4.2 Structure, materials, and fabrication of exemplary actuators

The primary materials (Figure S4) for fabricating the Double Bellows and the Triple Bellows are commercial PVC tubes (Yingli Plastic Co., Ltd, China), 3D-printed frames (DSM IMAGE8200 pro, WeNext Technology Co., Ltd., China), steel wires with a diameter of 0.5 mm (LXJMGSS, China), and AB epoxy resin glue (Deli Group, China). As presented in Figure S4c, we hypothesize that the tube is linearly elastic and experimentally obtain the stiffness using the universal testing machine (68SC-2, Instron, USA).
The Double Bellows is primarily composed of two commercial bellow-shaped PVC tubes (Figure 2a and Figure S4) caped with airtight adhesive at two ends. A pair of steel wires are introduced in the middle of the Double Bellows to keep the length of the central axis constant, which makes more length change of PVC tube convert to actuator bending. To avoid mechanical instability of the PVC tube when inflated, a group of constraint rings is inserted along the actuator body to bundle these PVC tubes and steel wires. Compared with the Double Bellows, the Triple Bellows comprises three PVC tubes arranged in an equilateral triangle (Figure 3a and Figure S4). Other structural features of the Triple Bellows keep the same as that of the Double Bellows. The tube length of both the Double Bellows and the Triple Bellows is 150 mm (To improve structure stiffness, the length in the applications is 100 mm).
The fabrication of these exemplary m-SPAs contains six steps: i) Cut PVC tubes; ii) Bundle the tube with the constraint rings; iii) Fabricate the end caps and seal the tubes with AB glue; iv) Insert and fix the steel wires; v) Waiting for the glue to cure. vi) Examine the airtightness of the actuator.