Advanced Artificial Muscle for Flexible Material‐Based Reconfigurable Soft Robots

Abstract Flexible material‐based soft robots are widely used in various areas. In many situations, the suitable soft robots should be rapidly fabricated to complete the urgent tasks (such as rescue), so the facile fabricating methods of the multifunctional soft robots are still in urgent needs. In this work, the origami structure is employed to design vacuum‐powered silicone rubber artificial muscles, which can perform multiple motions, including contraction, bending, twisting, and radial motions. Artificial muscles can be used for rapid reconfiguration of different soft robots, just like the “building bricks”. Based on these artificial muscles, four soft robots with different functions, including an omnidirectional quadruped robot, a flexible gripper, a flexible wrist, and a pipe‐climbing robot, are reconfigured to complete different tasks. The proposed origami artificial muscles offer a facile and rapid fabricating method of flexible material‐based soft robots, and also greatly improve the utilization rate of flexible materials.


Contents:
The fabrication of artificial muscles Performance test of artificial muscles Setup for deformation responses test

Pressure control system
The dynamic responses of artificial muscles The actuation sequences of pipe-climbing robots The actuation sequences of flexible wrist The actuation sequences of omnidirectional quadruped robots Table S1. Details of the soft building bricks robots in this paper. Table S2. A comparison of the average turning speed of pneumatic soft crawling robots.        Captions for Video S1 to S6.

Other supplementary materials for this manuscript include the following:
Video S1 (.mp4 format). The working principle of artificial muscles.
3 Video S2 (.mp4 format). The resistance to damage of artificial muscles.
Video S3 (.mp4 format). The motion of the modular quadruped robot.
Video S4 (.mp4 format). The motion of the flexible gripper.
Video S5 (.mp4 format). The pipe-climbing robot climbs in a crooked pipe.
Video S6 (.mp4 format). The flexible wrist adjusts the brightness of a bulb.

The fabrication of artificial muscles
All the artificial muscles were fabricated with elastomer casting. The molds (made of polylactic acid) for elastomer casting were manufactured using a 3D printer (Trianglelab, Dforce 300). As shown in Figure S1, two components of E630 silicone rubber (Shenzhen Hong Ye Jie Technology Co., Ltd.) and pigment were mixed, degassed, poured into the molds, and cured at 65 °C for 30 min. Then the artificial muscle body (including slanted sides, curved facets, and top facet) and the bottom facet were removed from the molds after they were cooled to room temperature. Finally, silicone adhesive (Smooth-On, Sil-Poxy) was used to stick silicone tube to the vent hole of the body and stick the body to the bottom facet.
In rigid connection, the fixing rings and rigid connectors were manufactured using a 3D printer. As for the soft connection, the part that sucker is embedded in was made of same material as the artificial muscles. The other part that is sucked by sucker was made of harder silicone rubber (Shenzhen Hong Ye Jie Technology Co., Ltd., E660).

Performance test of artificial muscles
In the deformation performance tests, we fabricated three identical artificial muscle samples for TCAM and TBAM and each sample was tested three times. The height of TCAM and TBAM is 40 mm, the pre-twisted angle of TCAM and TBAM is 45 °, the filling angle of TBAM is 30°. Acrylic sheets were glued to the top and bottom facets to facilitate the measurement. The pressure was regulated using a vacuum regulator (SMC, IRV 1000 regulator) and measured using a pressure sensor (Beijing Star Sensor Technology Co., Ltd., CYYZ31). In the damage tests, a needle with a diameter of 0.71 mm was used to prick artificial muscles.

Setup for deformation responses test
The setup used for the twisting angle and contraction deformation responses of a TCAM is shown in Figure S2a. The deformation process was recorded using two cameras at 60 frames per second. The two cameras were placed perpendicular to the twisting direction and contraction direction, respectively.
The setup used for the twisting angle and bending angle deformation responses of a TBAM is shown in Figure S2b. One camera, which was placed on top of the TBAM, was used to record the twisting process at 60 frames per second. The bending angle was recorded using an inclination sensor (MMA8451), whose data was transmitted to a laptop via the Arduino board (MEGA2560 R3).

