Active Contractility and Elastic Nonlinearity of Fiber Networks: From the Cytoskeleton to Blood Clots

Fibrous elastic networks occur ubiquitously in biological materials ranging from the subcellular 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, 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 contractile cells. In this presentation, we consider how the macroscopic response of an elastic fiber network to external shear and intrinsic contractility depends on fiber stiffness and connectivity. We model the fibers as elastic bonds in a disordered network which can stretch, bend, and buckle, while the contractile activity of cells/motors is represented by  force dipoles. We use our model to investigate the nonlinear elastic behavior of blood clots as revealed by rheological measurements. By inhibiting fibrin cross-linking in blood clots, we observe an anomalous softening regime in the macroscopic shear response as well as a reduction in platelet-induced clot contractility. Both these surprising observations are predicted and rationalized by the model based on the bending and buckling of fibers. We further predict that easy fiber bending should “screen out” force propagation, resulting in weaker macroscopic contractility and inter-dipole mechanical interactions that may be responsible for self-organized structures in both tissue and cytoskeleton. We conclude with ongoing work in characterizing the cooperative effects of multiple cells/myosin motors in driving network contraction.