Natural silk fibers show valuable changes in elastic modulus, E, and failure stress, sigma(f), upon treatment with an air flux of gaseous Si-alkoxides bearing a thin sol-gel layer of SiO2 on the fiber surface. These mechanical features are studied here as a function of the composition of gaseous flux and reaction time. Owing to the different behavior between original and treated fibers submitted to loading-unloading cycles, the maximum increase in E (about 50%) and sigma(f) (about 30%) are discussed in terms of intermolecular surface interactions of -O-Si-O- bridging groups and protein macromolecules. As an extension of this system, composed of a thin sol-gel SiO2 layer (0.05-0.1 mu m) on protein surfaces, the mechanical improvement of collagen + cell deposits upon deposition of sol-gel SiO2 is suggested.