  I put forward a continuum theory for active nematic gels, defined as fluids
or suspensions of orientable rodlike objects endowed with active dynamics, that
is based on symmetry arguments and compatibility with thermodynamics. The
starting point is our recent theory that models (passive) nematic liquid
crystals as relaxing nematic elastomers. The interplay between viscoelastic
response and active dynamics of the microscopic constituents is naturally taken
into account. By contrast with standard theories, activity is not introduced as
an additional term of the stress tensor, but it is added as an external
remodeling force that competes with the passive relaxation dynamics and drags
the system out of equilibrium. In a simple one-dimensional channel geometry, we
show that the interaction between non-uniform nematic order and activity
results in either a spontaneous flow of particles or a self-organization into
sub-channels flowing in opposite directions.
