Protein encapsulation in functionalized silica gel for bone reconstruction application

Over the last decades, temporary porous matrices, called scaffolds, have increasingly gained in importance for bone regeneration. These matrices serves for the support and control of the spatial organization of stem cells. Various materials have been proposed for their conception. Nevertheless, studies have shown a lack of cell differentiation and bone production at their surface. The aim of this study is the adjustment of the surface functionalization of hydroxyapatite via sol-gel coating of silica to promote the local sustained delivery of a model protein (i.e. Soybean Trypsin Inhibitor, STI). In this optic, the influence of the functional groups present at the surface of silica pores on the release kinetics of the protein and on its activity has been studied via two alternative methods: (1) the impregnation of already synthesized silica gels in the protein solution (i.e. ex situ method), and (2) the direct incorporation of the protein during the gel synthesis (i.e. in situ method). For the ex situ method, tetraethylorthosilicate (TEOS) was used as main silica precursor and 3-(2-aminoethylamino) propyltrimethoxysilane (EDAS) or phenyltrimethoxysilane (PTMS) as nucleating agents in alcoholic medium and under basic conditions. For the in situ method, tetramethylorthosicilate (TMOS) served as silica precursor after hydrolysis under acid conditions. The nucleating agents were also EDAS or PTMS. The results of the textural analysis revealed that ex situ gels presented a porous funnel-like structure (micro-, meso-, and macropores) while in situ gels presented a micro- and mesoporous structure. Regarding the in vitro release profile of STI, in both methods, the presence of phenyl caused a large but total entrapment inside the pores. For the calcined and amine modified ex situ samples, a burst over the 24 h of incubation was present followed by a plateau for the calcined samples and by a continuous release for the amine modified samples. The inhibitory activity decreased after 4 weeks of incubation to a low value (i.e. around 30 %) for the amine samples and was equal to 0 in the case of the calcined samples. These differences could be explained by the sign difference of the charges present at the surface of the pores (interaction of negatively charged STI with negative charges of calcined samples vs positive charges of amine modified samples). For the in situ samples, a continuous release up to 8 days could be observed. Interestingly enough, STI released after 4 weeks of incubation presented a high remaining activity (i.e. around 80 %).