Untethered magnetic actuators hold great promise for minimally invasive medicine, with potential applications in targeted drug delivery, remote surgery, and in vivo diagnostics. However, enabling these miniature robots to operate effectively in complex biological environments still faces a central dilemma: they must combine multimodal locomotion, morphological reconfiguration, and task-specific manipulation with simple, reliable, and predictable control. Conventional rigid magnetic actuators offer strong mechanical robustness and predictable motion under simple magnetic fields, but their fixed geometries and magnetization profiles limit functional versatility. Soft magnetic actuators, in contrast, can achieve richer deformation and reconfiguration through continuous body shape changes, but often suffer from reduced load capacity, excessive control complexity, and compromised operational stability. "Although recent hybrid designs based on multimaterial composites or stimuli-responsive materials have attempted to bridge this gap, they still face challenges such as slow response, complex fabrication, difficult miniaturization, and limited control precision." said the author Zhixian Chen, a researcher at Tsinghua University, "Therefore, how to retain the reliable control and mechanical robustness of rigid systems while introducing soft-robot-like reconfigurability has become a key challenge for designing next-generation multifunctional magnetic medical robots."
This study proposed a modular untethered magnetic actuator design framework based on revolute joints, connecting discrete rigid magnetic modules with nonmagnetic structural modules to preserve the mechanical robustness and predictable motion of rigid systems while introducing additional degrees of freedom for on-demand reconfiguration. The researchers first fabricated magnetic composites from NdFeB magnetic microparticles and PDMS, processed them into magnetic modules of different sizes, and magnetized them along radial or normal directions according to design requirements. Nonmagnetic functional structures were fabricated by high-resolution 3D printing and assembled with magnetic modules through interference fitting. Based on this framework, the team developed 4 articulated actuator prototypes, including the Magnetic Tweezer, Magnetic Mantis, Magnetic Pelican, and Magnetic Clip, enabling rolling, crawling, grasping, release, storage, and stirring under a single uniform rotating magnetic field. The researchers then validated the mechanism's stability and cross-scale feasibility through joint dynamic characterization, Euler–Lagrange mathematical modeling, and dimensionless scaling analysis. Finally, they tested navigation, cargo transport, targeted release, and local mixing in soft tubes, maze-like environments, vascular phantoms, and an ex vivo porcine stomach model to evaluate the platform's practical potential in complex biological environments.
The results showed that the revolute-joint-based modular design effectively overcomes the traditional trade-off between functional versatility and simple control in magnetic actuators. Joint dynamic experiments and mathematical modeling demonstrated that the articulated structure could generate stable and predictable opening, closing, and reconfiguration motions under a uniform rotating magnetic field. Dimensionless scaling analysis further confirmed that the system can remain controllable as its size decreases, indicating strong cross-scale potential. In functional tests, the Magnetic Tweezer automatically switched between rolling and crawling in soft tubes with different diameters and performed grasping, release, and modular tool-based operations. The Magnetic Mantis stably transported solid cargo and achieved rapid release through magnetic field switching. The Magnetic Pelican stored liquid payloads, released them at target sites, and enhanced mixing through in situ rotation. The Magnetic Clip demonstrated more complex programmable multistage operations, including rapid dual-payload release, sequential delivery, maze navigation, stepwise solid and liquid cargo release, and active stirring. Mechanical fatigue testing showed that the Magnetic Clip maintained normal opening and closing after 15,000 cycles. More importantly, the platform also performed well in complex biological environments: the Magnetic Clip successfully navigated across the mucus-covered, folded, and obstacle-rich surface of an ex vivo porcine stomach and completed targeted cargo delivery, indicating the potential of articulated untethered magnetic actuators for high-precision navigation and multifunctional operations in complex in vivo environments.
The main significance of this work lies in proposing an untethered magnetic actuator design that combines the reliable control of rigid robots with the reconfigurability of soft robots. By introducing revolute joints and modular assembly, the study transforms otherwise function-limited rigid magnetic modules into articulated systems capable of on-demand deformation, locomotion mode switching, and multistage task execution, while still being driven by a single uniform magnetic field. This substantially reduces control complexity. This structure-enabled functionality allows the actuators to perform navigation, grasping, release, storage, and stirring across different scales, and to achieve targeted delivery in a complex ex vivo biological environment, demonstrating potential for targeted drug delivery, minimally invasive surgery, and in vivo diagnostics. More importantly, the framework is modular and scalable, allowing task-specific actuators to be rapidly assembled from common components. "In the future, we will further verify its safety, biocompatibility, long-term stability, and precise positioning ability in real in vivo environments, and combine real-time imaging and closed-loop control systems to promote its transition from experimental verification to practical medical applications." said Zhixian Chen.
Authors of the paper include Zhixian Chen, Xiaoyu Zhao, Ying Liu, and Shengli Mi.
The authors acknowledge that they received no funding in support for this research.
The paper, "Articulated Untethered Magnetic Actuators for Multimodal and Cross-Scale Operations" was published in the journal Cyborg and Bionic Systems on May 19, 2026, at https://doi.org/10.34133/cbsystems.0560
