Mimicking silk spinning animals to make artificial fibres more green

Sheffield researchers discover that silkworms and spiders make silk by pulling rather than pushing, potentially leading to greener synthetic fibres. 

silkworm fibres
The caterpillar of the Bombyx mori silk moth pulls fibres from its silk-producing gland

The discovery could revolutionise how artificial fibres are manufactured, potentially improving both the properties of the fibres and the environmental performance of the industry.

Artificial fibres have been produced by industry for close to 80 years, and their manufacture has been dominated by a single method. Fibres are made by squeezing a liquid polymer precursor through a small hole and then exposing the liquid to changes in temperature or further chemical treatment in order to harden it into a solid thread. This has meant that the industry has been characterised by high energy usage and the use of potentially hazardous materials.

But the fibres that these filaments are intended to mimic do not require such conditions. Silk thread, produced by spiders and caterpillars, solidify at ambient temperatures outside the animal’s body and typically don’t require exposure to additional substances, leaving only water behind in the solidification process. The University of Sheffield team, from the Department of Materials Science and Engineering, believe that this might be connected with the physical process of how the fibre is produced.

In a paper in Nature Communications, the team describes how it studied Bombyx Mori silkworms to determine how they produced silk. “While it is easy to assume that silk is propelled out of the body like we see in comic books, we wanted to put that to the test,” said Dr Chris Holland, head of the department’s Nature Materials group. The paper’s lead author, PhD student Jamie Sparks, added: “We found that to spin silk by extrusion (pushing), means a silkworm would have to squeeze itself hard enough to generate more pressure than a firing diesel engine. This isn’t possible as the animal’s body would be unable to contain that pressure. It seems that you can’t squeeze silk like a tube of toothpaste.”

To test this further, the team adapted a rheometer, a device usually used to measure the viscosity of liquids, into a highly sensitive spinning wheel and used it to pull a silk fibre made from the liquid protein mix extracted from silkworms through simulated ducts. “By combining computer models with experimental data and practical measurements, we determined the forces needed to squeeze unspun silk down the animals’ silk gland and spin a fibre,” Holland said. The forces for this process, known as pultrusion, were much lower than those used for extradition, according to the paper.

The research also reveals some of the effects the production process has on the fibre’s properties, and, the team says, “identifies the requirement for the development of biomimetic artificial silk spinning devices based on pultrusion rather than extrusion.”

Sparks commented: “Silk is one of the most promising green biomaterials, and could be the perfect replacement for nylon and polyester-based clothing. Traditional production process for silk is both arduous and time-consuming, but if we can bypass that by mimicking nature in an industrial setting, we could improve not only silk, but also how we process our synthetic materials.”