Hair for a modern person is nothing more than an element of visual self-identification, part of the image and image. Despite this, these horny skin formations have several important biological functions: protection, thermoregulation, touch, etc. How strong is our hair? As it turned out, they are many times stronger than the hair of an elephant or a giraffe.
Today we will get acquainted with a study in which scientists from the University of California (USA) decided to check how the thickness of the hair and its strength correlate in different animal species, including humans. Whose hair turned out to be the strongest, what mechanical properties do different types of hair have, and how can this research help in the development of new types of materials? We learn about this from the report of scientists. Go.
Research basis
Hair, composed mostly of the protein keratin, is a horn formation in the skin of mammals. In fact, hair, wool and fur are synonymous. According to its structure, the hair consists of keratin plates, which are superimposed on each other, like dominoes that have fallen on top of each other. Each hair has three layers: the cuticle is the outer and protective layer; cortex - a cortical substance consisting of elongated dead cells (important for the strength and elasticity of the hair, determines its color due to melanin) and the medulla - the central layer of the hair, consisting of soft keratin cells and air cavities, which is involved in the transfer of nutrients to others layers.
If the hair is divided vertically, then we will get the supracutaneous area (rod) and subcutaneous (bulb or root). The bulb is surrounded by a follicle, the shape of which determines the shape of the hair itself: a round follicle is straight, an oval follicle is slightly curly, a kidney-shaped follicle is curly.
Many scientists suggest that due to technological progress, human evolution is changing. That is, some organs and structures in our body gradually become rudimentary - those that have lost their intended purpose. Such parts of the body include wisdom teeth, appendix and body hair. In other words, scientists believe that over time, these structures will simply disappear from our anatomy. Like it or not, it's hard to say, but for many ordinary people, wisdom teeth, for example, are associated with a visit to the dentist for their inevitable removal.
Be that as it may, a person needs hair, maybe they no longer play a crucial role in thermoregulation, but in aesthetics they are still an integral part. The same can be said about world culture. In many countries, from time immemorial, hair was considered the source of all strength, and their circumcision was associated with possible health problems and even failures in life. The sacred meaning of hair migrated from the shamanic rituals of the ancient tribes to more modern religions, the works of writers, artists and sculptors. In particular, female beauty was often closely related to how the hair of lovely ladies looked or was depicted (for example, in paintings).
Pay attention to how detailed the hair of Venus is depicted (Sandro Botticelli, The Birth of Venus, 1485).
Let's leave aside the cultural and aesthetic aspect of hair and begin to consider the study of scientists.
Hair, in one form or another, is found in many species of mammals. If for a person they are no longer so important from a biological point of view, then for other representatives of the animal world, wool and fur are vital attributes. At the same time, in their basic structure, human hair and, for example, an elephant are very similar, although there are differences. The most obvious of them is the size, because the hair of an elephant is much thicker than ours, but, as it turned out, not stronger.
Scientists have been studying hair and wool for a long time. The results of these works were implemented both in cosmetology and medicine, and in light industry (or, as the well-known Kalugina L.P. would say: “light industry”), or rather in textile. In addition, the study of hair greatly helped in the development of biomaterials based on keratin, which at the beginning of the last century was learned to isolate from animal horns using lime.
The keratin thus obtained was used to create gels that could be strengthened by the addition of formaldehyde. Later, they learned to isolate keratin not only from the horns of animals, but also from their wool, as well as from human hair. Substances based on keratin have found their way into cosmetics, composites, and even tablet coatings.
Nowadays, the industry of studying and producing durable and lightweight materials is rapidly developing. Hair, being such by nature, is one of the natural materials that inspire this kind of research. What is the tensile strength of wool and human hair, which is from 200 to 260 MPa, which is equivalent to a specific strength of 150-200 MPa / mg m-3. And this is practically comparable to steel (250 MPa/mg m-3).
The main role in the formation of the mechanical properties of the hair is played by its hierarchical structure, reminiscent of a nesting doll. The most important element of this structure is the inner cortex of cortical cells (about 5 µm in diameter and 100 µm in length) consisting of grouped macrofibrils (about 0.2–0.4 µm in diameter), which, in turn, consist of intermediate filaments (7.5 nm in diameter). ) embedded in an amorphous matrix.
