Auxetic Foam

This cylindrical foam piece might look innocuous from a distance, but it has the remarkable and counterintuitive property that if you stretch it lengthways, it doesn’t get any narrower in diameter. The term for this phenomenon – auxetic – comes from the Greek auxetikos, meaning ‘that which tends to increase’. The effect happens because of how the foam structure deforms when the material is stretched. The cell walls of the foam are made from a flexible polymer material, and when they flex, the geometry of the cell walls accommodates the motion without pushing or pulling at the neighbouring cell walls.

Normal foams can be transformed into auxetic foams. One method is to compress a polymer foam from all sides, and heat it above the polymer’s softening point. This relaxes the polymer molecules into a new configuration inside the foam and sets up a new ‘remembered’ shape for the air pockets when cooled. The foam becomes auxetic because this treatment encourages the cell walls to crumple inwards, rather than outwards.

Other sorts of matter can be auxetic too, including single molecules, crystals, or macroscopic structures like this foam. They can also be termed ‘negative Poisson’s ratio’ materials. The Poisson’s ratio describes the change in dimension of a material in one direction when stretched in a perpendicular direction. A foam like this that experiences no change in diameter when stretched lengthways would have a Poisson’s ratio of zero. Cork has a similar near-zero Poisson’s ratio, which makes it a good material for sealing wine bottles: it does not expand sideways when compressed from the top, so can be smoothly popped in and out of a bottle without getting jammed.

This auxetic foam is an example of a mechanical metamaterial; a structure which has different or additional mechanical properties to its constituent material. Mechanical metamaterials are useful in shock-absorbing applications such as body armour, helmets and packing material, thanks to their ability to absorb kinetic energy into their structure. They are also gaining in interest in architecture for making strong and lightweight structures, and in medicine for growing cells on bioscaffolds.