Manoeuvring In Vivo Devices with External Magnetic Fields

Harnessing the power of the magnet

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FIGURE I: Simulation of the magnetic fi eld with a permanent magnet (length = 100 mm, radius = 25 mm) with Br = 1.3 T. The field strength rapidly decays outside the magnet.

Permanent magnets are a tool of daily use in various applications such as fastening notes to a board or transporting goods by magnetic clamps. They are available off-the-shelf as mass produced products in various shapes and sizes. These consist of special alloys that can also be manufactured to custom specific shapes.

Currently, the strongest permanent magnets available on the market are made of a neodymium-iron-boron (NdFeB) alloy. They can be magnetised up to a residual magnetisation, Br, of approximately 1.3 Tesla (T).

Finding a solution to even a simple magnetostatic problem is a complex and difficult task. During the past years, research activities of Fraunhofer IBMT have focused on magnetic research for various biomedical applications such as generating a nuclear magnetic resonance compatible homogeneous permanent magnetic field and contact free manipulation of cells in sterile microfluidic environments.^2,3 Based on experience, a mixed approach of calculations, simulations and experiments in phantoms has been found to be most promising for evaluating the different options in magnetic manoeuvring.

Magnets produce a magnetic field (B) in which the strength on the magnet surface is much smaller than Br and reduces rapidly as the distance increases (Figure I). When bringing a small magnet into an external magnetic field, the smaller magnet will align along the field lines of the external field. As a result of the inhomogeneous magnetic fields, they will attract each other at opposite ends and create torque on the smaller magnet.

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FIGURE II: Magnetic fi eld and gradient strength of a bar magnet along the axis for distances ranging from 20-200 mm from the magnet surface.

The magnitude of the torque on the small magnet is directly proportional to the strength of the magnetic field at the position of the small magnet. Because the force is proportional to the difference of magnetic field strength over the length of the small magnet, it is essentially proportional to the derivative or gradient of the magnetic field strength. Both force and torque are proportional to the residual magnetisation and volume of the small magnet. At positions far away from the magnet, the strength of the magnetic field (B) is inversely proportional to the distance to the power of three  (Bx ∞ x^-3). Therefore, both torque (x^-3) and force (x^-4) on a small magnet reduce rapidly as the distance from the large magnet increases (Figure II). This reduction will not be intensified by normal biological tissue, because compared with electrical fields or ultrasound waves, magnetic fields do not experience attenuation from normal body tissue.

By adding differently shaped pole pieces (Figure III) on a magnet, the distribution of the magnetic field around the magnet and thus its properties can be manipulated. However, simulations showed that for the described application scenario pole pieces have only a marginal influence on the field strength at the point of interest.

In summary, the major design options for manoeuvring objects in vivo by external magnetic fields are:

• Magnetic material. Because the distances between the object to be manoeuvred and the external field are typically large compared with the size of the magnets and magnetic fields reduce rapidly with increasing distance, strong magnetic materials are necessary. Currently, NdFeB alloys represent the strongest magnetic materials; they are available off-the-shelf in different geometries and possess sufficient stability.
• Volume. The volume of the magnet determines its field strength. Typically, the dimensions of off-the-shelf products are limited. However, several magnets can be connected in an array to increase the effective size.
• Shape. The shape of the magnet has an influence on the magnetic field and the gradients. The gradient of an external field is responsible for the torque on an object.
• Alignment. The alignment of the internal magnet to the external field has an influence on the forces and torque at distinct positions in the external field.
• Distance. The distance between the magnets is the major factor influencing the ability to manoeuvre objects.

Risks of magnetic manoeuvring
Static magnetic fields penetrate without attenuation through normal body tissue and are considered harmless for field strengths of less than three T, providing no ferromagnetic implants such as steel plates or electronic implants such as hearing aids or pacemakers are involved.⁴

Magnetic manoeuvring is not compatible with magnetic resonance (MR) methods. The use of magnets in the vicinity or within a MR scanner is extremely dangerous. The strong magnetic fields in and around the scanner exert strong forces on the used permanent magnets.

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Figure III: The distribution of the magnetic fi eld around the magnet and its properties can be manipulated by adding differently shaped pole pieces.

NdFeB magnets are susceptible to corrosion and must be protected against oxidation; thus they are often coated or plated with nickel, tin, zinc or chromium. It is obvious that neither the alloy nor the coating materials are biocompatible and they are not certified for in vivo or implantable device applications. In some cases gold or polytetrafluoroethylene coatings are available. Thus, depending on the system and application, appropriate measures must be taken to protect the body from the magnetic material. Typically, magnets completely demagnetise when they are placed in an alternating magnetic field and at temperatures above the respective Curie temperature. NdFeB magnets are also available in high temperature grades. They have operating temperatures of up to 240 °C and can thus withstand standard sterilisation methods such as steam and hot air sterilisation. During medical procedures, potential forces on the object in vivo and subsequently on the tissue must be carefully controlled. Depending on the tissue, forces could exceed the tolerated stress and may lead to trauma.

Remote manoeuvring in capsule endoscopy

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Figure IV: Schematic view of the NEMO capsule with magnet and camera. The insert shows two versions of the PillCam (courtesy Given Imaging).

In the NEMO project,¹ which is under European Union Framework Programme (FP) 6, the above mentioned findings have been applied to capsule endoscopy. PillCam capsule endoscopy has proven to be an effective and valuable diagnostic tool for physicians to detect and diagnose diseases of the gastrointestinal tract. The PillCam capsules measure 11 mm x 26–31 mm and weigh less than 4 g (Figure IV). They contain microchip cameras, light emitting diodes acting as a flash, batteries, an antenna and a transmitter chip.

The disposable capsule is easily swallowed by the patient and captures images of the gastrointestinal tract and transmits the images to a data recorder worn by the patient as it travels naturally through the body. Images are then downloaded to a workstation and a video of the camera’s journey is produced and examined by the clinician.^5,6