The mechanical properties of hair, their sensitivity to temperature, humidity and deformation are a direct result of the interaction between the amorphous and crystalline components of the cortex. The keratin fibers of the human hair cortex typically have a high extensibility with a strain at break of more than 40%.
Such a high value is due to the unwinding of the structure а-keratin and, in some cases, its transformation into b-keratin, which leads to an increase in length (a full turn of the 0.52 nm helix is stretched to 1.2 nm in the configuration b). This is one of the main reasons why many studies have focused specifically on keratin in order to recreate it in a synthetic form. But the outer layer of hair (cuticle), as we already know, consists of plates (0.3–0.5 µm thick and 40–60 µm long).
Previously, scientists have already conducted a study of the mechanical properties of the hair of people from different age and ethnic groups. In this work, the emphasis was placed on studying the difference in the mechanical properties of the hair of different animal species, namely: man, horse, bear, wild boar, capybara, peccary, giraffe and elephant.
Results of the study
Image #1: Human hair morphology (А - cuticle; В - rupture of the cortex; showing the ends of the fibers, С - the fault surface, where three layers are visible; D - lateral surface of the cortex, showing the stretching of the fiber).
An adult hair is about 80-100 microns in diameter. With normal hair care, their appearance is quite holistic (1А). The internal component of human hair is the fibrous cortex. After a tensile test, it was found that the cuticle and cortex of a human hair broke differently: the cuticle was usually broken abrasively (crushed) and the keratin fibers in the cortex were detached and pulled out of the overall structure (1V).
In the picture 1S the fragile surface of the cuticle is clearly visible with the visualization of the layers, which are overlapping cuticle plates and have a thickness of 350–400 nm. The observed delamination at the fracture surface, as well as the brittle nature of this surface, indicate a weak interfacial bond between the cuticle and the cortex, as well as between the fibers within the cortex.
Keratin fibers in the cortex were stratified (1D). This suggests that the fibrous cortex is primarily responsible for the mechanical strength of the hair.
Image #2: Horsehair morphology (А - cuticle, some plates of which are slightly deviated due to lack of care; В - the appearance of the gap; С - details of the rupture of the cortex, where the torn cuticle is visible; D cuticle details).
The structure of horse hair is similar to human hair, except for the diameter, which is 50% larger (150 microns). Pictured 2А obvious damage to the cuticle can be seen, where many of the plates are not as closely connected to the shaft as they were on human hair. The horsehair rupture site contains both a normal fracture and a hair rupture (stratification of the cuticle plates). On 2V Both types of damage are visible. In areas where the plates are completely torn off, the interface between the cuticle and the cortex is visible (2S). Several fibers were torn out and flaked at the interface. Comparing the observational data with previous ones (human hair), these breakdowns indicate that horse hair did not experience the same high stress as human hair when the fibers in the cortex were stretched and completely detached from the cuticle. It can also be seen that some of the plates have become detached from the rod, which may be due to tensile stress (2D).
Image #3: Bear hair morphology (А - cuticle; В — damage at two points associated with the rupture area; С - cracking of the cuticle with delamination of fibers in the cortex; D - details of the structure of the fibers, several elongated fibers are visible from the overall structure).
Bear hair is 80 microns thick. The cuticle plates are extremely tightly attached to each other (3А), and in some areas it is even difficult to distinguish individual plates. This may be due to the friction of the hair on the neighboring ones. Under tensile stress, these hairs literally split with the appearance of long cracks (inset on 3B), which indicates that with a weak binding effect of the damaged cuticle, the keratin fibers in the cortex were easily stratified. Stratification of the cortex causes a tear at the cuticle, as evidenced by a zigzag fracture pattern (3S). This tension causes some fibers to be pulled out of the cortex (3D).
Image #4: Boar hair morphology (А - normal flat hair fracture; В - the structure of the cuticle demonstrates a poor state of integrity (clumping) of the plates; С - details of the gap at the interface between the cuticle and the cortex; D - fibers stretched out of the total mass and protruding fibrils).
Boar hair is quite thick (230 mm), especially in comparison with bear hair. The rupture of the boar's hair when damaged looks quite distinct (4А) perpendicular to the direction of tensile stress.
Relatively small exposed cuticle plates were torn off from the main body of the hair due to stretching of their edges (4V).
