Recent Advances of Auxetic Metamaterials in Smart Materials and Structural Systems

Auxetic metamaterials refer to materials and structures with extraordinary deformation, i.e., transverse expansion (contraction) under uniaxial tension (compression). In recent decades, a very wide range of innovative and functional performance has been discovered stemming from this extraordinary behavior. This desirable exhibition of adaptivity, programmability, and functionality provides great potential in soft intelligent systems. However, thus far, the mainstream research on auxetic metamaterials has focused on subjective design, monotonic mechanical properties, and passive tunability. This review provides a thorough overview of auxetic metamaterials from classical mechanical properties to intelligent applications, with the primary objective of proposing a new roadmap of auxetics for intelligent advances in this interdisciplinary field. The fundamental works are categorized in different configurations and mechanisms. In particular, the intelligent functional integration of shape morphing, actuation, sensing, multiphysical response, and inverse design is reviewed in detail. To accelerate the development of auxetics in smart materials and structural systems, the potential intelligent applications of auxetic metamaterials are generalized into soft robotics (outside the body), human–machine interaction (surrounding the body), and healthcare devices (inside the body). Finally, several significant research topics for intelligent auxetics are emphasized in theory, design, material choice, manufacturing technique, properties, and applications.