Methods, systems, and apparatus, including medium-encoded computer program products, facilitate the design and use of 3D printed auxetic structures. In one aspect, a system includes one or more computer storage media having instructions stored thereon; and one or more data processing apparatus configured to execute the instructions to perform operations including (i) receiving an input specifying a three dimensional (3D) model of a 3D structure that includes at least two different materials having a predefined arrangement with respect to each other to give the 3D structure a negative Poisson ratio, (ii) receiving an input regarding a change for the 3D structure, and (iii) modifying the predefined arrangement of the at least two different materials with respect to each other in response to the input regarding the change.