Protein encapsulation in functionalized and structured silica gel for bone reconstruction application

The goal of this work is to study the influence of the structured mesoporous silica (SMS), used as scaffolds for tissue engineering [1], on the encapsulation and on the release of a model bone morphogenetic proteins (BMP), i.e. Soybean Trypsin Inhibitor (STI).
First, SBA-15 silica samples were synthesized using tetraethyl orthosilicate (TEOS) as silica precursor in the presence of P123 as the surfactant in an acidic medium. In addition, a swelling agent was added to increase the pore size. Indeed, the common pore size for those mesostructured materials lies between 6 nm and 8 nm, while the STI has an average radius of 4 nm. The reactant addition sequence was also investigated. It was observed that when the swelling agent was added with the TEOS, a SMS was produced. On the contrary, if the swelling agent was added during the surfactant dissolution step, it resulted in an unstructured -but still mesoporous- silica.
Because of the high affinity of STI for hydrophobic surfaces [2], SMS were also produced using silica precursors containing phenyl groups (1,4-bis(triethoxysilyl)benzene, BTEB [3]). The BTEB samples exhibited two 2D-hexagonal structures with different wall thicknesses.
For samples synthesized with TEOS, the unstructured sample showed a higher protein loading and a higher protein release, which could be explained by a difference in the pore interconnectivity within the sample. In fact, a fast release of STI was observed during the first 24 h. Afterwards, the STI release slowed down and seemed to achieve a plateau. On the opposite, the SMS showed a steady release over time. Finally, the sample synthesized with BTEB did not show a significant release over the same period of time. This led us to the conclusion that the hydrophobicity of the silica surface plays a major role on the protein encapsulation and its release rate.