Magnetics Business & Technology - September/October 2019 - 10

FEATURE ARTICLE

Tech Tips: Magnetics in Medical Devices - a Practical Guide
By Nigel Syrotuck, Starfish Medical
(mainly Fe, Co, Ni), are ferromagnetic.
However, ferromagnetic materials aren't normally magnetized
because the micro-domains typically point in different, random
directions, canceling each other out. When a strong magnetic
field is presented (such as by a large electro-magnet), the
fields temporarily align and the material is attracted to that
strong magnet. The degree to which materials do this is called
magnetic permeability. If the magnetic field applied near the
ferromagnetic material is extremely strong (and other conditions
such as temperature are right), then the micro-domains will reorient permanently.

Permanent Magnets
More often than you might expect, magnetism plays a key role
in the medical device commercialization process. Permanent
magnets are used to affix components together to improve
cleanability and ingress protection-the magnets can be hidden behind sealed plastic or non-magnetic metal walls without
exposing a seam or grooves that collect dirt or biological debris.
Magnetic switches can also be used as interlocks to ensure
safety. Sometimes, magnets are behind the core technology of
a device and the commercialization team has to accommodate,
enhance or avoid interference with the magnetic field generated
by other device features.

When the micro-domains are permanently aligned, the material will become a permanent magnet. A very common type of
permanent magnet is a neodymium magnet, which is made up
of neodymium, iron, and boron. They are available in various
grades, with N52 being the strongest and most expensive.
They will lose their magnetism above their Curie temperature,
typically above 300oC. They are vulnerable to corrosion, so
most commercial versions are coated in a layer of nickel to
protect them. However, this layer tends to be brittle and chip
off, therefore alternate coatings may be more appropriate for
corrosive environments such as those often found in medical
applications.

In this article, practical magnetics from a mechanical engineering perspective will be discussed as it pertains to medical
devices.

Hysteresis

What Is Magnetism?

Magnetism is a class of physical phenomena characterized by
force fields. Magnetism isn't well defined by classical physics
and is regarded as a quantum effect - a characteristic which
makes it difficult for many people to really understand magnets.
That said, it's not too hard to imagine how typical magnetic
fields are created. Picture the crystal structure of a metallic object, where each atom is arranged in neat rows, and each atom
has a field of electrons spinning around it. If the right conditions are present, those magnetic fields line up, which result in
a micro-domain magnetic field - materials that can do this are
called ferromagnetic. Only a very small number of elements

Magnetics Business & Technology * September/October 2019

A soft magnetic material is one that is highly permeable but
doesn't easily retain a permanent magnetic field. A hard magnetic material is the opposite - it's easy to coerce into a permanent magnet. Even if not turned into a permanent magnet,
both soft and hard magnetic materials tend to stay magnetized
to some extent after being subjected to an external magnetic
field. This phenomenon is called hysteresis, and is a function
of energy dissipation - the more energy it takes to magnetize
a material the more likely it is to want to stay as a permanent
magnet. The degree to which hysteresis is present is called the
remanence of the material, and though it is an important factor
in the design of electromagnets, motors, transformers, and per-

www.MagneticsMag.com

http://www.MagneticsMag.com

Magnetics Business & Technology - September/October 2019

Table of Contents for the Digital Edition of Magnetics Business & Technology - September/October 2019