Gradient structure enabled robust silk origami with moisture responsiveness

Biomaterial-based origami functional films are fundamental to the development of implantable flexible electronics and wearable devices. However, the challenge of simultaneously achieving excellent strength, flexibility and functionality is yet to be fully explored. Inspired by the Fick’s law, we fabricate an origami pristine silk fibroin (SF) film via a straightforward yet robust strategy, using water and formic acid as the structural regulator. Benefiting from the asymmetric diffusion and the effect on the inter/intra-molecular hydrogen bonds of water, our strategy endows the SF film with gradient condensed structure from surface to interior, which has been both experimentally and theoretically confirmed. Owing to this unique structure with higher surficial crystallinity than interior, the SF films exhibit an excellent integration of strength, flexibility, and moisture-response functionality. The films are successfully origami-folded into a paper crane/boat without any damage, and can realize dynamic jump and moisture management functions. This work highlights a versatile path for exploiting biomaterial-based films combined with enhanced flexibility, strength, and additional functionalities.