The changes in the structure of 45S5 Bioglass® and its heat-treated versions upon SrO substitution were investigated by solid-state multinuclear magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray diffraction and differential scanning calorimetry. A series of the melt-derived glasses with SrO gradually substituted for calcium (0, 25, 50, 75 and 100%) on a molar basis was synthesised. The glass transition temperature, T_g, and crystallisation temperature decreased upon SrO substitution. Glasses with 75 and 100% of SrO substitution showed a minor crystalline phase. ³¹P MAS-NMR showed that phosphorus is present as orthophosphate in the entire glass series; ²⁹Si MAS-NMR data showed a predominantly Q² silicate network. A small step change in the position of the ³¹P and ²⁹Si MAS-NMR signals found between the glasses with 25 and 50% of SrO substitution is attributed due to an increase in the ring-type fraction of the Q² silicate glass over the mainly chain topology in the original 45S5 glass. On heat treatment combeite (Na₂Ca₂Si₃O₉) and combeite-type (Na₂CaSi₂O₆) silicate phases were identified. For glasses with increasing SrO contents the equivalent Sr²⁺ substituted phases were formed (viz. Na₂Sr₂Si₃O₉ and Na₂SrSi₂O₆) above 600 °C. The polymerisation degree of phosphorus increased and in addition to orthophosphate amorphous pyro- and meta-phosphate species were observed on heat treatment between 600 and 650 °C, the amount of which reduced with SrO substitution. Above 800 °C, changes in ³¹P MAS-NMR spectra suggested that several phosphate containing phases crystallised and the fraction of the remaining glass was below the detection limit. In contrast to the SrO containing glasses, in the original 45S5 composition a significant fraction of phosphorus remained amorphous even after heat treatment at temperatures above 800 °C.

It is well known that bone has excellent mechanical properties through its hierarchical structure and that design of new materials should take inspiration from its structure, especially those that will be used as synthetic bone grafts. However it has not yet been possible to mimic the complex structure. Here we present new organic–inorganic hybrid scaffolds produced by a sol–gel foaming process with the aim of producing materials that mimic the trabecular structure of bone and take inspiration from the nanocomposite structure. Poly(γ-glutamic acid) (γ-PGA) was chosen as the organic component because it is a polypeptide that can be functionalised and can degrade by enzyme action. The γ-PGA chains and bioactive silica were covalently bonded to each other (class II hybrid) through an organosilane coupling agent, glycidoxypropyl trimethoxysilane (GPTMS). This was key to obtaining a hybrid with good mechanical properties and homogeneous integration of inorganic and organic chains at the nanoscale. The effect of the degree of covalent coupling on mechanical properties and dissolution rate of the hybrids was investigated. Hybrids with a covalent coupling between 2 and 10 glutamic acid units per GPTMS molecule showed optimal mechanical properties and polymer release into TRIS-buffer solution. Polymer release was much slower from these hybrids than from those with lower amounts of covalent coupling. Monoliths of these compositions were not toxic to the SaOs-2 osteosarcoma cell line and supported cell attachment and growth. Foaming of the optimised hybrid composition was performed to produce porous 3D scaffolds. The modal pore and interconnect diameters were quantified to be 297 and 111 μm respectively, using micro-computed tomography (μCT) and image analysis, showing the pore network to mimic that of cancellous bone.

This paper presents the electrospinning of new silica/poly(L-lactic acid) (PLLA) based organic/inorganic sol–gel hybrid materials for skeletal defect regeneration. Here, hybrids are defined as inorganic and organic species interacting at the molecular level such that there are no distinguishable phases above the nanoscale. In previous hybrids, calcium has not been successfully incorporated into the silica network, limiting bioactivity. Here, an alkoxide source of calcium (calcium methoxyethoxide (CME)) was successfully used with a modified dual syringe reactive electrospinning (DSRS) setup to produce a calcium and silica containing PLLA sol–gel hybrid fibermat. Fibermats with continuous and homogeneous fibers were successfully produced that showed controlled and tailorable release of soluble silica in aqueous media. The CME-derived PLLA hybrid was observed to be hydrophilic and showed indication of bone-like apatite formation within 12 h of soaking in simulated body fluid (SBF). After soaking in SBF, the entire fiber surface was coated by a homogeneous thickness of apatite that had a plate-like crystal structure. The reversal of PLLA's hydrophobicity has tremendous implications on the biological properties of the material. All the electrospun materials were easy to handle and could be easily shaped into complex shapes. The tensile strength of the hybrid fibermats was found to be enhanced by the addition of silica, where a twofold increase in the tensile strength of 20 wt% silica containing PLLA fibermats compared to that of PLLA fibermats was observed.

Vaterite particles containing aminopropyl-functionalized silsesquioxane (SiV) were prepared as osteogenic devices for bone regeneration. The Si_xV particles (x = 0, 2.6 and 4.9 wt%) were synthesized by reacting a slurry of calcium hydroxide with carbon dioxide gas in the presence of γ-aminopropyltriethoxysilane (APTES), a source of soluble silica which would genetically enhance osteogenesis. The obtained Si_2.6V and Si_4.9V particles were monodispersed with a diameter of 1.4 and 1.5 μm, respectively. The Si_2.6V particles showed spherical morphologies. On the surface of the Si_4.9V particle small particles were aggregated, resulting in the formation of irregular textures. Transmission electron microscopy of a sectioned Si_2.6V particle revealed that the vaterite particles were present as lamellae with a length of 5–20 nm and surrounded by silsesquioxane from APTES. Moreover, the vaterite lamellae were relatively orientated to the c face of the unit lattice, where it is known to be highly polarized, compared to pure vaterite, due to the exposure of the uni-ionic plane with positive (Ca²⁺) or negative (CO₃ ²⁻) charge. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) revealed the co-existence of amorphous calcium carbonate (ACC) in the SiV particles. On contact with physiological pH buffer solution, the vaterite was transiently stabilized and subsequently dissolved and released after the dissolution of silsesquioxane from the particles. This stabilization time was significantly increased with the increase in silicon content. The vaterite was observed in Si_2.6V particles up to 3 h of soaking, which extended up to 12 h in Si_4.9V particles. The formation of the particles from the precursor gel was monitored by laser Raman spectroscopy and ATR-FTIR. During the initial 1 to 2 h of the aging step, maturation of ACC into vaterite and condensation of monomeric APTES molecules were found to begin simultaneously. These reactions proceeded up to 7 h of the analysis period. The condensation of hydrolyzed APTES is suggested to occur in the vicinity of growing vaterite, which might play a role in the enclosure of vaterite in silsesquioxanes.