Hair is a remarkable composite material that exhibits a wide spectrum of mechanical properties, ranging from the stiff quills of porcupines to the water-adapted fibers on capybaras, a mammal native to South America, and the largest living rodent in the world. This diversity is made all the more impressive by the fact that, regardless of species, hair is mostly comprised of the same constituent materials. Hair has attracted the attention of materials researchers, considering that a better understanding of its varied properties may lead to the improved design and manufacture of synthetic materials.

Toward this end, Wen Yang, a research scientist working in Marc Meyers’ laboratory at the University of California, San Diego, and her colleagues surveyed the tensile strength of hair samples taken from a myriad of sources. Their results, published in Matter, suggest a relationship exists between the structure, strength, and failure of hair fibers that is predicted by Weibull probability analysis. They conclude that the larger the diameter of a hair fiber, the more likely it is to have an internal flaw and be weaker for it.

The largest hair fibers tested were those taken from elephants and giraffes (with diameters of 330 µm and 370 µm, respectively) which proved to be among the weakest, with a breaking stress of approximately 125-150 MPa each. This was contrasted by human hair (50-100 µm in diameter) which showed a breaking stress ranging from 175 MPa to 275 MPa.

“On the weight basis, [hair] can be as strong as steel,” says Meyers. In highlighting this point, the researchers state that the density-normalized strength of human hair is between 150 MPa/Mg m^-3 and 200 MPa/Mg m^-3 which places it on par with structural steel alloys whose strength measures at approximately 250 MPa/Mg m^-3.

That human hair is strong is not news: Among other cases in history, it is believed that Macedonian siege equipment (such as the catapult) used human hair to generate torsional energy. The material underpinnings of this strength, however, are not well-understood.

In its simplest division, hair can be broken down into two layers: a brittle outer structure known as the cortical layer and a composite inner layer known as the cortex, the latter of which is most likely to determine hair’s tensile strength. This is reasoned by the fact that the cortex contains crystalline fibers made from a structural protein known as α-keratin. Here, α-keratin fibers are bundled together, joined by a multitude of reversible disulfide bonds, and embedded in an amorphous matrix.

Under strain, the helical protein domains in α-keratin fibers stretch, imbuing hair with its elastic properties. However, further stretching converts the helices into two-dimensional pleated sheets, resulting in the formation of β-keratin. This more rigid structure is then prone to fracture or delamination from the surrounding matrix.

Whether it is from an elephant or a human, though, all hair is comprised of α-keratin. So what, then, enables such a wide spectrum of mechanical properties?

“Hair is a fascinating material because slight changes in composition and chemical bonding can yield very different forms and behaviors,” says Malebogo Ngoepe, a researcher and lecturer in Mechanical Engineering at the University of Cape Town, South Africa, who was not involved in this study. Her work has taken a systems approach to understanding the factors associated with diverse hair types. In explaining her research interests, she stated that “[h]air is a seemingly simple structure but the more one delves into the detail, the richer it becomes.”

The findings reported by Yang and her colleagues suggest that hair has evolved to suit the varied needs of mammals, in part, through diversification of its hierarchical structure. For example, the researchers examined hair collected from a javelina (a medium-sized animal that looks similar to a wild boar) and found it to contain a porous cortex as well as be the weakest hair sample tested, exhibiting a breaking stress of just 50 MPa. The porous structure may be better able to withstand compression force, the researchers speculate, and its stiffness may serve as a deterrent to predators when raised on the javelina’s back.

Based on these and similar observations, it would seem that the hierarchical structure of hair fibers is critical to determining its mechanical strength. Future studies will need to dive further into hair’s many details to understand why exactly a larger diameter hair fiber translates to a weaker hair fiber and what implications that may hold for synthetic fiber development.

Read the article in Matter.