Cooperative active contractility in elastic cytoskeletal networks

Myosin motors in disordered actin networks produce contractile forces that drive cellular processes such as cell shape change, cell division as well as locomotion. In response to these active internal forces, the actin fibers undergo multiple deformation modes such as stretching, compression, bending, and buckling, that contribute to long-range and heterogeneous force transmission through the network. Here, we investigate the motor-driven macroscopic contraction of elastic fiber networks at different fiber bending rigidities and at different motor densities. The network is modelled as a diluted triangular lattice with a fixed circular boundary, while the contractile forces generated by myosin motors are modelled as isotropic force dipoles. We characterize the strain distribution and normal forces at the boundaries of the network for different configurations of dipoles. We find that networks with stiffer-to-bend fibers show more force chains and higher boundary stresses that scale linearly with bending stiffness. Moreover, interaction between dipoles leads to a non-linear scaling of boundary forces with dipole number. We also explore three different models of isotropic dipoles to find that local over-coordination near force dipoles can lead to stretching dominated networks even when the average coordination number is well below Maxwell's rigidity threshold. The results are relevant for active gel contraction in different biopolymer networks such as cells in the extra-cellular matrix.