Graphene-Oxide Seeds Nucleate Strong and Tough Hydrogel-Based Artificial Spider Silk

Abstract: Natural spider silk is composed of spun spidroin protein containing beta-sheet crosslinking sites drawn from an S-shaped spinning duct. It exhibits an excellent combination of strength (1150 ±200 MPa) and toughness (165 ±30 MJ∙m−3) that originates from its hierarchical structure, including crosslinking sites, highly aligned nano-aggregates, and a sheath-core structure. In this work, we prepared a hydrogel fiber that contains crosslinking sites, highly aligned nano-aggregates, and a sheath-core structure, by draw-spinning a bulk hydrogel composed of polyacrylic acid crosslinked with vinyl-functionalized silica nanoparticles (SNVs). The core-sheath structure was prepared by the water-evaporation-controlled self-assembly of the polyacrylic hydrogel, while nanometer-sized aggregates were formed by the self-assembly of polyacrylic acid chains. The addition of a tiny amount of graphene oxide (GO:0.01%), a 2D nanomaterial, enhanced the mechanical properties of the fiber (breaking strength:560 MPa; fracture toughness:200 MJ∙m−3; damping capacity:94%). In addition, we investigated the factors responsible for the mechanical properties of the gel fibers, including fiber diameter, drying time in air, relative air humidity, and stretching speed. A higher breaking strength and a lower fracture strain was obtained by decreasing the fiber diameter, increasing the drying time, or increasing the stretching speed, while a lower fracture strain and higher breaking strength were obtained by increasing the relative air humidity. Polarized optical and SEM images revealed that the GO-seeded material is better aligned and contains smaller nano-aggregates, with GO seeding found to play a key role in the formation of nano-aggregates and polymer-chain alignment. The prepared fiber exhibited excellent mechanical properties compared to gel fibers prepared by other methods (e.g., electro-, wet, dry, and microfluidic spinning, as well as templating, and 3D printing, etc.). Repeated mechanical testing involving stretch-release cycles to 70% strain at 20% relative humidity revealed that the fibers have an energy-damping capacity of 93.6%, which exceeds that of natural spider silk and many types of artificial fiber. The relaxed stretched fiber recovered its initial length when exposed to 80% relative humidity, while the fiber recovered its initial mechanical properties when stored for 2 h at room temperature. A yarn composed of three hundred of the prepared gel fibers was shown to lift a 3 kg object without breaking; the prepared fiber was also shown to absorb dynamic energy and lower the impact force of a falling object.