On the surface of the destruction zone, the stratification of fibers is clearly visible, it is also seen that they were very tightly connected to each other inside the cortex (4S). Only fibers at the interface between cortex and cuticle were exposed due to separation (4D), which revealed the presence of thick fibrils of the cortex (250 nm in diameter). Some of the fibrils protruded slightly due to deformation. It is assumed that they serve as a strengthening of the boar's hair.
Image #5: Elephant hair morphology (А — С) and giraffe (D — F). А - cuticle; В - stepped break of hair; С - voids inside the hair indicate where the fibers were torn out. D - cuticular plates; Е - even breakage of the hair; F - fibers torn from the surface in the area of the fracture.
The hair of an elephant calf can be about 330 microns thick, and in an adult it can reach 1.5 mm. The plates on the surface are difficult to distinguish (5А).Elephant hair is also prone to normal destruction, i.e. to a clean tensile fracture. Moreover, the morphology of the fracture surface demonstrates a stepped form (5V), possibly due to the presence of minor defects in the hair cortex. Some small holes can also be seen on the fracture surface, where reinforcing fibrils were probably located before the damage (5S).
Giraffe hair is also quite thick (370 µm), although the location of the cuticle plates is not so clear (5D). It is believed that this is due to their damage by various environmental factors (for example, rubbing against trees while feeding). Despite the differences, the giraffe's hair break was similar to that of the elephant (5F).
Image #6: Capybara hair morphology (А - double cuticular structure of the plates; В — rupture of the double structure; С - the fibers near the fracture boundary appear brittle and rigid; D are elongated fibers from the rupture zone of the double structure).
Capybara and peccary hair are different from all other hairs studied. In the capybara, the main difference is the presence of a double cuticle configuration and an oval hair shape (6А). The groove between the two mirror parts of the hair is necessary for faster removal of water from the animal's coat, as well as for better ventilation, which allows you to dry faster. When subjected to tension, the hair is divided into two parts along the groove, and each of the parts is destroyed (6V). Many fibers of the cortex are stratified and stretched (6S и 6D).
Image #7: Peccary hair morphology (А - the structure of the cuticle and the place of the gap; В — morphology of cortex destruction and details of its structure; С - closed cells (20 microns in diameter), the walls of which consist of fibers; D cell walls).
Bakers (family Tayassuidae, i.e. peccary) hair has a porous cortex, and the cuticle layer does not have clear plates (7А). The hair cortex contains closed cells 10–30 μm in size (7V), the walls of which are composed of keratin fibers (7S). These walls are quite porous, and the size of one pore is about 0.5–3 μm (7D).
As seen in the picture 7А, without the support of the fibrous cortex, the cuticle cracks along the line of rupture, and the fibers are stretched in some places. This hair structure is necessary for the hair to be more vertical, visually increasing the size of the animal, which may be a defense mechanism of the peccary. Peccary hair resists compression quite well, but it does not cope with stretching.
Having understood the structural features of the hair of different animals, as well as their types of damage due to tension, the scientists began to describe the mechanical properties.
Image #8: strain diagram for each hair type and scheme of experimental setup for data acquisition (strain rate 10-2 s-1).
As can be seen from the graph above, the reaction to stretching in the hair of different animal species was quite different. Thus, the hair of a human, a horse, a wild boar and a bear showed a reaction similar to the reaction of wool (not someone else's, but a textile material).
At a relatively high elastic modulus of 3.5–5 GPa, the curves consist of a linear (elastic) region followed by a plateau with slowly increasing stress up to a strain of 0.20–0.25, after which the hardening rate increases significantly up to a fracture strain of 0.40. Plateau area refers to unwinding а-helical structure of keratin intermediate filaments, which in some cases can (partially) turn into b-sheets (flat structures). Complete unwinding leads to a deformation of 1.31, which is much higher than at the end of this stage (0.20–0.25).
The crystalline filamentous part of the structure is surrounded by an amorphous matrix that does not transform. The amorphous part makes up about 55% of the total volume, but only on condition that the diameter of the intermediate filaments is 7 nm and that they are separated by 2 nm by amorphous material. Such accurate indicators have been derived in previous studies.
In the hardening stage of deformation, slip occurs between cortical fibers as well as between smaller structural elements such as microfibrils, intermediate filaments and an amorphous matrix.
Giraffe, elephant and peccary hair show a relatively linear hardening response with no clear distinction between plateaus and fast hardening regions (peaks). The elastic modulus is relatively low and is about 2 GPa.
