Combing Over What Makes Hair So Strong
Hair is one of the strongest materials—when stretched, hair is stronger than steel. A team of researchers collected and tested hair from eight different mammals including humans, javelinas, and capybaras to measure what gives hair its strength. The basic structure of hair is similar across species with an outer cuticle layer surrounding fibers, but each species’ hair structure accommodates different needs. Javelinas have stiffer fibers to allow them to raise their hair when it’s in danger. Their results, published in the journal Matter, found that thinner hair was stronger than thicker strands.
Engineer Robert Ritchie, who was one of the authors of that study, talks about the structure that gives hair its strength and how bio-inspired design can create better materials.
Invest in quality science journalism by making a donation to Science Friday.
Robert Ritchie is a professor of Materials Science and Mechanical Engineering at the University of California, Berkeley, and a Faculty Senior Scientist in the Materials Sciences Division at Lawrence Berkeley National Laboratory in Berkeley, California.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. A bit later in the hour, how diabetes patients are dealing with the high cost of insulin by looking for help online. We’ll talk about it. And we want to know that if you have diabetes and have had to borrow insulin from a friend or family member or have turned to social media for cheaper access to the drug, we want to hear from you. Give us a call– our number, 844-724-8255, 844-SCI-TALK. Or tweet us at @scifri. Once again, if you’ve had to get your insulin from social media or friends or family, we want to hear from you. 844-724-8255.
But first, what if I told you that one of the most advanced bioengineering factories is located on your head? Yeah, your hair is one of the strongest materials out there. When it’s stretched, hair is, believe it or not, stronger than steel. A team of researchers wanted to know what makes this furry fibrous so strong. They collected hair samples from humans, bears– javelina is another– mammals to examine the structure of hair. And what they found might surprise you, as it certainly surprised me. The results were published in the journal, Matter. And my next guest is here to comb over the results. See what I did with that, Robert? Robert Ritchie–
ROBERT RITCHIE: That’s terrible.
IRA FLATOW: Robert Ritchie is one of the authors of that study. He’s professor of Material Sciences and Engineering at the University of California, UC Berkeley, and a faculty senior scientist in the Materials Science Division, Lawrence Berkeley National Lab. He joins us by Skype. I’m sorry. I just couldn’t help myself. If you listen to this show, it comes with all the dad jokes, so.
ROBERT RITCHIE: Yes, I gather that.
IRA FLATOW: But I am– I have an engineering degree. I was never a practicing engineer, but I know that you are. And I can understand how you became interested in studying hair. Tell us about it.
ROBERT RITCHIE: Well, firstly, it’s not quite true that it’s stronger than steel. It’s the so-called specific strength, which is the strength normalized by the density divided by the density. If you’re building an airplane, you want lightweight material, so you generally have a strong material which is also lightweight. And so the specific strength is the important term, and hair has specific traits–
IRA FLATOW: Spoken like a true engineer, like a true engineer.
ROBERT RITCHIE: But it’s not strong– we’re not going to make airplanes–
IRA FLATOW: Yeah, no.
ROBERT RITCHIE: –out of hair. But this is a very interesting topic from the perspective of making new materials, designing new materials. Nature does things very, very differently, the way we do it. Nature has a very small palette of difficult materials to play with. And natural materials can’t be made in furnaces. It’s all done at room temperatures– near room temperature.
But the actual materials that nature makes is quite remarkable in terms of their properties, the combination of their properties, and how much better they are than what they’re made of. A seashell is– the mother of pearl– the abalone shell is something like 3,000 times tougher than what it’s made of, which is basically bricks of a mineral called aragonite, which is blackboard chalk, calcium carbonate.
So what we try to do is understand how nature makes things, what are nature’s secrets to making combinations of properties. And then the ultimate aim is to emulate them and make better synthetic materials. And so hair just is one of the materials we looked at in this category.
And properties of hair, as you mentioned at the beginning of the show, are pretty remarkable. We think of it as a cosmetic thing, but really, they have very, very strong properties. And they can be extended. You can pull a piece of hair about 40% of its length before it breaks, which is considerably better than a lot of the materials that we use in engineering.
IRA FLATOW: And I think I was also surprised to learn how actually the very structure of hair is different. It’s not just one little piece of fiber, right?
ROBERT RITCHIE: That’s right. I mean, and that’s another characteristic of natural materials. People talk about hierarchical structures, which means they have– if you look at them at the macro scale, normal eyesight, and then you look at them in a light microscope, so you can see round in the micro regime. You look at an electron microscope and you go down to the really almost [INAUDIBLE] regime, you see different structures.
So there’s structures within structures within structures in biological materials or natural materials. And hair has exactly that. I mean, the hair itself is a sort of about– human hair is about 50 to 100 microns in diameter and has a sort of what’s called a cuticle around it, which is a crispy bit which holds it in place.
But inside that, there are cells. And inside those cells are little fibers called microfibers. And inside the microfibers, there’s intermediate filaments. And inside the filaments, there’s the little– the sort of helical chains. And so there’s structure at four, five, six different dimensions. And that’s what really makes natural materials so amazing in their properties. It’s these very complicated structures that they can develop.
