Tufts University researchers have developed a method to create silk-based materials that refuse to stick to water, or almost anything else that contains water. In fact, the modified silk, which can be molded into shapes like plastic or applied to surfaces as a film, has non-stick properties that surpass those of non-stick surfaces typically used on cookware, and it could find applications stretching across a wide spectrum Range of consumer goods as well as medicine.
Silk is a natural fiber spun by moths and has been used to make durable and delicate fabrics – and surgical sutures to close wounds – for thousands of years.
More recently, scientists have learned to break down the fibers into their basic protein element – silk fibroin – and turn it into gels, films, sponges and other forms to make everything from implantable orthopedic screws to textile inks made in response change color to reflect body chemistry.
“What makes silk such a unique material is that not only can it take on a variety of shapes and forms, but you can easily alter its properties by chemically modifying the silk fibroin,” said Krishna Kumar, Robinson Professor of Silk Chemistry at Tufts.
“If we want to use silk fibroin to make orthopedic screws that are absorbed by the body at different rates, we modify the chemistry,” he said. “If we want to develop a blood sensor that detects oxygen, glucose or other blood components, we modify the chemistry. In this study, we modified silk fibroin to repel water, and we can do this in a way that can be tuned ‘the material to be more or less water-repellent.’
The advance was reported in the Journal ChemBioChem.
In order to turn silk into a water-repellent material, the surface of silk fibroin had to be coated with short chemical chains containing carbon and fluorine, called 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 for water and negatively and positively charged twigs on the silk attract water, a perfluorocarbon-coated silk protein allows the water little footing.
Perfluorocarbons even resist the attraction caused by other forces that typically bring molecules together. Changing the number and length of the perfluorocarbon chains on the silk protein can adjust how “non-sticky” it behaves. Luke Davis, assistant professor of chemistry, specified the level of fluorine required on the surface of silk to exhibit non-stick properties.
The chemical synthesis takes place under mild conditions, so the manufacturing process could be safer, both for the workers and the environment, unlike the manufacturing of other non-stick substances. Safer manufacturing and a renewable biological material source meet two criteria for sustainability.
Tufts researchers measured non-stick properties by observing how water beaded off the material’s surface – much like water beaded off a waxed car. In fact, on non-stick floss, which was formed into rods with the highest percentage of perfluorocarbons, the water rolled up into droplets that were even more rounded than on Teflon.
It is not just water that will bead off the non-stick floss, but any substance that has water as its main ingredient, which can include various foods, blood, cells and tissues. Although not tested in this study, perfluorinated materials are also known to repel oils.
“Modifying medical devices to prevent harmful interactions with water and other biologics has the potential to maintain strength and integrity for as long as needed,” said Julia Fountain, a graduate student in Kumar’s lab and a co-author of the article. “Silk is already relatively inactive for the immune system, so tuning its ability to fight off cells or other substances could become even more useful.”
The benefits of highly nonstick surfaces go far beyond medical applications. While there are concerns about chemicals being absorbed into the body from commercially available non-stick coatings, silk-based non-stick surfaces may offer an alternative option that can be evaluated for relative safety.
One could also imagine car windows that rainwater rolls off without a wiper, coatings on metals that help prevent rust, or on fabrics to make them easier to clean.
“The success we’ve had modifying silk to repel water extends our successes chemically modifying silk for other functionalities – such as the ability to change color, conduct electrical charge, or exist in a biological environment or mine it,” said David Kaplan, Stern Family Professor of Engineering at Tufts. “As a protein, silk lends itself well to modular chemistry – the ability to fit different functional components into a natural scaffold.”