Unlike other species, capybara hair exhibits a response characterized by rapid strengthening upon which successive stresses are applied. This observation is associated with the unusual structure of the capybara hair, or rather with the presence of two symmetrical parts and a longitudinal groove between them.
Previous studies have already been conducted that suggested that Young's modulus (longitudinal elasticity modulus) decreases with increasing hair diameter in different animal species. In these works, it was noted that the Young's modulus of the peccary is significantly lower than that of other animals, which may be due to the porosity of the structure of its hair.
It is also curious that the bakers have both black and white patches on their hair (bicolor). Tensile breaks occur most often in the white area of the hair. The increased resistance of the black area is due to the presence of melanosome, which is found exclusively in black hair.
All these observations are indeed unique, but the main question remains - does the size of the hair play a role in its strength?
If we describe hair in mammals, then we can highlight the main facts that are known to researchers:
- in most species, the hair is thicker in the central part and tapers towards the end; the wool of wild animals is thicker due to their habitat;
- changes in the diameter of hairs of one species shows that the thickness of most hairs varies within the general range of thickness for a given species of animal. The thickness of the hairs in different representatives of the same species may differ, but what influences this difference is still unknown;
- different types of mammals have different thicknesses of hair (no matter how trite it may sound).
By summing up these publicly available facts and the data obtained during the experiments, the scientists were able to compare all the results to form the dependences of the thickness of the hair and its strength.
Image #9: Relationship between hair thickness and hair strength in different animal species.
Because of the differences in hair diameter and extensibility, the scientists set out to see if their tensile stresses could be predicted based on Weibull statistics, which can specifically account for differences in sample size and resulting defect size.
It is assumed that the hair segment with volume V comprises n volume elements, and each unit volume V0 has a similar distribution of defects. Using the weakest link assumption, at a given voltage level σ chance P maintaining the integrity of this hair segment with volume V can be expressed as a product of additional probabilities of maintaining the integrity of each of the elements of the volume, namely:
P(V) = P(V0) · P(V0)… · P(V0) = · P(V0)n
where is the volume V contains n volume elements V0. With increasing voltage P(V) naturally decreases.
Using the two-parameter Weibull distribution, the probability of destruction of the entire volume can be expressed as:
1 - P = 1 - exp [ -V/V0 (σ/p0)m]
where σ is the applied voltage, σ0 is the characteristic (reference) strength, and m is the Weibull modulus, which is a measure of the variability of properties. It should be noted that the probability of destruction increases with the increase in the sample size. V at constant voltage σ.
On the chart 9А the Weibull distribution of experimental breaking stresses for human and capybara hair is shown. Curves for other species were predicted using Formula #2 with the same m value as for human hair (m = 0.11).
The average diameter used was: wild boar 235 µm, horse 200 µm, peccary 300 µm, bear 70 µm, elephant hair 345 µm, and giraffe 370 µm.
Based on the fact that the breaking stress can be determined at P(V) = 0.5, these results show that the breaking stress decreases with increasing hair diameter in different species.
On the chart 9V predicted breaking stresses at 50% failure probability are shown (P(V) = 0.5) and the average experimental breaking stress for different types.
It becomes clear that as the hair diameter increases from 100 to 350 mm, its breaking stress decreases from 200–250 MPa to 125–150 MPa. The results of the Weibull distribution model are in excellent agreement with the results of actual observations. The only exception is peccary hair, as it is extremely porous. The actual strength of the peccary hair is lower than that shown by the Weibull distribution simulation.
For a more detailed acquaintance with the nuances of the study, I recommend looking at и to him.
Finale
The main conclusion of the above observations is that thick hair is not the same as strong hair. True, as the scientists themselves say, this statement is not a discovery of the millennium, since similar observations were made in the study of metal wire. The point here is not even in physics, mechanics or biology, but in statistics - the larger the object, the more scope for defects.
Scientists believe that the work we have reviewed today will help their colleagues create new synthetic materials. The main problem is that despite the development of modern technologies, they are not yet able to create something similar to the hair of a person or an elephant. After all, to create something so small is already a challenge, not to mention its complex structure.
As we can see, this study showed that not only spider silk is worthy of scientists' attention as an inspiration for future ultra-strong and ultra-light materials, but human hair can also surprise with its mechanical properties and amazing strength.
Thank you for your attention, stay curious and have a good week everyone guys. 🙂
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Source: habr.com