Isabel Izquierdo-Barba

Revisiting silica based ordered mesoporous materials : medical applications

The bioactivity behaviour of SBA-15, MCM-48, MCM-41 mesoporous materials, is revisited in this paper. The influence of their different textural and structural properties on apatite formation is outlined and strategies to modify their kinetics are proven and discussed. On the basis of the experimental data showing the feasibility of control of the bioactivity kinetics on mesoporous materials, together with their controlled drug release abilities, new possibilities for tissue engineering developments are proposed.

In vitro calcium phosphate layer formation on sol‐gel glasses of the CaO‐SiO2 system

A glass with a composition of SiO(2) 80% and CaO 20% (in mol %) was prepared by the sol-gel method, and its in vitro bioactivity was studied by soaking it in simulated body fluid (SBF) at 37 degrees C. The formation of a calcium phosphate layer on the glass was analyzed by determining the ionic concentrations in solution and by studying the surface of the glass with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and electron diffraction (ED) after it had been in SBF for varying periods of time. The composition of this bioactive glass, formed of only two components, allowed us to monitor the formation process of the hydroxycarbonate apatite (HCA) layer. The bioactive behavior of this glass indicates that the presence of phosphorous in the glass composition is not an essential requirement for the development of a HCA layer. In this case, the layer is formed because of the phosphorous present in the in vitro assay solution.

Structure and functionalization of mesoporous bioceramics for bone tissue regeneration and local drug delivery.

This review article describes the importance of structure and functionalization in the performance of mesoporous silica bioceramics for bone tissue regeneration and local drug delivery purposes. Herein, we summarize the pivotal features of mesoporous bioactive glasses, also known as ‘templated glasses’ (TGs), which present chemical compositions similar to those of conventional bioactive sol–gel glasses and the added value of an ordered mesopore arrangement. An in-depth study concerning the possibility of tailoring the structural and textural characteristics of TGs at the nanometric scale and their influence on bioactive behaviour is discussed. The highly ordered mesoporous arrangement of cavities allows these materials to confine drugs to be subsequently released, acting as drug delivery devices. The functionalization of mesoporous silica walls has been revealed as the cornerstone in the performance of these materials as controlled release systems. The synergy between the improved bioactive behaviour and local sustained drug release capability of mesostructured materials makes them suitable to manufacture three-dimensional macroporous scaffolds for bone tissue engineering. Finally, this review tackles the possibility of covalently grafting different osteoinductive agents to the scaffold surface that act as attracting signals for bone cells to promote the bone regeneration process.

Influence of P2O5 on crystallinity of apatite formed in vitro on surface of bioactive glasses

Two sol-gel glasses with 80 mol % SiO2 were prepared in the system SiO2-CaO-P2O5; the first one had 3 mol% P2O5 in its composition, and the second one was P2O5 free. The in vitro behavior of glasses was studied by soaking them in simulated body fluid for 7 days at 37 degrees C. After the in vitro test, the study by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, electron diffraction, and transmission electron microscopy showed an apatite-like layer had formed on the surface of both glasses. However, for identical soaking time, the apatite crystals formed on the surface of the glass containing P2O5 in the composition were larger. Therefore, the presence of P2O5 in the sol-gel glass composition promotes the crystal growth of the apatite.

Incorporation of antimicrobial compounds in mesoporous silica film monolith

Incorporation of the antimicrobial peptide LL-37 (LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES), as well as low molecular weight antimicrobial chlorhexidine, into mesoporous silica was obtained using an EISA one-pot synthesis method. FTIR confirmed efficient encapsulation of both LL-37 and chlorhexidine into mesoporous silica, while XRD and TEM showed that antimicrobial agent incorporation can be achieved without greatly affecting the structure of the mesoporous silica. The modified mesoporous silica released LL-37 and chlorhexidine slowly, reaching maximum release after about 200 h. The release rate could also be controlled through incorporation of SH groups in the pore walls, adding to pore hydrophobicity and reducing the release rate by about 50% compared to the unmodified mesoporous silica. Mesoporous silica containing either LL-37 or chlorhexidine displayed potent bactericidal properties against both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. While chlorhexidine-loaded mesoporous silica displayed an accompanying high toxicity, as judged from hemolysis, LDH release, and MTT assay, the corresponding material containing LL-37 showed very low toxicity by all these assays, comparable to that observed for mesoporous silica in the absence of antibacterial drug, as well as to the negative controls in the respective assays. Mesoporous silica containing LL-37 therefore holds potential as an implantable material or a surface coating for such materials, as it combines potent bactericidal action with low toxicity, important features for controlling implant-related infections, e.g., for multi-resistant pathogens or for cases where access to the infection site of systemically administered antibiotics is limited due to collagen capsule formation or other factors.

Biomimetic Apatite Deposition on Calcium Silicate Gel Glasses

In order to understand the Biomimetic apatite formation mechanism on gel glasses, a glass (in mol-%) SiO2 80%—CaO 20% (80S20C) was prepared by the sol-gel method and its behaviour in a simulated body fluid (SBF) was studied. To study the role of phosphorous in the in vitro apatite formation, a gel glass (in mol-%) SiO2 80%—CaO 17%—P2O5 3% (80S17C3P) was prepared comparing its behaviour in SBF with that of 80S20C. In both studies, a protocol without renovation of SBF (static) was used. To mimic the conditions in the living organisms, an in vitro protocol with continuous renovation of solution (dynamic) was proposed. To check the feasibility of dynamic protocol, 80S20C and 80S17C3P were studied in dynamic and results compared with obtained in static. Static studies of 80S20C allowed us to verify that phosphorus is not essential for bioactivity because the apatite-like layer was formed from the phosphorous in SBF. However, a 3 mol-% of P2O5 in 80S17C3P gel glass favoured apatite crystallization. In dynamic, complete assays were performed with ionic concentrations and pH in solution almost equal to human plasma. After 7 days in dynamic, apatite crystals and crystalline aggregates were larger than in static. Besides, compositional variations were observed in the newly formed layer as a function of the protocol. In static, the layer formed in both glasses contained calcium and phosphorous, (Ca/P molar ratio = 1.6) and silicon. In dynamic, the layer did not contain silicon and the Ca/P molar ratio was 1.2. Differences in composition and pH of assay solution, 8 in static and 7.3 in dynamic could explain these variations. In static, an apatite close to stoichiometric could be formed. In dynamic, a mixture of calcium deficient apatite and other calcium phosphates could constitute the layer.

