Talking about teeth in people is most often associated with caries, braces and sadists in white coats who only dream of making beads out of your teeth. But jokes aside, because without dentists and established oral hygiene rules, we would eat only crushed potatoes and soup through a straw. And everything is to blame for evolution, which gave us far from the most durable teeth, which still do not regenerate, which probably indescribably pleases the representatives of the dental industry. If we talk about the teeth of representatives of the wild, then majestic lions, bloodthirsty sharks and extremely positive hyenas immediately come to mind. However, despite the power and strength of their jaws, their teeth are not as amazing as those of sea urchins. Yes, this ball of needles under water, stepping on which you can ruin a good part of your vacation, has quite good teeth. Of course, there are not many of them, only five, but they are unique in their own way and are able to sharpen themselves. How did scientists identify such a feature, how exactly does this process proceed and how can it help people? We learn about this from the report of the research group. Go.
Research basis
First of all, it is worth getting to know the main character of the study - Strongylocentrotus fragilis, in human terms, with a pink sea urchin. This type of sea urchin is not very different from its other counterparts, with the exception of a more flattened shape at the poles and a glamorous color. They live quite deep (from 100 m to 1 km), and they grow up to 10 cm in diameter.
The "skeleton" of a sea urchin, which shows five-ray symmetry.
Sea urchins are, no matter how rude it may sound, right and wrong. The former have an almost perfectly round body shape with pronounced five-beam symmetry, while the latter are more asymmetric.
The first thing that catches your eye when you see a sea urchin is its quills that cover the entire body. In different species, the needles can be from 2 mm up to 30 cm. In addition to the needles, the body has spheridia (organs of balance) and pedicellaria (processes that resemble forceps).
All five teeth are clearly visible in the center.
To depict a sea urchin, you first need to stand upside down, since its mouth opening is located on the lower part of the body, but the other holes are on the upper. The mouth of sea urchins is equipped with a chewing apparatus with a beautiful scientific name "Aristotle's lantern" (it was Aristotle who first described this organ and compared it in shape with an antique portable lantern). This organ is equipped with five jaws, each of which ends in a sharp tooth (the Aristotelian lantern of the investigated pink hedgehog is shown in picture 1C below).
There is an assumption that the durability of the teeth of sea urchins is ensured by their constant sharpening, which occurs through the gradual destruction of the mineralized tooth plates to maintain the sharpness of the distal surface.
But how exactly does this process proceed, which teeth need to be sharpened and which not, and how is this important decision made? Scientists have tried to find answers to these questions.
Results of the study
Image #1
Before revealing the dental secrets of sea urchins, consider the structure of their teeth in general.
On the pictures 1Πβ1S the hero of the study is shown - a pink sea urchin. Like other sea urchins, representatives of this species get their mineral components from sea water. Among the skeletal elements, the teeth are highly mineralized (by 99%) with magnesium-enriched calcite.
As we discussed earlier, hedgehogs use their teeth for scraping food. But besides this, with the help of their teeth, they dig holes for themselves, in which they hide from predators or bad weather. Given this unusual use of teeth, the latter must be extremely strong and sharp.
On the image 1D microcomputed tomography of a segment of a whole tooth is shown, making it clear that the tooth is formed along an elliptical curve with a T-shaped cross section.
Cross section of the tooth (1E) shows that the tooth is composed of three structural regions: primary laminae, calculus region, and secondary lamellae. The stone area consists of fibers of small diameter, surrounded by an organic shell. The fibers are encased in a polycrystalline matrix composed of magnesium-rich calcite particles. The diameter of these particles is about 10-20 nm. The researchers note that the concentration of magnesium is not uniform throughout the tooth and increases closer to its end, which provides its increased wear resistance and hardness.
Longitudinal section (1F) of the calculus of the tooth shows the destruction of the fibers, as well as the separation, which occurs due to delamination at the interface between the fibers and the organic shell.
Primary veneers are usually composed of calcite single crystals and are located on the convex surface of the tooth, while secondary veneers fill the concave surface.
