Microrobots rarely have on-board power; energy and control signals are delivered remotely. The most common scheme uses magnetised structures (often helical) driven by a rotating or gradient magnetic field, which in the low Reynolds number regime (viscosity-dominated) converts rotation into forward motion. Alternative propulsion is catalytic (decomposition of a fuel such as hydrogen peroxide), phototactic (light), acoustic (ultrasound) and bio-hybrid (bacteria or cells acting as a motor). Navigation is performed in closed loop under medical imaging (MRI, ultrasound, fluorescence). Motion must be non-reciprocal to bypass the constraint of the scallop theorem.
Enables tasks in spaces inaccessible to macroscopic robots and tools โ inside blood vessels, tissues, body lumens or microchannels โ with single-cell-scale precision and without invasive surgery.
The sub-millimetre physical chassis, often helical, spherical or porous, fabricated from polymers, metals or hydrogels.
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Mechanism converting external energy into motion: magnetic (helical structures), catalytic, acoustic, phototactic or bio-hybrid.
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Apparatus generating the controlling field/energy: magnetic coils (Helmholtz/Maxwell), light sources or ultrasound transducers.
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Real-time tracking of the microrobot's position via MRI, ultrasound or fluorescence, enabling closed-loop control.
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The task payload carried: a drug, a gripper, a sensor or biopsy material.
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Biocompatible, often biodegradable materials limiting toxicity and allowing removal after the task.
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No room for batteries and motors forces external power delivery or energy harvesting from the environment.
Precise steering and tracking of a micrometre-scale object inside the body is difficult.
Materials and fuels may be toxic to tissues.
Reciprocal motion yields no net displacement (scallop theorem).
After completing the task the microrobot must be removed or safely degraded.
Richard Feynman's lecture "There's Plenty of Room at the Bottom" outlined the idea of machines and manipulation at the micro/nano scale.
E.M. Purcell described swimming in the viscosity-dominated regime and formulated the scallop theorem, setting the rules for microrobot propulsion.
An ETH Zurich team demonstrated helical microswimmers steered by a rotating magnetic field.
The review "Microrobots for Minimally Invasive Medicine" systematised propulsion, control methods and medical applications.
A UC San Diego team demonstrated nanomotors operating inside a living organism, bringing medical applications closer.
Computer-designed bio-hybrid microrobots built from frog cells, capable of locomotion and self-healing, were presented.
Size of the microrobot (from below 1 mm down to the micrometre scale), determining the physics of motion and feasible applications.
Choice of propulsion: magnetic, catalytic, acoustic, phototactic or bio-hybrid.
Remote-control modality: magnetic field, light, chemistry or ultrasound.
Selection of materials for biocompatibility and biodegradability.
Type and amount of carried payload (drug, sensor, gripper).
