Mechanical design of a meta-film with giant in-plane dimensional transitions

Thin films have demonstrated remarkable potential across natural systems, daily applications, and industrial technologies. Mechanically bistable free-standing/substrate-free films that are capable of retaining distinct in-plane dimensions could unlock unprecedented multifunctional capabilities. However, their realization remains challenging. While multistable microstructured materials—such as those with block-like or geometrically constrained configurations—exhibit global stability and allow reversible shape/dimensional transformations under simple cyclic loading (e.g., tension-compression or bidirectional torsion), free-standing thin films display asymmetric responses that limit in-plane dimension transformations during simple cyclic loading. Specifically, in-plane tensile loads induce significant in-plane deformation, whereas in-plane compression triggers buckling-dominated out-of-plane displacements. To address this limitation, we propose a paradigm-shifting strategy leveraging bistable structure for pure tension-driven in-plane dimensional transitions of films. A bistable unit cell is designed to enable controlled structural transformations through bidirectional tensile actuation, circumventing compression-induced instability. The energy landscape of the system is mapped to identify stable configurations and transition thresholds. Finite element simulations elucidate the deformation pathways, while prototype experiments validate the reversible switching mechanism under programmed pure tensile loading. These results provide a foundation for the development of adaptive and programmable thin films, offering potential applications in fields such as adaptive optics, wearable electronics, and programmable metamaterials.