Autonomous waves and global motion modes in living active solids

Elastic active matter—also called an active solid—consists of self-propelled units embedded in an elastic matrix and it resists deformation. This shape-preserving property and the intrinsic non-equilibrium nature make active solids an attractive potential component for self-driven devices, but their mechanical properties and emergent behaviour remain poorly understood. Here, using a biofilm-based living active solid, we observe self-sustained elastic waves with wave properties not seen in passive solids, such as power-law scaling of wave speed with activity. Under isotropic confinement, the active solid develops two topologically distinct global motion modes that can be selectively excited, with a step-like frequency jump at the transition between the two modes. Our findings reveal spatiotemporal order in elastic active matter and may guide the development of solid-state adaptive or living materials.