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Electrolytic vascular systems for energy-dense robots


Modern robots lack the multifunctional interconnected systems found in living organisms and are consequently unable to reproduce their efficiency and autonomy. Energy-storage systems are among the most crucial limitations to robot autonomy, but their size, weight, material and design constraints can be re-examined in the context of multifunctional, bio-inspired applications. Here we present a synthetic energy-dense circulatory system embedded in an untethered, aquatic soft robot. Modelled after redox flow batteries, this synthetic vascular system combines the functions of hydraulic force transmission, actuation and energy storage into a single integrated design that geometrically increases the energy density of the robot to enable operation for long durations (up to 36 hours). The fabrication techniques and flexible materials used in its construction enable the vascular system to be created with complex form factors that continuously deform with the robot’s movement. This use of electrochemical energy storage in hydraulic fluids could facilitate increased energy density, autonomy, efficiency and multifunctionality in future robot designs.

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Code availability

The Arduino code used to control the pumps within the robot is available from the corresponding author upon reasonable request.

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The datasets generated and analysed during this study are available from the corresponding author upon reasonable request.


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This work was supported by the US Office of Naval Research, grant number N00014-17-1-2837.

Reviewer information

Nature thanks Herbert Shea and Wei Wang for their contribution to the peer review of this work.

Author information

R.F.S. and J.H.P. conceived the idea of the study. C.A.A., S.C., J.H.P. and R.J. planned and carried out experiments. L.A.A., J.H.P. and R.F.S. supervised the project. All authors discussed the results and contributed to the final manuscript.

Correspondence to Robert F. Shepherd.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Replica moulding of robot parts.

Left, one half of the robot’s silicone is cast in a 3D-printed mould. Right, the finished part. A two-part mould is used in the fabrication process to generate the pleats and fluidic channels of the final robot.

Extended Data Fig. 2 The effects of design alternatives on the energy density of the synthetic vascular system.

a, Figure 4f is reproduced here for comparative purposes, showing the impact of the volume fraction and energy density of the hydraulic fluid on the overall increase in system energy density. b, The effects of ZnI2 catholyte concentration on the overall increase in system energy density for our robot. The estimated energy density values at certain concentrations (solid red circles) are shown11.

Extended Data Fig. 3 Cell capacity and cycling efficiency data for the pelvic fin cell.

A pelvic fin cell, filled with 5 ml of ZnI2 catholyte, was subjected to n = 19 charging and discharging cycles to examine how cell capacity, coulombic efficiency and battery longevity are affected by reuse and recharging of the battery cells.

Extended Data Fig. 4 Buckling test for fluid-filled battery actuator cell.

Water, serving as a substitute for the ZnI2 catholyte solution, was injected into the battery cell composite depicted in Fig. 3 to examine how inclusion of incompressible fluids affected the bending stiffness of the flow battery cells during buckling testing (n = 8). The maximum buckling force and the bending stiffness increased by approximately 10% compared to the cell tested without fluid. Error bars show 1σ uncertainties.

Extended Data Table 1 Bending stiffness of the battery cell testing blank

Supplementary information

Video 1

Tail fin actuation. Tail fin actuation is demonstrated while the robot is submerged in a salt water tank. This actuation is initiated by the reversible pumping of ZnI2 catholyte solution from one side of the tail to the other.

Video 2

Pectoral fin actuation. Pectoral fin actuation is demonstrated while the robot is submerged in a salt water tank. This actuation is initiated when catholyte solution contained in the left and right dorsal fins is pumped into the corresponding pectoral fin, thereby inflating them and pushing them outward from the body.

Video 3

Underwater locomotion. The robot swims in a salt water tank via tail fin actuation. The robot moves at a rate of 1.56 body lengths per minute against a current.

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Fig. 1: A lionfish-inspired robot powered by a multifunctional zinc iodide RFB.
Fig. 2: Assembly of the fin cell and actuation.
Fig. 3: Testing the bending stiffness of the battery cells.
Fig. 4: Characterization of the pelvic fin battery cells.
Fig. 5: Synthetic vascular system schematic and swimming demonstration.
Extended Data Fig. 1: Replica moulding of robot parts.
Extended Data Fig. 2: The effects of design alternatives on the energy density of the synthetic vascular system.
Extended Data Fig. 3: Cell capacity and cycling efficiency data for the pelvic fin cell.
Extended Data Fig. 4: Buckling test for fluid-filled battery actuator cell.


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