Protein encapsulation in functionalized sol-gel silica: Effect of the encapsulation method on the release kinetics and the activity

Over the last few years, bone repair has increasingly gained in importance. In recent years, considerable attention has been given to the administration of therapeutic biomolecules to promote tissue regeneration. The aim of this work is the study of the influence of functional groups present at the surface of silica pores and the textural structure on the release kinetics of a model protein (i.e. Soybean Trypsin Inhibitor, STI) and on the preservation of its inhibitory activity. For this purpose, two alternative methods have been investigated: i) impregnation of presynthesized silica gels with the protein solution (i.e. impregnation method), ii) direct incorporation of the protein during the synthesis of the gel (i.e. in situ method). Regarding the impregnation method, a fast release of STI was observed when incubated in physiological conditions (i.e. burst followed by a plateau for the calcined samples or by a sustained release for the dried sample) while the in situ method allowed a better control of the release rate of this protein over the first 24 h. This difference in release kinetics could be explained in terms of the expected geometry of the pores (open versus closed porosity). Interestingly, no significant change in the activity of the protein was noticed for the silica synthesized via the in situ method, while a partial or total loss of inhibitory activity was measured after four weeks of incubation for the impregnated silica.