Symmetry Breaking Metamaterial Sleeve Actuators

Inflatable soft actuators hold promise for applications that require large and complex deformations. Although they adapt readily to their environment and are inherently safe, they still require a large amount of tethers which limit their applicability. To overcome this limitation, researchers have exploited richness in the energy landscape, expressed in a nonlinear pressure–volume (PV) inflation characteristic, to sequence the motion of multiple actuators with only a few supply lines. However, designing actuators for a certain PV curve is an uphill task. Here, metamaterials, that consist of cylindrically tessellated unit cells, are explored to create a sleeve with nonlinear force–displacement characteristics. When combined with an inflatable core, these metamaterial sleeves imbue nonlinear characteristics to the actuator as a whole, making the PV characteristic tunable via the unit cell's geometry. Where previously, similar architectures were used for shape programming and shape retention, here, the symmetry‐breaking properties are analyzed and exploited to create metamaterial sleeve actuators that display a nonreciprocal bending motion; bending to the right during inflation, and to the left during deflation. Finally, the motion nonreciprocity of such actuators is used to make a quadruped robot walk.