When you stretch a material, it normally becomes thinner.  Take the case of a rubber elastic band.  Most materials behave like this.  Yet, a small number of materials can become thicker when they are stretched! (see Fig. A) An example is the tendons which connect muscle to bone. These materials are known as ‘auxetics’, from the Greek auxetos, meaning ‘that which may be increased’.

This behaviour is found in several naturally occurring materials such as ceramics (e.g. natrolite and alpha-cristobalite), metals (e.g. arsenic and cadmium) and biological systems (e.g. cat skin, salamander skin and cow teat skin) among others.  It is also possible to manufacture certain auxetic materials including foams and yarns. 

While not common, this behaviour is especially interesting since the resulting materials have unique and superior mechanical properties. For example, they can bend into a dome-shaped structure and show enhanced mechanical properties such as increased indentation, hardness and shear modulus. These properties have made it possible for auxetics to be studied for use in applications such as personal protective equipment (helmets, bulletproof vests and knee pads).

Interestingly, Nike has started to incorporate auxetic materials in some of its footwear to provide better comfort to endurance athletes (Fig. B).  Auxetics might also benefit persons who are bedridden by reducing the occurrence of pressure sores while also reducing discomfort for office workers who spend long hours sitting down. 

Auxeticity mainly depends on the internal geometry a material has and the way this geometry deforms when stress is applied. Thus, a macro or microstructure material may exhibit conventional behaviour or auxetic behaviour, depending on how the microstructure is arranged. For example, it is possible to find both conventional and auxetic polyurethane foam.

To date, man-made auxetic materials whose behaviour is due to the geometry and its deformation at a molecular scale do not exist, although numerous systems have been proposed. Such artificial materials would find applications in specialised fields such as the production of some auxetic biomedical devices. 

In this regard, a team of researchers from the University of Malta are looking into possible ways of producing an auxetic material for biomedical applications by using computer simulations followed by experimental work.  Furthermore, 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 lead the way to reduced hospital-induced infections and more comfort for the patient.     

Ruben Gatt is an associate professor at the Metamaterials Unit within the Faculty of Science of the University of Malta. Maria Cardona is a Marie Curie post-doctoral fellow at the University of Malta funded through the project AMPLIFI Grant Agreement: 101026382.

Sound Bites

•        Till now, financial and technological constraints in the production methods have limited the use of auxetic polyurethane foam to the academic world and some 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 polyurethane foam. This means there is no post-processing and it can be produced at large, industrial scales.

More information can be found here.   

•        Nature has evolved several complicated mechanisms to sustain the intricate mechanisms of life.  A team of researchers from the University of Padova has devised a way to generate an inhomogeneous matrix that imitates processes occurring in the cytoplasm of the cell.  This enables a chemical reaction to occur only in this matrix for a limited period of time.  The novel methodology can be used by other researchers as they seek to develop further complex reaction-diffusion networks. 

Reference: Chen R., Das K., Cardona MA., Gabrielli L., Prins LJ. J. Am. Chem. Soc. 2022 144 (4), 2010-2018.

For more sound bites, listen to Radio Mocha: Mondays at 7 pm on Radju Malta and Thursdays at 4 pm on Radju Malta 2 (https://www.fb.com/RadioMochaMalta).

DID YOU KNOW?

•        When a salamander moves abruptly, for example, if it is attacked by a predator, it can show auxetic behaviour and swell. This shocks the predator and prevents it from hurting it excessively. 

•        The company Petit Pli Ltd has produced auxetic clothes which can grow with the individual.  Imagine how useful this is for pregnant women and growing children!

•        Auxetics have also been proposed to produce furniture which can be made bigger simply by pulling it.

For more trivia, see www.um.edu.mt/think.