The scale matters: A review on stimuli-responsive microrobots categorized by scale for biomedical applications

Microrobots based on responsive materials have pioneered a new paradigm for disease treatment. However, the field currently lacks clear scale-dependent design principles, often conflating robots ranging from micrometers to centimeters in size. This review addresses this conceptual ambiguity by proposing a well-defined multi-scale classification based on the characteristic dimensions of microrobots—millimeter scale, sub-millimeter scale, micrometer scale, and nanometer scale. Starting from responsive materials, we systematically analyze how each scale dictates fundamental choices in design feature, manufacturing technology, driving mechanism, and control strategy, which are inherently governed by scale-dominated physical principles. Furthermore, the application of scale-determined operation modes of microrobots across different stages of medical intervention is explored, clarifying how size empowers each group with unique functionalities. This review clarifies the concept of scale and examines the literature on robots developed between 1 μm and 1 cm, excluding those at the nanoscale.