US Patent 4,668,557
United States Patent and Trademark Office
University of Iowa Research Foundation (Iowa City, IA, US)
An open cell foam structure that has a negative Poisson's ratio. Structure can be created by triaxially compressing a conventional open-cell foam material and heating the compressed structure beyond the softening point to produce a permanent deformation in the structure of the material. The structure thus produced has cells whose ribs protrude into the cell resulting in unique properties for materials of this type.
Lakes, Roderic S. Polyhedron cell structure and method of making same. US Patent 4,668,557 filed July 18, 1986 and issued May 26, 1987. https://ir.uiowa.edu/patents/53