Artificial hagfish protein fibers with ultra-high and tunable stiffness.

Stiff fibers are used as reinforcing phases in a wide range of high-performance composite materials. Silk is one of the most widely studied bio-fibers, but alternative materials with specific advantages are also being explored. Among these, native hagfish (Eptatretus stoutii) slime thread is an attractive protein-based polymer. These threads consist of coiled-coil intermediate filaments (IFs) as nano-scale building blocks, which can be transformed into extended β-sheet-containing chains upon draw-processing, resulting in fibers with impressive mechanical performance. Here, we report artificial hagfish threads produced by recombinant protein expression, which were subsequently self-assembled into coiled-coil nanofilaments, concentrated, and processed into β-sheet-rich fibers by a "picking-up" method. These artificial fibers experienced mechanical performance enhancement during draw-processing. We exploited the lysine content to covalently cross-link the draw-processed fibers and obtained moduli values (E) in tension as high as ∼20 GPa, which is stiffer than most reported artificial proteinaceous materials.