Nanostructured mesoporous silicas for bone tissue regeneration

The research on the development of new biomaterials that promote bone tissue regeneration is receiving great interest by the biomedical scientific community. Recent advances in nanotechnology have allowed the design of materials with nanostructure similar to that of natural bone. These materials can promote new bone formation by inducing the formation of nanocrystalline apatites analogous to the mineral phase of natural bone onto their surfaces, i.e. they are bioactive. They also stimulate osteoblast proliferation and differentiation and, therefore, accelerate the healing processes. Silica-based ordered mesoporous materials are excellent candidates to be used as third generation bioceramics that enable the adsorption and local control release of biological active agents that promote bone regeneration. This local delivery capability together with the bioactive behavior of mesoporous silicas opens up promising expectations in the bioclinical field. In this review, the last advances in nanochemistry aimed at designing and tailoring the chemical and textural properties of mesoporous silicas for biomedical applications are described. The recent developed strategies to synthesize bioactive glasses with ordered mesopore arrangements are also summarized. Finally, a deep discussion about the influence of the textural parameters and organic modification of mesoporous silicas on molecules adsorption and controlled release is performed.

Preparation of 3-D scaffolds in the SiO2-P2O5 system with tailored hierarchical meso-macroporosity.

Herein we report for the first time the synthesis of three-dimensional scaffolds in the binary system SiO2-P2O5 exhibiting different scales of porosity: (i) highly ordered mesopores with diameters of ca. 4 nm; (ii) macropores with diameters in the 30-80 μm range with interconnections of ca. 2-4 and 8-9 μm; and (iii) ultra-large macropores of ca. 400 μm. The hierarchical porosity of the resulting scaffolds makes them suitable for bone tissue engineering applications. The chemical nature and mesoporosity of these matrices would allow these scaffolds to act as local controlled delivery systems of biologically active molecules, such as certain drugs to treat bone pathologies. The synthetic method consists of the combination of a single-step sol-gel route in the presence of a surfactant as the mesostructure directing agent and a biomacromolecular polymer such as methylcellulose as the macrostructure template followed by rapid prototyping technique. An exhaustive study of the aging process as well as of the rheological properties of the slurry after methylcellulose addition has been carried out to obtain hierarchical meso-macroporosity. This study allows the establishment of the time period in which the slurry presents appropriate viscosity to be extruded during the rapid prototyping once the ink is prepared. The setting up of this manufacture process at the laboratory level is important from the industrial point of view when the large-scale production of scaffolds for bone tissue repair and regeneration is targeted.

In vitro structural changes in porous HA/β-TCP scaffolds in simulated body fluid

Porous scaffolds of biphasic calcium phosphate (hydroxyapatite/beta-tricalcium phosphate (beta-TCP)) have been fabricated and changes induced both in phase composition and porous architecture by immersion in simulated body fluid (SBF) under static and orbital stirring conditions have been studied. The starting porous scaffolds exhibit a low and randomized micro- and mesoporosity, an interconnected macroporosity centered at 100 and 0.6microm, a fractal connectivity of D=2.981 and total percent porosity of ca. 80%. After immersion for up to 60days the micro- and mesoporosity increase slightly, which could be attributed to dissolution of the beta-TCP phase confirmed by transmission electron microscopy. The effects of the change in the porous framework with SBF immersion time favor the bioactive behavior of the tested materials, inducing a nucleation and growth of a nanocrystalline apatite phase as the interconnected macroporosity centered at 0.6microm is reduced. The macroporosity centered at 100microm is still stable after 60days in SBF. Therefore, these biphasic calcium phosphate porous scaffolds combine bioactive behavior with the stability of interconnected macroporosity over large periods of soaking time in SBF under static and orbital stirring conditions.

Effect of the continuous solution exchange on the in vitro reactivity of a CaO‐SiO2 sol‐gel glass

The in vitro reactivity of a sol-gel glass with a composition in mol % of 80% SiO(2)-20% CaO (80S20C) was studied by soaking in a simulated body fluid (SBF). To model the continuous flow of body fluids, in this article a protocol for in vitro tests with continuous exchange of the assay solution (continuous) is proposed. The in vitro behavior of 80S20C in continuous is compared with that without SBF exchange (static). In static, remarkable variations in ionic concentration and pH of solution were detected after a few minutes of soaking. However, exchange of solution with 1 mL/min flow allowed us to maintain SBF ionic concentration and pH almost constant and close to plasma. Glass surface before and after soaking was studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and electron diffraction (ED). After soaking, a calcium phosphate layer formed in both cases on the glass surface. However, some differences were observed as a function of the in vitro protocol used. In static, faster formation of the phosphate layer was detected in the first 6 h by FTIR, but for higher soaking times the situation was equivalent in both cases. After 7-day assay in continuous, XRD, SEM, TEM, and ED studies showed larger crystalline aggregates and apatite crystals on the newly formed layer. The use of a continuous protocol allowed us to perform complete in vitro studies with an ionic concentration and pH in solution almost identical to physiological fluids.