What are Auxetic Materials?
Auxetics are materials with a negative Poisson ratio – when you stretch them, they get fatter instead of thinner. One might wonder how this is even physically possible. The answer is in their underlying structure. Imagine a foam made out of millions of tiny bow-tie shaped cells, connected to one another. If you pull on the sides of the material, the bow ties expand into squares, expanding on the transverse plane as well as the plane parallel to the stretching action. Because the phenomenon is caused by the macrostructure or microstructure of the material and not the chemical composition of the material itself, many common materials can be put in auxetic arrangements, though materials that are flexible and stretchy work best. The whole auxetics field is relatively new. The concept of materials with a negative Poisson ratio was first published in Science in 1987 by Rod Lakes of the University of Iowa, who continues to be a leader in the nascent field. The term “auxetic” was not used to r
1. Auxetic materials get fatter when pulled and narrower when compressed. Mathematically this is described in Poisson’s ratio, which measures the changes in shape which result from the uniaxial loading of materials. In the positive ratio structure above, pulling on the connectors (fibrils) between the hinge points (nodules) produces a thinner structure. In the negative ratio structure with the “bowtie” shape, pulling on the fibrils enlarges the shape. A good existing example of this is PTFE, which shows the nodule and fibril structure, and demonstrates auxetic behaviour. 2. Auxetic materials occur in a wide variety of sizes, from large engineering structures to the molecular level, and there are also naturally occurring auxetic biomaterials (including some forms of skin). 3. A major breakthrough occurred in 2001/ 2002, when Dr Kim Alderson and her researcher Ginny Simkins developed a novel melt extrusion process to produce the world’s first reported auxetic filaments.
