  Solids built out of active components can exhibit non-reciprocal elastic
coefficients that give rise to non-Hermitian wave phenomena. Here, we
investigate non-Hermitian effects present at the boundary of two-dimensional
active elastic media obeying two general assumptions: their microscopic forces
conserve linear momentum and arise only from static deformations. Using
continuum equations, we demonstrate the existence of the non-Hermitian skin
effect in which the boundary hosts an extensive number of localized modes.
Furthermore, lattice models reveal non-Hermitian topological transitions
mediated by exceptional rings driven by the activity level of individual bonds.
