Handed Shearing Auxetics

Handed shearing auxetics (HSAs) are a new kind of metamaterial that allow us to have compliant linear actuation through direct rotational drives. Auxetic materials are those with a negative Poisson's ratio, meaning that they expand perpendicularly when pulled in tension (unlike most materials which contract when pulled). Shearing auxetics expand with a bias rather than just isotropically expanding. By adding chirality to a shearing auxetic pattern, we can create new structures that have a strong coupling between twisting and extension, as well as structures with selective stiffness based on the pairing of chirality. We have demonstrated the potential of HSAs by using it to create a compliant linear actuator and as a directly servo-driven soft robotic manipulator.

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Paper: Handedness in shearing auxetics creates rigid and compliant structures

Published in Science, DOI: 10.1126/science.aar4586

Abstract: In nature, repeated base units produce handed structures that selectively bond to make rigid or compliant materials. Auxetic tilings are scale-independent frameworks made from repeated unit cells that expand under tension. We discovered how to produce handedness in auxetic unit cells that shear as they expand by changing the symmetries and alignments of auxetic tilings. Using the symmetry and alignment rules that we developed, we made handed shearing auxetics that tile planes, cylinders, and spheres. By compositing the handed shearing auxetics in a manner inspired by keratin and collagen, we produce both compliant structures that expand while twisting and deployable structures that can rigidly lock. This work opens up new possibilities in designing chemical frameworks, medical devices like stents, robotic systems, and deployable engineering structures.

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Paper: Compliant Electric Actuators Based on Handed Shearing Auxetics

Published in 2018 IEEE-RAS International Conference on Soft Robotics (RoboSoft), 2018.

Abstract: In this paper, we explore a new class of electric motor-driven compliant actuators based on handed shearing auxetic cylinders. This technique combines the benefits of compliant bodies from soft robotic actuators with the simplicity of direct coupling to electric motors. We demonstrate the effectiveness of this technique by creating linear actuators, a four degree-of-freedom robotic platform, and a soft robotic gripper. We compare the soft robotic gripper against a state of the art pneumatic soft gripper, finding similar grasping performance in a significantly smaller and more energy-efficient package.