Submillimeter‑Scale Untethered Magnetic Actuators Enabling on‑Demand Manipulation and In Situ Viscosity Sensing

Untethered magnetic actuators offer a promising platform for minimally invasive surgery due to their compact size, mechanical compliance, multifunctionality, and remote controllability. However, the structural simplicity imposed by current construction strategies limits their operational versatility to leverage established surgical paradigms, and systematic studies of locomotion across the variable viscosities of bodily fluids remain scarce. Herein, we introduce an interference-fit assembly strategy that couples stereolithographic 3D-printed microstructures with customized magnetic pixels to fabricate submillimeter-scale untethered magnetic actuators. Theoretical and experimental analyses in model-fluid environments spanning physiologically relevant viscosity ranges demonstrate that, once above the critical magnetic flux density required for stable motion, the locomotion velocity scales linearly with the driving frequency. The velocity-frequency slope exhibits a hyperbolic dependence on ambient viscosity, enabling semi-quantitative viscosity sensing concurrently with motion and functional execution. Furthermore, we showcase multifunctional on-demand manipulation capabilities: enhanced diffusion and directional transport with qualitative viscosity sensing; in situ quantitative viscosity measurement inspired by a rotational viscometer; mechanical fragmentation via continuous rotation; and a rigid-soft synergistic magnetic gripper for targeted cargo capture and delivery. Collectively, this strategy establishes a versatile and scalable paradigm for designing multifunctional, untethered, submillimeter‑scale magnetic actuators with prospective potential for biomedical applications.