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Human hair is a remarkably strong material

The strength of human hair is comparable to steel: due to the hierarchical structure of proteins comprising human hair it tends to be resistant to tearing easily.

Healthy and shiny hair is considered an important beauty feature. But human hair can do more than just look good - it is also remarkably strong. Researchers have now investigated whether the hair of other mammals is as tear-resistant as human hair, or perhaps even stronger. Their results were surprising.

The new research revealed a surprising correlation: Contrary to intuitive assumptions, thin hair tends to be stronger than thicker hair. Elephant hair, for example, breaks significantly faster than its human counterpart. The reason: depending on their diameter, hair breaks differently.

A single human hair is quite thin: Europeans have an average diameter of only 0.06 to 0.08 millimetres. Asian hair is somewhat thicker in comparison, at 0.08 to 0.12 - but even that still seems rather small. Nevertheless, these thin threads of keratin are surprisingly strong. This means that 500 to 1,000 of our hairs can easily bear the weight of a human being. Their so-called tear strength is between 200 and 260 megapascals.

How strong is a strand of human hair?

If the different density of the two materials is taken into account, the strength of human hair is even in the order of magnitude of steel. Researchers are convinced that this property is due to the special structure of our hair: the cuticle layer, called the cuticle, forms the outer covering of the hair. This contains the cortex, which consists of many bundles of fibers. These bundles are composed of a large number of keratin fibres, the fibrils. In turn, even smaller fibrils are embedded in each fibril.

This hierarchical structure, together with a special connection of the individual fibres, makes the hair extremely resistant. But what does this actually look like in other animals - is their hair just as strong, or perhaps even stronger than ours?

Scientists led by Wen Yang from the University of California at San Diego have now addressed this question. For their study they examined hair samples from eight different mammals: In addition to human hair, they examined the fur of horses, bears, wild boar, elephants and giraffes, as well as the hair of collared peccaries and capybaras. As expected, the thickness of a single hair varied considerably from species to species. For example, an elephant hair has a diameter of 0.35 millimeters - about four times as thick as its human counterpart.

Yang's team used a special stress test to check the tensile strength of these animal hairs. A machine gradually pulled the hairs apart until they tore. Intuitively, one would expect thicker hair to be stronger, said Yang. But the tests revealed that this was not true. Instead, the strength actually tends to decrease with increasing thickness. For example, elephant hair is only half as thick as the hair on our head. A similar correlation was also found when comparing different hairs of one and the same species. As the researchers found, thin child hair withstood greater tension than thicker adult hair. Therefore, interspecies differences in hair structure cannot explain the observed differences. But how do they come about?

Why thin hair is stronger than thick hair?

To solve this mystery, the scientists then examined the hair under an electron microscope. They found that the hair structure of many mammalian species is similar in many respects - although there are surprising differences. Despite the comparable structural composition, hair seems to break in different ways depending on its thickness. This is what the experiments revealed: Hairs whose diameter exceeds 0.2 millimetres break in the form of a simple fracture - a clean break is produced. Thinner hairs such as those of humans, bears and horses behave differently. Hairs of these species do not break smoothly, but unevenly. The scientists compare this process with the way a tree branch can break in a storm, for example.

As a result of mechanical stress, zigzag-shaped cracks develop within the material, Yang explains. These cracks spread and in some biological materials, the final breakage only occurs when the small cracks meet. Such materials can withstand greater stress than material that forms a normal fracture, said Yang. The interesting thing is that what at first glance appears to be less than intuitive is already familiar to materials researchers from a different context. Even with materials such as metal wires, thicker is sometimes weaker than thinner. That's a statistical matter. A thicker piece has a higher probability of having a defect in one place says Yang's colleague Robert Ritchie. It's surprising to see this now even with a material like hair.

Human hair as a model for new materials?

The researchers hope that their insights into the structure and resistance of hair will one day help in the design of synthetic materials. Natural materials such as hair have been created over thousands of years of evolution - they are very well developed. We want to learn from this and one day create products with similar properties, Yang emphasizes. Materials structured like hair could be interesting for many different applications, the researchers explain. However, the researchers still have a long way to go before they can produce materials as thin and strong as hair, that have a special hierarchical structure.