Unconventional Materials - Auxetic Foams

Rachel Enright, 15 March 2017

Auxetic foams are defying what we thought we knew about elastic materials. Product designers here at Bayly Group are always excited about new materials and new technologies. When we came across this one, we just had to share it!

Most materials become thinner through the middle when we stretch them. We know this from experience as well as education. In materials science, we call the ratio of stretching to thinning the Poisson's Ratio (PR). In conventional materials, this is a positive number: e.g. The PR for steels is about 0.3, while the PR for rubbers tends to be around 0.5.

Auxetic foams are materials which have a negative Poisson’s Ratio. This means they get thicker, expanding and increasing in volume when we stretch them. This goes against everything we were taught. It also opens a world of possibilities for exciting new applications.

The unique internal structure of auxetic foams causes this unusual behaviour. You can see the difference between conventional materials and auxetic materials in the image to the right. Conventional materials' hexagonal structure gets squashed when you stretch the material. Auxetic materials' 'interlocking hourglass' structures expand when you stretch it. 

Five Potential Benefits

1. Low density
Auxetic foams are generally lower in density, meaning you can make lighter weight products.

2. Effective volume increase under tension
As explained above, the structure causes a volume increase when stretched. We are yet to see many people taking advantage of this unique property but we are keen to see what people come up with!

3. High energy absorption (both impact and acoustic)
By transferring loads through their unique, less rigid structure, auxetic foams can absorb more impact energy than regular materials. This could have a massive impact on the design of body armour and helmets, as well as other applications. Similarly, auxetics can better absorb acoustic energy (sound vibration). Auxetics are especially good at insulating against low frequency sound.

4. High fluid absorption
When you twist a regular, water-logged sponge, the water squeezes out. An auxetic foam sponge would behave very differently, twisting but retaining the water. This is because the pores don't squash and close when you stretch them. This makes it a great material for biomedical applications such as drug delivery and blood vessel replacement. The main benefits are the lack of ‘choking’ or constriction when bent, stretched or twisted.

5. Porous and Breathable
Again, because pores can remain open when stretched, we can make very breathable materials. This is especially great for outdoor clothing or sportswear. This porosity can also have big implications for the medical industry. Auxetics could make excellent mechanical lungs. Bone implants made from auxetics could also increase healing as living tissue grows into the pores.

Existing Applications

Footwear soles which expand during walking or running allow increased flexibility (developed by Grima and Evans).

GORE-TEX is a sandwich of different materials with an auxetic layer at its centre. This comes in the form of expanded polytetrafluoroethylene, or E-PTFE (aka Teflon). It prevents the ingress of large, liquid particles and allows vapour to pass through. This creates breathable, waterproof materials.

Filters are not always perfectly stationary and perfectly installed. The minor stretching and warping they experience closes the pores of regular materials, drastically reducing performance. When auxetic foams marginally increase their pore size in response to stretching, performance, the filters maintain high performance in diverse conditions.

What's next?

This material development is at the forefront of material science. As a result, we are just at the beginning of discovering its potential applications. This is an exciting time and an exciting space to be watching! What potential applications can you think of?

If you are looking for designers to develop your next product or would like more information on developing a new product, contact us here in Melbourne on +61 3 9413 9000 or send us an email at This email address is being protected from spambots. You need JavaScript enabled to view it.

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For further explanation of auxetic materials, check out the video below