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Self-powered soft robot in the Mariana Trench

Nature volume 591, pages 66–71 (2021)Cite this article

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Abstract

The deep sea remains the largest unknown territory on Earth because it is so difficult to explore1,2,3,4. Owing to the extremely high pressure in the deep sea, rigid vessels5,6,7 and pressure-compensation systems8,9,10 are typically required to protect mechatronic systems. However, deep-sea creatures that lack bulky or heavy pressure-tolerant systems can thrive at extreme depths11,12,13,14,15,16,17. Here, inspired by the structure of a deep-sea snailfish15, we develop an untethered soft robot for deep-sea exploration, with onboard power, control and actuation protected from pressure by integrating electronics in a silicone matrix. This self-powered robot eliminates the requirement for any rigid vessel. To reduce shear stress at the interfaces between electronic components, we decentralize the electronics by increasing the distance between components or separating them from the printed circuit board. Careful design of the dielectric elastomer material used for the robot’s flapping fins allowed the robot to be actuated successfully in a field test in the Mariana Trench down to a depth of 10,900 metres and to swim freely in the South China Sea at a depth of 3,224 metres. We validate the pressure resilience of the electronic components and soft actuators through systematic experiments and theoretical analyses. Our work highlights the potential of designing soft, lightweight devices for use in extreme conditions.

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Fig. 1: Deep-sea field test of pressure-resilient soft robot with onboard power and control.
Fig. 2: Pressure resilience of polymer-encapsulated electronics.
Fig. 3: Design and fabrication of soft robot.
Fig. 4: Swimming experiments in a pressure testing chamber and a deep lake.

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Data availability

All data are available from the corresponding authors upon reasonable request.

Code availability

Finite element analysis was performed using ABAQUS.

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Acknowledgements

We acknowledge the support of the following: National Key R&D Program of China 2017 YFA0701100, National Natural Science Foundation of China (92048302, 11822207, 11525210, 91748209, 21636008, 41876179, 52075476, 91948303 and 21676244), Zhejiang Provincial Natural Science Foundation of China (R18A020004), China Postdoctoral Science Foundation (2020M671820), Zhejiang University, Zhejiang Lab Research Funds (113006-Z31801, 115002-AA2003), Fundamental Research Funds for the Central Universities, and Dr Li Dak Sum & Yip Yio Chin Fund for Stem Cell and Regenerative Medicine. The mechatronic equipment is supported by Shenzhen Dinghai Advanced Material Technology and Shaoxing Haivoo Power Supply Technology. We thank the crews of cruises from Guangzhou Marine Geological Survey, Westlake University and Shanghai Ocean University.

Author information

Author notes

  1. These authors contributed equally: Guorui Li, Xiangping Chen, Fanghao Zhou

Authors and Affiliations

  1. State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou, China

    Guorui Li, Mingqi Zhang, Zheng Chen, Wei Song, Tiefeng Li, Shaoxing Qu & Zhilong Huang

  2. Zhejiang Lab, Hangzhou, China

    Guorui Li, Yiming Liang, Wei Song, Tiefeng Li, Jason Gu & Shiqiang Zhu

  3. Center for X-Mechanics, Department of Engineering Mechanics, Zhejiang University, Hangzhou, China

    Guorui Li, Xiangping Chen, Fanghao Zhou, Shunyu Yin, Yi Xu, Xuxu Yang, Guoyong Mao, Zheng Jia, Haofei Zhou, Tiefeng Li, Shaoxing Qu, Zhilong Huang & Wei Yang

  4. Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou, China

    Xiangping Chen, Xunuo Cao, Zhen Zhang, Mingqi Zhang, Shunyu Yin, Yi Xu, Xuxu Yang, Guoyong Mao, Zheng Jia, Haofei Zhou, Shaoxing Qu, Zhilong Huang & Wei Yang

  5. State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou, China

    Fanghao Zhou, Youhua Xiao, Yingwu Luo & Tao Xie

  6. Institute of Process Equipment, College of Energy Engineering, Zhejiang University, Hangzhou, China

    Fanghao Zhou & Zhongbin Xu

  7. Ocean College, Zhejiang University, Zhoushan, China

    Xunuo Cao, Hongbo Fan, Zheng Chen, Wei Song & Shiqiang Zhu

  8. Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya, China

    Baosheng Wu & Shunping He

  9. College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha, China

    Wenjing Yang

  10. Shanghai Engineering Research Center of Hadal Science and Technology, Shanghai Ocean University, Shanghai, China

    Binbin Pan

  11. National Center for International Research of Subsea Engineering Technology and Equipment, Dalian Maritime University, Dalian, China

