We use electrospinning to fabricate sperm-shaped magnetic microrobots with a range of diameters from 50 µm to 500 µm. The variables of the electrospinning operation (voltage, concentration of the solution, dynamic viscosity, and distance between the syringe needle and collector) to achieve beading effect are determined. This beading effect allows us to fabricate microrobots with similar morphology to that of sperm cells. The bead and the ultra-fine fiber resemble the morphology of the head and tail of the sperm cell, respectively. We incorporate iron oxide nanoparticles to the head of the sperm-shaped microrobot to provide a magnetic dipole moment. This dipole enables directional control under the influence of external magnetic fields. We also apply weak (less than 2 mT) oscillating magnetic fields to exert a magnetic torque on the magnetic head, and generate planar flagellar waves and flagellated swim. The average speed of the sperm-shaped microrobot is calculated to be 0.5 body lengths per second and 1 body lengths per second at frequencies of 5 Hz and 10 Hz, respectively. We also develop a model of the microrobot using elastohydrodynamics approach and Timoshenko-Rayleigh beam theory, and find good agreement with the experimental results.

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This work addresses the magnetic-based control of a helical robot and the mitigation of the magnetic forces on its dipole moment during radial steering using rotating permanent magnets. A magnetic system with two synchronized permanent magnets that rotate quasistatically is used to move the helical robot (length and diameter of 12.5 mm and 4 mm, respectively). We experimentally demonstrate that using two synchronized permanent magnets for radial steering of a helical robot achieves higher motion stability, as opposed to propulsion using single rotating dipole field. The two synchronized dipole fields decrease the lateral oscillation (average peak-to-peak amplitude) of the helical robot by 37%, compared to the radial steering using a single dipole field at angular velocity of 31 rad/s. We also show that driving the helical robot using two synchronized rotating magnets achieves average swimming speed of 2.1 mm/s, whereas the single rotating dipole field achieves average swimming speed of 0.4 mm/s at angular velocity of 31 rad/s for the rotating permanent magnets. The proposed configuration of the helical propulsion allows us to decrease the magnetic forces that could cause tissue damage or potential trauma for in vivo applications.

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