High Frequency Ultrasound Imaging of Tumbling Magnetic Microrobots
The diminutive size of microrobots makes them advantageous for minimally invasive operations and precise, localized treatment. One such application is aiding in localized drug delivery for colorectal cancer as microrobots could offer reduced patient trauma, lower risk of side effects, and higher drug retention rates. In this study, we evaluate the abilities of a magnetic microrobot in a variety of conditions using a high frequency ultrasound system. Under the influence of an external rotating magnetic field, the microrobot tumbles end-over-end to propel itself forward. Cytotoxicity tests demonstrated the constituent materials of polydimethylsiloxane (PDMS) and SU-8 were nontoxic to murine fibroblasts. Then, we quantified robot locomotion in an ex vivo porcine colon, testing the materials, the tumbling orientation, and three magnet rotation frequencies. Significant differences were found between materials and tumbling orientation, revealing that SU-8 lengthwise microrobots were the fastest with an average velocity of 2.12±0.25mm/s at a frequency of 1Hz. With this finding, the next tests were completed at 1Hz frequency with SU-8 lengthwise microrobots. We used in vitro agarose gels to maneuver the robot through a variety of trajectories, tested the microrobots in situ and in vivo murine colons as well. Average velocities were calculated for all tests with the in vivo murine colon tests finding an average velocity of 2.07±0.05mm/s. Finally, the microrobots were coated with a fluorescein payload and were shown to release a payload over a one-hour time period. These findings suggest microrobots are promising for targeted drug delivery and other in vivo biomedical applications.