Active force driven shape change in contractile actomyosin gels

Living systems exhibit shape changes from cellular to tissue scale during morphogenesis. In cell cytoskeleton, such shape changes are often driven by myosin motors that actively generate mechanical forces. We are motivated by in vitro experiments on contractile actomyosin gels comprising networks of actin filaments crosslinked by fascin crosslinkers, and contracted by myosin II motors. These gels with same composition but different initial geometry contract under myosin activity and spontaneously self-organize into different classes of 3D shapes: domes, saddles, and wrinkles. We propose a continuum theoretical model based on active contractile stresses, elastic strain and orientational order of actin fibers which directs the active stress, while the gradient of active stress drives non-uniform contraction in the gel disk leading to out-of-plane buckling. We present analytic solutions for steady-state displacement in a 2D annular geometry for axisymmetric distribution of active stress. The results qualitatively agree with strain profiles extracted from Particle Imaging Velocimetry (PIV) for domed vs wrinkled gels. We next analyze the dynamics of contraction based on a poroelastic model to show how different strain profiles result for thicker and thinner gels based on non-uniform contraction rates determined by differential rates of fluid outflow at the basal vs lateral surfaces of the gels. Overall, we study how active mechanical forces drive 3d shape formation in biological matter.