There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Microrobots have the potential to revolutionize many aspects of medicine. These untethered, wirelessly controlled and powered devices will make existing therapeutic and diagnostic procedures less invasive and will enable new procedures never before possible. The aim of this review is threefold: first, to provide a comprehensive survey of the technological state of the art in medical microrobots; second, to explore the potential impact of medical microrobots and inspire future research in this field; and third, to provide a collection of valuable information and engineering tools for the design of medical microrobots.

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Microrobots for Minimally Invasive Medicine

For biomedical applications, such as targeted drug delivery and microsurgery, it is essential to develop a system of swimmers that can be propelled wirelessly in fluidic environments with good control Here, we report the construction and operation of chiral colloidal propellers that can be navigated in water with micrometer-level precision using homogeneous magnetic fields The propellers are made via nanostructured surfaces and can be produced in large numbers The nanopropellers can carry chemicals, push loads, and act as local probes in rheological measurements

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Controlled propulsion of artificial magnetic nanostructured propellers.

Inspired by the natural design of bacterial flagella, we report artificial bacterial flagella (ABF) that have a comparable shape and size to their organic counterparts and can swim in a controllable fashion using weak applied magnetic fields. The helical swimmer consists of a helical tail resembling the dimensions of a natural flagellum and a thin soft-magnetic “head” on one end. The swimming locomotion of ABF is precisely controlled by three orthogonal electromagnetic coil pairs. Microsphere manipulation is performed, and the thrust force generated by an ABF is analyzed. ABF swimmers represent the first demonstration of microscopic artificial swimmers that use helical propulsion. Self-propelled devices such as these are of interest in fundamental research and for biomedical applications.

Artificial bacterial flagella: Fabrication and magnetic control

A new type of micro swimming mechanism is proposed for microrobots working in water. It is composed of a small magnet attached to a spiral wire. An external alternating magnetic field causes the magnet to rotate due to magnetic torque. As a result, the mechanism can swim propelled by waves traveling along the spiral. The swimming velocity increases linearly with increasing excitation frequency, and the increasing rate depends on the shape of the spiral. The experimental velocity agrees with the calculation result based upon Lighthill's theory (1975, 1976).

Micro swimming mechanisms propelled by external magnetic fields

In this paper, the authors propose applying a magnetic actuator to a capsule endoscope. Capsule-shaped magnetic actuators were fabricated and in vitro tests were examined. The magnetic actuator was composed of a capsule dummy, a permanent magnet inside the capsule, and a spiral structure. The actuator was rotated and propelled wirelessly by applying an external rotational magnetic field. It was demonstrated that the actuator with 1-mm height spiral could move at 5 mm/s. In addition, the torque required to start rotation was measured. The actuator has great potential for wireless operation of the capsule endoscope.

Fabrication of magnetic actuator for use in a capsule endoscope

Magnetic micro-machines capable of swimming through liquid or gel were fabricated. The micro-machines were driven by an external rotating magnetic field and featured a screw-shaped structure and permanent magnet. The machines could swim under condition of a Reynolds number ( Re ) of 10 −7 , and were able to run through agar or a bovine tissue sample using the same principle. Their running behavior was dependent on the frequency and strength of the external field, and on the surrounding media. These machines have great potential for medical applications in the human body.

Swimming micro-machine driven by magnetic torque

Magnetic micromachines for medical applications

The spiral-type magnetic micro-machine, which is driven by a rotating magnetic field, is characterized by wireless operation for swimming. The machine has to swim under a lower Reynolds number environment, when the machine is downsized and working inside the human body. In this study, using various silicone oils whose kinematic viscosity ranged from 1 to 5/spl times/10/sup 5/ mm/sup 2//s, the swimming performance of the machine at low Reynolds number was examined. The machine composed of a cylindrical NdFeB magnet could swim in oils under condition of Re=10/sup -7/. This Reynolds number is the same as that of a micro-machine with micron size swimming in water. In addition, the machine could turn by controlling the external rotational magnetic field, and therefore the swimming direction of the machine could be controlled. Using these principles, a magnetic micromachine, which can run in a gel, was also fabricated.

Kazushi Ishiyama

Papers

Micro swimming mechanisms propelled by external magnetic fields

Fabrication of magnetic actuator for use in a capsule endoscope

Swimming micro-machine driven by magnetic torque

Magnetic micromachines for medical applications

Spiral-type micro-machine for medical applications