Soft Actuation of Structured Cylinders through Auxetic Behavior

There has been a recent upsurge in the study of auxetic, or negative Poisson’s ratio, materials that derive functionality from mechanical instabilities of the underlying structure. One of the simplest auxetic designs comprises an elastomeric slab patterned with an array of voids. Under uniaxial compression, the slender ligaments contiguous to the voids buckle cooperatively, resulting in the shrinkage in the orthogonal direction, and hence a negative Poisson’s ratio. This mechanism has also been implemented in patterned spherical shells that can significantly reduce their volume under loading, while remaining spherical. We regard this self-folding inducedbybucklingas aminimalmechanismfor soft actuation, with a single radial degree of freedom. Soft actuators, as opposed to those with rigid parts, have been gaining prominence in robotics. Their ability for distributed deformation with many degrees of freedom enhances flexibility, dexterity, and environmental adaptivity. A variety of mechanisms have been explored for soft actuation, including: electro-active polymers, electroor magneto-active fluids, shape-memory alloys, inflatable architectures, and granular jamming. Still, realizing soft structures with many degrees of freedom, such as in the arms of an octopus, remains challenging given the large number of embedded activators required. Here, we introduce a novel design for soft mechanical structures that exploit the auxetic behavior of cylindrical shells that are patterned with an array of voids, to achieve reversible flexural and twisting motion, as shown in Figure 1a and b, respectively. Depressurizing our elastomeric structures allows for reversible control of the resulting motion, on demand. Below a critical pressure, there is cooperative local buckling of the thin ligaments that neighbor the voids, which themselves can be selectively plugged with inclusions. Avoid with (or without) an inclusion is regarded as a mechanical pixel, or mexel, that can be off (or on) to preclude (or enable) local buckling. The underlying auxetic behavior causes a