Created soft chameleon material with programmable properties

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Russian scientists took part in the development of a material that is close to human skin in terms of softness, and resembles chameleon skin in terms of its ability to change color. It is a polymer consisting of several types of monomer units and can be useful for creating a biological implant. This is reported by an article in Science. The research was supported by a grant from the Russian Science Foundation (RSF).

Materials for biological implants should not differ in their mechanical properties – elasticity, stiffness, resistance to deformation – from ordinary biological tissue. Otherwise, serious mechanical stresses will arise at the border between them, which can damage the surrounding tissues. To prevent this from happening, the synthetic substance must deform in the same way as the tissues next to it. At the same time, body tissues are very diverse in their mechanical characteristics.

“If you butcher a chicken for dinner, you saw that the adipose tissue is very soft, its mechanical properties – elasticity, the rigidity of the material – are very weak, mechanical stress – only a couple of hundred pascals. On the other hand, there is leather, it is easy to stretch, smooth or squeeze – it is very soft. But if you try to compress or stretch even more, nothing will work, that is, the modulus of elasticity can increase by several orders of magnitude, ”explains co-author of the article Dmitry Ivanov, head of the laboratory of engineering materials science at the Faculty of Fundamental Physical and Chemical Engineering, Moscow State University. Lomonosov.

For living tissue, softness in the initial state and significant strengthening upon deformation are common properties, but they have not yet been imitated in synthetic materials. Often you have to choose between softness and elasticity. So, various types of rubber and silicone can be quite elastic, but not as soft as needed.

“As a child, did you play with dinosaurs, which you throw into the water, and they grow or hatch from eggs? – Dmitry Ivanov gives an example. “It was a polymer called a hydrogel. It is composed of monomer units and has hydrophilic moieties that love water. When water penetrates into this polymer, hydrophilic fragments will absorb water and it will swell. “

The hydrogel can absorb up to 99% of water, becoming soft like living tissue. But it has several important disadvantages. It lacks mechanical strength, it breaks down very easily – like jelly that falls apart if you squeeze it in your hands or try to stretch it. In addition, it is very dependent on the presence of moisture and solvents, it can dry out or swell too much, and the materials for implants must be stable: not change their size, not soak in physiological fluids. Until now, it has not been possible to create a synthetic material that will have the softness of living tissues, but upon deformation it becomes tens and even thousands of times more rigid.

Scientists have invented such a material, making it stable, soft and durable. Its properties, including color, can be controlled through its structure. Like the changing color of the chameleon, the color of the new material is structural. This means that it is due not to a chemical effect (pigment dye), but to a physical one – diffraction (deflection of light waves with the appearance of color). In a chameleon, these effects are due to guanine nanocrystals in the skin cells. When the lizard is agitated or agitated, the distance between the crystals changes, and the interference gives different shades of color in the visible region. Like the chameleon, the new material can change its color due to physical effects. By itself, it has a bluish tint, but mechanical action affects its supramolecular structure, and the color also changes.

The new material does not consist of a mixture of different molecules, does not need additives and solvents. In the structure of just one molecule, you can program both the mechanical properties and the color of the material. It is a long chain – a polymer consisting of several types of monomer units.

These monomers are grouped into blocks. In the center there is a block that resembles a bottle brush: it has a rod, from which a lot of stiff but flexible bristles depart. The bristles should be very frequent, then the bonds between them will keep them sticking out, preventing them from “sticking” to the rod. In the English-language literature, such structures are called bottlebrush, that is, “bottle brushes”. End (or, scientifically, terminal) blocks are simple chains, not like brushes. Blocks from the middle and at the ends “do not like” each other and stratify into two phases: the end blocks are twisted into balls with a diameter of 20-40 nanometers, between which “brushes” are located. By adjusting the stiffness and length of the “brushes” and the size of such balls, you can program the mechanical properties of the material and its color.

Schematic representation of copolymer chains and their self-assembly into a supramolecular structu
re Dmitry Ivanov

Thanks to this structure, the material can completely imitate the mechanical properties of pig skin and approach human skin. At the same time, since there are no solvents in it, it does not absorb physiological fluids and does not dry out in air. It is too early to say whether it will be possible to create soft tissues and skin specifically from this material, but the technology of controlling mechanical properties through the structure of polymer chains can be useful for this pu

rpose. Examples of the selection of materials for reproducing deformation curves of pig skin. Dmitry Iva
nov Source: indicator.ru