Magnetically actuated momentum-driven millirobots.

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.