Preparation and Motion Study of Magnetically Driven Micro Soft Robot Mimicking the Cownose Ray

In narrow, unstructured underwater environments such as environmental monitoring and minimally invasive medical procedures, micro soft robots exhibit unique advantages due to their flexible movement capabilities and small size. At the same time, applying bionic technology to the structural design of micro soft robots can significantly improve their swimming performance. However, limited by their miniaturization, these robots are difficult to power internally and usually adopt a wireless power supply method. This study designs and fabricates a magnetically responsive, cownose ray-inspired micro soft robot based on the swimming principle of the cownose ray. The robot is made of a certain proportion of NdFeB and PDMS. Then, a three-dimensional Helmholtz coil is used to generate an oscillating harmonic magnetic field to conduct swimming experiments on the robot, exploring the influence of magnetic field parameters on the robot’s swimming performance. The experimental results show that the swimming speed is the fastest at B = 5 mT and f = 11 Hz, reaching 5.25 mm/s, which is about 0.5 body lengths per second. In addition, by adjusting the current direction and frequency of the coil, the robot can perform different swimming modes such as straight swimming, turning swimming, and directional swimming. By employing a stepwise adjustment method, the impact of response errors on the robot’s trajectory can be effectively reduced. This study demonstrates a method for magnetically driven micro soft robots, laying a foundation for the application of wireless-driven robots in underwater narrow spaces.

💡 Research Summary

The paper presents a magnetically driven micro‑soft robot that mimics the swimming mechanism of the cownose ray, targeting applications in confined underwater environments such as environmental monitoring and minimally invasive medical procedures. Because internal power sources are impractical at the millimetre scale, the authors adopt a wireless magnetic actuation strategy. The robot is fabricated from a composite of neodymium‑iron‑boron (NdFeB) particles embedded in a polydimethylsiloxane (PDMS) matrix. After systematic testing of various NdFeB loadings, a 75 wt % composition is selected as it provides the highest saturation magnetization and sufficient remanence while preserving the flexibility required for large‑amplitude fin deformation.

The biomimetic design replicates the flat, wide body of a cownose ray using a NACA0018 airfoil profile (length = 11.34 mm, width = 20.56 mm, thickness = 1.5 mm). Two pectoral fins—front and rear—are shaped as near‑square plates (≈9.7 mm × 1 mm × 0.12 mm) to increase contact area with water and to enable the characteristic phase‑delayed up‑and‑down flapping observed in the biological counterpart. The fins and body are molded via a multi‑step process that includes 3‑D printed molds, ultrasonic dispersion of magnetic particles, laser cutting for fine features, and a final magnetization step using an external permanent magnet to align the NdFeB domains.

Actuation is achieved with a three‑dimensional Helmholtz coil system capable of generating a uniform oscillating magnetic field up to 8 mT. By feeding sinusoidal currents of varying frequency (5–15 Hz) and amplitude, the authors induce a torque on the magnetized composite that causes the fins to oscillate in a coordinated wave‑like motion. Systematic experiments reveal that the optimal swimming performance occurs at a magnetic flux density of 5 mT and a frequency of 11 Hz, yielding a forward speed of 5.25 mm s⁻¹—approximately half a body length per second. Below 2 mT the torque is insufficient for propulsion, while above 8 mT the excessive torque limits fin deformation, reducing efficiency.

Beyond straight swimming, the robot’s trajectory can be steered by altering the phase relationship between the three coil axes. Reversing the current direction produces backward motion; introducing a 90° phase shift generates turning maneuvers. The authors also propose a “stepwise adjustment” method in which the coil currents are calibrated iteratively to compensate for initial mismatches, reducing trajectory error from an average of 12 % to under 4 %.

The study situates its contribution within a broad literature of bio‑inspired soft robots, noting that previous cownose‑ray‑type prototypes relied on rigid linkages, onboard batteries, or complex multi‑stage mechanisms that limited flexibility and scalability. By contrast, the presented design achieves multi‑modal locomotion with a single, fully wireless magnetic drive, while maintaining a compact, fully soft architecture suitable for narrow, cluttered spaces.

Limitations include testing only in a quiescent water tank, which does not capture the hydrodynamic complexities of real underwater or intravascular environments. Long‑term durability of the NdFeB/PDMS composite under repeated high‑frequency actuation, as well as magnetic retention over extended periods, remain open questions. Future work is suggested to explore three‑dimensional maneuverability using more sophisticated coil configurations, integration of onboard sensors for closed‑loop control, and performance validation in turbulent or confined flow conditions.

Overall, the paper delivers a comprehensive workflow—from material selection and magnetization to robot fabrication, magnetic field generation, and performance evaluation—demonstrating that magnetically driven micro‑soft robots can achieve biologically inspired propulsion, precise steering, and adaptable swimming modes without any tethered power source. This advances the state of the art in wireless micro‑robotics and opens pathways for practical deployment in environmental and biomedical applications.