4D metallic metamaterials for bone implants via biodegradation

4D scaffolds offer transformative potential for bone implants. Yet their application to metallic materials is constrained by the scarcity of suitable alloys and the requirement for harsh external stimuli to trigger shape change. Here, we introduce 4D metallic metamaterials driven by controlled biodegradation that combine biodegradable constraints with biometals of higher corrosion potential. We show that upon electrochemical degradation of the constraint, the metamaterials recover their original geometry-via stretching, bending, or expansion-generating programmable recovery forces tuned through structural design parameters. We demonstrate that when turned into scaffolds for bone implants, the 4D metallic metamaterials are cytocompatible and promote bone regeneration through the synergistic effects of bioactivity and mechanical stimulation in vivo. This strategy establishes a paradigm in 4D shape transformation of metal via metamaterial design, enabling bioactive, self-recovering implants with broad applicability across biomedical engineering.