Ultrastiff Bioinspired Protein–Carbon Nanotube Hybrid Sponge with Shape Memory Effects

Natural protein-based biomaterials with complex hierarchical structures often have incredible and even counterintuitive mechanical properties. Understanding and utilizing the conformational transition mechanisms of natural proteins will further guide the design of natural-inspired biomaterials. In this study, a small static-force-induced spatiotemporal "freezing" phenomenon of silk fibroins confined in porous carbon nanotube sponges has been investigated. The "freezing" silk fibroins not only bring the shape memory effect to elastic carbon nanotube sponges but also enable them to prop up heavy objects with loads exceeding 10,000 times their own weight. Also, the protein/CNTS hybrid achieves an ultrastiffness (over 10 MPa) and superelastic shape recovery (recovery strain >90%). Both experimental and numerical results indicate that the secondary conformational transition of silk fibroin plays a key role, where more α-helices/random coils transform into β-sheets under both confinement and low pressure. Our work reports a conformational transition mechanism of silk fibroin in a confined space, which provides guidance for constructing protein-based biological smart materials with potential applications in textiles, medicine, architecture, and other research fields.