Mechanical heterogeneity and directional cues in the fibrous extracellular matrix

When cells contract or migrate within the three-dimensional extracellular matrix, they pull on the fibers surrounding them, generating displacements in the fibrous network. Though it is known that fibrous materials deform primarily by bending of fibers, the implications of fiber bending on the mechanics of the network are not fully clear. This is especially true for forces applied at small length scales, such as those of a cell. At these scales, typically tens of microns, the fiber network is highly random and nonlinear. This presentation will discuss experiments that quantify the mechanics of fibrous materials at small length scales under large deformations, like those induced by a contracting cell. Our experiments use microspheres made of poly(N-isopropylacrylamide), an active hydrogel that contracts when heated. With these particles, we can apply well-controlled displacements of large magnitude at scales of tens of microns. This presentation will demonstrate our use of these particles to quantify mechanical heterogeneity and anisotropy in matrices made of collagen fibers, with a key result being that the mechanics in random fiber networks are far more heterogeneous than often appreciated. This presentation will also describe use of these microspheres to study force-induced alignment and densification of fibers into distinct bands, similar in appearance to those that occur on the boundary of a tumor. Our experiments provide evidence that band formation results at least in part by an elastic phase transition resulting from an instability due to compression softening of the fiber network.