MRI-based navigation and propulsion of millirobots is a new and promising approach for minimally invasive therapies. The strong central field inside the scanner, however, precludes torque-based control. Consequently, prior propulsion techniques have been limited to gradient-based pulling through fluid-filled body lumens. This paper introduces a technique for generating large impulsive forces that can be used to penetrate tissue. The approach is based on navigating multiple robots to a desired location and using self-assembly to trigger the conversion of magnetic potential energy into sufficient kinetic energy to achieve penetration. The approach is illustrated through analytical modeling and experiments in a clinical MRI scanner.

Actuators that are powered, imaged, and controlled by magnetic resonance (MR) scanners could inexpensively provide wireless control of MR-guided robots. Similar to traditional electric motors, the MR scanner acts as the stator and generates propulsive torques on an actuator rotor containing one or more ferrous particles. Generating maximum motor torque while avoiding instabilities and slippage requires closed-loop control of the electromagnetic field gradients, i.e., commutation. Accurately estimating the position and velocity of the rotor is essential for high-speed control, which is a challenge due to the low refresh rate and high latency associated with MR signal acquisition. This paper proposes and demonstrates a method for closed-loop commutation based on interleaving pulse sequences for rotor imaging and rotor propulsion. This approach is shown to increase motor torque and velocity, eliminate rotor slip, and enable regulation of rotor angle. Experiments with a closed-loop MR imaging actuator produced a maximum force of 9.4 N.

Actuators that are powered, imaged, and controlled by Magnetic Resonance (MR) scanners offer the potential of inexpensively providing wireless control of MR-guided robots. Similar to traditional electric motors, the MR scanner acts as the stator and generates propulsive torques on an actuator rotor containing one or more ferrous particles. The MR scanner can control three orthogonal gradient fields. Prior work demonstrated control of a single actuator rotor. This paper proposes and demonstrates independent, simultaneous control of n rotors. The controller relies on inhomogeneity between rotors, such as ensuring no rotor axes are parallel. This paper provides easily-implemented velocity and position controllers with global asymptotic convergence, and optimization techniques for implementation. Code for simulations and control laws is available online.

Control of multiple magnetic particles inside the human body may have many potential applications, such as drug delivery in places where conventional procedures cannot reach. A natural candidate for powering as well as tracking these magnetic particles is the MRI scanner. Although it offers the means to control the particles, it also poses difficulties: the magnetic field is applied uniformly to the group, thus making the independent control of each particle a challenging issue, while the actuation and the tracking are interleaved which can result to delays in actuation and measurement. The closed-loop control of a group of millimeter-scale particles, immersed in fluid, driven by the MRI scanner is studied in this paper. More specifically, this problem is presented in a unified manner, handling such issues as delays, constraints, as well as disturbances, and results in a robustly stable controller. We also propose a condition that effectively answers when the system should be in tracking mode versus actuation mode. In addition to theoretical results, the capability of the proposed controller is illustrated through simulation results.

This paper reports the successful navigation of Magnetotactic Bacteria (MTB) towards regions located inside a solid tumor using a computer controlled set of magnetic coils. MTB uses two flagella bundles connected to rotary molecular motors as a propulsion system enabling them to reach swimming velocities of 300µm·s−1without external source of power. Acting like autonomous microrobots, they can be remotely controlled by an appropriate magnetic guidance system as their swimming direction is predominantly determined by the direction of the ambient magnetic field. In order to cope with the harsh environment of the solid tumor and to bypass the lack of knowledge of the internal vessels architecture forming the route to the tumor, fundamental MTB motion properties are taken into account in addition to their ability to swim along the magnetic field. The studies revealed the presence of these bacteria in the necrotic zone of a solid tumor. Preliminary results suggest that not only the magnetic guidance can help enhancing the uniform distribution of MTB inside the tumor for therapeutic or diagnostic purposes, but the experimental data showed that they could perform accurately and efficiently under computer control, many of the tasks previously envisioned for future synthetic microrobots of only 1 to 2 micrometers in diameter and designed to operate in the human microvascular network.

A real-time positioning and tracking technique for untethered devices or robots magnetically propelled by a clinical magnetic resonance imaging (MRI) system is described. The local magnetic field induced by the device, composed of a ferromagnetic material, is used as a signature to localize the device on three one-dimensional projections. A high-precision 3D circular-motion system was used to assess the precision and accuracy of this method. The integration of this technique inside propulsion and imaging MRI sequences was also achieved to demonstrate the feasibility of this tracking scheme in a closed-loop control scheme. Finally, in vivo tracking during automatic navigation of an untethered device in the carotid artery of a living animal is demonstrated.

Experimental data and proofs of concepts are used to show the feasibility of providing the basic components and functionalities required for the implementation of intelligent untethered 150 × 300µm bacterial microrobots capable of sophisticated collective tasks under computer supervision and coordination. More specifically, we show that it is possible to embed within such microrobots, photovoltaic cells supplying ∼4µW necessary to power an internal microelectronic circuit providing embedded intelligence with the capability to communicate commands and data wirelessly to an external computer. We also show that such data or commands transmitted wirelessly could be used to instruct an external computer to send a swarm of flagellated bacteria to move such microrobots towards a specific target based on various sensory information acquired with specific sensors embedded in each microrobots. Similar to chemotaxis used by several species of flagellated bacteria, the algorithms used to move such microrobots could be governed by a larger range of sensory means, leading to what we refer to here as sensotaxis-based hybrid microrobots. The possibility of transmitting a request to a central computer to send a swarm of flagellated magnetotactic bacteria to provide propulsion and steering in order to move accurately to desired locations would allow such microrobots to perform collective tasks. A simple example suggesting the possibility of implementing accurate collective tasks by such hybrid microrobots is demonstrated experimentally where a microstructure emulating a V-shaped microrobot is moved and rotated autonomously using a swarm of approximately 3000 flagellated bacteria towards another similar V-shaped microstructure to form the character ‘M’ as in Microrobot.

A novel magnetic resonance (MR)-tracking method specifically developed to locate the ferromagnetic core of an untethered microdevice, microrobot, or nanorobot for navigation or closed-loop control purpose is described. The tracking method relies on the application of radio-frequency (RF) excitation signals tuned to the equipotential magnetic curves generated by the magnetic signature of the object being tracked. Positive contrast projections are obtained with reference to the position of the magnetic source. A correlation function performed on only one k-space line for each of the three axes and corresponding to three projections, is necessary to obtain a 3-D location of the device. In this study, the effects of the sphere size and the RF frequency offset were investigated in order to find the best contrast noise ratio (CNR) for tracking. Resolution and precision were also investigated by proper measurement of the position of a ferromagnetic sphere by magnetic resonance imaging (MRI) acquisition and by comparing them with the real position. This method is also tested for a moving marker where the positions found by MRI projections were compared with the ones taken with a camera. In vitro and in vivo experiments show the operation of the technique in tortuous phantom and in animal models. Although the method was developed in the prospect of new interventional MR-guided endovascular operations based on miniature untethered devices, it could also be used as a passive tracking method using tools such as catheters or guide wires.