Structured for strength

Ever looked at your hair and studied its split ends, fretting over the apparent damage? A new study suggests that those split ends actually allow your hair to endure greater tension before it breaks.

Hair provides protection for many mammals. A strand of hair has a cortex of keratin fibres (that are also in your skin, teeth and nails) surrounded by cuticle scales in a layered structure.

The exact hair structure, however, varies in different mammals depending on its function: for example, as ventilation, to signal aggression, or rapid drying.

Consequently, the diameter of hair can vary from as fine as 80 μm (micrometres) in humans up to 350 μm in elephants and giraffes.

A new study published in the journal Matter compared hair from eight mammalian species – human, horse, bear, boar, javelina, capybara, elephant and giraffe – to explore the relationship between hair diameter and strength.

The study reveals that thin hair tends to be stronger than thick hair because of the way it breaks. For instance, despite being four times thicker than human hair, elephant hair is only half as strong.

191212 hair closeup

Protein fibres in the human hair cortex delaminate when the hair breaks, suggesting human hair fractures in a shear mode. Credit: Wen Yang

 “We were very surprised by the result because, intuitively, we would think thick hair is stronger,” says lead author Wen Yang, a nanoengineering researcher at the University of California, San Diego.

“Natural materials have undergone thousands of years of evolution so, to us, these materials are very well developed. We hope to learn from nature and develop synthetic products with comparable properties.”

The researchers tied individual strands of hair to a machine that gradually pulled them until they broke.

A scanning electron microscope revealed that although most hairs share a similar structure, they broke in different ways.

Hairs with a diameter greater than 200 μm, such as those of boars, giraffes and elephants, tend to have a clean break – similar to what happens if a banana breaks in the middle.

Hairs that are thinner than 200 μm, such as those of humans, horses and bears, break in a shear mode. The break is uneven, such as when a tree branch snaps in a storm.

This distinction occurs because the structural elements in different hairs interact uniquely.

“Shearing is when small zig-zag cracks are formed within the material as a result of stress,” Yang says.

“These cracks then propagate [spread], and for some biological materials, the sample isn’t completely broken until the small cracks meet. If a material shears, it means it can withstand greater tension and thus is tougher than a material that experiences a normal fracture.”

Co-author Robert Ritchie says that the notion of thick being weaker than thin is not unusual – it is also seen in the study of brittle materials such as metal wires.

“This is actually a statistical thing… a bigger piece will have a greater possibility of having a defect,” says Ritchie.

“It’s a bit surprising to see this in hair, as hair is not a brittle material, but we think it’s because of the same reason.”

The researchers believe their findings could help scientists design better synthetic materials, but Yang says her team’s bio-inspired material manufacture is still at its infancy.

“There are many challenges in synthetic materials we haven’t had a solution for, from how to manufacture very tiny materials to how to replicate the bonds between each layer as seen in natural hair,” she says.

“But if we can create metals that have a hierarchical structure like that of hair, we could produce very strong materials, which could be used as rescue ropes and for constructions.”

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