An interesting animation by Adrian Rossiter of a 3D model showing 2D auxetic behaviour.

http://www.antiprism.com/misc/jit_cubo_oct4.gif

For those of you interested in the geometry of light frameworks and models for extremely porous materials, here are a couple of links. The first one contains two 3D objects. You need Java to view them.

http://people.bath.ac.uk/rg247/swansea/javaview/gyroid_146_007.html

http://people.bath.ac.uk/rg247/swansea/gyroid_146_007_n.png