Abstract

Designing millirobots capable of navigating high-friction environments remains a significant challenge due to limitations in force output and the absence of efficient transmission mechanisms at small scales. In this study, we introduce a magnetically inner actuated millirobot capable of generating a thrust force exceeding 15 N with a body weight of 5.82 g for moving across diverse frictional terrains. The inner-actuated millirobot features a dual-coil array positioned at each end of a plastic skeleton and a permanent magnet accommodated in the center channel of the body. When powered by a 0.5 A current, the internal magnetic interaction propels the magnet to a velocity of 2.10 m/s within 17 ms, striking the end wall to produce a powerful instantaneous thrust that overcomes friction forces. Experimental results demonstrate the millirobot’s ability to operate in viscous oil, traverse sand and granular media, and transport cargo exceeding 300 times its body weight. Furthermore, the magnetically inner actuated millirobot shows promising potential for accessing confined tubular environments. This magnetically inner-actuated design, leveraging momentum conservation for propulsion, enables millirobots with high force capacity for high-friction and confined-space applications.

Data availability

The original experimental data generated in this study have been deposited in the Figshare database under accession code: 10.6084/m9.figshare.30598439.

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Acknowledgements

This work is financially supported by the Research Grant Council (RGC) of Hong Kong under Grants 11212321, 11217922, and 21212720; and in part by the Science, Technology and Innovation Committee of Shenzhen under Grant SGDX20210823104001011, and the Start-up Research Fund of Central South University under Project No. 502044037.

Author information

Authors and Affiliations

Department of Data and Systems Engineering, The University of Hong Kong, Hong Kong SAR, China

Min Wang & Jun Liu 1.

College of Mechanical and Electrical Engineering, Central South University, Changsha, China

Min Wang 1.

Department of Mechanical Engineering, City University of Hong Kong, Hong Kong SAR, China

Min Wang, Wenlong Wu, Zeju Zheng, Wei Dai, Tianyi Wu, Rui Liu, Steven Wang & Jun Liu 1.

Academy of Electronic Science and Technology of CETC, Beijing, China

Wenlong Wu 1.

Thrust of Sustainable Energy and Environment, Hong Kong University of Science and Technology (Guangzhou), Guangzhou, China

Zeju Zheng 1.

Department of Biomedical Engineering, City University of Hong Kong, Hong Kong SAR, China

Yuxuan Xiang & Jiachen Zhang 1.

Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong SAR, China

Zuankai Wang

Authors

1. Min Wang
2. Wenlong Wu
3. Zeju Zheng
4. Wei Dai
5. Tianyi Wu
6. Rui Liu
7. Yuxuan Xiang
8. Steven Wang
9. Jiachen Zhang
10. Zuankai Wang
11. Jun Liu

Contributions

M.W. and J.L. conceived and designed the study. W.W. and Z.Z. assisted with robot fabrication and figure preparation. M.W. and J.L. conducted the experiments and analyzed the data. M.W., Y.X., and J.Z. developed the kinematic modeling. W.W., T.W., R.L., W.D., and S.W. assisted with the experiments and data analysis. M.W. wrote the manuscript with input from all authors. J.L. provided constant feedback and suggestions throughout all experiments. J.L., Z.W., and J.Z. supervised the study and edited the manuscript. All authors discussed the results and reviewed the manuscript.

Corresponding authors

Correspondence to Jiachen Zhang, Zuankai Wang or Jun Liu.

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Nature Communications thanks Islam Khalil, Louis William Rogowski and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

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Wang, M., Wu, W., Zheng, Z. et al. Magnetically actuated momentum-driven millirobots. Nat Commun (2025). https://doi.org/10.1038/s41467-025-67936-6

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Received: 11 September 2024

Accepted: 12 December 2025

Published: 27 December 2025

DOI: https://doi.org/10.1038/s41467-025-67936-6