Bioactivity in ordered mesoporous materials

Abstract

Mesoporous MCM-41 materials seem adequate for their use as bioactive materials since they exhibit high specific surfaces constituted by siloxane and silanol groups, and present a high porosity, which are important conditions for bioactivity. Even though, no signs of bioactivity are shown after 2 months of in vitro assays, revealing the need of some other element such as cations that can act as defect generators, promoting the apatite layer nucleation. The addition of a small amount of glass to a MCM-41 material induce the formation of an apatite-like layer starting from the glass surface, covering after 7 days of assay the whole surface of the biphasic material MCM-41-glass.

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 ($>1000{\text{m}}^2{\text{g}}^{-1}$), 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.