Karl Vollmers

Modeling and Control of Untethered Biomicrorobots in a Fluidic Environment Using Electromagnetic Fields

This paper investigates fundamental design, modeling, and control issues related to untethered biomedical microrobots guided inside the human body through external magnetic fields. Proposed areas of application for these microrobots include sensing, diagnosis, and surgical procedures in intraocular, cardiovascular, and inner-ear environments. A prototype microrobot and steering system are introduced. Fluid drag experiments performed on the prototype robot show that the 950 × 400 μ m elliptical shape has a spherical equivalent diameter of 477 μ m. Drag forces combined with saturation magnetization (5 × 10 5 A/m) of the prototype indicate that the required magnetic field gradients for application inside the vitreous humor and blood vessels are on the order of 0.7T/m.

Small, Fast, and Under Control: Wireless Resonant Magnetic Micro-agents

Primary challenges in the building of untethered submillimeter sized robots include propulsion methods, power supply, and control. We present a novel type of microrobot called MagMite that utilizes a new class of wireless resonant magnetic micro-actuator that accomplishes all three tasks. The term MagMite is derived from Magnetic Mite—a tribute to the underlying magnetic propulsion principle and the micro-scale dimensions of the robot. The device harvests magnetic energy from the environment and effectively transforms it into inertia-and impact-driven mechanical force while being fully controllable. It can be powered and controlled with oscillating fields in the kilohertz range and strengths as low as 2 mT, which is only roughly 50 times the average Earth magnetic field. These microrobotic agents with dimensions less than 300 μm × 300 μm × 70 μm and a total mass of 30—50 μg are capable of moving forward, backward and turning in place while reaching controllable speeds in excess of 12.5 mm s—1 or 42 times the robot’s body length per second. The robots produce enough force to push micro-objects of similar sizes and can be visually servoed through a maze in a fully automated fashion. The prototype devices exhibit an overall degree of flexibility, controllability, and performance unmatched by other microrobots reported in the literature. The robustness of the MagMites leads to high experimental repeatability, which in turn enabled us to successfully compete in the RoboCup 2007 and 2009 Nanogram competitions. In this work it is demonstrated how the robots exhibit a plethora of driving behaviors, how they can operate on a host of unstructured surfaces under both dry and wet conditions, and how they can accomplish fully automated micromanipulation tasks. Various micro-objects ranging from beads to biological entities have been successfully manipulated. To the same end, multi-agent studies have shown great promise to be used in cooperative tasks.

Wireless resonant magnetic microactuator for untethered mobile microrobots

Power and propulsion are primary challenges in building untethered submillimeter robots. We present a class of actuators utilizing wireless resonant magnetic actuation which accomplishes both tasks with a high degree of control. The actuator harvests magnetic energy from the environment and transforms it to impact-driven mechanical force. It can be powered and controlled with oscillating fields in the kilohertz range and strengths as low as 2mT. The wireless resonant magnetic microactuator was incorporated in microrobots, which measure 300×300×70μm3, that are capable of moving forward, backward, and turning in place while reaching speeds in excess of 12.5mm∕s.

Design and control of in-vivo magnetic microrobots

This paper investigates fundamental design, modeling and control issues related to untethered biomedical microrobots guided inside the human body through external magnetic fields. Immediate application areas for these microrobots include cardiovascular, intraocular and inner-ear diagnosis and surgery. A prototype microrobot and steering system are introduced. Experimental results on fluid drag and magnetization properties of the robots are presented along with an analysis of required magnetic fields for application inside blood vessels and vitreous humor.

