Knitting Shape‐Memory Alloy Wires for Riding a Robot: Constraint Matters for the Curvilinear Actuation

To allow a linear one‐way shape‐memory alloy (SMA) wire to have a deterministic curvilinear two‐way actuation, the SMA wire can be bonded on a curved substrate or prestrained soft materials to create mismatched SMA‐wire composites. These composites have recently been applied to soft robots to achieve rapid periodic untethered locomotion. Herein, by knitting bare SMA wires on scattered holes, it is shown that the substrate‐free knitted SMA wires can periodically ride robots forward, where a deterministic spatial curvilinear actuation can be achieved in which its in‐between heated configuration and the final stable pattern is sensitive to the prescribed hole constraints. By arranging “knitting‐constraints,” the 3D‐printed and origami robots with a pair of SMA wires, respectively, are ridden. The SMA wires have an angle parameter with a variation range of 30° for the 3D‐printed robot and that of 8° for the origami robot, where the angle parameter is defined to indicate the performance of the knitted SMAs. Both the robots can displace up to 5 mm in one motion cycle with an average speed of 20 and 15 mm min−1, respectively. The motion efficiencies of the two robots are close to 1 when taking the possible backward movement in one forward‐moving cycle into consideration. A generalized method for achieving reciprocating spatial curvilinear actuations to ride robots with shape‐memory alloy (SMA) wires is proposed, where the SMA wires are knitted through elaborately prescribed scattered constraints. This study presents a novel insight into the SMA‐based actuation technology with an application demonstration on riding lightweight or soft robots.