Imitation with teeth

Airplane wings, satellites, and possibly car bumpers - are just some of the devices that will be designed, perhaps, in the future on the basis of biological materials that nature has given them unusual strength: they are arranged as a structure within a structure - similar to Russian "babushka" dolls

When it comes to sophistication, nature beats us time and time again. For example, when it comes to material planning, we keep copying from him. Airplane wings, satellites, and possibly car bumpers - are just some of the devices that will be designed, perhaps, in the future on the basis of biological materials that nature has given them unusual strength: they are arranged as a structure within a structure - similar to Russian "babushka" dolls.

To produce such durable synthetic materials, the natural structure must be studied in detail. Weizmann Institute of Science scientists recently took an important step in this direction. Prof. Daniel Wagner and the post-doctoral researcher Dr. Benny Bar-On, from the Department of Materials and Surfaces in the Faculty of Chemistry, built a new mathematical model, which clarifies the mechanical properties of many strong biological materials - such as teeth, bones, oysters, fish scales, horns Moose, turtle shells, and more.

All of these are called "composite materials", because they are built from two components: hard mineral elements embedded in a soft organic substrate, such as protein. The combination between the two creates a structure similar to that of bricks and plaster, which gives composite materials their durability. Prof. Wagner and Dr. Bar-On took into account another fact: in some of the composite materials, the bricks and the plaster are themselves composite materials, built from the same components, but 100 times smaller, or 1,000 times smaller. This point of view added a new level of complexity, which explains, for example, how our teeth serve us well, for decades.

Dentin, the inner material of the tooth, is a composite material made up of thin mineral tubes that are "glued" together by a collagen "plaster". The mineral itself is fragile, but when combined with collagen it becomes extraordinarily strong. The new model of Prof. Wagner and Dr. Bar-On takes into account that the collagen "plaster" that wraps the mineral tubules of the dentin, which are several micrometers wide (several millionths of a meter), contains not only collagen. It turns out that it is itself a composite material, which is made up of mineral crystals several nanometers in size (billionths of a meter), wrapped in protein - also nanosized. "The structure resembles a Russian doll - we opened one 'doll', and we discover another composite structure inside," says Dr. Bar-On. The model takes into account that the "bricks" of the mineral dentin are placed not only in a way that is offset from one another, as in a brick wall, but also in a different spatial layout: the large bricks, which form the structure, are placed in the shape of a fan, and the smaller bricks face in different directions - something which adds hardness and durability to dentin in many directions, and varying density at different depths of the tooth.

The model provides an explanation for the findings from Prof. Wagner's previous studies, according to which the dentin is softer at the top of the tooth, and harder at the base, close to the gums. The differences in density ensure shock absorption, and prevent the spread of cracks in the tooth enamel. In addition, the reference to changes in density resolves a controversy that has prevailed so far around dentin: so far, very different values ​​have been measured for the hardness of dentin, but no explanation for the phenomenon has been found. It is now clear that the measurements were made in different layers of the dentin, without knowing that its density, and consequently its stiffness, varies from one layer to another. The natural design of the dentin can be an inspiration for synthetic structures with composite components, which can be "mechanically adjusted".

Dentin, like other multi-scale biological materials, is created in nature in a miraculous way - through self-organization. The new model of Prof. Wagner and Dr. Bar-On may lead to the production of such composite materials in a synthetic way. The model was described in the last year and a half in four scientific articles, in the journals Journal of the Mechanics and Physics of Solids and Journal of Biomechanics. The model proved to be extremely reliable, both when tested against numerical simulations, and against physical measurements of dentin and other biological materials, made in various laboratories around the world - including Prof. Wagner's laboratory at the Weizmann Institute. The current model is a significant leap forward compared to previous models describing the structure and mechanics of bones, developed by Prof. Wagner in the 90s, in collaboration with Prof. Steve Weiner from the Department of Structural Biology at the Weizmann Institute of Science.

In addition to parts of airplanes, satellites and cars, materials inspired by nature may contribute to the improvement of many devices, for which resistance to damage is an essential feature. These include, for example, armored personnel carriers, armored cars, and particularly durable doors. The materials of the future based on materials with structures of many orders of magnitude are expected to be resistant to erosion as well.