In the picture 1G one can see an array of curved primary plates lying parallel to each other. The image also shows fibers and a polycrystalline matrix filling the space between the plates. keel (1H) forms the base of the transverse T-section and increases the bending stiffness of the tooth.
Since we know what structure the tooth of the pink sea urchin has, we now need to find out the mechanical properties of its components. For this, compression tests were carried out using a scanning electron microscope and the method nanoindentation*. Samples cut along the longitudinal and transverse orientations of the tooth participated in nanomechanical tests.
Nanoindentation* β check of the material by the method of indentation into the surface of the sample of a special tool β the indenter.
Data analysis showed that the average Young's modulus (E) and hardness (H) at the tooth tip in the longitudinal and transverse directions are: EL = 77.3 Β± 4,8 GPa, HL = 4.3 Β± 0.5 GPa (longitudinal) and ET = 70.2 Β± 7.2 GPa, HT = 3,8 Β± 0,6 GPa (transverse).
Young's modulus* - a physical quantity that describes the ability of a material to resist tension and compression.
Hardness* - the property of the material to resist the introduction of a more solid body (indenter).
In addition, recesses were made in the longitudinal direction with a cyclic additional load to create a model of ductile damage for the stone area. On 2Π the load-displacement curve is shown.
Image #2
The modulus for each cycle was calculated based on the Oliver-Farr method using unloading data. The indentation cycles showed a monotonic decrease in modulus with increasing indentation depth (2V). Such a deterioration in stiffness is explained by the accumulation of damage (2C) as a result of irreversible deformation. It is noteworthy that the development of the third occurs around the fibers, and not through them.
The mechanical properties of the tooth constituents were also assessed using quasi-static micropillar compression experiments. A focused ion beam was used to fabricate micrometer-sized pillars. To assess the strength of the connection between the primary plates on the convex side of the tooth, micropillars were fabricated with an oblique orientation relative to the normal interface between the plates (2D). Pictured 2E a microcolumn with an inclined interface is shown. And on the chart 2F the results of shear stress measurement are shown.
Scientists note an interesting fact - the measured modulus of elasticity is almost half that of indentation tests. This discrepancy between indentation and compression tests is also noted for tooth enamel. At the moment, there are several theories explaining this discrepancy (from environmental influences during tests to contamination of samples), but there is no clear answer to the question of why the discrepancy occurs.
The next step in the study of the teeth of the sea urchin was wear tests carried out using a scanning electron microscope. The tooth was glued to a special holder and pressed against a substrate of ultrananocrystalline diamond (3Π).
Image #3
The scientists note that their version of the wear test is the opposite of what is usually done when a diamond tip is pressed into a substrate of the material under study. Changes in the wear test methodology allow a better understanding of the properties of microstructures and tooth components.
As we can see in the pictures, when the critical load is reached, chips begin to form. It is worth considering that the force of the βbiteβ of the Aristotelian lantern in sea urchins varies depending on the species from 1 to 50 newtons. In the test, a force from hundreds of micronewtons to 1 newton was applied, i.e. from 1 to 5 newtons for the entire Aristotelian lantern (since there are five teeth).
In the picture 3B(i) small particles (red arrow) are visible, formed as a result of wear of the stone area. As the stone area wears and contracts, cracks at the interfaces between the plates can originate and propagate due to compression-shear loading and stress buildup in the area of ββthe calcite plates. Snapshots 3B(ii) ΠΈ 3B(iii) show the places where the fragments broke off.
For comparison, two types of wear experiments were carried out: with a constant load corresponding to the beginning of yield (WCL) and with a constant load corresponding to the yield strength (WCS). As a result, two variants of tooth wear were obtained.
Wear test video:
Stage I
Stage II
Itap III
Stage IV
In the case of a constant load in the WCL test, compression of the area was observed, however, no chipping or other damage to the plates was noticed (4A). But in the WCS test, when the normal force was increased to maintain the nominal contact voltage constant, chipping and falling out of the plates were observed (4V).
