Can synthetic silk beat nature's own?

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A silkworm (Bombyx mori).

For the first time, people can do what only spiders and silkworms can do: spin silk fibers to specific specifications.

The new technique opens the door to customizing fibers to use in medicine, engineering and – of course – clothing. It relies on a special device to build silk protein chains that are then spun together into fibers with predicted and controlled strength, rigidity and width.

The researchers will continue to refine the method that may lead to a cheaper and more directed silk product than spiders and silkworms currently provide. The innovative method to make synthetic silk is explained in the journal Biomacromolecules.

Context

Silk is a very attractive material for clothing and other textiles due its smooth, cool and comfortable qualities. Manufacturers also find silk tantalizing. Silk's mechanical properties – it is as strong as steel yet six times lighter – and ecofriendliness – it isbiocompatible and biodegradable – make it an attractive material for biomedical and engineering applications. In fact, silk is used for suture threads and in tissue grafts.

In all the world, only silkworms and spiders can make silk. A mysterious spinning process produces the strong threads that are weaved together into webs and cocoons. The animals make the unique silk protein and spin it together in just the right way to form a special structure that gives the fiber its desired properties. The invertebrates do this with apparent ease. More so, they can control its properties, depending on where and for what purpose the silk thread will be used.

Obtaining natural silk for consumer uses, then, comes with a set of problems. The properties of natural silk – its strength, width, length – is determined by the specific species of spiders and worms. Basically, people get what the animals produce. It is also expensive and laborious to keep the silk-producing creatures – normally silkworms – and harvest the product.

So, there is need for a rapid method to fabricate silk and control its mechanical properties. The first step in this process has already been solved. A variety of other organisms – ranging from bacteria to goats – have been genetically modified and can produce silk proteins. The type of silk protein can be easily varied. It can also be produced in large quantities. However, the second challenge of how to spin those proteins into fibers with the desired properties remains.

What did they do?

Researchers from Boston, Mass., and Göttingen, Germany, used a microfluidic device that mimics the silkworm glands by extruding silk proteins from an aqueous solution. The solid device is infused with two crossing channels similar to the thickness of a human hair. Liquids can flow through the channels, which come together to allow a very small point of mixing. Through one channel flows the aqueous silk protein solution. Through the two intersecting channels flows an acidic solution of polyethylene glycol. Where the two channels meet, the fluids mix. Polyethylene glycol is a benign molecule  – a polymer – that envelops the active drug ingredients and acts as a lubricant in a variety of medicines.

Several different flow...

speeds and ratios of silk protein to polyethylene glycol solution were tested. The ensuing mechanical properties of the spun fiber – such as strength, strain and stress failure – were measured.

What did they find?

The device created a single strand of silk fiber with very well-defined properties. The fiber's properties were altered by varying the spinning conditions, such as flow speed and the speed with which the dry fiber was drawn out of the device.

A small amount of original material was required – as little as 50 microliters, which is about the volume of a drop of water. This makes it a very interesting tool to screen different processing parameters and determine their effects on the spun fiber properties.

The authors related the flow characteristics inside the device to the fiber's mechanical properties – strength, rigidity – and the internal structure formed by the silk proteins inside the fiber. They were able to control the fiber diameter and fiber strength. They even could vary the diameter of the fiber as it was spun.

While they did not yet match the properties of natural silk in this work, the control they showed over properties is very promising.

What does it mean?

The researchers designed a device that can rapidly produce synthetic silk fibers from silk proteins. The spinning conditions are easily controlled to determine the effect on the properties of the spun fibers.

This new technique opens up the possibility to customize fiber properties so they be used for specific applications. Of several methods being investigated to produce synthetic silk, this one is closest to the natural process, so it offers a very clean method to produce silk fibers on a large scale. No real waste is generated, and the polyethylene solution can be recycled.

More generally, the process is an example of how green chemists can develop a clean manufacturing process straight away and avoid creating pollution problems right from the start.

This work has a great potential to improve understanding of how to make synthetic silk fibers and possibly make them stronger, softer, more stretchable or thinner than natural silk. Different silk proteins can be produced from bacteria, goats and other animals – or even synthetically in the lab – so it is now possible to study the effect of spinning conditions and variations on the silk fibers that are produced. The small starting volume reduces the amount of silk protein required making the tests cheaper and faster to run.

The authors investigated only two processing parameters, but their new device can easily be used to screen all others quite rapidly. The process will no doubt increase understanding of both synthetic and natural silk production.  More importantly, the device may produce better, designer silk fibers.

Resources Silkworm information. Center for Insect Science Education Outreach. University of Arizona. http://www.environmentalhealthnews.org/ehs/newscience/spinning-silk-better-than-nature/