Research on mesoporous materials for biomedical purposes has experienced an outstanding increase during recent years. Since 2001, when MCM-41 was first proposed as drug-delivery system, silica-based materials, such as SBA-15 or MCM-48, and some metal-organic frameworks have been discussed as drug carriers and controlled-release systems. Mesoporous materials are intended for both systemic-delivery systems and implantable local-delivery devices. The latter application provides very promising possibilities in the field of bone-tissue repair because of the excellent behavior of these materials as bioceramics. This Minireview deals with the advances in this field by the control of the textural parameters, surface functionalization, and the synthesis of sophisticated stimuli-response systems.

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A series of ordered mesoporous SiO 2 -CaO-P 2 O 5 sol-gel glasses which are highly bioactive has been synthesized through evaporation-induced self-assembly in the presence of a nonionic triblock copolymer, EO 20 PO 70 EO 20 (P123), template. By keeping constant the SiO 2 + P 2 O 5 /P123 ratio, the influence of the CaO precursor, Ca(NO 3 ) 2 ‚4H 2 O, on the mesostructure has been determined. After calcination at 700°C, ordered mesoporous glasses are obtained, showing structures that evolve from 3D-cubic to 2D-hexagonal when the CaO content increases. The mesoporous glasses are highly bioactive compared with conventional ones, due to the increased textural characteristics supplied by the template. The bioactivity tests point out that the surface area, porosity, and 3D-structure become more important than chemical composition during the apatite crystallization stage in these materials, due to the very high textural parameters obtained. IntroductionSince 1991, when scientists of Mobil Oil Corporation synthesized the silica-based MCM-41 molecular sieve, 1-2 highly ordered mesoporous materials have attracted the attention of many scientists, mainly due to their potential technological applications. Mesoporous materials are characterized as having high surface area, pore volume, and pore size, with narrow pore diameter distribution. For this reason, applications in the field of catalysis, lasers, sensors, solar cells, etc. have been proposed and/or developed. [3][4][5][6][7][8] Recently, these materials have been proposed for their application in biomaterials science. 9 Due to their textural properties of surface and porosity, ordered mesoporous materials have shown to be excellent candidates for two biomedical applications: local drug delivery systems 10-12 and bone tissue regeneration. 13-15 Actually, silica-based mesoporous materials are able to incorporate high dosages of drugs into the mesopores. On the other side, mesoporous materials can be synthesized with analogous chemical composition to highly bioactive sol-gel glasses. These materials are able to bond to living bone when implanted through the formation of a nonstoichiometric carbonated hydroxyapatite of nanometrical size (CHA). 16,17 This bioactiVe bond ensures the implant osteo-integration, and its degradation products promote the bone tissue regeneration.Increasing the specific surface area and pore volume of bioactive glasses greatly accelerates the CHA formation and therefore enhances the bioactive behavior. 18 In this sense, highly ordered mesoporous materials provide very promising possibilities in the field of bone tissue regeneration. Moreover, these materials can be loaded with osteogenic agents promoting the new bone formation in vivo and can also be applied as scaffolds for bone tissue engineering. 19,20 Nonionic block copolymers are an interesting class of structure-directing agents whose self-assembly characteristics lead to ordered mesostructures. 21-25 They have the advantage that their ordering properties can be tuned by adjusting * Corres...

The local structures of highly ordered mesoporous bioactive CaO−SiO2−P2O5 glasses were investigated for variable Ca contents. 1H NMR revealed a diversity of hydrogen-bonded and “isolated” surface silanols as well as adsorbed water molecules. The structural roles of Si and P were explored using a combination of 29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques; the proximities of Si and P to protons were studied through cross-polarization-based experiments, including 1H−29Si and 1H−31P hetero-nuclear two-dimensional correlation spectroscopy. The results are consistent with SiO2 being the main pore-wall component, whereas P is present as a separate amorphous calcium orthophosphate phase, which is dispersed over the pore wall as nanometer-sized clusters. The excess Ca that is not consumed in the phosphate phase modifies the silica glass network where it associates at/near the mesoporous surface. This biphasic structural model of the pore wall leads to the high accessibility of both Ca and P to body fluids, and its relation to the experimentally demonstrated high in vitro bioactivities of these materials is discussed.

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

An array of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments is applied to explore the surface reactions of a mesoporous bioactive glass (MBG) of composition Ca0.10Si0.85P0.04O1.90 when subjected to a simulated body fluid (SBF) for variable intervals. Powder X-ray diffraction and 31P NMR techniques are employed to quantitatively monitor the formation of an initially amorphous calcium phosphate surface layer and its subsequent crystallization into hydroxycarbonate apatite (HCA). Prior to the onset of HCA formation, 1H → 29Si cross-polarization (CP) NMR evidence dissolution of calcium ions; a slightly increased connectivity of the speciation of silicate ions is observed at the MBG surface over 1 week of SBF exposure. The incorporation of carbonate and sodium ions into the bioactive orthophosphate surface layer is explored by 1H → 13C CPMAS and 23Na NMR, respectively. We discuss similarities and distinctions in composition−bioactivity relationships established for traditional melt-prepared bioglasses compared to MBGs. The high bioactivity of phosphorus-bearing MBGs is rationalized to stem from an acceleration of their surface reactions due to presence of amorphous calcium orthophosphate clusters of the MBG pore wall.

By exploiting 1H and 31P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, we explore the proton and orthophosphate environments in biomimetic amorphous calcium phosphate (ACP) and hydroxy-apatite (HA), as grown in vitro at the surface of a 10CaO–85SiO2–5P2O5 mesoporous bioactive glass (MBG) in either a simulated body fluid or buffered water. Transmission electron microscopy confirmed the presence of a calcium phosphate layer comprising nanocrystalline HA. Two-dimensional 1H–31P heteronuclear correlation NMR established predominantly 1H2O↔31PO43– and O1H↔31PO43– contacts in the amorphous and crystalline component, respectively, of the MBG surface-layer; these two pairs exhibit distinctly different 1H→31P cross-polarization dynamics, revealing a twice as large squared effective 1H–31P dipolar coupling constant in ACP compared with HA. These respective observations are mirrored in synthetic (well-crystalline) HA, and the amorphous calcium orthophosphate (CaP) clusters that are present in the pristine MBG pore walls: besides highlighting very similar local 1H and 31P environments in synthetic and biomimetic HA, our findings evidence closely related NMR characteristics, and thereby similar local structures, of the CaP clusters in the pristine MBG relative to biomimetic ACP.

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