Jonathan Lao

New Strontium-based Bioactive Glasses: Physicochemical Reactivity and Delivering Capability of Biologically Active Dissolution Products

The development of bone tissue regeneration calls for biomaterials able to release biologically active substances in a controlled manner after implantation. In this context, strontium-doped bioactive glasses are of major interest; their key property relies on the increased kinetics of surface reactions, along with the release of critical concentrations of ionic dissolution products capable of stimulating cellular responses. In this paper, we report a complete evaluation of the in vitro reactivity of new SiO2–CaO–SrO and SiO2–CaO–P2O5–SrO bioactive glasses. In contact with simulated acellular physiological fluids, these materials induce the formation of a calcium phosphate surface layer that closely resembles to the biological apatite present in bones. The two most commonly used materials shapes for clinical applications – glass particles and glass bulk – were studied and provide us with concordant results. Compared to strontium-free materials, the dissolution of SiO2–CaO–SrO and SiO2–CaO–P2O5–SrO glasses is reduced. However, the surface layer is more quickly transformed into a bone-like apatite phase, according to the kinetics of evolution of the Ca/P atomic ratio. Evidences of the presence of Sr at the glass/biological fluids interface were obtained, along with the demonstration that this element is released in physiological concentrations into the biological environment. Knowing the well-recognized beneficial effects of strontium on cell activity and bone remodeling, this crucial result gives high hopes for the development of innovative applications based on Sr-doped glasses in the treatment of osteoporosis and tissue engineering.

87Sr solid-state NMR as a structurally sensitive tool for the investigation of materials: antiosteoporotic pharmaceuticals and bioactive glasses.

Strontium is an element of fundamental importance in biomedical science. Indeed, it has been demonstrated that Sr(2+) ions can promote bone growth and inhibit bone resorption. Thus, the oral administration of Sr-containing medications has been used clinically to prevent osteoporosis, and Sr-containing biomaterials have been developed for implant and tissue engineering applications. The bioavailability of strontium metal cations in the body and their kinetics of release from materials will depend on their local environment. It is thus crucial to be able to characterize, in detail, strontium environments in disordered phases such as bioactive glasses, to understand their structure and rationalize their properties. In this paper, we demonstrate that (87)Sr NMR spectroscopy can serve as a valuable tool of investigation. First, the implementation of high-sensitivity (87)Sr solid-state NMR experiments is presented using (87)Sr-labeled strontium malonate (with DFS (double field sweep), QCPMG (quadrupolar Carr-Purcell-Meiboom-Gill), and WURST (wideband, uniform rate, and smooth truncation) excitation). Then, it is shown that GIPAW DFT (gauge including projector augmented wave density functional theory) calculations can accurately compute (87)Sr NMR parameters. Last and most importantly, (87)Sr NMR is used for the study of a (Ca,Sr)-silicate bioactive glass of limited Sr content (only ~9 wt %). The spectrum is interpreted using structural models of the glass, which are generated through molecular dynamics (MD) simulations and relaxed by DFT, before performing GIPAW calculations of (87)Sr NMR parameters. Finally, changes in the (87)Sr NMR spectrum after immersion of the glass in simulated body fluid (SBF) are reported and discussed.

In vitro reactivity of Cu doped 45S5 Bioglass® derived scaffolds for bone tissue engineering

Cu-doped 45S5 bioactive glasses with varying Cu contents were fabricated and used to process 3D porous scaffolds via the foam replica technique. Cu-doping results in the weakening of the glass network and a decrease in its glass transition temperature. Acellular in vitro studies revealed very high bioactivity independent of Cu doping as indicated by the fast formation of a carbonated hydroxyapatite layer (CHA) on scaffold surfaces after immersion in simulated body fluid (SBF). The kinetics of the glass-ceramic scaffolds transition to an amorphous calcium phosphate layer (ACP) and the crystallisation of CHA were explored by FT-IR and SEM analyses. The elemental distribution in the scaffold/fluid interface region was monitored by the advanced micro-PIXE-RBS (particle induced X-ray emission/Rutherford backscattering spectrometry) method. Cu-containing glasses showed slower release of Si, Ca and P from the scaffold periphery, whereas traces of Cu were found incorporated in the CaP layer on the scaffold surface. Cu release kinetics from the scaffolds in SBF were found to depend on culturing conditions while highest Cu concentrations of ∼3.1 ppm and ∼4.6 ppm under static and quasi-dynamic conditions, respectively, were observed. Since Cu exhibits potential angiogenic and osteogenic properties, the Cu-containing scaffolds are suggested as promising materials for bone tissue engineering applications.

Imaging physicochemical reactions occurring at the pore surface in binary bioactive glass foams by micro ion beam analysis.

In this work, the physicochemical reactions occurring at the surface of bioactive sol-gel derived 3D glass scaffolds via a complete PIXE characterization were studied. 3D glass foams in the SiO(2)-CaO system were prepared by sol-gel route. Samples of glass scaffolds were soaked in biological fluids for periods up to 2 days. The surface changes were characterized using particle induced X-ray emission (PIXE) associated to Rutherford backscattering spectroscopy (RBS), which are efficient methods to perform quantitative chemical maps. Elemental maps of major and trace elements at the glass/biological fluids interface were obtained at the micrometer scale for every interaction time. Results revealed interconnected macropores and physicochemical reactions occurring at the surface of pores. The micro-PIXE-RBS characterization of the pores/biological fluids interface shows the glass dissolution and the rapid formation of a Ca rich layer with the presence of phosphorus that came from biological fluids. After 2 days, a calcium phosphate-rich layer containing magnesium is formed at the surface of the glass scaffolds. We demonstrate that quantities of phosphorus provided only by the biological medium have a significant impact on the development and the formation of the phosphocalcic layer.

