Untethered Biomimetic Soft Robots by Kirigami of Thin-Film Polymer and 3D-Printed Silicone Actuators

<b><i>Background. </i></b>Untethering soft robotics from electrical or pneumatic power is one of the grand challenges in the field. The major difficulty of untethered operation is in maintaining a sufficiently high power-to-weight ratio. Untethered operation requires that the robot must have sufficient mechanical power to carry its structural weight and additional payloads such as power sources (<i>i.e.</i>, batteries), actuators (<i>i.e.</i>, pneumatic pumps, valves), and microcontrollers. Existing demonstration of untethered soft robots usually consists of weighty slabs of pneumatic networks actuators (PneuNets) made out of silicone rubber that further reduces the power-to-weight ratio. Furthermore, the fabrication of these PneuNets actuators relies heavily on replica molding, making them unsuitable for rapid prototyping.<br/><b><i>Contribution. </i></b>To address these challenges, we developed a unique approach to fabricate pneumatically-driven actuators by combining 3D printing and <i>kirigami</i>-based thin-film polymers. Our approach drastically reduced the overall weight of the actuating units (~20 g) that can be combined with lightweight pneumatic pumps and a control board. The developed actuating units are modular, enabling rapid prototyping of soft actuators that offer customizability and complex locomotion.<br/><b><i>Techincal achievement. </i></b>To realize the lightweight soft robots, the weight of the structure was reduced using PVC sheets (thickness = 0.18 mm) 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. Using silicone sheets (thickness = 0.8 mm) (Dragon Skin™ 20, Smooth-on, Easton, PA, USA) and direct ink writing (DIW) of silicone adhesive (Sil-poxy, Smooth-on, Easton, PA, USA), we fabricated standardized silicone balloons that served as actuation modules for all the demonstrated locomotion. 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 permits 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) an amphibious turtle crawling and swimming and (2) an inchworm climbing up a branch. 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 such as a small pneumatic pump (8 g), micro pneumatic 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 (but not limited to) mimicking the locomotion of a turtle and an inchworm. Importantly, new structures and locomotion can be rapidly prototyped by redesigning the PVC sheets and repositioning the modular actuators. Ultimately, we envisage potential applications of the developed actuators in rehabilitation, disaster relief, and space exploration.