  We present a Landau type theory for the non-linear elasticity of biopolymer
gels with a part of the order parameter describing induced nematic order of
fibers in the gel. We attribute the non-linear elastic behavior of these
materials to fiber alignment induced by strain. We suggest an application to
contact guidance of cell motility in tissue. We compare our theory to
simulation of a disordered lattice model for biopolymers. We treat homogeneous
deformations such as simple shear, hydrostatic expansion, and simple extension,
and obtain good agreement between theory and simulation. We also consider a
localized perturbation which is a simple model for a contracting cell in a
medium.
