Mechanical force driven shape change in contractile poroelastic actomyosin gels

Shape change is ubiquitous in living systems across scales and fundamental to morphogenesis. At the cellular level, these changes are driven by myosin motors that actively generate mechanical forces within the cytoskeleton. Inspired by in vitro contractile cytoskeletal gels composed of actin filaments, fascin crosslinkers and myosin motors, we investigate how active forces drive 3d shape change. Despite identical composition, these gels spontaneously form domes, saddles, and wrinkles, depending on initial geometry. The disk-shaped gel behaves as a poroelastic material, where active contraction of the elastic actomyosin network induces fluid outflow. Using a 2D poroelastic model with catch-bond myosin kinetics and heterogeneous active stresses, we capture how stress gradients drive in-plane contraction. Our results qualitatively capture trends of gel velocity profiles observed from quantitative particle image velocimetry (PIV). Thick gels contract slowly and show azimuthal alignment of fibers at gel boundary. The anisotropic contraction at the boundary is expected to form domes. In contrast, thin gels remain isotropic but contract most strongly in an inner annular region due to the spatial gradient in myosin activity. Overall, our results show how active stresses, fiber alignment, catch bond kinetics and poroelastic effects, together determine the contraction and shape change dynamics of biological active gels.