  Contractile biopolymer networks, such as the actomyosin meshwork of animal
cells, are ubiquitous in living organisms. The active gel theory, which
provides the thermodynamic framework for these materials, has been mostly used
in conjunction with the assumption that the microstructure of the biopolymer
network is based on rigid rods. However, experimentally, the actin network
exhibits entropic elasticity. Here we combine an entropic elasticity kinetic
theory, in the spirit of the Green and Tobolsky model of transiently
crosslinked networks, with an active flux modelling biological activity. We
determine this active flux using Onsager reciprocal relations and interpret the
corresponding microscopic dynamics. We obtain a closed model of the macroscopic
mechanical behaviour. We show how this model can be written using the framework
of multiplicative strain gradient decomposition, which is convenient for the
resolution of such problems.