Pressure control system
All the modular robots mentioned in the manuscript were controlled using the pressure control system shown in Figure S3. The control signals, generated by the Arduino board (MEGA2560 R3), were transmitted to relays that control the solenoid valves (Kamoer, KVP04 valve) directly. The three-way solenoid valves determine the actuating and releasing of artificial muscles. Vacuum and the atmospheric air (or pressurized air) are connected to the two inlet ports of solenoid valve, respectively, and the artificial muscle is connected to the outlet port.
The pressurized air is generated by the air compressor (Jaguar, FB-36/7 compressor) and adjusted using a pressure-regulating valve (Delixi, AFR2000 series). The vacuum is produced by a vacuum pump (VALUE, V-I240SV pump) and regulated using a vacuum regulator (SMC, IRV 1000 regulator). The soft robots are controlled according to the actuation sequences in Figure S5-7.

The dynamic responses of artificial muscles
The dynamic responses of artificial muscles are studied by repeating actuation ten times (the period is 4 s) to observe their shape recovery performance before and after the operation. The artificial muscles have good shape recovery and deformation performances in these 10 cycles, as shown in Table S3-4.
The shape recovery performance of TCAM: T ri is the twisting recovery index, C ri is the contraction recovery index.
The shape recovery performance of TBAM: T ri is the twisting recovery index, B ri is the bending recovery index.

The actuation sequences of pipe-climbing robots
Actuating and releasing the scalable module and two supporting modules in a sequence enables the robot walk along the pipe, as illustrated in Figure S5b-d and Video S5. When the robot is climbing forward, the diameter of the lower supporting module decreases first (S1), followed by the contraction of the scalable module in S2. The robot moves along the pipe and drives the lower supporting module moving forward. In S3, the lower supporting module is connected to atmospheric pressure and returns to its original shape, effectively attaching to the pipe. Then the upper supporting module is evacuated and its diameter is decreased (S4).
The scalable module is connected to atmospheric pressure and returns to its original shape in S5 to drive the upper supporting module moving forward. In S6, the upper supporting module is connected to atmospheric pressure and its diameter increases. Repeating S1 to S6, the robot can continuously climb forward in the pipe. Similarly, the robot can climb backward by repeating S1 to S6 according to Figure S5c-d. The artificial muscles of the pipe-climbing robot weigh 25g, the pipe-climbing robot weighs 440g (17.6 times of the weight of the artificial muscle), the robot can work normally while climbing up a vertical pipe, which indicates that the artificial muscles have good load capacity.

The actuation sequences of flexible wrist
The actuation sequences of turning up (clockwise rotation) and turning down (anticlockwise rotation) the light are shown in Figure S6c-e and Video S6. When the flexible wrist is used to turn up the light, a vacuum of 70 kPa is applied to the TCAM-1 to initialize the twisting action (S0). Then the flexible gripper holds the knob tightly when a high-pressure air of 90 kPa is applied (S1). In S2, we applied -70 kPa to actuate the TCAM-2 and atmospheric pressure to relieve the twisting of the TCAM-1. Both of the two TCAMs offer torque to twist the knob clockwise. In S3 and S4, the gripper loosens the knob and the flexible wrist returns to its initial state. Continuous clockwise rotation by repeating S1-S4 can turn up the light.
Similarly, continuous anticlockwise rotation ( Figure S6d-e) can turn down the light.

The actuation sequences of omnidirectional quadruped robots
The quadruped robot can achieve omnidirectional movement using crawling gaits (Video S3).
As shown in Figure S7c, two adjacent feet are left up (S2, S6) and then dragged (S3) or pushed (S7) forward simultaneously by the contraction or elongation of the scalable modules.
Repeating S1 to S8, the robot can continuously move forward. Other crawling gaits, such as moving backward, left, and right are shown in Figure S7d and Figure S7g. The average walking speed is 10.7 mm/s, equivalent to 3.4 times of its body length per minute.
Besides omnidirectional movement, turning movement is also realized in quadruped robot utilizing rotating gaits. As shown in Figure S7e, the two feet that are in a diagonal position are left up (S3, S7) first. Then each raised foot is pushed and dragged by the two adjacent scalable modules simultaneously (S4, S8). Repeating S3 to S10, the robot can continuously turn clockwise. Similarly, the anticlockwise rotating gait can be achieved according to the actuation sequence in Figure S7f and Figure S7h. The average turning speed is 18.0 °/s which is faster than most soft crawling robots [1][2][3][4][5] , as shown in Table S2.  Robertson et al. [2] Yes 3.5 2017 Waynelovich et al. [3] No 1.73 2016 Zou et al. [4] Yes 1.63 2018 Tolley et al. [5] No 0.20 2014