Programming active metamaterials using topological defects

Active metamaterials are able to modify their own shape, providing an exciting range of potential applications. Typically, an active metamaterial contains components able to generate force dipoles, the arrangement of which determines its macroscopic behaviour. In order to fully harness the power of these materials, we must understand how macroscopic morphological changes are determined by the specific orientations and positions of the active components. We show that by imposing topological defects in the arrangements of the force generating components, a thin active sheet is able to realise a range of morphologies. By changing the slope angle, a single spiral defect can lead to positive, negative or zero local Gaussian curvature. In addition, we predict a morphological transition that is controlled by the thickness of the sheet. As a proof of principle, we confirm the existence of this transition by performing an experiment on reconstituted actomyosin gels. Finally, we apply our theoretical analysis to surfaces with different topology. These principles can be applied
in the design of programmable active mechanical metamaterials that form the basis for autonomous soft robots.