    Jiaoyi Hou & Weifeng Zou

Authors
  1. Guorui Li
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  2. Xiangping Chen
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Contributions

T.L., S.Q., S.H., Z.H., T.X., J.G., S.Z. and Wei Yang conceived the concept and wrote the manuscript. G.L., X. Chen, F.Z., Y. Liang, Y. Xiao, Z.Z., M.Z., B.W., H.F., Z.C., J.H., W.Z., Z.X., B.P. and W.S. carried out the experiments. G.L., X. Chen, F.Z., W.Z., Y. Liang and B.P. carried out the field tests. T.L., G.L., F.Z., X. Chen, Wenjing Yan, S.Y., Y. Xu, X.Y., Z.J., G.M. and H.Z. conducted the mechanical analysis. Y. Xiao, Y. Luo and T.X. guided the material fabrication. T.L., Z.H., T.X., Y.L., J.G., S.Z. and Wei Yang directed the project. All authors analysed and interpreted the data.

Corresponding author

Correspondence to Tiefeng Li.

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Competing interests

T.L. is applying for patents related to the described work. The other authors declare that they have no competing interests.

Additional information

Peer review information Nature thanks Kaitlyn Becker, Cecilia Laschi, Brennan Philips and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Field test in the deep sea.

a, The soft robot was mounted onto a deep-sea lander to reach the bottom of the Mariana Trench (11.33° N, 142.19° E, depth 10,900 m). b, c, The soft robot has an embedded high-voltage amplifier (b) and a 2,500 mA h lithium-ion battery (c), generating the actuating a.c. voltages of 7 kV, 8 kV and 9 kV at 0.5 Hz in sequence. d, e, The experimental processes were recorded by the video cameras (d) and LED lights (e) protected by pressure vessels. f, Throughout field tests in the South China Sea, the soft robot was grasped by a robotic arm on the ROV HAIMA, reaching the bottom of the South China Sea (depth of 3,224 m). After being released, the soft robot demonstrated free-swimming locomotion with flapping fins, actuated by an onboard a.c. voltage of 8 kV and 1 Hz. This process was recorded by the video cameras and LED lights on the ROV.

Extended Data Fig. 2 Pressure test of centralized and decentralized electronics.

a, Top and bottom views of a centralized electronic circuit, in which electronics such as microchips, transformer, capacitors and crystal oscillator are densely packed on a PCB. The electronics are encapsulated in a soft silicone matrix for the pressure test. b, c, After pressure test at 110 MPa, we removed the silicone matrix and investigated the functioning of the electronics. The crystal oscillator, which contains an air gap, collapses (b), and the MCU connection fails (c). d, Top view of decentralized electronics. A calibrated MSI clock from the STM32L4 microprocessor is used to replace the crystal oscillator. e, Electronic function tests of each stage (illustrated in Extended Data Fig. 5) for decentralized electronics. Blue curve is the battery output (channel 1; 1 V per grid), green curve is the boost circuit output (channel 4; 5 V per grid), cyan curve is the ‘flyback’ circuit output (channel 2; 500 V per grid), and purple curve is the final output (channel 3; 2 kV per grid). The results indicate that the decentralized electronics maintained their function after the 110-MPa pressure test.

Extended Data Fig. 3 Pressure resilience for polymer-encapsulated electronics.

a, FEA of detaching electronics. The electronic component A is modelled as a linear elastic unit with an elastic modulus of 120 GPa and a Poisson’s ratio of 0.35. Component B is modelled as a linear elastic unit with an elastic modulus of 22 GPa and a Poisson’s ratio of 0.39. The encapsulating silicone matrix C is modelled as a hyper-elastic unit with a shear modulus of 0.2 MPa and a bulk modulus of 2,000 MPa. The stress distribution at the interface illustrates the stress mitigation effect of detaching (components separated by a distance of 2 mm). The blue and red dashed lines highlight the paths on the interfaces used for stress investigation. The relation between the distance to the centre and the maximum shear stress is plotted to illustrate the effect of stress mitigation. The dimensions of components A and B are both 10 mm × 10 mm × 2 mm. b, A symmetric FEA model is built to simulate two adjacent electronic components on a PCB. The distance between the two components (d) varies from 0.4 mm to 2.4 mm. The highlighted red dashed line indicates the path for shear stress investigation. c, The shear stress distributions of the nodes on the path (red dashed line) with different d are plotted. The result shows that distancing neighbouring components on the PCB reduces the interfacial shear stress.

