Active contraction in biological fiber networks
Large-scale force generation is essential for biological functions such as cell motility, embryonic development, wound healing and muscle contraction. In these processes, forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale active stresses. While fiber networks are well characterized macroscopically, this stress generation by microscopic active units is not well understood. In this Thesis, I present a comprehensive theoretical and numerical study of force transmission in elastic fiber networks. I show that the linear, small-force response of the networks is remarkably simple, as the macroscopic active stress depends only on the geometry of the force-exerting unit. In contrast, as non-linear buckling occurs around these units, local active forces are rectified towards isotropic contraction, making the local geometry of force exertion irrelevant. This emergent contractility is amplified by non-linear force transmission through the network. This stress amplification is reinforced by the networks’ disordered nature, but saturates for high densities of active units. Our predictions are quantitatively consistent with experiments on reconstituted tissues and actomyosin networks, and that they shed light on the role of the network microstructure in shaping active stresses in cells and tissue.