Nano-physicist Bodil Holst's interest in polar bear fur began while she was watching a German quiz show.
"I learned that polar bears are invisible in infrared cameras, meaning their fur has the same temperature as the surroundings," says Holst, of the University of Bergen in Norway. She also knew that polar bears jump into frigid water when they hunt, coming back onto land to eat their prey. Most mammalian hair can freeze when it gets wet in cold temperatures — think human beards on a frosty winter day. But, Holst noticed, polar bear fur did not freeze after getting wet.
"I was very puzzled," she says. "When they go into the water and out again, why do they not get covered in ice?" The work could eventually lead to more environmentally friendly alternatives to existing anti-ice chemicals.
Hair grease is the secret, it turns out. And it's partly what the fur has — and what it lacks.
Polar bear fur contains a cocktail of greasy compounds that make it exceptionally resistant to freezing, Holst and her colleagues reported last week in Science Advances.
Holst initially thought that polar bear fur might have structural properties that prevent ice from forming. She and her colleagues used high-powered microscopes to zoom in on the fur, but "we couldn't really see anything special, they just looked normal," Holst says. "We started to suspect, there's more to this than structure."
While the team was handling the fur, they noticed it was very greasy. When the team washed the hair, it largely lost its anti-icing properties.
"We realized that this was down to polar bear hair grease, effectively," Holst says.
The team then did a suite of molecular analyses to identify what specifically about hair grease might prevent ice from forming. They found high levels of certain compounds that are especially resistant to freezing, specifically because ice has a harder time sticking to them.
Polar bear fur also lacked a compound called squalene, the researchers found. Squalene is abundant in other marine mammals and has properties that make ice stick to it easily.
This combination makes polar bear fur highly resistant to freezing, Holst says. Lab tests showed that it performed about as well as fluorinated ski waxes, which have been banned in Norway for environmental reasons. "That was quite amazing, that polar bear fur does just as well as these very superior skiing waxes," Holst says.
To be extra sure that these anti-ice properties are unique to polar bear fur grease, as opposed to any hair grease, one of Holst's Ph.D. students took matters into his own hands.
"He didn't wash his hair for quite some time and then shaved and made a little mat of his own greasy hair," says Holst. "He tested the anti-icing properties and could see very clearly that human hair, whether you wash it or not, isn't very good at anti-icing."
Holst hopes that the research might inspire new approaches to concocting anti-icing materials that could be used for ski waxes, lubricants and even plane de-icing fluids. But she stresses that her team isn't the first to identify that polar bear fur has such special anti-icing properties.
"We didn't discover it," she says. "It's been known to Arctic people for centuries."
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