Extended Data Fig. 4 Fabrication of the soft robot.

a, A SBAS DE membrane (initial thickness 1.5 mm) is clamped onto a stretching machine. b, The SBAS DE membrane is pre-stretched (3 × 3) and attached to four square brackets made of acrylonitrile butadiene styrene (border length of 13.2 cm and border width of 2 cm) to keep the pre-stretch. Then we cut off the four brackets, each with a pre-stretched DE membrane, to fabricate the DE actuator. c, The carbon grease electrode is brushed on the pre-stretched DE membrane. An aluminium foil feed line is placed on the electrode to connect it with the external circuit. d, Two frames are combined to protect the internal electrode. e, The laser-cut PET elastic frame (region indicated by blue dashed line, thickness of 0.35 mm, also shown in Fig. 3a) is glued to the DE membrane in one side. f, Bracing units (red dashed lines: (1) acrylic board with thickness 1 mm, (2) carbon fibre plate with thickness 0.3 mm and (3) PET with thickness 0.35 mm) are glued on the opposite side of the membrane. g, The precursor (Dragon Skin 20) is poured into the 3D-printed mould to encapsulate the electronics. h, The as-fabricated soft robot.

Extended Data Fig. 5 High-voltage amplifier and electronics for control.

The electronic system of soft robot is mainly composed of a high-voltage amplifier and electronics for control. The high-voltage output is achieved by a three-stage circuit. The first stage is a boost circuit, which is dominated by a MP3431 chip to boost a 3.7-V battery voltage to 15 V. The second stage is the ‘flyback’ circuit, which boosts the voltage from 15 V to a maximum amplitude of 3 kV through a transformer. The third stage is a voltage-multiplier circuit, which can multiply the voltage output of the second stage, and finally produces the final voltage (maximum amplitude of 10 kV). The MCU tunes the voltage output with amplitude in the range of ~2–10 kV and frequency in the range of ~0.5–3.0 Hz.

Extended Data Fig. 6 Actuation performance experiments of VHB and SBAS.

a, b, The circular actuators of VHB (a) and SBAS (b) are composed of two DE membranes (thickness 1.5 mm) with pre-stretch of 3 × 3, fixed on a rigid ring. A carbon grease electrode is sandwiched by the two DE membranes with an initial radius (R) of 1.25 cm as the active region. The carbon grease electrode is connected to the power source as the high-voltage end. The external water surrounding the other sides of the DE membranes is connected to the power source as the ground end. When a high voltage is applied, the active region expands to the actuated radius of r. The area strain is calculated as (r2 − R2)/R2. c, The experimental set-ups for the performance test of the DE actuator. The circular actuators are installed in a pressure chamber and connected to an external high-voltage power source (Trek 610E). The voltage-induced area strain of the DE circular actuator is recorded from the top by a camera protected by a metallic vessel. With an identical actuating voltage of 8 kV at 1 Hz, we tested the actuation performances of VHB and SBAS under various temperatures (2.7 °C, 5.0 °C, 10.0 °C, 15.0 °C, 20.0 °C and 25.0 °C) and hydrostatic pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, 100 MPa and 110 MPa). d, The SBAS film (thickness of 1.5 mm) after solution casting and solvent evaporation process (320 g SBAS is dissolved in 3,000 ml tetrahydrofuran; the casting area is 1 m × 1 m). e, AFM phase image of the SBAS film (the bright islands indicate polystyrene nanodomains with high stiffness). f, The schematic shows that owing to the thermodynamic incompatibility of different blocks, the physically crosslinked SBAS elastomer has a sea–island microstructure via microphase separation of polystyrene blocks (blue) and poly(butyl acrylate) (orange). g, GPC curves of molecular weight distribution indicate the block-by-block chain extension process (black, then red, and finally the blue curve) during the synthesis of the triblock coploymer SBAS. PSt, polystyrene; PnBA, poly(n-butyl acrylate). h, Dynamic mechanical analysis of VHB (red) and SBAS (black). The glass transition temperature of the material can be determined from the tipping points on the curve of storage modulus versus temperature. SBAS material has a lower Tg (−17.2 °C) than VHB (0.3 °C). The blue dashed line indicates the temperature close to the Mariana Trench (2.7 °C). At this temperature, the VHB is close to its glass transition and has higher storage modulus than SBAS, resulting in its reduced actuation in low temperature and high pressure (Fig. 3d).

