Magnetic Surface Microrollers for Endovascular Navigation

Mobile microrobots offer great promise for minimally invasive targeted medical applications to overcome the current limitations in cargo delivery thanks to their precise controllability. The circulatory system represents the ideal route for the navigation and delivery of microrobots; however, blood flow impairs the propulsion of microrobots. Magnetic surface rolling microrobots, or “surface microrollers” emerged as an attractive microrobotic platform for navigation in blood vessels utilizing decreased flow velocities on the blood vessel walls. In this work, we assess the navigation potential of the surface microrollers in blood vessels for future drug delivery applications. First, we started developing the microrobotic platform from scratch. Then, we characterized their locomotion characteristics, which yielded the fastest magnetic microrobot in its size scale in the literature, corresponding to 80 body lengths per second. Then, its upstream locomotion performance in physiological blood flows was characterized in microfluidic chips; the results have shown significant potential for navigation in blood flows. Furthermore, the antibody-modified surface microrollers could also recognize the cancer cells during active locomotion and deliver chemotherapeutics to cancer cells, showing the platform's multi-functionality. Having confirmed the system's potential, we explored the hydrodynamic barriers for locomotion in physiological blood flows in detail. The hydrodynamic barriers can be classified under three main topics: 1) Surface microtopography effect, 2) Confinement effects, and 3) Flow rate/velocity effects. The surface microtopography of the vessel walls, in the same size scale of surface microrollers (3-8 μm), unexpectedly creates a significant barrier for locomotion. Even though the locomotion is high-speed and unimpeded on flat surfaces, the surface microtopography effect can entirely stop the locomotion of spherical microrollers due to unfavorable hydrodynamic interactions between microroller and surface microtopography, revealed in computational fluid dynamics (CFD) simulations. We have demonstrated that rod-shaped microrollers render minimal interaction with the neighboring boundaries, thus smoothly locomoted on such textured surfaces. The problem can also be solved by using bigger-sized spherical microrollers, and then the surface microtopography effect disappears. Next, we investigated the physical confinement effect on the surface microrollers, which would be encountered in small blood vessels such as capillaries and venules. Our experiments demonstrated that the locomotion efficiency of spherical microrollers drastically decreases in confined spaces, even causing reverse locomotion in circular confinements. The CFD analysis revealed that the impeded locomotion is due to out-of-plane rotational flows rather than translational flows generated during locomotion. Hence, a slender microroller design, generating smaller rotational flows, outperforms spherical microrollers in confined spaces. Last, we characterized different flow rate/velocity effects on surface microrollers. In a CFD model, we investigated the upstream locomotion performance of surface microrollers in main blood vessel types in the systemic circulation. The results have demonstrated that the microroller locomotion halted in the small vessels due to increased flow and confinement effects. In contrast, the locomotion seemed fully and partially possible in the venous and arterial flow. Having explored the limitations and possibilities, we have demonstrated the upstream locomotion of biocompatible and high-performance microrollers in venous flow under real-time imaging, confirming the previous findings. Overall, we comprehensively investigated the potential of magnetic surface microrollers in blood vessels for their future drug delivery applications.