IRA FLATOW: And what your team discovered is that contrary to what we may think, thicker hair is not necessarily stronger than thinner hair, which was surprising to me.
ROBERT RITCHIE: Yes. I mean, it does sort of defy conventional wisdom initially. But we think this is similar to what was first discovered by Leonardo da Vinci in the Middle Ages. He was doing experiments on wires. And he found that long wires were not as strong as short wires. And there’s a statistical reason for this. In materials that have little cracks in– and hair is not an exception here– the bigger the material, the thicker the hair or the longer the sample, the higher probability of finding a defect in there.
So if you have– so it’s a statistical volume type effect. And it’s very common in ceramics. If you take a piece of ceramic like alumina and you take a big sample or a small sample, the small sample will generally be stronger. Because there’s a lower probability of finding a defect. And we use something called Weibull statistics, which is a form of statistics quite well known. And it fit perfectly into that kind of model. So yeah, it does seem strange, but in light of that, it’s not so strange.
IRA FLATOW: So and your team hand-picked and hand-plucked these hair samples from different animals.
ROBERT RITCHIE: Well, yes. I worked with my colleague in San Diego. It’s a guy called Mark Meyers. He’s a professor down there, and he tends to pick up roadkill wherever he can, I think. But at any rate, he is Brazilian. He brought a lot of these samples back. He has good ties with the San Diego Zoo in cases like that. So we didn’t handpick them all. I think some of the hair came from some of the people in this project, the human hair. But the other ones came from the source and so forth.
IRA FLATOW: So give us a rundown on how hair strength compares across the species. For example, is elephant hair stronger than human hair? Who’s got the best strength?
ROBERT RITCHIE: Well, human hair is up there. Bear hair and boar hair looked pretty strong. Elephant hair is near the bottom because it’s very, very thick. It’s several hundred microns in diameter compared to human hair, which is generally under 100 microns. But of course, hair has different functions, right? On certain animals, it’s a protective layer, so to speak, whereas in humans, it’s more cosmetic. And of course, it holds heat into the head.
But so there’s a functionality to that, which can– so it’s not only the strength. We looked at this javelina hair, which is this Central American– it’s sort of a mammal pig-like mammal. And that tends to, when it gets angry, wants its hard to stand up on its neck as a sort of defense mechanism. So that has slightly different properties in that it’s more porous. And so it can be elevated. So the strength is not the only story here because it depends on the function of the hair.
IRA FLATOW: Interesting.
ROBERT RITCHIE: But the human hair is up there. It’s one of the strongest of the hairs that we looked at, certainly.
IRA FLATOW: You also study your fish scales as material to make armor. I mean–
ROBERT RITCHIE: Well, again, I mean, we look at the structure of fish scales because they are a lightweight armor. They’re a very effective lightweight armor in fish. And so we thought there may be some lessons to be learned that we can– from nature, which we can apply to making real armor. The interesting thing, we looked at this arapaima fish, which is this– I think it’s the largest freshwater fish. It populates these lakes in the Amazon, which have piranhas in. And it can basically survive piranha attacks. And the piranha teeth are the sharpest of any fish tooth as far as I know, so.
The way it’s done, again, it’s very clever what nature does. You need a hard outer layer to stop the penetration of the tooth. But if the whole scale was made of that, it would simply shatter, like if you drove a nail into a piece of glass. So the underbelly of this hard layer in this scale is a much tougher material that can absorb all the deformation and so forth associated with the attack.
And so if we did that, we’d take a piece of armor. We’d put a layer in, and then we’d put a hard surface layer on top of it. And there’d be an interface between them, which is always a weak link that nature grades it. There’s this beautiful gradient in properties of these scales from the very hard outer layer to the tougher inner layer. It’s just beautifully done.
IRA FLATOW: And have we been able to take any of that knowledge and make something practical for we human?
ROBERT RITCHIE: Well, the trouble is that when we make a material, we take a big chunk of metal, and we beat it and we cut it up and so forth. That’s called top down. We start from the top and work down. But nature goes bottom up. It starts from molecules and atoms and builds up these complicated structures. And so that’s difficult for synthetic processing to actually emulate.
So the possible future of this, I think, is that we hear a lot about 3D printing and how it’s going to revolutionize the world. 3D printing is very much in its infancy, but we can’t really guarantee necessarily good material when we use 3D printing. But that has the means to do bottom-up processing, to start from a small scale and then build up on top of that. So I think once this processing technique, additive manufacturing, whatever you call it, becomes more mature, we have a much better possibility of being able to emulate these very complicated hierarchical–
IRA FLATOW: Interesting.
ROBERT RITCHIE: –structures that nature makes.
IRA FLATOW: Well, Dr. Ritchie, thank you. Fascinating. I see you hate what you’re doing. So let me– Robert Ritchie is professor of Material Science and Engineering at the University of California, Berkeley, and faculty senior scientist, Material Science Division at the famous Lawrence Berkeley National Lab.