Intrinsically colored artificial silk fibers made from mini-spidroin fusion proteins

Spider silk is an extraordinary natural material that combines strength, extensibility, and toughness in a lightweight, protein-based fiber. While the recombinant production of spidroins has advanced, the creation of silk fibers with additional intrinsic functionalities, such as color, remains a major challenge. Here, we report the rational design, expression, and spinning of intrinsically red-colored artificial silk fibers. A mini-spidroin variant was engineered as a fusion with the red fluorescent protein mCherry, expressed at high yields (20 g/L) in E. coli fed-batch fermentations, and purified under native conditions. Although the presence of the mCherry globular domain reduced spinnability when used alone, blending with wild-type mini-spidroins enabled continuous wet spinning into robust, fluorescent fibers. Our biomimetic spinning approach preserved the correct folding of the mCherry domain within the fiber, resulting in stable red coloration and fluorescence, while maintaining mechanical properties comparable to those of other recombinant silks. This work establishes a scalable, sustainable strategy for fabricating colored bio-based fibers, opening avenues for environmentally friendly textiles and functional biomaterials that may reduce reliance on synthetic and chemically dyed fibers.