In targeted therapy, clusters of drug carriers (nanoparticles and microparticles) could be in contact with a surface such as the lumen of blood vessels and the interior of the gastrointestinal tract. We study the motion characteristics of clusters of microparticles when they slide on a surface under the influence of weak oscillating magnetic fields (less than 11 mT). The oscillating magnetic fields exert a magnetic torque on the microparticles and allow them to oscillate, and hence overcome the static friction and slide on a surface. We characterize the frequency response of clusters of microparticles by applying oscillating magnetic fields with a frequency range of 0 Hz to 55 Hz, in the presence of a constant magnetic field gradient (0.9 T/m). Clusters of 3 to 4 and 5 to 9 microparticles achieve maximum sliding speeds of 1100 µm/s and 1150 µm/s, at oscillating magnetic fields of 30 Hz. In addition, we experimentally demonstrate closed-loop motion control of the clusters with maximum position error of 20 µm. Furthermore, we show that the magnetic field gradient required to drive a cluster of microparticles (with 3 to 4 microparticles) decreases by 75% in the presence of oscillating magnetic fields from 5 Hz to 50 Hz.

We experimentally demonstrate that using oscillating weak magnetic fields a sperm-shaped microrobot (which we refer to as MagnetoSperm) can swim using flagellar propulsion and slide on a surface under water. The sperm morphology allows the MagnetoSperm to mimic the locomotion mechanism of a living sperm cell. The MagnetoSperm is designed and developed with a magnetic head and a flexible tail to provide a magnetic dipole moment and propulsion, respectively. The head oscillates under the influence of controlled oscillating weak magnetic fields (~5 mT). This oscillation generates a thrust force in the flexible tail, and hence allows the MagnetoSperm to overcome the drag and friction forces during swimming and sliding on a surface, respectively. Point-to-point open- and closed-loop control of the MagnetoSperm are accomplished using an electromagnetic system under microscopic guidance. This motion control is done in two cases, i.e., swimming in water and sliding on a surface. At oscillating magnetic field of 5 Hz and 45 Hz, the MagnetoSperm swims at an average swimming speed of 32 μm/s (0.1 body lengths per second) and 158 μm/s (0.5 body lengths per second), respectively. At the same frequencies, the MagnetoSperm slides on the bottom of a petri-dish at an average speed of 21 μm/s (0.07 body lengths per second) and 6 μm/s (0.02 body lengths per second), respectively.