  We present theoretical and experimental studies of the elastic response of
fibrous networks subjected to uniaxial strain. Uniaxial compression or
extension is applied to extracellular networks of fibrin and collagen using a
shear rheometer with free water in/outflow. Both axial stress and the network
shear modulus are measured. Prior work [van Oosten et al., Scientific Reports,
2015, 6, 19270] has shown softening/stiffening of these networks under
compression/extension, together with a nonlinear response to shear, but the
origin of such behaviour remains poorly understood. Here, we study how uniaxial
strain influences the nonlinear mechanics of fibrous networks. Using a
computational network model with bendable and stretchable fibres, we show that
the softening/stiffening behaviour can be understood for fixed lateral
boundaries in 2D and 3D networks with comparable average connectivities to the
experimental extracellular networks. Moreover, we show that the onset of
stiffening depends strongly on the imposed uniaxial strain. Our study
highlights the importance of both axial strain and boundary conditions in
determining the mechanical response of hydrogels.
