The world is saturated with water-resistant materials crafted from fluorocarbons such as polytetrafluoroethylene, a subgroup of per- and polyfluoroalkyl substances (PFAS) known as “forever chemicals”. This poses a pressing challenge. PFAS accumulate harmfully in the environment, imperilling health. To counter this, researchers propose a novel alternative: bioengineered protein found in spider silk that are capable of forming remarkably water-resistant films under specific conditions. This discovery, triggered by chance and humidity, could potentially replace conventional water-repellent coatings.
From your rain jacket to the coating on pills, water-resistant films are everywhere – but unfortunately, many are made from fluorocarbons, a type of PFAS.
PFAS are human-made substances that contain an unusually strong bond between carbon and fluoride. Since that bond does not break down over time, PFAS accumulate in the environment, with harmful effects on human and animal health. PFAS are a potential carcinogen according to the United States Environmental Protection Agency and have been identified in drinking-water, soil and air around the world – and even in the blood of newborn babies, suggesting that exposure starts within the womb.
With pressure mounting to phase out fluorocarbons, scientists are turning to nature for alternatives. New research, published in March in Langmuir suggests a promising candidate: bioengineered protein found in spider silk form a surprisingly water-resistant film under certain conditions.
Luck and humidity
This was really a “chance finding”, says co-author Markus Linder, Professor of Biomolecular Materials at Aalto University in Espoo, Finland.
Teemu Välisalmi, a PhD student in Linder’s lab and the lead author, was hard at work on another project – using various types of animal and human proteins to build organic scaffolds that the researchers hoped could facilitate growing mammal cells in the lab. Then, a rainy day changed the course of his research.
“When you make silk films at low humidity, the films always turn out to be hydrophilic” – meaning they are easily drenched and likely to dissolve in water, which is good for building scaffolds. “And then one day it was raining, and the humidity climbed to about 90% and the films turned hydrophobic” – meaning they repelled water, Välisalmi explains. This was totally unexpected, he says.
The protein that caught Välisalmi’s eye was a modified version of one found in the silk of the European garden spider (Araneus disdematus), a common arachnid found all across Europe and North America.
To test the film’s properties, Välisalmi needed this bioengineered spider protein in bulk. By injecting RNA – essentially assembly instructions for spider silk protein – inside Escherichia coli cells, Välisalmi duped the bacteria into building the modified protein for him.
Then, Välisalmi dissolved the protein into a solution before carefully dropping a tiny puddle on a glass slide. As the water evaporated, a film was left behind in the puddle’s footprint.
Material scientists assess water-repellent films by their wettability – more specifically, by their contact angle, a measure of how high water beads up off the surface of the material. Think of the difference between when water spills on a table versus on a duck’s back. On the duck’s highly hydrophobic feathers, it is raised high up off the surface. To their surprise, tiny droplets deposited on the dried film formed contact angles of more than 120 degrees.
“The contact angles are really very high,” Linder says. “They are higher than people were able to make with protein before.”
But what makes the film so repellent to water? The protein’s structure is key, Linder says. In its natural state, it comprises repeating hydrophobic and hydrophilic components bookended by globular regions – blobby, folded-up wads of protein.
These globular regions seem to be important in forming the structure of the film – without them, the naked middle sections jumble up like a haystack, whereas “when you get a crack in these films, there are these tiny nano-sized fibres – indicating that there has to be some kind of structural assembly,” Välisalmi says.
By manipulating the amount of salt in the solution, the team realised that they could alter the film’s hydrophobicity. The researchers theorise that “somehow the salt makes some kind of bed underneath the silk protein, and the hydrophobic regions of the silk come to the surface,” Välisalmi explains.
But not ready for the market
There are still kilometres to go before bioengineered spider silk protein can coat a pill. “This study did not go all the way to making a durable version that could actually be used in practice,” Linder says. “I think if we had, we would not have just published this – we would have started a company,” he adds with a chuckle.
The film seemed to lose its hydrophobicity after its first contact with water. Välisalmi’s droplets formed high-contact-angle beads, which were effectively repelled by the film while the droplet evaporated. But even with the naked eye, he could see a dark outline of where the droplet had been – that initial contact with water changed the film’s properties. Additional drops of water in the affected area did not have the same high contact angles.
Välisalmi believes that “when these films are wetted, somehow the salt comes back to the surface and it loses its hydrophobicity because of it,” he says. After that first wetting, the salt bed falls apart.
Remedying this will be a challenge, Linder says. “The next steps are difficult.”
This means that the modified spider silk protein is not a total breakthrough, but it is also not a dead end, Linder explains. It is a promising lead that deserves further examination.
Although more study is required to pin down how the protein’s internal hydrophobic regions are structured, the team learned that the amorphous globular regions are a bioengineer’s playground.
“Regardless of what we put on the ends – regardless of what their function is,” having any sort of globular region helped to form the structure of the film, Linder says. “It just made handling this silk protein easier.” Since the globular regions can be made from whatever material the bioengineers choose, this is a ripe opportunity for add-ons that could further reinforce the film’s durability or hydrophobicity.
“Overall, this research shows that by studying biological molecules, in principle, these problems are solvable,” Linder says. “Drawing inspiration from nature is a feasible route to sustainable solutions.”