The emergence of contractility in biological fiber networks

Large-scale force generation is essential for biological functions such as cell motility, embryonic development, and muscle contraction. In these processes, active forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale contractile stresses. I will present a comprehensive theoretical study of force transmission in these networks. While the linear response to small forces is remarkably simple, taking into account the nonlinear properties of the filaments yields strikingly counter-intuitive effects such as the reversal of extensile forces into contractile ones. These forces are furthermore amplified as they induce buckling on large scales in the network, resulting in large tensile stresses at the macroscopic scale. We develop a new experimental technique, termed Nonlinear Stress Inference Microscopy, to measure stresses in biological media, and show that it reveals this force amplification in reconstituted tissues. These results shed light on the role of the network microstructure in shaping active stresses in cells and tissue