On the internal architecture of lightweight negative Poisson's ratio (auxetic) metastructures: a review

Development of lightweight materials with enhanced mechanical properties has been a long-standing challenge in science and engineering. Lightweight auxetic metastructures (AMSs) provide attractive solutions to this problem. AMSs' negative Poisson's ratio is a unique characteristic which results in very interesting practical properties such as high energy absorption and improved toughness. Different properties of metastrcutures, including anisotropy, are dependent, in addition to their original material, on the unit cell shape and geometrical features which have very high variations. Over the past few years, researchers have developed AMSs with various unit cells, either introducing new internal architecture or enhancing and optimizing the existing ones. Despite the progress made in this field, there is no comprehensive review of the AMS unit cells. This review describes the cellular AMSs associated with a classified set of metastructures, comprising more than 100 distinct auxetic repetitive unit cells. Besides, achievable range of negative Poisson's ratios are provided for different categories of AMSs. Finally, the future perspective of the research field and potential developments and applications in this field are discussed.