Structured porous metamaterial

The invention relates to a structured porous metamaterial. The structured porous metamaterial comprises a three-dimensional matrix of at least one repeating base unit, wherein the matrix is formed from an array of at least eight base units, each base unit comprises a platonic solid including at least one shaped void, each base unit has void geometry tailored to provide a porosity of between 0.3 and 0.97; or provide the metamaterial with a response comprising at least one of a Poisson's ratio of 0 to -0.5 when under tension and compression; or negative linear compression (NLC), negative area compression (NAC), zero linear compression (ZLC), or zero area compression (ZAC) behaviour when under pressure.