Excitable Nonreciprocal Solids

Emergent nonreciprocal interactions can give rise to states that are impossible to realize in equilibrium. Here, using a combination of experiments, analysis, simulations, and theory, we demonstrate the existence of two stable nonreciprocal states, one static and one oscillatory, in chiral active solids composed of living starfish embryos. The two states are characterized by two distinct mechanisms of chiral symmetry breaking – the spinning of individual embryos and the precession of their axis of rotation. Using mechanical perturbations, we show that the oscillatory state can be selectively excited from the static state over a narrow range of applied compression rates. The dispersion relation shows that a quadratic acoustic band and a flat optical band are enhanced in the oscillatory state. Our results demonstrate how we can selectively excite work-generating cycles in odd elastic living metamaterials.