  Collagen is the most abundant extracellular-network-forming protein in animal
biology and is important in both natural and artificial tissues, where it
serves as a material of great mechanical versatility. This versatility arises
from its almost unique ability to remodel under applied loads into anisotropic
and inhomogeneous structures. To explore the origins of this property, we
develop a set of analysis tools and a novel experimental setup that probes the
mechanical response of fibrous networks in a geometry that mimics a typical
deformation profile imposed by cells in vivo. We observe strong fiber alignment
and densification as a function of applied strain for both uncrosslinked and
crosslinked collagenous networks. This alignment is found to be irreversibly
imprinted in uncrosslinked collagen networks, suggesting a simple mechanism for
tissue organization at the microscale. However, crosslinked networks display
similar fiber alignment and the same geometrical properties as uncrosslinked
gels, but with full reversibility. Plasticity is therefore not required to
align fibers. On the contrary, our data show that this effect is part of the
fundamental non-linear properties of fibrous biological networks.
