Twisted-and-Coiled Actuators with Free Strokes Enable Soft Robots with Programmable Motions

Various actuators (e.g., pneumatics, cables, dielectric elastomers, etc.) have been utilized to actuate soft robots. Besides widely used actuators, a relatively new artificial muscle-twisted-and-coiled actuators (TCAs)-is promising for actuating centimeter-scale soft robots because they are low cost, have a large work density, and can be driven by electricity. However, existing works on TCA-actuated soft robots in general can only generate simple bending motion. The reason is that TCAs fabricated with conventional methods have to be preloaded to generate a large contraction, and thus cannot actuate soft robots properly. In this work, an upgraded technique is presented to fabricate TCAs that can deliver 48% free strokes (contraction without preloading). We first compare the static performance of TCAs with free strokes with conventional TCAs, and then characterize how will the fabrication parameters influence the TCAs' stroke and force capability. After that, we demonstrate that such TCAs can actuate centimeter-scale soft robots with programmable motions (gripping, twisting, and three-dimensional bending). Finally, we combine those motions to demonstrate a soft robotic arm that can perform a pick-and-place task. We expect that TCAs with free strokes can enable miniature soft robots with rich three-dimensional motions for both locomotion and manipulation. Because TCAs are electrically driven, we can also potentially develop untethered soft robots by carrying onboard batteries and control circuits.