科学美国人60秒:头发的力量超乎想象

This is Scientific American's 60-second Science, I'm Susanne Bard.
All mammals have hair—commonly called fur when it's on your cat or a koala. And the thickness of individual hairs varies from species to species. For example, elephant hairs are more than four times thicker than a strand from an adult human.
"Normally, when the animal is larger, the hair tends to be thicker."
University of California, San Diego, materials scientist Wen Yang. She's interested in how biological structures like hair hold up under stress. That interest comes from a desire to design better synthetic materials.
Yang's team tested the tensile strength of hair from eight different mammal species, including humans. They subjected those hairs to increasing levels of tension until the fibers broke. The researchers assumed that thick hair—from giraffes, elephants and boars, for example, would be more robust. But they were wrong.
"Interestingly, we find the thinner hair, actually, is stronger."
In fact, adult human hair was some of the strongest in the study. And children's hair—the thinnest of all—was even more durable.
Yang's team took a closer look at the hair with a scanning electron microscope. All hair is made of the protein keratin. The microscope views revealed specific patterns of breakage in the keratin fibers, which are composed of a protective outer cuticle and an inner cortex that provides strength.

The microscope images showed that the cortex of thicker hair tends to snap when broken, leaving a clean, even break. But the inner cortex of thinner hair breaks off less evenly. Yang compares the process to what happens when a powerful gust of wind blows a tree down. In those cases, the trunk tends to shear at an angle. And that kind of break is a sign of strength—it actually takes more tension to cause shearing than a clean break.
The researchers found that the cutoff point between these two types of breakage was about 200 micrometers—about two and a half times thicker than human hair.
The study is in the journal Matter.
Yang is now attempting to mimic the complex structure of hair in the lab with 3-D printing. Once the technology is perfected down to the nanoscale, these artificial fibers could be used to create a variety of strong materials for use in construction or even rescue operations—such as nearly indestructible ropes.
Until then, we'll just have to be content with the knowledge that fewer than 1,000 human hairs can support an entire person's weight. Think of all the strength that's left on the hair salon floor.
Thanks for listening for Scientific American's 60-second Science. I'm Susanne Bard.