Mechanical and Strain-Sensing Responses in Functional ZnO Tetrapod - Silk Fibroin Composites: A Detailed Investigation of the Roles of Filler Size and Shape.

In this study, functional composites were developed by dispersing ZnO tetrapods (ZnO-T) in a solution of silk fibroin (SF) and formic acid (FA). Micro- and nanoparticles with different shapes were observed in the SF matrix after evaporation of the solvent, depending on ZnO-T filler content. The secondary structure of SF, as well as the shape (e.g., spherical, linear, or branched) and aspect ratio of the fillers, were investigated by FTIR spectroscopy, optical and scanning electron microscopy. While FTIR spectroscopy indicates that the secondary structure of SF is not affected by the size and shape of ZnO-T, the observed variation in effective elastic modulus measured for composites is correlated with different filler particle aspect ratios, predicting the formation of a stiff interface. We observed that branched tetrapodal fillers exhibit significant potential for optimization of composites' mechanical properties in comparison to their spherical and linear counterparts. By combining ZnO microparticles with SF, we fabricated a stretchable strain sensor that demonstrated an appreciable improvement in the fractional change of electrical resistivity and gauge factor with respect to neat SF films.