  Collagen is the most abundant extracellular-matrix protein in mammals and the
main structural and load-bearing element of connective tissues. Collagen
networks show remarkable strain-stiffening properties which tune the mechanical
functions of tissues and regulate cell behaviours. Linear and non-linear
mechanical properties of in-vitro disordered collagen networks have been widely
studied using rheology for a range of self-assembly conditions in recent years.
However, a one to one correlation between the onset of macroscopic network
failure and local deformations inside the sample is yet to be established in
these systems. Here, using shear rheology and in-situ high-resolution boundary
imaging, we study the yielding dynamics of in-vitro reconstituted networks of
uncrosslinked type-I collagen. We find that in the non-linear regime, the
differential shear modulus ($K$) of the network initially increases with
applied strain and then begins to drop as the network starts to yield beyond a
critical strain (yield strain). Measurement of the local velocity profile using
colloidal tracer particles reveals that beyond the peak of $K$, strong
strain-localization and slippage between the network and the rheometer plate
sets in that eventually leads to a detachment. We generalize this observation
for a range of collagen concentrations, applied strain ramp rates, as well as
different network architectures obtained by varying the polymerization
temperature. Furthermore, by fitting the stress vs strain data with a continuum
affine network model, we map out a state diagram showing the dependence of
yield-stain and -stress on the reduced persistence length and mesh size of the
network. Our findings can have broad implications in tissue engineering,
particularly, in designing highly resilient biological scaffolds.