Bioactive glass–gelatin hybrids: building scaffolds with enhanced calcium incorporation and controlled porosity for bone regeneration

Thanks to their active promotion of bone formation, bioactive glasses (BG) offer unique properties for bone regeneration, but their brittleness prevents them from being used in a wide range of applications. Combining BG with polymers into a true hybrid system is therefore an ideal solution to associate toughness from the polymer and stimulation of bone mineralization from the glass. In this work, we report the synthesis and characterization of hybrid scaffolds based on SiO2-CaO bioactive glass and gelatin, a hydrolyzed form of bone type-I collagen. Incorporation of calcium ions, known to trigger bone formation and cellular activity, into the hybrid structure was achieved at ambient temperature through careful control of chemistry of the sol-gel process. Thorough characterization of the materials highlights the effect of grafting an organoalkoxysilane coupling molecule to covalently link networks of BG and gelatin, and proves it a successful means to take control over the degradation and bioactive properties of hybrids. Importantly, BG-gelatin hybrids are synthesized in a process fully conducted at ambient temperature that allows obtaining open-porous scaffolding structures, with well-controlled and tuneable porosity with regards to both pore and interconnection sizes. Mechanical properties of the scaffolds under compression are similar to that of trabecular bone and their apatite-forming ability is even higher than that of pure BG scaffold foams.

Influence of Glass Scaffolds Macroporosity on the Bioactive Process

Little is known about the ideal morphology for three-dimensional (3D) porous scaffolds to be used in bone tissue engineering. The present study will supply useful data about the dependence of the mineralization process upon macroporous features of bioactive glass scaffolds. It also points out the difficulty in distinguishing between the bioactive properties of scaffolds if using common characterization techniques often considered as standard techniques to assess in vitro bioactivity. Here, two bioactive glass foams with different porosities (porous diameters and interconnection sizes) were successfully synthesized by varying the surfactant quantity in the sol-gel foaming process. The two foams had porosities apparently sufficient to serve as a bone tissue engineering scaffold and exhibited no significant difference when studied for the releasing or the taking up of ionic species when immersed in simulated body fluid (SBF). However, thanks to microion beam analysis, it was possible to highlight key differences in the mineralization reaction taking place at the surface of the pores. It is clearly evident that the homogeneity of reaction inside the 3D-scaffolds is particularly dependent upon porosity. In particular, it is demonstrated that inadequate porous features can result in limited circulation of the fluid inside the pores. Careful attention must be paid to the pore size distribution and interconnection sizes when designing scaffolds for bone tissue engineering, in order to induce homogeneous mineralization inside the porous material and for the scaffold to be efficiently alimented with nutrients or growth factors while allowing a free circulation of the bone cells.

Green and safe in situ templating of bioactive glass scaffolds for bone tissue engineering

This communication reports a new process for the synthesis of bioactive glass foams. This process is based on the use of gelatin as a template during the foaming of a sol, and the gelled gelatin template formed in situ maintains the foam structure during further condensation of the glass network.

Influence of mesostructuration on the reactivity of bioactive glasses in biological medium: a PIXE-RBS study

Building mesostructured biomaterials is a challenging and exciting task that has attracted much attention because of their use as drug carriers or drug delivery systems. In the case of bioactive materials, the mesostructuration can also deeply impact their physico-chemical properties and the reactivity. In this study, we show how highly ordered mesoporosity influences the early steps of the biomineralization process and the reactivity in binary (SiO2–CaO) and ternary (SiO2–CaO–P2O5) bioactive glasses. Conventional porous sol–gel glasses were synthesized using a classical route, while mesostructured glasses were developed using a non-ionic surfactant. Textural properties of these materials have been characterized. The in vitro biomineralization process was followed, using Particle Induced X-ray Emission (PIXE) associated to Rutherford Backscattering Spectrometry (RBS), which are efficient methods for a highly sensitive multi-elemental analysis. Elemental maps of silicon, calcium and phosphorus were obtained at a micrometer scale and revealed for the first time a bulk reactivity for mesostructured glasses. This is a major advantage over conventional glasses, for which the first steps of biomineralization are limited to the periphery of the material. Their enhanced bioactivity combined with their possible use as drug-delivery systems make them promising candidates for bone regeneration.

Bioavailability of Strontium Ions from Bioactive Glasses In Vivo: A Micro-PIXE Study of Trace Elements at the Bone Interface

Studying the local release of strontium traces in vivo is of key interest, but calls for highly sensitive techniques besides providing an excellent (micronic) spatial resolution. In this context nuclear microprobes such as the PIXE (Particle-Induced X-ray Emission) technique, appear as powerful tools of investigation. Here, the in vivo behaviour of a Sr-delivering bioactive glass has been investigated through micro-PIXE analyses in connection with histological studies. New bone formation is observed after 6 weeks of implantation in rabbit femoral condyle. Interestingly, Sr traces are detected over a large area at the site of implantation, demonstrating the efficient release of Sr osteo inductive ions from the glass and their diffusion over several ten microns through the tissues. From its inorganic composition and content in traces of interest such as Zn, neo formed bone seems of higher quality for Sr-delivering particles compared to Sr-free particles, evidencing the positive effect of Sr in vivo.