Forced reeling from Bombyx mori reveals the mechanically toughest silk fiber at the natural spinning speed

Domesticated silkworms produce silk fibers in a mild condition, where humidity and temperature are controlled at a constant level. The natural silk spinning mechanisms of silkworms have not yet been elucidated. Previous studies have demonstrated that wild silkworms have evolved to spin mechanically and structurally robust silk fibers irrespective of reeling speeds. Here, we examined the influence of spinning speeds on the physical properties of silk fibers that are forcibly reeled from the domesticated silkworm, Bombyx mori (B. mori). We found that the B. mori maintained physical properties of the spun fibers in a narrower range of reeling speeds compared with wild silkworms. When the reeling speed was higher than the threshold of 60 mm/s, the molecular alignment of the silk fibers increased, while the ductility decreased. The toughness of silk fibers was highest when the reeling speed was close to the natural spinning speed of 10 mm/s. The B. mori silkworms might have less resistance to changes of the reeling speeds than wild silkworms that live in the wild. This study is not only useful for clarifying the biological system underlying the silk spinning of silkworms but can also contribute to considering spinning conditions aimed for producing synthetic silk.