Active curved-crease origami metamaterials for large folding ratio and tunable stiffness

Active metamaterials capable of autonomous shape change and on-demand property reconfiguration under environmental stimuli represent a rapidly growing class of intelligent structures. Origami-based designs are particularly attractive owing to their capacity for large deformations, programmable geometry, and kinematic reconfigurability. In existing active origami, however, stimuli-responsive materials are embedded only in the narrow crease regions, and therefore actuating capability and mechanical performance are predominantly governed by crease rotation, limiting the achievable folding ratio and performance tunability. To overcome these limitations, curved-crease origami is newly introduced as an active metamaterial design wherein folding is driven through panel bending. Folding kinematic analyses demonstrate that the elastic strain energy of the panels and creases is simultaneously minimized when circular arc creases are combined with orthogonal generators. Building on this geometric principle, a panel-driven actuation framework is established using high-modulus bimetallic strips. Further analysis confirms that this minimum-energy curved-crease origami configuration achieves a high conversion efficiency from actuation strain of material to active structural deformation strain. Circular sheets with curved zigzag patterns are then proposed that exhibit predictable thermally induced self-folding from a planar sheet into a compact wrapped cylindrical shape. By introducing Euler spiral creases, self-wrapping over multiple turns is realized with an area folding ratio of up to 19.1. Further extension to multilayer architectures yields active metamaterials that demonstrate three-dimensional shape transformations. The coupling of a soft mode dominated by crease folding and a stiff mode involving simultaneous crease folding and panel bending enables a stiffness tunability spanning three orders of magnitude, the widest range in active metamaterials to date. Therefore, this work broadens the design space of active metamaterials and offers new opportunities for applications in soft robotics, deployable structures, and medical devices.