When you stretch a material, it normally becomes thinner. Take the case of a rubber band. Most materials behave this way. However, a small number of materials can become thicker when stretched! (see Fig. A) An example is the tendons that connect muscle to bone. These materials are known as “auxetics”, from the Greek auxetos, which means “that which can be increased”.
This behavior is found in several natural materials such as ceramics (eg natrolite and alpha-cristobalite), metals (eg arsenic and cadmium) and biological systems (eg cat skin, salamander skin and cow teat skin), among others. It is also possible to make some auxetic materials, including foams and wires.
Although not common, this behavior is particularly interesting because the resulting materials have unique and superior mechanical properties. For example, they can bend into a domed structure and exhibit improved mechanical properties such as increased indentation, hardness, and shear modulus. These properties have made it possible to study auxetics for applications such as personal protective equipment (helmets, bulletproof vests and knee pads).
Interestingly, Nike has started incorporating auxetic materials into some of its shoes to provide better comfort for endurance athletes (Fig. B). Auxetics could also benefit bedridden people by reducing the occurrence of pressure sores while reducing discomfort for office workers who spend long hours sitting.
Auxeticity mainly depends on the internal geometry of a material and how this geometry deforms when a stress is applied. Thus, a material with macro or microstructure can exhibit conventional behavior or auxetic behavior, depending on the arrangement of the microstructure. For example, it is possible to find both conventional and auxetic polyurethane foam.
To date, artificial auxetic materials whose behavior is due to geometry and its deformation at the molecular scale do not exist, although many systems have been proposed. Such artificial materials would find applications in specialized fields such as the production of certain auxetic biomedical devices.
In this regard, a team of researchers from the University of Malta is investigating possible ways to produce an auxetic material for biomedical applications using computer simulations followed by experimental work. Additionally, they are designing these materials to include antimicrobial groups, with the aim of producing new auxetic materials with antimicrobial properties to be incorporated into biomedical devices such as catheters. These newly designed materials would pave the way for reduced hospital-induced infections and greater patient comfort.
Ruben Gatt is Associate Professor in the Metamaterials Unit of the Faculty of Science at the University of Malta. Maria Cardona is a Marie Curie Post-Doctoral Fellow at the University of Malta funded by the AMPLIFI Project Grant Agreement: 101026382.
• Until now, financial and technological constraints in production methods have limited the use of auxetic polyurethane foam to the academic world and to certain very niche applications. A Maltese company, Smart Materials, in collaboration with the University of Malta, has developed a revolutionary method for the production of auxetic polyurethane foam using a one-pot reaction in the same way one would produce any other type of foam. of polyurethane. This means that there is no post-processing and it can be produced on an industrial scale.
More information can be found here.
• Nature has developed many complex mechanisms to support the complex mechanisms of life. A team of researchers from the University of Padua has developed a way to generate an inhomogeneous matrix that mimics processes occurring in the cell’s cytoplasm. This allows a chemical reaction to occur only in this matrix for a limited period of time. The new methodology can be used by other researchers looking to develop other complex reaction-diffusion networks.
Reference: Chen R., Das K., Cardona MA., Gabrielli L., Prins LJ. Jam. Chem. Soc. 2022 144 (4), 2010-2018.
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DID YOU KNOW?
• When a salamander moves suddenly, for example if attacked by a predator, it may exhibit auxetic behavior and bloat. This shocks the predator and prevents it from excessively injuring it.
• Petit Pli Ltd has produced auxetic garments that can grow with the individual. Imagine how useful this is for pregnant women and growing children!
• Auxetics has also been offered to produce furniture that can be extended simply by pulling it out.
For more information, see www.um.edu.mt/think.
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