The onboard-motor bottleneck of medical microrobots: what the bacterial flagellar motor teaches

Perspective article, not peer reviewed (v2). Medical microrobots are today mostly driven by external magnetic fields; an onboard rotary motor remains an open problem, although bacteria have one: the flagellar motor, about 50 nm wide, measured at up to 1,700 rev/s (about 102,000 rpm) in Vibrio alginolyticus. The article summarises three routes to power a microrobot (external field, biohybrid, onboard synthetic motor) and quantifies the flagellar motor from a published torque-speed curve: about 5.8 fW for the chimeric Na+ motor in E. coli, i.e. a power density with a median of 22 kW/L (90% of draws between 8 and 67 kW/L) depending on the assumed envelope, compared with 3.3-9.1 kW/L for commercial traction motors of 2017 and the 50 kW/L U.S. DOE 2025 target. It adds wild-type E. coli and Vibrio scenarios, per-stator-unit figures, a three-dimensional map of power density versus envelope size, a comparison with other machines (skeletal muscle, MEMS micromotors, a rocket turbopump under an assumed envelope), and the Stokes-drag power of a spherical microrobot: a motor of this order covers slow swimming at tens of um/s, but not fighting a 1 mm/s flow. The estimates are indicative and rest on explicit assumptions listed in the paper and the supplement (code, results, README). French version included. Prepared with the assistance of Claude (Anthropic).