A silk moth. Photo: Zivya.
A silk moth. Photo: Zivya.

Researchers at Tufts University have developed a method to make silk-based materials that can repel water – and almost anything else containing water for that matter. In fact, the modified silk, which can be molded into forms like plastic or coated onto surfaces as a film, has non-stick properties that surpass those of non-stick surfaces typically used on cookware. As such, it could find use in applications that extend into a wide range of consumer products, as well as medicine.

Silk is a natural fiber spun by moths and has been used for thousands of years to make durable and fine fabrics—and surgical sutures to close wounds. More recently, scientists have learned to break down the fibers to their basic protein element—silk fibroin—and reconstitute it as gels, films, sponges and other forms to create everything from implantable orthopedic screws to textile inks that change color in response to body chemistry.

“What makes silk such a unique material is that not only can it take on a wide range of forms and shapes, but one can easily change its properties by chemically modifying the silk fibroin,” said Krishna Kumar, professor of chemistry at Tufts.

“If we want to make orthopedic screws that are absorbed by the body at different rates using silk fibroin, we modify the chemistry. If we want to create a blood sensor that detects oxygen, or glucose, or other blood components, we modify the chemistry. In this study, we modified silk fibroin to repel water, and we can do it in a way that can ‘tune’ the material to be more or less water repellent.” Kumar and his colleagues report their advance in a paper in ChemBioChem.

To turn silk into a water-repellent material, the researchers covered the surface of the silk fibroin with short chemical chains containing carbon and fluorine, known as perfluorocarbons. These chains are very stable and do not react with other chemicals, nor do they interact with proteins and other biological chemicals in the body.

While the natural surface of the silk protein acts like a magnet to water – with both negatively and positively charged branches on the silk attracting water – a silk protein covered with perfluorocarbons leaves little for the water to grab on to.

Perfluorocarbons even resist attraction caused by other forces that typically bring molecules together. By changing the number and length of the perfluorocarbon chains on the silk protein, the researchers could control how ‘unsticky’ it was. Luke Davis, assistant professor of chemistry at Tufts, established the level of fluorine required on the silk surface for it to exhibit non-stick behavior.

The chemical synthesis is performed under mild conditions, which means that the manufacturing process for the silk surface could be safer than for other non-stick substances, both for workers and the environment. Safer manufacturing and a renewable biological source of material checks off two boxes for sustainability.

The Tufts researchers measured the non-stick behavior of the silk material by observing how water beads up on its surface– like how water rolls off a waxed car. On non-stick silk with the highest level of perfluorocarbons, which was molded into bars, the water rolled up into more spherical drops than they do on Teflon.

It’s not just water that rolls off the non-stick silk, but any substance that has water as a major component, which could include various foods, blood, cells and bodily tissue. Although not tested in this study, perfluorinated materials are also known to repel oils.

“Modifying medical devices to prevent detrimental interactions with water and other biologics has the potential to preserve strength and integrity for as long as they are needed,” explained Julia Fountain, a graduate student in Kumar’s lab and co-author of the paper. “Silk is already relatively inert to the immune system, so tuning its ability to repel cells or other substances could make it even more useful.”

The advantages of highly non-stick surfaces go well beyond medical applications. While there is concern regarding chemicals absorbed by the body from commercially available non-stick coatings, silk-based non-stick surfaces may offer an alternative option that can be explored for its relative safety.

The silk surfaces could also be used to produce automotive windshields where rainwater just rolls off without using wipers, coatings on metals that help to prevent rust, and fabrics that are easier to clean.

“The success we had with modifying silk to repel water extends our successes with chemically modifying silk for other functionalities – such as the ability to change color, conduct electrical charge, or persist or degrade in a biological environment,” said David Kaplan, professor of engineering at Tufts. “As a protein, silk lends itself well to modular chemistry – the ability to ‘plug in’ different functional components on a natural scaffold.”

This story is adapted from material from Tufts University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.