Newly developed silkworm silk is 70% more durable than spider silk, scientists say

Silk is stronger than steel if used correctly. Although spiders play a leading role in silk production, new research led by Tianjin University shows that silkworms can be made 70 percent stronger than spider silks by removing a sticky outer layer and manually spinning the silk.

Published in Matter on October 6, "Our finding reverses the previous perception that silkworm silk cannot compete with spider silks on mechanical performance," says senior author Zhi Lin, a biochemist at Tianjin University about the research.

Silkworm silk has traditionally been used in fashion as a source of luxury robes and royal apparel, but today, silk-based materials are more likely to be found in biomedicine as a material for stitches and surgical mesh.

Silkworms are more durable now

Having silk from silkworms is the easiest way, but the products are not as durable as that of spiders.

"Dragline silk is the main structural silk of a spider web. It is also used as a lifeline for a spider to fall from trees," says Lin. Silkworms, on the other hand, use their softer silks for the construction of their cotton-ball-like cocoons during their transformation into their moth forms.

Lin's team wanted to use common silkworms since they are more available and manageable. They were motivated by the successful artificial spinning of spider egg case silk, a close relative of silkworm silk that has been demonstrated to spin easily.

Silkworms' natural silk fiber is made up of a core fiber wrapped in silk glue, which interferes with commercial fiber spinning. To get around this, the researchers boiled silk from the common silkworm Bombyx mori in a chemical bath that could dissolve this glue while also minimizing the degradation of silk proteins. The silk was then solidified in a bath of metals and sugars to improve its spinning properties.

"Since silkworm silk is very structurally similar to egg case spider silk, which has previously been demonstrated to do well in a mix of zinc and iron baths, we thought to test this alternative method to avoid hazardous conditions used elsewhere," says Lin.

"Sucrose, a form of sugar, may increase the density and viscosity of the coagulation bath, which consequently affects the formation of the fibers."

"We hope that this work opens up a promising way to produce profitable high-performance artificial silks," Lin also adds.

Abstract:

Silks are protein-based biopolymers with unique combinations of different physical properties. Silkworm silk is the most well-studied due to its abundance and good mechanical behaviors. However, silkworm silk exhibits a weaker strength than the strongest natural silk, spider dragline silk, which is difficult to be produced on a large scale. Here, a facile strategy is developed for spinning superstrong silk fibers in a metal-ion coagulation bath using regenerated silkworm fibroins. The tensile strength of the artificial silk reaches ∼2.0 GPa, more than 70% higher than the average tenacity of dragline silks. It also exhibits an average Young’s modulus of ∼43 GPa, significantly higher than any known natural silks. Structural investigation indicates that the extraordinary strength may be ascribed to high crystallinity and small nano-crystallites formed in the artificial fiber. The incorporation of Zn2+ ions into the fiber during spinning and post-spinning drawing processes may also contribute to its excellent mechanical properties.