Extended Data Fig. 7 Free swimming of the soft robot in a pressure chamber.

a, b, A high-pressure hydraulic pump (a) pumps water into a pressure chamber (b) with inner diameter 80 cm and height 2 m, to simulate the 110-MPa condition in the deep sea. c, d, The soft robot was first magnetically attracted by a electromagnet (c) and located at the left side of the pressure chamber (d). The electromagnet with electronic timer was encapsulated in a soft silicone matrix with the same decentralized design for pressure resilience. It can robustly operate in 110 MPa pressure to attract and release the soft robot. e, The experimental time-controlled sequence for the soft robot and the electromagnet. When the soft robot was released by the electromagnet, it was then actuated by a high-voltage signal (dashed blue line shows releasing point). f, After being released, the soft robot free-swam in the pressure chamber with an actuating voltage of 7 kV at 1 Hz.

Extended Data Fig. 8 Field exploration by the soft robot in a deep lake.

a, The soft robot was gripped by an ROV to be lowered and then released in a deep lake. Multiple tests were conducted at depths of 8 m and 70 m. b, Free-swimming robot (side view) after release from the ROV.

Extended Data Table 1 Material properties for FEA
Full size table
Extended Data Table 2 Molecular information of triblock copolymer SBAS
Full size table

Supplementary information

Video 1

: Flapping actuation of soft robot in the Mariana Trench at the depth of 10,900 m. This video shows deep sea field test of soft robot in the Mariana Trench. Mounted on a deep sea lander, the soft robot reached the bottom of Mariana Trench. At the depth of 10,900m in the Mariana Trench, the DE driven soft robot kept flapping its fins. The front view and side view of the soft robot were recorded by the deep sea cameras and LED lights in anti-pressure shells.

Video 2

: Free swimming of soft robot in deep sea at the depth of 3,224 m. This video shows the deep-sea free swimming of soft robot in the South China Sea. The soft robot was grasped by a robotic arm on ‘HAIMA’ ROV and reached the bottom of the South China Sea (depth of 3,224 m). After the releasing, the soft robot was actuated with an on-board AC voltage of 8 kV at 1 Hz and demonstrated free swimming locomotion with its flapping fins. The front view and side view of swimming process were recorded by the cameras and LED lights on the ROV. This video shows the potential of soft robots in deep-sea exploration.

Video 3

: FEA simulation of soft robot flapping actuation under the pressure of 110 MPa. This video shows the actuating mechanism of soft robot using FEA software ABAQUS 6.13. The resting state and actuating state under a hydrostatic pressure of 110 MPa are shown in the video. When a voltage is applied on the membrane, the bending angle of the wings decreases. Actuated with a cyclic voltage, the wing flaps periodically.

Video 4

: Soft robot swimming in circle in pressure chamber. This video shows the swimming experiment in circle in a pressure chamber. The soft robot was attached to a passive, rotatable and low-friction rod. The hydrostatic pressures were 0 MPa and 110 MPa. Actuated with on-board AC voltages of 9 kV, 8 kV and 7 kV at 2 Hz, the soft robot flapped its wings and swam in circles. The pressure responsible foam (green dyed) was compressed from a diameter of 6 cm (0 MPa) to 1.4 cm (110 MPa).

Video 5

: The free swimming tests of soft robot. This video shows free swimming experiment in of the soft robot under hydrostatic pressures of 0 MPa (in a pool) and 110 MPa (in a chamber). The soft was attached on a time-controlled electromagnet and released. After the releasing, the soft robot was actuated with an on-board AC voltage of 7 kV at 1Hz and demonstrated a free swimming locomotion.

Video 6

: Soft robot free swimming in deep lake. This video shows field exploration of the robot in a deep lake (depth ~70 m). The soft robot was carried by a gripper on ‘Blue ROV’ and released at depth of 8 m and 70 m. After the releasing, the soft robot was actuated with an on-board AC voltage of 8 kV at 1Hz. The free swimming of soft robot was recorded by the internal camera and lights on the ROV, which further confirms the robustness of the robot in field exploration.

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Li, G., Chen, X., Zhou, F. et al. Self-powered soft robot in the Mariana Trench. Nature 591, 66–71 (2021). https://doi.org/10.1038/s41586-020-03153-z

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