Study of elephant, capybara, human hair finds that thicker hair isn't always stronger

December 11, 2019

Despite being four times thicker than human hair, elephant hair is only half as strong--that's just one finding from researchers studying the hair strength of many different mammals. Their work, appearing in a paper publishing December 11 in the journal Matter, shows that thin hair tends to be stronger than thick hair because of the way that it breaks.

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

Previous studies have found that human hair has strength comparable to that of steel when adjusted for density. This is because of hair's hierarchical structure: human hair is composed of an outer layer called the cuticle that wraps around an inner cortex made of many small fibers linked by chemical bonds. Within each fiber, there are even smaller fibers embedded. This structural design allows hair, which is made of proteins, to be resistant to deformation.

Yang and her team, including researchers from the Meyers and Ritchie groups at University of California, San Diego, and University of California, Berkeley, were curious if hair from other animals shares similar characteristics. They collected hair samples from eight different mammals, including humans, bears, boars, horses, capybaras, javelinas, giraffes, and elephants. These hairs vary in thickness: human hair is as thin as 80 μm in diameter, while those of elephants and giraffes are over 350 μm in diameter.

The researchers tied individual strands of hair to a machine that gradually pulled them apart until they broke. To their surprise, they found thin hair was able to endure greater tension before it broke compared to thick hair. This also applied to hairs from the same species. For example, thin hair from a child was stronger than thicker hair from an adult.

By studying the broken hairs using a scanning electron microscope, the team found 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 break in a normal fracture mode, a clean break similar to what would happen 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, like when a tree branch is snapped in a storm. The distinction in cracking path is because the structural elements in different hairs interact differently.

"Shearing is when small zig-zag cracks are formed within the material as a result of stress," Yang says. "These cracks then propagate, 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."

"The notion of thick being weaker than thin is not unusual, and we have found that happening when studying brittle materials like metal wires," says co-author Robert Ritchie at the University of California, Berkeley. "This is actually a statistical thing, which is a bigger piece will have a greater possibility of having a defect. 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 that their findings could help scientists design better synthetic materials. But Yang says her team's bio-inspired material manufacture is still at its infancy. Current technologies are not yet able to create materials that are as fine as hair and have a sophisticated hierarchical structure.

"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," Yang 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."
-end-
This work was supported by the Multidisciplinary University Research Initiative to the University of California Riverside, the Air Force Office of Scientific Research, and the Powell Foundation through the Jacobs School of Engineering at UCSD.

Matter, Yang et al.: "On the strength of hair across species" https://www.cell.com/matter/fulltext/S2590-2385(19)30234-6

Matter (@Matter_CP), published by Cell Press, is a new journal for multi-disciplinary, transformative materials sciences research. Papers explore scientific advancements across the spectrum of materials development--from fundamentals to application, from nano to macro. Visit: https://www.cell.com/matter. To receive Cell Press media alerts, please contact press@cell.com.

Cell Press

Related Elephants Articles from Brightsurf:

How do giraffes and elephants alter the African Savanna landscape?
Through their foraging behavior across the diverse topography of the African savanna, megaherbivores may be unknowingly influencing the growth and survival of vegetation on valleys and plateaus, while preserving steep slopes as habitat refugia.

New findings highlight threatened status of forest elephants
Conservation efforts for the African forest elephant have been hindered by how little is known the large animal, according to researchers.

Researchers study elephants' unique interactions with their dead
Stories of unique and sentient interactions between elephants and their dead are a familiar part of the species' lore, but a comprehensive study of these interactions has been lacking -- until now.

A chronicle of giant straight-tusked elephants
About 800,000 years ago, the giant straight-tusked elephant Palaeoloxodon migrated out of Africa and became widespread across Europe and Asia.

Capturing elephants from the wild hinders their reproduction for over a decade
Capturing elephants to keep in captivity not only hinders their reproduction immediately, but also has a negative effect on their calves, according to new research.

Sisters improve chances of reproduction in Asian elephants
Researchers at the University of Turku found that the presence of a maternal sister was positively and significantly associated with annual female reproduction in a population of working elephants in Myanmar.

Future of elephants living in captivity hangs in the balance
Scientists at the University of Sheffield and University of Turku are looking at ways to boost captive populations of Asian elephants without relying on taking them from the wild.

Wildlife tourism may negatively affect African elephants' behavior
Increasing numbers of tourists are interested in observing wildlife such as African elephants, and income generated from tourism potentially aids in the protection of animals and their habitats.

Sex differences in personality traits in Asian elephants
Scientists from the University of Turku, Finland, have found that male and female Asian elephants differ in their personality.

New welfare tool to help improve the lives of elephants in human care
Zoos and safari parks in the UK are using a special new tool to help them more successfully monitor the wellbeing of elephants in their care, thanks to a study led by The University of Nottingham.

Read More: Elephants News and Elephants Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.