The effect of water on regenerated silkworm silk fibers has been studied and compared with that of water on natural silkworm silk fibers. Regenerated fibers are spun from an N-methylmorpholine-N-oxide (NMMO) fibroin solution through a wet-spinning process, leading to fibers with two distinct tensile behaviors, labeled as brittle and ductile, respectively. Regenerated fibers show a significant contraction when immersed in water. Contraction increases further after drying. In contrast, natural silkworm silk fibers show a negligible contraction when submerged in water. Regenerated fibers tested in water are considerably more compliant than samples tested in air, though their stiffness and tensile strength are significantly reduced. It has been shown that the tensile properties of brittle regenerated fibers can be modified by a wet-stretching process, which consists of deforming the fiber while immersed in water. Regenerated wet-stretched fibers always show a ductile behavior independent from their initial tensile behavior.
High-performance regenerated silkworm Bombyx mori silk fibers with new properties that mimic those of spider silk can be produced through a wet spinning process modified with an immersion postspinning drawing (IPSD) step. IPSD fibers show the ability to recover from irreversible deformation, and their tensile behavior can be tailored repeatedly, features solely exhibited until now by natural spider silk. It is further shown that the new properties emerge from a microstructure that is closer to that of spider than to natural silkworm silk. This work demonstrates that processing plays a role at least comparable to that of the amino acid sequence in the final properties of the material. The spinning process does not only modify the mechanical parameters of the fiber but also can even prompt the emergence of new properties, opening a wide range of new applications for regenerated silk fibers. It also represents a significant change of the paradigm in the field of biomimetics, given that it relaxes the condition of copying the natural protein sequences as close as possible to recover the outstanding properties of the natural materials.
Regenerated silkworm fibers spun through a wetspinning process followed by an immersion postspinning drawing step show a work to fracture comparable with that of natural silkworm silk fibers in a wide range of spinning conditions. The mechanical behavior and microstructure of these high performance fibers have been characterized, and compared with those fibers produced through conventional spinning conditions. The comparison reveals that both sets of fibers share a common semicrystalline microstructure, but significant differen-
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