Electrochemical Codeposition of Magnetic Particle-Ferromagnetic Matrix Composites for Magnetic MEMS Actuator Applications

This paper reports on a new electrochemical codeposition technique for microelectromechanical systems ~MEMS! actuator application. Various magnetic composite materials can be obtained by incorporating hard magnetic particles with electrodeposited magnetic ~either soft or hard ferromagnetic materials! metal matrices. The incorporated magnetic particle investigated is barium ferrite (BaFe12O19), and the metal matrices are electroplated nickel, CoNiMnP, and an electroless plated nickel-phosphorus alloy. Scanning electron microscopy results show that BaFe12O19 particles can be successfully incorporated into all the investigated metal matrices. The resulting magnetic properties show that materials produced by the electrochemical codeposition technique have a high coercivity ( Hc) of up to 2120 Oe and a maximum magnetic energy density, (BH) max ,o f up to 8.0 KJ/m 3 . This

Magmites - wireless resonant magnetic microrobots

Primary challenges in the building of untethered sub-millimeter sized robots include power supply, propulsion methods, and control. We present a novel type of microrobot termed Magmite that utilizes a new class of wireless magnetic actuator which accomplishes all three tasks. The device harvests magnetic energy from the environment and effectively transforms it into mechanical propulsion while being fully controllable. This microrobotic agent with dimensions less than 300 mum times 300 mum times 70 mum is capable of maneuvering with 3 degrees of freedom. A specially prepared substrate allows for adjustable speeds exceeding 12.5 mm/s or 42 times the robots body length per second (see accompanying video). It is powered by oscillating fields in the kHz range and strengths as low as 2 mT - roughly 50 times the average Earth magnetic field.

Analysis and design of wireless magnetically guided microrobots in body fluids

The active guidance of magnetic particles inside biological organisms for drug delivery, cell separation, and protein manipulation has been actively pursued in biomedical research for many years. Recent advances in the integration of magnetic materials in MEMS are enabling the convergence of these two technologies towards the realization of wireless magnetically guided biomedical microrobots. This paper discusses some of the fundamental design issues for a biomedical microrobot that is actively steered in body fluids using magnetic fields. The effects of miniaturization on magnetic, fluid drag, and gravity/buoyancy forces and on control stability are analyzed. The advantages of using hard magnetic materials and the state of the art in the integration of hard magnetic materials into MEMS devices are discussed. The analysis indicates that untethered microrobots capable of being navigated under external control within biological organisms including the human body is a realistic goal.

Visual servoing and characterization of resonant magnetic actuators for decoupled locomotion of multiple untethered mobile microrobots

Wireless resonant magnetic micro-actuators have been previously described as highly effective propulsion mechanisms for untethered mobile microrobots. The discussion thus far has been primarily relegated to a characterization of stationary devices and the de facto observation of their mobility. Before applications of microrobots can be more fully explored, devices are required that can operate reliably and repeatably in a host of operating environments. In this paper, we analyze the in situ performance of resonant magnetic actuators for microrobotic locomotion to better understand their durability, substrate requirements, and driving characteristics.

Measuring the Magnetic and Hydrodynamic Properties of Assembled-MEMS Microrobots

Microrobots experience physical phenomena that are difficult to model analytically and that are not completely captured with macro-scale prototypes. In this paper we present a reconfigurable robotic measurement system to characterize the magnetic and hydrodynamic properties of assembled-MEMS microrobots. The system consists of a powerful permanent magnet that is position controlled with a linear stage. The magnetic field is accurately characterized. Precision sensors are used to measure magnetic force as a function of applied field. The system is first used to validate an existing model for the magnetic force on a soft-magnetic ellipsoid. Next, the magnetic force on a soft-magnetic assembled-MEMS microrobot as a function of the applied field is measured experimentally. Finally, a vision tracking system is integrated with the setup to measure the hydrodynamic properties of the microrobot. The coefficient of viscous friction for the microrobot is obtained experimentally.

Guidance of magnetic intraocular microrobots by active defocused tracking

Current laparoscopic techniques for intraocular surgery require that the vitreous humor is removed and at least three cannulas are inserted through the sidewalls of the eye. This paper investigates an alternate intraocular surgical technique based on the use of wireless microrobots guided by external magnetic fields. Issues investigated include the effects of magnetic and viscous drag forces faced by magnetic microrobots in the vitreous humor and the 3D visual servoing of these microrobots using a single microscope view. A new active defocused tracking method is proposed for visually servoing the microrobot along the optical axis of the microscope. This method uses a purposely defocused view of the microrobot to unambiguously resolve depth while servoing. Experimental results demonstrating the method with a microrobot visually servoed in 3D at 60 Hz using a single microscope view are presented.