Image #4
These observations are confirmed by the plot (4S) measurements of the compression area and the volume of chipped plates depending on the sliding length (sample over diamond during the test).
This graph also shows that in the case of WCL no chips are formed even if the sliding distance is greater than in the case of WCS. Inspection of compressed and chipped plates for 4V allows you to better understand the mechanism of self-sharpening of sea urchin teeth.
The area of ββthe compressed area of ββthe stone increases as the plate breaks off, causing part of the compressed area to be removed [4B(iii-v)]. Microstructural features such as the bond between stone and slabs facilitate this process. Microscopy has shown that the fibers in the calculus are bent and penetrate through the layers of plates in the convex part of the tooth.
On the chart 4S there is a jump in the volume of the chipped area when the new plate is detached from the tooth. It is curious that at the same moment there is a sharp decrease in the width of the oblate region (4D), which indicates the process of self-sharpening.
Simply put, these experiments have shown that while maintaining a constant normal (not critical) load during wear tests, the tip becomes blunt, while the tooth remains sharp. It turns out that the teeth of hedgehogs are sharpened during use, if the load does not exceed the critical one, otherwise damage (chips) may occur, and not sharpening.
Image #5
In order to understand the role of tooth microstructures, their properties and their contribution to the self-sharpening mechanism, a nonlinear finite element analysis of the wear process was carried out (5Π). To do this, images of a longitudinal section of the tip of the tooth were used, which served as the basis for a two-dimensional model consisting of stone, plates, keel and interfaces between plates and stone.
Image 5B-5H are contour plots of the Mises criterion (plasticity criterion) at the edge of the stone and slab area. When a tooth is compressed, the calculus undergoes large viscoplastic deformations, accumulates damage and shrinks (βflattensβ) (5B ΠΈ 5C). Further compression induces a shear band in the stone, where most of the plastic deformation and damage accumulates, tearing off part of the stone, bringing it into direct contact with the substrate (5D). Such fragmentation of the stone in this model corresponds to experimental observations (split fragments on 3B(i)). Compression also results in delamination between the plates as the interface elements are subjected to mixed loading resulting in decohesion (peeling). As the contact area increases, the contact stresses increase, causing the initiation and propagation of a crack at the interface (5Bβ5E). Loss of adhesion between the plates reinforces the kink, which causes the outer plate to disengage.
Scratching exacerbates interface damage resulting in plate removal when the plate(s) undergo splitting (where cracks deviate from the interface and penetrate the plate, 5G). As the process continues, the fragments of the plate are detached from the tip of the tooth (5H).
It is curious that the simulation very accurately predicts chipping in both the stone and plate regions, which scientists have already noticed during observations (3B ΠΈ 5I).
For a more detailed acquaintance with the nuances of the study, I recommend looking at ΠΈ to him.
Finale
This work once again confirmed that evolution was not very supportive of human teeth. Seriously, in their study, scientists were able to examine in detail and explain the mechanism of self-sharpening of the teeth of sea urchins, which is based on the unusual structure of the tooth and the correct load on it. The plates covering the hedgehog tooth peel off under a certain load, which allows you to keep the tooth sharp. But this does not mean that sea urchins can crush stones, because when critical load indicators are reached, cracks and chips form on the teeth. It turns out that the principle βthere is power, no mind is neededβ certainly would not bring any benefit.
One might think that the study of the teeth of the inhabitants of the deep sea does not bring any benefit to man, except for the satisfaction of insatiable human curiosity. However, the knowledge gained during this study can serve as the basis for the creation of new types of materials that will have properties similar to the teeth of hedgehogs - wear resistance, self-sharpening at the material level without external assistance, and durability.
Be that as it may, nature holds many secrets that we have yet to reveal. Will they be helpful? Perhaps yes, perhaps not. But sometimes, even in the most complex research, sometimes itβs not the destination that matters, but the journey itself.
Friday off-top:
Underwater forests of giant algae serve as a gathering place for sea urchins and other unusual ocean dwellers. (BBC Earth, voice-over - David Attenborough).
Thanks for watching, stay curious and have a great weekend everyone! π
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