What's so strange about hanging a person? Your hair is much stronger than an elephant's hair.

Key Points

★ Theoretically, a head of hair is enough to lift a truck.

★ The thicker the hair is, the stronger it is. Human hair is stronger than elephant hair which is much thicker.

Recently, a video of a woman hanging herself on a subway handrail with her hair went viral, with everyone marveling at the high quality of this young lady's hair.

However, this behavior should not be imitated, because the woman is a professional aerialist circus performer who often hangs herself with her hair, but this requires a lot of training and special hairstyle requirements. Ordinary people who rashly try it may cause serious injuries. Afterwards, the woman also apologized, saying that she just wanted to shoot a funny video and didn't expect to cause trouble to everyone.

So, how strong can hair really be? Does the thickness of hair have anything to do with its strength? Let's talk about this question.

01

Our hair can lift a truck

In materials science, we usually use strength to measure the tensile/compressive properties of a material. It indicates the maximum force that a material can stably bear per unit cross-sectional area. It is an inherent property of the material and has nothing to do with its size.

For example, if we pull a steel wire and a potato wire of the same thickness with the same force, the potato wire breaks first but the steel wire does not. We can conclude that the tensile strength of the steel wire is greater than that of the potato wire.

Steel wire (left) and potato shreds (right) | pixabay, Tuchong

By this standard, researchers found that human hair is very strong, even comparable to steel!

Calculations show that 500 to 1,000 hairs can support the weight of an adult. A normal person has nearly 100,000 hairs on their head, which together can support 5 tons, enough to lift a large truck!

Even in daily life, we can easily find that when we pull out hair, we pull it out by the roots, and it rarely breaks in the middle, which shows that human hair is really strong. If you tie all the hair together and pull hard, the scalp may be torn off, but the hair will not break.

02

The thicker the hair, the better? No.

Most animals have hair on their bodies. Humans have hair, sweat hair, and beards, monkeys have body hair, and sheep have wool. The thickness of hair varies between animals, and the thickness of hair between adults and juveniles of the same species is also different.

But what is the relationship between hair strength and thickness? Scientists conducted an experiment and the results were a bit surprising.

Elephant hair (top) and human hair (bottom) | pixabay

Human hair is relatively thin among mammals (about 0.06 mm in diameter), while giraffe and elephant hair is relatively thick. Scientists at the University of California, USA, collected hair from different animals and studied its strength.

They were surprised to find that as the diameter of the hair increased, its strength decreased, that is, the thicker the hair, the weaker its strength. Although elephant hair is six or seven times thicker than human hair, its strength is only half that of human hair.

So the question is, what factors determine the strength of hair?

03

What determines the strength of hair? The cortex is absolutely dominant

From a microscopic perspective, the structure of hair is multi-layered. For example, human hair contains stratum corneum, medulla, cortex, microfibrils, fibrils, polypeptide chains, etc.

Human hair structure | Baidu Encyclopedia

The cuticle, also known as the epidermis, is arranged like fish scales from the root to the end of the hair and expands when exposed to warm water or alkali. It helps the hair resist external physical damage and accounts for about 5-15% of the total weight of the hair.

The cortex is located inside the epidermis and is composed of protein cells and pigment cells. It is the main body of hair and accounts for about 80% of the total hair composition. It contains fibrous cortical cells, which give hair elasticity and toughness. The physical and chemical effects of hair (such as breakage and curling) are attributed to this layer.

The medulla layer is located in the center of the hair and only accounts for 0-5% of the hair. It mainly plays a supporting role in the hair and has little effect on the hair quality.

Among all the layers, the cortex is the most important for the strength of animal hair. The outer cuticle only ensures the integrity of the hair.

The study found that when thinner hair (less than 0.2mm in diameter) is stretched, the fibers inside its cortex are mainly damaged by mutual shear slippage - this method can usually withstand greater external forces; while when the hair is thicker, vertical fractures mainly occur inside the cortex and can only withstand smaller external forces.

Hair (cortex) shear slip and vertical fracture, the former can withstand greater tension | Image provided by the author

Scientists believe that this difference is caused by the size of the defects in the hair relative to the size of the hair. Because the cells in the hair are not perfectly arranged (defective), when the hair is thin, the defects are large relative to the diameter of the hair, which is prone to shear slip; when the hair is thick, the defects are small relative to the size of the hair, which is more prone to vertical breakage. This is why the thicker the hair, the weaker the strength.

Of all the hairs tested, the thinner the hair, the stronger it is. But there are exceptions to everything, and wild boars are an exception. Research has found that wild boar hair is as thick as elephant hair, but less than half as strong. This is because wild boar hair is special, probably to make it easier for humans to shave it when eating, and there are many pores in it, which greatly reduces its tensile strength.

Wild boar: I heard you want to eat me? | pixabay

04

Natural hair is very strong and is a good material for bionics

Why do scientists bother with hair? It's also for application needs.

The cortex of most animal hairs has high-strength protein fibers, which give it tensile strength comparable to that of metals. If we can mimic the composition and structure of natural hair and create synthetic materials with sufficient strength, it is expected to replace metals in a non-toxic, harmless, and biologically stable environment, which has broad prospects in biomedicine.

In addition, scientists have found that melanosomes also make hair stronger, so dark hair (such as black hair) is stronger than light hair (such as white hair). This is also a research direction for high-strength synthetic materials.

So, now you know how important black hair is?

Author: Lin Qiang, Mechanical Engineering, Shanghai Jiao Tong University

Editor | Ding Zong

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