In the design of microrobots, a helical geometry is pivotal to overcome the time-reversal constraints of the scallop theorem. The helical geometry enables the microrobots to propel themselves forward in viscous fluids with a corkscrew like motion when they are allowed to rotate. It is physically advantageous for microrobots to swim with near-zero angle of attack much like buoyant microorganisms, allowing high thrust for forward propulsion. This type of propulsion is not possible as the non-buoyant microrobot drifts downward due to gravity. Here, we analyze the stability problem of controlling magnetically driven helical microrobots to achieve bounded straight runs without drift in a low-Reynolds-number regime. We demonstrate periodic active suspension solutions, that facilitate helical propulsion with minimal angle of attack and zero drift. We theoretically predict unique control inputs, for a given helical microrobot geometry and magnetic composition (i.e., 62% Ni and 24% Au Wt%), which can be generated with rotating field and field-gradient pulling. Using microrobots fabricated of denser-than-water soft-magnetic body (4870 kg•m−3), we find that the microrobot is allowed to swim with near-zero angle of attack of 8.3° ±5.2° (mean ±s.d.), outperforming conventional gravity compensation methods.

Until now, the potential of untethered magnetic robots (UMRs), propelled by external time-periodic magnetic fields, has been hindered by the limitations of wireless manipulation systems or noninvasive imaging techniques combined. The need for simultaneous actuation and noninvasive localization imposes a strict constraint on both functionalities. This study addresses this challenge by substantiating the feasibility through experimental validation, showcasing the direct teleoperation of UMRs within a fluid-filled lumen. This teleoperation capability is facilitated by a scalable X-ray-guided robotic platform, extendable to match the dimensions required for in vivo applications, marking a noteworthy advancement. Our methodology is demonstrated by teleoperating a 12-mm-long screw-shaped UMR (5 mm in diameter) within a bifurcated lumen, filled with blood. This navigation is achieved using controlled rotating magnetic fields, guided by real-time X-ray Fluoroscopy images. Incorporating a two-degree-of-freedom control system, we demonstrate the operator’s capability to use X-ray Fluoroscopy images to keep the UMR coupled with the external field during wireless teleoperations, resulting in a success rate of 76.6% when moving along the intended pathways, with a mean absolute position error of 1.6 ± 2.1 mm.

This paper explores the application of X-ray-guided magnetic fields for the wireless control of untethered magnetic robots (UMRs) within cerebral vascular phantoms. With a focus on addressing challenges associated with strokes and brain aneurysms, the study aims to enhance neurosurgical procedures by improving precision and maneuverability. Experimental findings showcase the feasibility and effectiveness of this innovative approach in navigating UMRs, characterized by a screw-shaped body and a ferromagnetic core, through complex vascular structures. Cone-beam computed tomography is employed to determine the tomography and provide various reference trajectories for the UMR inside the cerebral vascular phantom. Our motion control experiments show that the X-ray-guided magnetic fields enable the UMR to move along any intended path with an average success rate of 89%, allowing the UMR to move between the left and right common carotid artery to the left and right internal and external carotid artery.

To date, all previous research in the wireless magnetic actuation of untethered helical devices has achieved motion stability using feedback control in vitro. However, feedback control systems are likely to be affected by the increased sensory uncertainty during in vivo trials. In this study we investigate the input-output boundedness of an interconnection between a helical device and a single rotating magnet actuator in low-Reynolds-number regime. Using the resistive-force theory, the interconnection is expressed in terms of all possible input-output pairs. Inputs representing the actuation frequency, pitch angle, lateral speed, and field strength are analyzed numerically and experimentally. We demonstrate input-output boundedness of the states of the helical device during circular and straight runs in open-loop, and we demonstrate bounded input-output propulsion without orienting the angle of attack (the often used input to swim horizontally without vertical drift) of the helical device to counteract gravity. Our results are important for a number of minimally invasive applications and tasks requiring improved control authority for stable runs of helical devices without drift due to gravity and without feedback control and restricted configuration imposed on the helical device's motion.

One approach to the wireless actuation and gravity compensation of untethered helical magnetic devices (UHMD) is through swimming with a non-zero angle of attack (AoA). This configuration allows us to counteract gravity, so that for a given desired path, we can move the UHMD controllably without drifting downward under its own weight. This study seeks to investigate the use a reduced-order model of the complex 6-degrees-of-freedom model of UHMDs in low Reynolds-number regime. A one-dimensional model representing the relative position of the UHMD with respect to an actuator rotating permanent magnet is used to predict a gap which yields bounded behavior of the open-loop system. Using geometric representation of the reduced-order model, the local bounded behavior of the UHMD with near-zero AoA is attributed to periodic active magnetic suspension, which dominates near-zero AoA. Our numerical results are verified experimentally and bounded behavior of the UHMD demonstrates the capability to swim with near-zero AoA (6.3° ± 2.2°) without drifting downward. With this actuation strategy, it is unlikely that the orientation of the UHMD will be needed during noninvasive localization, making the control system dependent on only its position with respect to a prescribed trajectory. This strategy will also provide a computational advantage in adjusting the gap between the UHMD and a robotically controlled rotating permanent magnet actuator.