Active deformation and self-organization in elastic fiber networks*

Fibrous networks occur ubiquitously in living matter ranging from the cytoskeleton to the extracellular matrix. Due to their disordered structure and the propensity of slender fibers to bend and buckle,  these biomaterials exhibit unique mechanical properties such as elastic nonlinearity and rigidity transitions, non-affine deformation modes, and long-range but heterogeneous force transmission. These  properties emerge from the collective response of individual fibers to external stress as well as to intrinsic active stresses created by myosin motors. In this presentation, we consider how such actively generated forces are transmitted through the network and how this depends on fiber stiffness and connectivity. These mechanical forces can mediate long-range effects in the macroscopic deformation of the network as well as drive the self-organization of its structural components. We model the fibers as elastic bonds in a connected network which can stretch, bend, and buckle, while the contractile activity of myosin motors is represented by  force dipoles. We predict numerically that predominant fiber bending screens out force propagation, resulting in weaker macroscopic contractility and inter-dipole mechanical interactions. Further, we predict an atypical fiber buckling-induced softening regime under intermediate external shear, before the well-characterized stiffening regime. Both these predictions are supported by experiments on crosslinker-inhibited fibrin in platelet-contracted blood clots and has indirect support in in vitro actomyosin networks. Finally, we consider the 3D deformation of initially flat, contractile actomyosin gels to show how active motor-generated forces align and restructure the network, which in turn redirects the active forces. This feedback between force generating units and network elasticity provides insight into the spontaneous shape changes shown by cells and tissues during biological processes such as wound healing and morphogenesis.