## 3 Methods and Results  
### Actuator Characteristics  
Figure 9a shows the setup for testing the characteristics of the actuators. A 1 m-long tube with a 3 mm outer diameter connects the folded hydraulic actuator to a syringe pump. A pressure sensor (SparkFun MS5803-14 BA) close to the syringe pump measures the pressure inside the actuator, with the sensor data recorded via an Arduino Uno microprocessor, which also controls the syringe pump. Figure 9b provides a detailed diagram of the actuator test platform. The folded actuator is fixed to both a fixed pad and a rotating pad which is rotated by the unfolding of the actuator. The cable, fixed onto and wound around a cylindrical rod connected to the rotating pad, is attached to a cart-like slider. A weight is used to pretension the cable. The pad rotation leads to the cable displacement, which is obtained by measuring the distance of two retro-reflective markers fixed on the bench and the slider, respectively. The positions of the two reflective markers were recorded by the OptiTrack (NaturalPoint, Corvallis, OR, USA) optical motion tracking system. The rotation angle of the rotating pad is calculated as the cable displacement divided by the radius of the cylindrical rod. An assumption made in the test is that the rotation angle of the actuator is equivalent to the pad rotation angle. Before testing, any air bubbles in the hydraulic lines were purged. Figure 8: Robot fabrication. (**a–c**) The scaffold, triangular support, and actuator plastic film pieces were welded according to the patterns and cut either by a laser cutter or manually. (**d**) A photograph of how to manually weld the actuators and the triangular support onto the scaffold. (**e**) A photograph of how to manually weld edges to generate a hollow structure. (**f**) Lastly, the structure can be pressurized and formed into a hexagonal prism. Figure 7: The welding and cutting patterns for producing (**a**) the scaffold, (**b**) the triangular support, and (**c**) the actuators. The maximum water volume within the folded actuator is 2 mL, allowing the actuator to fully straighten. Based on the kinematics of the robot, the actuator's rotation angle is between 90\({}^{\circ}\) and 180\({}^{\circ}\) given the specified dimensions of each component in Figure 7. Therefore, testing was conducted within the rotation angle range of 90\({}^{\circ}\) to 180\({}^{\circ}\). The water volume varies from 1.2 mL to 2 mL while the actuator's rotation angle ranges from 90\({}^{\circ}\) to 180\({}^{\circ}\) under a 10 N\(\cdot\)mm payload. The payload torque applied to the actuator is calculated as the product of the weight's gravity and the radius of the cylindrical rod. To enhance repeatability from the outset of data collection, the chamber underwent a break-in process during fabrication, in order for desirable creases to appear. Figure 10a illustrates the relationships between the water volume inside the actuator and the rotation angle of the actuator under different payloads. The data presented in the curves represent the average of three actuator samples. Notably, there is a larger hysteresis observed when the payload is small, attributable to the stiffness of the actuator, which makes folding difficult when the actuator straightens. Moreover, increased payload induces actuator deformation, leading to a decrease in the actuator's rotation angle output due to its inherent flexibility. The folded actuator was driven through 100 full cycles with a 15 N\(\cdot\)mm payload. The repeatability test results are shown in Figure 10b, demonstrating the actuator's high repeatability. Figure 10c demonstrates the maximum pressures within the actuator across one actuation cycle at different payloads. The maximum pressure inside the actuator exhibits linearity with the payload when it exceeds 20 N\(\cdot\)mm. Additionally, the burst pressure of the actuator was approximately 200 kPa. The rotation angle of the folded actuator can be determined through the inverse kinematics model.