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.