Abstract
Magnetic microrobots are untethered mobile robots at micro- and sub-micro scales and have attracted considerable attention in recent years due to their remarkable potential applications in various biomedical applications. Driven by an external magnetic field, magnetic microrobots can navigate through small and complex biological regions to perform localized diagnosis, precise delivery of drugs/cells at predetermined destinations, or other tasks for precise treatment. Microrobots made with biocompatible and degradable materials disappear after their mission is completed. To overcome the limitation of individual microrobots, multiple microrobots form a microswarm, which has many advantages, such as improved adaptability to complex bio-environments and enhanced functionality and carrying capacity. In this study, magnetically powered microrobots and their driven methods are investigated. The study is performed from the following aspects.First, the design, modeling, and simulation of magnetically driven mechanisms for microrobots are investigated. Successful transformation of the input magnetic energy into microrobot motion is a key issue to enable the application of magnet-driven microrobots. A time-varying magnetic field, which means that the direction of the magnetic field changes over time, is utilized in this study. Magnetic energy can be produced by either a uniform magnetic field or a gradient magnetic field depending on the application. In a uniform magnetic field, the microrobot is rotated by magnetic torque when the magnetic dipole moment of the microrobot is not aligned with the external field. Propelling force is then generated due to the interaction between the microrobot and ambient liquid. This mechanism is used to drive an individual multi-segment microrobot in this study. In the magnetic gradient field, the microrobot is magnetized and dragged by gradient force, and the design of the microrobot is not constrained by its shape. When the magnetic gradient field is rotated, an equivalent centripetal force that combines magnetic and fluid-structure interactions is generated and can be used to converge the microrobot swarm toward the targeted site.
Second, a torque-driven magnetic microrobot with a multi-segment structure is proposed. This microrobot includes a magnetized head and several non-magnetized rigid body segments actuated by an external oscillating magnetic field. The components of the microrobot are linked together by rigid mechanical joints with an angle-limiting mechanism, thereby forming simplified and discrete wave locomotion in an environment with a low Reynolds (Re) number. The motion of this multi-segment structure with different segment numbers is analyzed, and the swimming locomotion involving the segment interaction of the microrobots and ambient liquid is characterized. Theoretical and experimental studies indicate that the number of segments is an important factor that affects the athletic capability of microrobots. At least three segments are needed to enable microrobots to move forward, and having four segments results in the best comprehensive performance. On the basis of this analysis, the geometric parameters of the four-segment microrobot are further optimized, and experiments verify the enhancement of its motion capacity in a regime with a low Re number. This study provides a unique design method of torque-driven multi-segment microrobots. It characterizes the driven mechanism and motion performance of microrobots with different segment numbers, based on which the ideal number of segments is recommended for the first time.
Lastly, a new magnetic driven mechanism that uses a rotating gradient-based magnetic field to drive a magnetic microrobot is presented. The rotation of the magnetic field can be realized by sequentially energizing each coil of the electromagnetic coil system. With this method, an equivalent centripetal force directed toward a specific target site is generated, and it can make the microrobot swarm converge to this target site from different directions without relying on real-time visual guidance. The location of the target site can be adjusted by changing the current inputs of magnetic coils. This study shows that automatically controlling microrobots toward the desired position without sufficient support from real-time imaging feedback is possible. Experiments are performed to demonstrate the effectiveness of the proposed approach. The successful development of this new actuation method is an important step toward the realization of microrobot delivery in real applications and opens up a new dimension in precision targeted therapy.
In summary, this study demonstrates that the magnetic actuation method and magnetically powered microrobots can be effectively utilized in diverse biomedical applications. The success of this study provides new solutions for magnetic microrobot design and potentially benefits the exploration of various biomedical applications in the future.
| Date of Award | 2 Mar 2022 |
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| Original language | English |
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| Supervisor | Dong SUN (Supervisor) |