  We present a theory of active, permeating, polar gels, based on a two-fluid
model. An active relative force between the gel components creates a
steady-state current. We analyze its stability, while considering two polar
coupling terms to the relative current: a permeation-remodeling term, which
describes network deformation by the solvent flow, and a permeation-alignment
term, which describes the alignment of the polarization field by the network
deformation and flow. Novel instability mechanisms emerge at finite wave
vectors, suggesting the formation of periodic domains and mesophases. Our
results can be used to determine the physical conditions required for various
types of multicellular migration across tissues.
