Using Silk Proteins to Create Nonstick Surfaces

Researchers have found a way to create nonstick surfaces that are water repellent through the use of a naturally-derived material: moth silk. A team at Tufts University developed a method to fabricate silk-based materials that won't stick to water nor anything containing it, making it a useful invention for medical, automotive, and numerous other consumers applications, researchers said. The material could even be used as a safer way to create nonstick coatings to replace Teflon, which can emit hazardous chemicals at high temperatures.

Silk is a natural fiber spun by moths that's been used for thousands of years to fabricate both durable and fine fabrics—and not just the one commonly used to make clothing. Another of its lesser-known uses is as surgical sutures to close wounds.

A team led by Krishna Kumar, a professor of chemistry at Tufts, modified moth silk into its most basic protein element—silk fibroin—and turned it into various gels, films, sponges, and other forms. They have already used this technique to great effect to develop products such as implantable orthopedic screws and  textile inks that change color in response to body chemistry.

Their most recent exploration of moth silk led them to a new modification to create yet another use for the versatile material, Kumar said. This modified silk—which can be molded into forms like plastic or coated onto surfaces as a film—has shown a nonstick performance that surpasses those typically used in cookware that are developed with synthetic chemicals, such as Teflon.

“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,” he said in an article in Tufts Now.

Sustainable Way to Repel Water

To turn silk water-repellent involved some clever chemistry, researchers said. They covered the surface of the silk fibroin with short and stable chemical chains containing carbon and fluorine, called perfluorocarbons. These chains don't react with other chemicals, nor do they interact with proteins and other biological chemicals in the body.

In its natural state, the surface of a silk protein attracts water like a magnet—with negatively and positively charged branches on the silk attracting water. However, researchers found that a silk protein covered with perfluorocarbons allows nothing for the water to attach itself to, they said.

In fact, perfluorocarbons even resist attraction caused by other forces that tend to cause molecules to join together, researchers said. This means that by changing the number and length of perfluorocarbon chains on the silk protein, researchers can adjust how "unsticky" it behaves. As part of their research, Luke Davis, an assistant professor of chemistry at Tufts, established the level of fluorine required on the material's surface to exhibit nonstick behavior.

"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 explained in the article.

The process promises a more sustainable way to create nonstick surfaces for two reasons, researchers said. One is obviously the use of a naturally sourced material to create water-repellent products.

The other is that the chemical synthesis that changes the property of the silk to be water-repellent occurs under mild conditions, they said. This means the manufacturing process to create products using the material is potentially safer for workers and the environment than processes that use toxic chemicals for nonstick surfaces, they said.  

Testing the Silk

Researchers measured the nonstick ability of the material by observing how water beads up on the surface of the material, finding that on nonstick silk molded into bars using the highest level of perfluorocarbons, the water rolled up into drops that are rounded even tighter than on Teflon, they said.

Moreover, it's not just water that rolls off the nonstick silk, but any substance that has water as a major component. This could include various foods, blood, cells, and tissues, researchers noted, making the water-repellent material well-suited to medical applications, they said.

“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,” Julia Fountain, a graduate student in Kumar’s lab, said in the article. “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.” Other applications include any instance in which a nonstick coating or surface is needed, providing a more natural alternative to Teflon and other widely used but potentially harmful chemical coatings, researchers said.

Researchers published a paper on their work in the journal ChemBioChem and, based on the success of the research, plan to continue to explore the various uses of silk fibroids, they said.

“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,” David Kaplan, a professor of engineering at Tufts, said in the article. “As a protein, silk lends itself well to modular chemistry—the ability to ‘plug in’ different functional components on a natural scaffold.”