Printed miniature robotic actuators with curvature-induced stiffness control inspired by the insect wing.

Stimuli-responsive actuating materials offer a promising way to power insect-scale robots, but a vast majority of these material systems are too soft for load bearing in different applications. While strategies for active stiffness control have been developed for humanoid-scale robots, for insect-scale counterparts for which compactness and functional complexity are essential requirements, these strategies are too bulky to be applicable. Here, we introduce a method whereby the same actuating material serves not only as the artificial muscles to power an insect-scale robot for load bearing, but also to increase the robot stiffness on-demand, by bending it to increase the second moment of area. This concept is biomimetically inspired by how insect wings stiffen themselves, and is realized here with manganese dioxide (MnO2) as a high-performing electrochemical actuating material printed on metallized polycarbonate films as the robot bodies. Using an open-electrodeposition printing method, the robots can be rapidly fabricated in one single step in ~15 minutes, and they can be electrochemically actuated by a potential of ~1 V to produce large bending of ~500° in less than 5 s. With the stiffness enhancement method, fast (~5 s) and reversible stiffness tuning with a theoretical increment by ~4000 times is achieved in a micro-robotic arm at ultra-low potential input of ~ 1 V, resulting in an improvement in load-bearing capability by about 4 times from ~10 μN to ~41 μN.