Chainmail metamaterials assembled from 3D-printed discrete units with vacuum-programmable stiffness for adaptive locomotion

：Granular jamming chainmail metamaterials provide a reversible route for coupling large deformation with load-bearing capability, yet their macroscopic stiffness and is mainly discussed in terms of vacuum level or overall topology. Here, we shift the design focus to interunit contact morphology and demonstrate how point-, line-, and surface-dominated contacts can be programmed through discrete building-block geometry. Five classes of mechanically interlocked units, including circular-ring, semi-ellipsoidal, cubic, compressive–torsional, and compressive–dilational units, were fabricated by fused deposition modeling and assembled into chainmail metamaterials (CMMs). After encapsulation in a flexible airtight envelope, the CMMs undergo vacuum-induced densification and jamming, allowing continuous stiffness regulation without changing the base material. Quasistatic bending and dynamic impact tests show that all architectures exhibit monotonic stiffness enhancement with increasing vacuum level, while their stiffness hierarchy is governed by the local contact mode. In particular, cubic CMMs develop surface-dominated contacts that form stable force-transmission pathways, leading to the highest flexural stiffness and strong deformation resistance under impact. By contrast, circular-ring and other curved units retain greater mobility and conformability, making them suitable for deformation-dominant functions. Finite element simulations further verify that contact morphology controls interunit sliding, rotational constraint, and stress transfer during loading. Finally, ring and cubic CMMs were integrated into variable-stiffness wheels to examine how planar unit behavior is translated into a curved tire configuration. The wheels switch between a compliant obstacle-negotiation state and a stiff load-bearing state through vacuum regulation, while the curved assembly reveals an additional geometry-dependent packing effect. This work establishes contact morphology, rather than vacuum pressure alone, as a key design variable for scalable jamming chainmail metamaterials and adaptive locomotion systems.