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Volume 13
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This is an early access version, the complete PDF, HTML, and XML versions will be available soon.
Article

Elastic-Snapping–Driven Butterfly Stroke: A Soft Robotic Fish

by
Lin Tian
1,
Ruo-Pu Chen
1,
Yu Zhao
1,
Zhi-Peng Wang
1,
Jiao Jia
2,
Weifeng Yuan
3,
Xi-Qiao Feng
4,5 and
Zi-Long Zhao
1,*
1
Institute of Solid Mechanics, School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China
2
School of Aviation, Beihang University, Beijing 100191, China
3
Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, School of Manufacturing Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China
4
Institute of Biomechanics and Medical Engineering, Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
5
Mechano-X Institute, Tsinghua University, Beijing 100084, China
*
Author to whom correspondence should be addressed.
Machines 2025, 13(12), 1078; https://doi.org/10.3390/machines13121078
Submission received: 31 October 2025 / Revised: 18 November 2025 / Accepted: 21 November 2025 / Published: 24 November 2025
(This article belongs to the Special Issue Innovations in Soft Robotics: Enhancing Safety, Performance, and Dexterity)
Versions Notes

Abstract

The locomotion of fish provides inspiration for designing efficient and agile underwater robots. Potamotrygon motoro propels itself by generating traveling waves along its pectoral fins. Inspired by its graceful swimming stroke, we design and fabricate a robotic fish, where the snap-through instability of elastic curved rods is exploited to produce the undulatory fin motion. In this design, the rotary input of two motors is transformed smoothly and continuously to controllable wave-like fin deformation. By changing the initial fin shape, motor speed, and friction at the releasing end, the propulsion performance and the maneuverability of the robotic fish can be significantly improved. The physical prototype of the robotic fish is fabricated, and its swimming performance is measured. Its maximum swimming speed reaches 0.76 BL/s, and it can achieve small-radius turns with a maximum angular speed of 1.25 rad/s. In contrast to the multi-actuator systems, the proposed dual-motor, elastic-snapping–driven design is featured by simple structural construction, low energy consumption, excellent maneuverability, and superb adaptation to environments. Our robotic fish holds promising applications in such areas as environmental monitoring, underwater inspection, and ocean exploration. The propulsion strategy presented in this work may pave a new way for the design of shape-morphing robots as well as other soft machines at multiple length scales.
Keywords: elastic instability; snap-through; pectoral fin propulsion; robotic fish elastic instability; snap-through; pectoral fin propulsion; robotic fish

Share and Cite

MDPI and ACS Style

Tian, L.; Chen, R.-P.; Zhao, Y.; Wang, Z.-P.; Jia, J.; Yuan, W.; Feng, X.-Q.; Zhao, Z.-L. Elastic-Snapping–Driven Butterfly Stroke: A Soft Robotic Fish. Machines 2025, 13, 1078. https://doi.org/10.3390/machines13121078

AMA Style

Tian L, Chen R-P, Zhao Y, Wang Z-P, Jia J, Yuan W, Feng X-Q, Zhao Z-L. Elastic-Snapping–Driven Butterfly Stroke: A Soft Robotic Fish. Machines. 2025; 13(12):1078. https://doi.org/10.3390/machines13121078

Chicago/Turabian Style

Tian, Lin, Ruo-Pu Chen, Yu Zhao, Zhi-Peng Wang, Jiao Jia, Weifeng Yuan, Xi-Qiao Feng, and Zi-Long Zhao. 2025. "Elastic-Snapping–Driven Butterfly Stroke: A Soft Robotic Fish" Machines 13, no. 12: 1078. https://doi.org/10.3390/machines13121078

APA Style

Tian, L., Chen, R.-P., Zhao, Y., Wang, Z.-P., Jia, J., Yuan, W., Feng, X.-Q., & Zhao, Z.-L. (2025). Elastic-Snapping–Driven Butterfly Stroke: A Soft Robotic Fish. Machines, 13(12), 1078. https://doi.org/10.3390/machines13121078

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