Facile and Universal Method to Obtain Negative Poisson's Ratio via Support‐Free 3D Printing Tetrahedron‐Framework

Negative Poisson's ratio (NPR) structures, based on concave shapes, can improve mechanical properties effectively. Via methods of compression, vacuum, supercritical, and so on, foams can obtain reentrant cells and NPR properties, but they may suffer mass‐ or heat‐transfer gradient, causing uneven cells and performance. 3D printing technology can fabricate uniform NPR structures, whereas the common 3D‐printed concave shapes own downward concave points that should be supported, and removing the support materials complicates the preparation severely. Thus, to propose a facile method for building NPR structures, a 3D‐repeating tetrahedron‐framework with 2D NPR origami facets is designed. One edge of the tetrahedron is designed to be vertical, and thus all the facets are not horizontal. Then, the concave points on the facets are obliquely downward instead of totally downward so that the framework can be 3D‐printed without support. The Poisson's ratio of that structure is controlled from −0.8 to 0.4 via adjusting the facet thickness, and the mechanism is confirmed by differential geometry calculation and finite element analysis. Notably, improvement in specific modulus is found in different materials with that structure, and the support‐free 3D printing strategy can further be used to design customized shapes (such as lightweight aerial vehicle devices and NPR soles here).