We report here the first experimental measurements of
non-
linear
rheological material properties of hagfish slime, a hydrated
biopolymer/biofiber network, and develop a microstructural constitutive
model to explain the observed
non-
linear viscoelastic behavior. The
linear
elastic modulus of the network is observed to be G' approximate to 0.02
Pa for timescales 0.1 s <= t <= 10 s, making it one of the
softest elastic biomaterials known.
Non-
linear
rheology is examined via simple shear deformation, and we observe a
secant elastic modulus which strain-softens at large input strain while
the local tangent elastic modulus strain-stiffens simultaneously. This
juxtaposition of simultaneous softening and stiffening suggests a
general network structure composed of
non-
linear elastic strain-stiffening elements, here modeled as finite extensible
non-
linear
elastic (FENE) springs, in which network connections are destroyed as
elements are stretched. We simulate the network model in oscillatory
shear and creep, including instrument effects from rotational inertia.
The network model captures the simultaneous softening of the secant
modulus and stiffening of tangent modulus as the model enters the
non-
linear viscoelastic regime.