Shape change and orientational order in contractile active gels

Biological active gels comprising cytoskeletal filaments and molecular motors can show complex shape changes driven by mechanical forces. Here, we consider a general theoretical model for the feedback between active forces (generated by molecular motors such as myosin) and orientational order of fibers (such as actin bundles). The model combines active and elastic stresses with strain-induced alignment leading to orientational order which in turn directs the active contractile forces. We are motivated by in vitro experiments on contractile actomyosin gel disks with same composition but different initial geometry, that spontaneously self-organize into various shapes like dome, saddle and wrinkled, with varying degrees of actin alignment at the gel boundary. By solving the model analytically in a 2D annular geometry for axially symmetric contraction, we show the spontaneous emergence of orientational order and strain gradients. By combining linear stability analysis with numeric solution of the coupled dynamics of orientational order and elastic deformation, we show how such deformation and ordering arises from an initial random, undeformed state. The resulting in-plane distribution of strains may indicate the complex pathway of 3D shape selection. Overall, we predict how active force driven alignment in elastic materials determines shape change.