  Through mechanical forces, biological cells remodel the surrounding collagen
network, generating striking deformation patterns. Tethers-tracts of high
densification and fiber alignment-form between cells, thinner bands emanate
from cell clusters. While tethers facilitate cell migration and communication,
how they form is unclear. Combining modeling, simulation and experiment, we
show that tether formation is a densification phase transition of the
extracellular matrix, caused by buckling instability of network fibers under
cell-induced compression, featuring unexpected similarities with martensitic
microstructures. Multiscale averaging yields a two-phase, bistable continuum
energy landscape for fibrous collagen, with a densified/aligned second phase.
Simulations predict strain discontinuities between the undensified and
densified phase, which localizes within tethers as experimentally observed. In
our experiments, active particles induce similar localized patterns as cells.
This shows how cells exploit an instability to mechanically remodel the
extracellular matrix simply by contracting, thereby facilitating
mechanosensing, invasion and metastasis.
