Bioactivity in ordered mesoporous materials
Graphical abstract
Introduction
Since Hench et al. discovered Bioglass in 1971 [1], many studies have been carried out to develop new bioactive materials for their application in hard tissue reparation and/or substitution [2], [3], [4]. These bioactive materials are characterized by forming a stable join with tissues in a biological environment by means of formation of an apatite layer on their surface. Hence, it seems necessary to study the mechanism of the layer formation in order to understand the behavior of these materials, and to develop new materials having those kinds of properties. Although this mechanism is not completely elucidated, the presence of silanol groups in the surface seems to be necessary, and the hydrated silica formed afterward in the body [5], [6], [7], [8] would induce the nucleation of the apatite. Among the factors that influence the mechanism of the apatite layer formation are: the synthesis method [9], pH, thermal treatment [10] and, specially, the textural properties [11], [12], [13], [14], [15]. In fact, a study carried out by Pereira and Hench [12] revealed that there are some requisites indispensable for the layer formation, such as the porosity of the material and the negative charge of its surface. Besides, Li et al. [9] proved that the apatite layer was formed on silica gels but not on dense silica glasses or quartz. In addition, the layer formation is enhanced by the presence of pores bigger than 2 nm, a direct relation existing between both pore size and volume and the nucleation rate of the layer, in which the pores act as nucleation sites for apatite nucleation [12].
Taking into account these considerations and previous studies that reveal the bioactivity of pure silica materials [10], [11], [12], MCM-41 materials subject of study in our group, seem to be adequate to be used as new bioactive materials. These compounds were first synthesized by the Mobil Oil Company in 1991 [16], [17] and have been widely studied due to their numerous applications in fields such as catalysis, ion exchange or sensors [18], [19], [20] and recently drug delivery systems [21], [22], [23]. Mesoporous MCM-41 are composed of silica, showing a hexagonal array of cylindrical mesopores with a high porosity and BET specific area (), which confer them the necessary requisites of composition and texture to behave as bioactive materials. In fact, taking into account the large surface constituted by siloxane and silanol groups with negative density charge, and the high quantity of mesopores bigger than 2 nm in the structure, a bioactive behavior towards physiological fluids is predictable and even high kinetics of reactivity could be expected. In fact, some positive results have been found for photocalcined ordered mesoporous silica coatings [24].
Despite these considerations, in vitro assays carried out in order to study the in vitro behavior of these materials have shown no signs of bioactivity after two months of assay. Consequently, it becomes necessary to add some component that could induce the bioactivity in MCM-41. Taking into account the experience of our group in bioactive glasses [25], [26], [27], [28], and the previous usage of them as accelerators of the bioactivity of some other materials [29], MCM-41 has been combined with 10 wt% of a bioactive glass in order to study the bioactive behavior of the mixture.
Therefore, the aim of this work is to study MCM-41 mesoporous material and its mixture with 10% of glass as new bioactive materials by means of in vitro assays in simulated body fluids, comparing the behavior of both materials.
Section snippets
Experimental
Mesoporous material MCM-41 named as M(100%) was synthesized by sol–gel method as previously reported [21] from silica precursor tetraethyl orthosilicate (TEOS) and using hexadecyltrimethylammonium as directing agent. This surfactant was removed by calcination (1 h at 550 °C under nitrogen followed by 3 h in air). The material was compacted by uniaxial (2.75 MPa) and isostatic pressure (3 MPa) to obtain disks (13 mm in diameter and 5 mm in height) used in the bioactivity assay.
To obtain the
Materials characterization
Powder XRD patterns of materials M(100%) and M(90%)–G(10%) are shown in Fig. 1. M(100%) shows the characteristic reflections (100), (110) and (200) of a hexagonal array of mesopores, with spacings of 39 Å for powder samples and 37 Å for compacted ones. The reflections (100), (110) and (200) were also detected in M(90%)–G(10%), verifying that this material maintained the hexagonal array of mesopores, with spacings of 40 and 35 Å for powder and compacted samples, respectively. The
Conclusions
The mesoporous material MCM-41, even apparently having all the requisites needed to behave as bioactive material, does not show signs of bioactivity after a month of assay. Nevertheless, the addition of a small amount of glass induces the formation of an apatite-like layer on its surface, revealing the bioactive behavior of this material.
Acknowledgment
The authors are grateful for financial support provided by CICYT MAT02-0025.
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2018, Ceramics InternationalCitation Excerpt :This is in contrast to Hench's bioactivity mechanism [18], since there are no cations that can exit from the amorphous silicate network to activate the apatite-forming mechanism. This issue is still controversial and a partial explanation was attributed to the role played by the unique textural properties of mesoporous materials: in fact, their ultrahigh specific surface area (above 100 m2/g) seems to govern the material reactivity in aqueous media more than the composition [133,134]. The fabrication of hierarchical systems comprising pure-silica mesoporous microspheres (SBA-15, MCM-41) embedded in bioactive glass-ceramic macroporous scaffolds was reported in the attempt to develop multifunctional materials able to treat infections (through the controlled release of drugs from the mesopores) and promote bone bonding/regeneration at the same time [135–138].