Crawl, Climb, Perch and Fly—Origami, Kirigami and 3D Printing of Soft Robots for Versatile Locomotions

<b><i>Background. </i></b>Untethering soft robotics from electrical or pneumatic power is one of the major challenges facing the field. Maintaining a sufficiently high power-to-weight ratio is the key problem of untethered operations. Untethered operations necessitate that the robot has sufficient mechanical power to carry its structural weight as well as additional payloads such as power sources (i.e. batteries), actuators (i.e. pneumatic pumps, valves), and microcontrollers. Existing demonstrations of untethered soft robots typically involve weighty slabs of pneumatic network actuators (PneuNets) made from silicone rubber, which further decreases the power-to-weight ratio. In addition, the fabrication of these PneuNets actuators heavily relies on replica molding, which renders them unsuitable for rapid prototyping.<br/><b><i>Contribution. </i></b>Combining 3D printing and kirigami-based thin-film polymers allowed the development of a novel method for the fabrication of pneumatically-driven actuators. Our technique effectively decreased the overall weight of the actuating units (20 g), which can be combined with lightweight pneumatic pumps and a control board. The developed actuating units are modular, allowing for rapid prototyping of soft actuators that are customizable and capable of complex locomotions (i.e., crawl, climb, perch and fly).<br/><b><i>Techincal achievement. </i></b>To realize the lightweight soft robots, the weight of the structure was reduced using PVC sheets (0.18-mm thick) with predesigned incisions by a cutting plotter. A single sheet of material is prone to twisting and bending. However, a rolled sheet with a curved surface gains anisotropic structural rigidity without adding extra weight. Selectively rolling sections of the PVC sheet aided in the reinforcement of the overall rigidity of the structure. The reduction of the overall weight of the actuators was further achieved by making lightweight pneumatic balloons. Direct ink writing (DIW) of silicone adhesive on silicone sheets (0.8-mm thick) allowed fabricating silicone balloons that served as standardized actuation modules for all the demonstrated locomotions. Next, to convert the expansion of the silicone balloons to bending motion, we took advantage of the mechanical properties of the curved surfaces; analogous to the cross-sectional curvature found in the blade of a tape measure (<i>i.e.,</i> spring return pocket tape measures). The slight cross-sectional curvature keeps the blade rigid when extended. Crucially, the cross-sectional curvature permitted anisotropic motions of the film actuator. By sandwiching a silicone balloon between two sheets of PVC in this curved configuration, we reproducibly demonstrated bending motion when the balloon was pneumatically expanded. Harnessing the bending motion and using the <i>kirigami</i>-patterned structural frame, we prototyped soft robots mimicking the locomotion of (1) crawling turtle, (2) climbing inchworm, (3) perching bat, and (4) flying and foldable wing of a ladybug. Importantly, the overall weight of the structure (<i>kirigami</i> of PVC sheets using a cutting plotter) and silicone balloons in both instances weighed &lt; 20 g but could carry a payload of &gt; 100 g, which is a substantial weight reduction compared to the existing soft robots system. As such, the soft robot was capable of carrying additional payloads (e.g., pneumatic pump (8 g), valves (5 g each), battery (~15 g), and microcontroller (~7 g)) to achieve untethered operation.<br/><b><i>Significance.</i></b> Overall, we designed a method of fabricating lightweight, untethered soft robots capable of mimicking locomotion such as crawling, climbing, perching and flying. Advantageously, our approach allows designing locomotions and prototyping new structures only using the PVC sheets and repositioning the modular actuators. Ultimately, we envisage potential applications of the developed actuators in rehabilitation, disaster relief, and space exploration.