3D self-locking granular metamaterials.

Mechanical metamaterials derive unique properties from microarchitectures, yet most designs remain static once fabricated. Here, we propose a three-dimensional (3D) self-locking granular metamaterial composed of rigid ellipsoidal particles with flexible hook-like appendages inspired by Xanthium seed burs. The hooks interlock to form self-confined structures that fail via sequential stepwise unhooking under tension, greatly enhancing ductility. Under shear, the material transitions from an initially low-resistance, fluid-like state to a rigid, solid-like state once hooks engage, exhibiting a sliding-to-locking transition. In compression and impact, energy is dissipated through layer penetration and collapse of internal voids, with kinetic energy converted into heat via interhook friction, yielding superior energy absorption. Unlike foams, the structure can protect fragile payloads without external packaging. Because the hooks deform elastically rather than plastically, the units remain intact and reusable after loading. This work demonstrates the 3D self-locking granular metamaterial without external confinement, enabling adaptive and reusable protective materials.