Topological mechanical metamaterials are artificial materials that harbour robust motion at edges. The lattice of gears shown in this video exhibits such a motion, which is confined to one edge of the system while the rest of the lattice, including the other edge, remains rigid. The difference in flexibility of the left and right edges originates from a topological index (the "polarization") that characterizes the lattice mode spectrum. Akin to the number of holes in a donut or pretzel, this index captures the topology of the underlying material.
Topological design of geared metamaterials
Anne S. Meeussen, Jayson Paulose, and Vincenzo Vitelli