Aquatic caddisfly larva spin a sticky silk tape used underwater to construct a protective composite stone case. Caddisworm silk fibers are drawn on-demand from fluid precursors stored in the posterior region of the silk gland. Fibers begin to form in the gland at a cuticular narrowing at the entrance into the short (2-3 mm) anterior conducting channel leading to the spinneret. The caddisworm silk comprises a thin adhesive peripheral coating on a tough viscoelastic core fiber. The thin adhesive layer contains glycoproteins and a heme-peroxidase in the peroxinectin subfamily (Pxt). Pxt catalyzes dityrosine cross-linking in the fiber periphery and may catalyze covalent adhesive cross-links to surface-active natural polyphenolic compounds. The major component of the silk core, H-fibroin, contains around 13 mol% phosphoserines (pS) in repeating (pSX)n motifs, wherein X is usually hydrophobic, and n is 4 or 5. The (pSX)n motifs form β-domains crossbridged and stabilized by multivalent metal ions, predominantly Ca2+ in natural fibers. During loading, the Ca2+/(pSX)n β-domains reversibly rupture to reveal hidden length and dissipate strain energy. The tough fibers can be strained to more than 100% of their initial length before fracture. The work of extension to failure, -17.3±6.2 MJ/m3, is higher than articular cartilage. Silk fibers cycled to 20% elongation completely recover their initial stiffness, strength, and hysteresis within 120 min as an elastic covalent network guides the post-yield recovery of the Ca2+/(pSX)n β-domains.
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