Magnetic untethered millirobots, which are actuated and controlled by remote magnetic fields, have been proposed for medical applications due to their ability to safely pass through tissues at long ranges. For example, magnetic resonance imaging (MRI) systems with a 3-7 T constant unidirectional magnetic field and 3D gradient coils have been used to actuate magnetic robots. Such magnetically constrained systems place limits on the degrees of freedom that can be actuated for untethered devices. This paper presents a design and actuation methodology for a magnetic millirobot that exhibits both position and orientation control in 2D under a magnetic field, dominated by a constant unidirectional magnetic field as found in MRI systems. Placing a spherical permanent magnet, which is free to rotate inside the millirobot and located away from the center of mass, allows the generation of net forces and torques with applied 3D magnetic field gradients. We model this system in a 3D planar case and experimentally demonstrate open-loop control of both position and orientation by the applied 2D field gradients. The actuation performance is characterized across the most important design variables, and we experimentally demonstrate that the proposed approach is feasible.

Untethered robots miniaturized to the length scale of millimeter and below attract growing attention for the prospect of transforming many aspects of health care and bioengineering. As the robot size goes down to the order of a single cell, previously inaccessible body sites would become available for high-resolution in situ and in vivo manipulations. This unprecedented direct access would enable an extensive range of minimally invasive medical operations. Here, we provide a comprehensive review of the current advances in biomedical untethered mobile milli/microrobots. We put a special emphasis on the potential impacts of biomedical microrobots in the near future. Finally, we discuss the existing challenges and emerging concepts associated with designing such a miniaturized robot for operation inside a biological environment for biomedical applications.

One major challenge for untethered micro-scale mobile robotics is the manipulation of external objects in the robot's three-dimensional (3D) work environment. Here, we present a method to use the capillary force at a solid-liquid-gas interface to reversibly attach objects to a mobile magnetic microrobot. This is accomplished by the addition of a cavity in the hydrophobic microrobot, in which an air bubble is captured when the microrobot is placed in a water environment. The extension of the air bubble from the cavity is adjusted dynamically by controlling the pressure of the workspace environment. A peak switching ratio between the maximum and minimum gripping forces of 14:1 is shown for controlled attachment/detachment experiments, which allows for reliable pick-and-place operation. This work introduces an analytical capillary adhesion model and demonstrates control of the bubble size for pick-and-place gripping. A proof-of-concept demonstration of 3D manipulation in a fluidic environment shows the potential of capillary gripping for future use in confined environments such as inside microfluidic devices for transportation or assembly of hydrophobic objects.

A major challenge for untethered micro-scale mobile robotics is the control of many agents in the same workspace for distributed operation. In this work, we present a new method to independently control multiple sub-mm microrobots in three dimensions (3D) using magnetic gradient based direct pulling as the 3D motion generation method. This is accomplished through the use of geometrically or magnetically distinct microrobots which assume different magnetization directions in a rotating magnetic field. Such diversity in design allows for different magnetic forces to be exerted on each, enabling path following with less than 370μm mean path deviation for a set of two microrobots of size 350μm and 1500μm. This addressability method could be used for the 3D control of a team of microrobots inside microfluidic channels or in the human body for localized therapy or diagnostics.