Bioinspired Photo-Cross-Linking of Stretched Solid Silks for Enhanced Strength

In this study, solid fibroin fibers (FFs) were directly cross-linked by employing a ruthenium-mediated redox pair under visible light at room temperature for the first time. The chemical cross-link through dityrosine connection was confirmed by Fourier-transform infrared (FTIR) spectroscopy, fluorescence spectra, and a solubility test. The resultant cross-link density of fibers was calculated based on their swelling ratio evaluation in LiBr solution. Further applying stretch to the fibers during irradiation increased the fiber strength to higher values. The break stress and Young's modulus of photo-cross-linked 15% stretch FFs reached a 60-90% increase in comparison to the original FFs in dry and wet conditions. This approach constitutes an easy and straightforward strategy for strengthening FFs, which is scalable industrially to enhance FFs in a wide range of applications.