Application Cases

Experiment Name: Verification of Exceptional Environmental Adaptability to Narrow Unstructured Liquid Environments and Outstanding 3D Controllability Under Magnetic Field Excitation

Research Direction:In clinical practice, natural orifices often provide medical instruments with invasive access to various target tissues. These body cavities (e.g., the urinary and digestive systems) are typically filled with liquid and facilitate the transport of substances within the body or between the body and the outside environment. Compared to traditional open or minimally invasive surgeries, biocompatible magnetically driven millirobots can transport various therapeutic agents and safely traverse natural orifices in a non-invasive manner to perform medical tasks such as targeted drug delivery and tissue diagnosis. However, executing medical tasks safely, reliably, and efficiently in such narrow, liquid-filled environments poses strict requirements on the robot's locomotion performance.

Carangiform fish can swim at high speeds and with great agility in complex natural waters. By mimicking their morphology and kinematic features, researchers have designed an untethered carangiform-like magnetic milliswimmer capable of agile 3D gravity-defying navigation in the narrow fluid environments found within the body. The body curvature distribution generated by this milliswimmer is very similar to that of fish, using magnetic torque to drive its body and mimicking the principle by which fish generate torque through muscle contraction. This biomimetic design endows the milliswimmer with carangiform-like swimming behavior and outstanding swimming performance. It is equipped with strong thrust, enabling it to swim freely underwater in 3D against gravity without relying on auxiliary buoyancy structures and to swim smoothly upstream in narrow, curved, and variable-diameter tubular environments. Additionally, this design provides the milliswimmer with high swimming speed and excellent equivalent locomotive performance, with a speed U up to 20BLs−1 (similar to fish’s ηU83.49BLmg−1MT−1s−1), which is more than six times higher than previously reported values. Moreover, this design endows the milliswimmer with excellent cruising maneuverability, allowing it to easily avoid obstacles (specifically, the milliswimmer has a minimum cruising turning radius r of 0.05BL and a maximum cruising turning speed ω of up to 4737°s−1 when the external driving magnetic field strength remains constant). Importantly, this milliswimmer has strong environmental adaptability. Due to its high degree of freedom, it can flexibly adjust its swimming posture to adapt to liquid-filled, unstructured, narrow environments. Like most fish, its negative buoyancy enables it to adapt to various liquid environments with densities higher than water. Under joystick control and automatic visual navigation, the milliswimmer exhibits excellent controllability, capable of hovering at designated positions and navigating arbitrary 3D trajectories while overcoming gravity. Finally, by combining common medical imaging techniques, the potential clinical application of the milliswimmer in ex vivo porcine urinary systems has been demonstrated.

Experiment Objective:To determine whether precise control of the magnetic field’s strength and direction can achieve precise control of liquid in three-dimensional space, providing arguments and groundwork for subsequent experiments.

Testing Equipment:ATA-6223 Power Amplifier, CCD, Data Acquisition Card, PC, Magnetometer, Magnetic Field Generating Device, Micro-Injection Pump

Experiment Process:To mimic the kinematic features of fish, the milliswimmer is driven to generate strong thrust in water, enabling it to swim freely in three dimensions against gravity and external resistance. This is mainly because of its larger workspace, which is suitable for ex vivo experiments and other experiments requiring such capacity. However, the high inductance of the larger workspace system necessitates the use of a resonant circuit to generate sufficient high-frequency driving current. The static and dynamic magnetic fields generated by the system are calibrated using a magnetometer (CH-3600). The system primarily consists of a magnetic field generating device, three power amplifiers (ATA-6223), a data acquisition card, a PC, three charge-coupled devices (CCD) cameras, and three lenses. The magnetic system can generate a magnetic field in any direction in space. To monitor the flow field generated by the milliswimmer during swimming, PIV technology was employed. Neutrally buoyant nylon seed particles were uniformly dispersed in the experimental water environment and illuminated with a 532nm laser. The motion of the particles was recorded from the side or top using a high-speed CCD camera and analyzed using openpiv157. A micro-injection pump was used to provide fluid input to the curved glass tube, and by controlling the horizontal propulsion speed of the injection pump, the average fluid velocity at the inlet of the curved glass tube could be adjusted.

Figure 1-1 Block Diagram of the Potential Application of Microparticles in Ex Vivo Porcine Urological System Organs

Experiment Results:It illustrates how the milli-swimmer, under constant magnetic field magnitude and driving frequency, successfully navigates obstacles and follows predefined trajectories, including “M,” “S,” and “R” shapes (MSR), using different swimming maneuvers controlled by a joystick. Initially, the milli-swimmer swims in mode 1, following the “M”-shaped trajectory (Figure 1-2b). When approaching the horizontally narrow gap obstacle 1 (length 2×width 0.3 mm, as shown in the inset of Figure 1-2c), the milli-swimmer transitions from mode 1 to mode 2 by adjusting the roll angle θ to 90°, successfully traversing the narrow horizontal gap. Subsequently, the swimmer continues swimming along the S-shaped trajectory in this mode (Figure 1-2c). When approaching the inclined 45° narrow gap obstacle 2 (length 2×width 0.3 mm, as shown in the inset of Figure 1-2d), the milli-swimmer adjusts the roll angle θ to 45°, successfully crossing the narrow gap obstacle 2. Afterward, by rotating the milli-swimmer at a roll frequency f of 1 Hz, it adopts mode 3 and moves forward along the R-shaped trajectory (Figure 1-2d). Finally, the milli-swimmer effortlessly transitions from mode 3 back to mode 1, easily avoiding the circular obstacle and navigating through the water. Throughout the process, the average manual operation error of the milli-swimmer is 0.45 mm (0.12BL).

Figure 1-2 Controllability and Environmental Adaptability of the Milli-Swimmer. a) Schematic illustration of the milli-swimmer smoothly following the predefined trajectory (MSR) and adjusting its swimming mode to traverse narrow gaps. b) “M”-shaped trajectory. c) Obstacle 1, characterized by a horizontally narrow gap (length 2 mm, width 0.3 mm, see inset) and S-shaped trajectory. d) Obstacle 2, an inclined 45° narrow gap (length 2 mm, width 0.3 mm, as shown) and an R-shaped trajectory. e) Under vision-based automatic control, the milli-swimmer uses three swimming maneuvers to track fish-shaped and MSR trajectories. The figure shows the statistical error distribution of the tracking. f) Under vision-based automatic control, the milli-swimmer can hover at designated points in space and on specified planes, exhibiting varying degrees of error in each of the three swimming postures. Error bars represent the standard deviation (N=3). g) Statistical error distribution of the milli-swimmer tracking a helical trajectory under vision-based automatic control. h) Behavior of the milli-swimmer freely swimming around a complex 3D network structure (coral-like) by switching between three swimming postures. i) Schematic illustration of the milli-swimmer swimming in a 3D network. ii) Video snapshots of the milli-swimmer continuously swimming within the 3D structure. Scale bar 2 mm.

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