Gear-based metamaterials for extraordinary bandgap tunability

Metamaterials can be engineered with tunable bandgaps to adapt to dynamic and complex environments, particularly for controlling elastic waves and vibration. However, achieving wide-range, seamless, reversible, in situ, and robust tunability remains challenging and often impractical due to limitations in bandgap mechanisms and design principles. Here, we introduce gear-based metamaterials with unprecedented bandgap tunability. Our approach leverages Taiji planetary gear systems as variable-frequency local resonators, which allows the metamaterial to seamlessly modulate its bandgap's center frequency by three to seven times (e.g., shifting from 250–430 to 1400–2000 Hz), surpassing existing methods. Notably, this is achieved without pre-deformation or major changes to its static stiffness in the wave propagation direction, ensuring robust in situ tunability and smooth control even under heavy static loads. This enables adaptable wave manipulation for versatile smart platforms.