Stiffness-tunable single-DOF mechanical metamaterials based on bistable chiral units

Stiffness-tunable metamaterials are promising for mechanical systems that require on-demand stiffness adaptation in variable environments. To address this need, this paper proposes a stiffness-tunable single-degree-of-freedom (DOF) mechanical metamaterial based on bistable chiral units. It enables quasi-linear, multi-level stiffness tuning with zero mechanical holding power at the bistable unit level and supports digital control for in-situ tuning, while keeping the envelope unchanged. Each bistable chiral unit is designed as a rigid-elastic coupling self-equilibrated module, comprising a rigid planar rhombic four-revolute-joint (4R) mechanism and a spine-inspired elastic chiral structure. The bistable chiral unit exhibits distinct stiffness in its two stable states without changing the configuration of the rhombic 4R mechanism. We arrange the bistable chiral units into an array to form a metamaterial, which features a single-DOF mechanism array constructed through rhombic 4R mechanisms formed at the junctions. By constraining deformation to a predefined pathway, this single-DOF mechanism array enables a deterministic mapping between the stable states of bistable chiral units and the overall stiffness of the metamaterial, achieving quasi-linear grading in stiffness modulation through switching the stable states of the units. Furthermore, by integrating actuators into the bistable chiral units, we demonstrate on-demand and in-situ tuning of the metamaterial's stiffness. Experimental evaluations of this system demonstrate a 10-level quasi-linear stiffness gradient with a maximum change ratio of 4.89. Overall, this work introduces a feasible strategy for achieving stiffness-tunable properties by arranging bistable chiral units into a single-DOF metamaterial, laying a solid foundation for the future design of smart mechanical systems.