SpeakerDr. Russell J. Stewart, Host: Herb Waite
Date and LocationWednesday November 19, 2014 11:00am
Caddisflies are aquatic insects that share a common silk-spinning ancestor with terrestrial moths and butterflies, including the domesticated silk moth. Caddisfly larvae use their silk like a pressure sensitive adhesive tape to assemble composite structures underwater. A peripheral layer of glycoproteins likely mediates interfacial adhesion to wet surfaces. Single silk fibers are viscoelastic, display strain rate dependence, large strain cycle hysteresis, and self-recover 99% of their initial stiffness and strength within 120 min. A multi-network structural model is proposed in which caddisfly silk viscoelasticity is attributed to two independent, self-recovering, Ca2+ crosslinked networks acting in parallel. The networks can be separated by decreasing the pH of the test solution. The first network is attributed to Ca2+-complexed phosphoserine motifs in H-fibroin, the second to Ca2+ complexed carboxylate groups in the N-terminus of H-fibroin and an elastic PEVK-like protein. The functional group assignments were corroborated by ATR-FTIR spectroscopy. Reversible rupture of the Ca2+-crosslinked domains at a critical stress results in pseudo-plastic deformation. Slow refolding of the Ca2+-crosslinked domains directed by an elastic covalent network results in nearly full recovery of fiber length, stiffness, and strength. Reversible deformation of the fiber backing protects against irreversible rupture at the adhesive interface. The fiber toughening and self-recovery mechanisms are similar to those of synthetic super tough multi-network hydrogels.