We derive the constitutive equations of an active polar gel from a model for
the dynamics of elastic molecules that link polar elements. Molecular binding
kinetics induces the fluidization of the material, giving rise to Maxwell
viscoelasticity and, provided that detailed balance is broken, to the
generation of active stresses. We give explicit expressions for the transport
coefficients of active gels in terms of molecular properties, including
nonlinear contributions on the departure from equilibrium. In particular, when
activity favors linker unbinding, we predict a decrease of viscosity with
activity - active thinning - of kinetic origin, which could explain some
experimental results on the cell cortex. By bridging the molecular and
hydrodynamic scales, our results could help understand the interplay between
molecular perturbations and the mechanics of cells and tissues.Comment: 6 pages, 2 figure
