioactive glasses and inorganic/organic hybrids have great potential as biomedical implant materials. Sol–gel hybrids with interpenetrating networks of silica and biodegradable polymers can combine the bioactive properties of a glass with the toughness of a polymer. However, traditional calcium sources such as calcium nitrate and calcium chloride are unsuitable for hybrids. In this study calcium was incorporated by chelation to the polymer component. The calcium salt form of poly(γ-glutamic acid) (γCaPGA) was synthesized for use as both a calcium source and as the biodegradable toughening component of the hybrids. Hybrids of 40 wt.% γCaPGA were successfully formed and had fine scale integration of Ca and Si ions, according to secondary ion mass spectrometry imaging, indicating a homogeneous distribution of organic and inorganic components. 29Si magic angle spinning nuclear magnetic resonance data demonstrated that the network connectivity was unaltered with changing polymer molecular weight, as there was no perturbation to the overall Si speciation and silica network formation. Upon immersion in simulated body fluid a hydroxycarbonate apatite surface layer formed on the hybrids within 1 week. The polymer molecular weight (Mw 30–120 kDa) affected the mechanical properties of the resulting hybrids, but all hybrids had large strains to failure, >26%, and compressive strengths, in excess of 300 MPa. The large strain to failure values showed that γCaPGA hybrids exhibited non-brittle behaviour whilst also incorporating calcium. Thus calcium incorporation by chelation to the polymer component is justified as a novel approach in hybrids for biomedical materials

A unified in vitro evaluation for apatite-forming ability of bioactive glasses and their variants

The aim of this study was to propose and validate a new unified method for testing dissolution rates of bioactive glasses and their variants, and the formation of calcium phosphate layer formation on their surface, which is an indicator of bioactivity. At present, comparison in the literature is difficult as many groups use different testing protocols. An ISO standard covers the use of simulated body fluid on standard shape materials but it does not take into account that bioactive glasses can have very different specific surface areas, as for glass powders. Validation of the proposed modified test was through round robin testing and comparison to the ISO standard where appropriate. The proposed test uses fixed mass per solution volume ratio and agitated solution. The round robin study showed differences in hydroxyapatite nucleation on glasses of different composition and between glasses of the same composition but different particle size. The results were reproducible between research facilities. Researchers should use this method when testing new glasses, or their variants, to enable comparison between the literature in the future

Tailoring the nanoporosity of sol-gel derived bioactive glass using trimethylethoxysilane

Sol-gel derived bioactive glasses are thought to have high potential as materials for bone regeneration and drug delivery devices. They bond to bone and have a controllable degradation rate. Their unique nanoporosity provides high surface area and exposes a high concentration of surface hydroxyl groups. Protein adsorption, degradation rate and cellular response are known to be affected by nanotopography, therefore it is important to be able to produce glasses with a range of pore sizes. In this study, the modal nanopore diameters of glasses with the bioactive composition 70 mol% SiO2 and 30 mol% CaO (70S30C) were successfully increased from 12 to 30 nm by adding specific amounts of trimethylethoxysilane (TMES) during the sol-gel process. The mechanism of the nanoporosity modification was studied with transmission electron microscopy (TEM), nitrogen sorption and Si-29 magic angle spinning (MAS) solid-state NMR spectroscopy. Solid-state NMR was used to investigate how the modification processes affected the atomic scale structure of the glass, such as Q structure and network connectivity, which was related to the changes in nanostructure using combinations of nitrogen sorption and TEM. The TMES was found to inhibit the fusion of the nanoparticle structural components of the glasses, causing an increase in pore size

Role of pH and temperature on silica network formation and calcium incorporation into sol–gel derived bioactive glasses

Bioactive glasses and inorganic/organic hybrids have great potential as implant materials. Bioactive glasses can bond to bone through the formation of a bone-like hydroxycarbonate apatite (HCA) layer and stimulate bone growth via their dissolution products. The brittle nature of these glasses can be combined with the toughness of a biodegradable polymer by forming a hybrid through the sol–gel process. However, for polymer incorporation, lower temperatures and milder pH conditions are required rather than the current method which uses pH 2 were mesoporous. This indicates a difference in gel formation about the isoelectric point of silicic acid, which was confirmed by 29Si solid state NMR. When immersed in simulated body fluid (SBF), the glasses stabilised at 600 °C were more porous, yet had a slower ion release rate than the gels dried at 40 °C. All gels and glasses formed an HCA layer in SBF; however, calcium was only incorporated into the silica network after stabilisation at 600 °C and thus a new way of incorporating calcium at low temperatures must still be found. This work is an important foundation for hybrid synthesis as raising the pH of the sol–gel process from pH < 1 to pH 5.5 was found to have no adverse effects on silica network formation and thus polymer can be incorporated into the sol–gel process at milder pH conditions without the concern of acid catalysed polymer degradation by chain scission

A Unified in Vitro Evaluation for Apatite-Forming Ability of Bioactive Glasses and Their Variants

The aim of this study was to propose and validate a new unified method for testing dissolution rates of bioactive glasses and their variants, and the formation of calcium phosphate layer formation on their surface, which is an indicator of bioactivity. At present, comparison in the literature is difficult as many groups use different testing protocols. An ISO standard covers the use of simulated body fluid on standard shape materials but it does not take into account that bioactive glasses can have very different specific surface areas, as for glass powders. Validation of the proposed modified test was through round robin testing and comparison to the ISO standard where appropriate. The proposed test uses fixed mass per solution volume ratio and agitated solution. The round robin study showed differences in hydroxyapatite nucleation on glasses of different composition and between glasses of the same composition but different particle size. The results were reproducible between research facilities. Researchers should use this method when testing new glasses, or their variants, to enable comparison between the literature in the future

Chemical characterisation and fabrication of chitosan–silica hybrid scaffolds with 3-glycidoxypropyl trimethoxysilane

Chitosan has been explored as a potential component of biomaterials and scaffolds for many tissue engineering applications. Hybrid materials, where organic and inorganic networks interpenetrate at the molecular level, have been a particular focus of interest using 3-glycidoxypropyl trimethoxysilane (GPTMS) as a covalent crosslinker between the networks in a sol–gel process. GPTMS contains both an epoxide ring that can undergo a ring opening reaction with the primary amine of chitosan and a trimethoxysilane group that can co-condense with silica precursors to form a silica network. While many researchers have exploited this ring-opening reaction, it is not yet fully understood and thus the final product is still a matter of some dispute. Here, a detailed study of the reaction of GPTMS with chitosan under different pH conditions was carried out using a combination of solution state and solid state MAS NMR techniques. The reaction of GPTMS with chitosan at the primary amine to form a secondary amine was confirmed and the rate was found to increase at lower pH. However, a side-reaction was identified between GPTMS and water producing a diol species. The relative amounts of diol and chitosan–GPTMS species were 80 and 20% respectively and this ratio did not vary with pH. The functionalisation pH had an effect on the mechanical properties of 65 wt% organic monoliths where the properties of the organic component became more dominant. Scaffolds were fabricated by freeze drying and had pore diameters in excess of 140 μm, and tailorable by altering freezing temperature, which were suitable for tissue engineering applications. In both monoliths and scaffolds, increasing the organic content disrupted the inorganic network, leading to an increase in silica dissolution in SBF. However, the dissolution of silica and chitosan was congruent up to 4 weeks in SBF, illustrating the true hybrid nature resulting from covalent bonding between the networks