Conventional materials such as rubber bands become thinner as they are stretched. So for the past 30 years researchers have been attempting to mimic the so-called auxetic properties of naturally-occurring materials such as cat skin, human tendons, and the protective layer in mussel shells, which become thicker upon stretching.
However, until now they have only been successful by structuring conventional materials using complex engineering processes such as 3D printing, which can be time consuming and costly. The processes can also lead to weaker, porous products.
The new non-porous material, which has been discovered by researchers at Leeds University, has inherent auxetic stretching properties, according to Dr Devesh Mistry, who led the research.
“This is not something that needed to be engineered in any particular way, you don’t need to 3D print this kind of behaviour,” said Mistry.
The researchers discovered the synthetic material, which has been published in the journal Nature Communications, while investigating Liquid Crystal Elastomers.
“Liquid crystals by themselves are probably best described as an ordered fluid, so you essentially have molecules with a rod-like shape, which are packed together in a particular alignment,” said Mistry. “But unlike a crystalline solid the molecules are able to flow past one another, so they act very much like a liquid,” he said.
When these liquid crystals are linked with polymer chains to form rubbery networks, the material becomes auxetic at a molecular level, giving it a completely new set of mechanical properties.
For example, the materials exhibit so-called soft elasticity, in which they can be elastically stretched without energy cost, said Mistry.
The materials can also undergo large shape transformations – up to around a 400 per cent change in length, for example - through the application of a stimulus such as heat.
More research is needed to understand what drives the auxetic behaviour, and how it can be applied commercially. But the researchers are already investigating potential applications for the material.
“[Auxetic materials] are known particularly for their shock-absorbing, tear-resisting or fracture resisting properties,” said Mistry. “So we are looking into those type of application areas, such as biomedical devices, next generation sportswear or sports devices, or protection.”
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