Structural Evolution of Biomedical Microrobots: From Rigid to Soft to Rigid-Soft Integrated Systems.

Owing to their small size, micron-scale accuracy, rapid response, and biocompatibility, microrobots are emerging as a promising tool for biomedical applications, especially in targeted drug delivery and minimally invasive microsurgery. The architectural configuration of microrobots governs their locomotion performance, environmental adaptability, and functional integration capacity. Although existing reviews have systematically organized this field by actuation strategies, material categories, or application scenarios, a comprehensive summary centered on the structural evolution paradigm remains conspicuously absent. This review aims to fill this gap by systematically tracing the evolutionary trajectory of microrobot structures from rigid architectures through soft configurations to rigid-soft integrated systems. First, the foundational principles underlying structural evolution were introduced, encompassing the connotation of structure, fluid dynamics constraints, and the drivers of structural innovation. Then, rigid microrobot architectures were systematically elucidated, including geometric asymmetric and surface asymmetric designs. Subsequently, soft microrobot structures, covering both predefined deformation structures and dynamically reconfigurable architectures, were discussed. Finally, the rigid-soft integrated systems reconciling compliance and performance through spatial heterogeneity or temporal stiffness modulation were systematically surveyed. This evolutionary trend reflects a transition from optimizing individual performance parameters toward achieving balanced functional synergy across distinct task phases. Based on the current research progress, this review also presents future research directions in data-driven structural optimization, reconfigurable architectures, and autonomous structural intelligence, offering strategic guidance for next-generation microrobot design.