Defect-driven shape transitions in elastic active nematic shells

Active matter is characterized by its ability to induce motion by self-generated stress. In the case of a solid, such motion can lead to shape transformations. The stress-generating components can be anisotropic endowing the material with mesoscopic orientational order. It is currently unknown how the specific postions and orientations of these active constituents influence morphological changes. We study theoretically the effects of imposing topological point defects in the arrangements of the stress-generating components on the morphology of elastic active nematic shells. We show that topological defects of charge +1 are uniquely capable of increasing, reducing or maintaining the intrinsic curvature of the shell. These changes depend on the nature of the active stress and the phase angle of the defect. We apply our theory to experiments conducted on contracting actomyosin sheets. By combining defects of different charges, we can generate shells with arbitrary complexity. We confirm this flexibility by reproducing the shape of the freshwater polyp Hydra, in which topological defects have been associated with morphological features of the animal. In addition to understanding morphogenetic processes, these principles can be applied to the design of programmable active mechanical metamaterials that form the basis of autonomous soft robots.