Magnetics in Medical Devices: A Practical Guide

Permanent Magnets
The most 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, but that temperature is typically above 300^oC. 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 are more appropriate for corrosive environments.

Hysteresis
Even if not turned into a permanent magnet, ferromagnetic materials have a tendency to stay magnetized to some extent after being subjected to an external magnetic field. This phenomenon is called hysteresis. 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 and permanent magnets, it tends to be an insignificant effect in medical devices—except where magnetics are used as sensors.

All About Metals
(Note that ferromagnetism is described here, as opposed to the other types of magnetism (para, dia, and anti-ferromagnetism), which tend to be both weaker (i.e. less practical) and more complex.)

Alloys with iron (e.g. steel), cobalt, and nickel are generally magnetic. These are commonly found in general structural parts, implants, and instruments respectively. Some alloys common in medical devices, such as certain types of stainless steels, chrome-cobalt implants, and nitinol (shape memory alloy wire) contain these elements but aren’t magnetic due to their resultant crystalline structure. The chart below shows very common medical device metals and their magnetism, though it’s highly dependent on the manufacturing, treatment, and handling processes used to produce a part. 

Common Name:

200 Series Stainless Steel

300 Series Stainless Steel (austenitic)

400 Series Stainless Steel

Duplex Steel

17-4PH

Stainless Steel Welds, casts

Plain Carbon Steel

Cobalt Chrome

Nitinol

Includes:

201, 202, 205, etc.

301,302, 303, 304, 321, 347 18/8, A2

316, Sometimes 18/8

310, 305

Ferritic: 430 and 446

Martensitic

2XXX series

630, 1.4542 (EU)

N/A

Numerous Grades

Nitinol 55 and Nitinol 60

410, 420, 440

416

Corrosion Resistance:

Fair

Good

Excellent

Excellent

Good

Good

Fair

Good

Good

Various

Poor, unless coated

Excellent

Good

Main Characteristics:

Cheap, Hard, Strong

Biocompatible, autoclavable, weldable 310 is more stable at high temperatures but more expensive than 400 series

Wear resistant

Wear resistant

Machinable

Strong, harder to machine

Cheaper, lighter, high strength

Different alloys

Cheap and strong

Inert, hard, biocompatible

Bendable, reverts shape

Relative Magnetism: ***

Slightly magnetic (1.004-3**)

Slightly magnetic1 (1.004-3**)

Non-magnetic2 (~1.003-1.01) Especially LN types

Non-magnetic1 (~1)

Magnetic2 1000-1800*

Magnetic2 750-950*

Magnetic: 750*

Magnetic, (halfway between austenitic and ferritic alloys)

Magnetic (similar to martensitic alloys)

Can be fairly magnetic3

Magnetic ~1004

Non-magnetic, Typically safe for MRI

Non-magnetic (<1.002)5

* when annealed to increase magnetic properties
**unless annealed to remove magnetic effects
***…as a general rule. Actual magnetism is relative and highly dependent on processing (such as working, welding, or annealing). In units of μ/μ₀ (relative permeability) where vacuum is defined as exactly 1.

Modeling Magnetic Fields
Ferromagnetic metals that don’t emanate a unified magnetic field (e.g., a “non-magnetized” piece of iron) interact with magnetic fields emanating from permanent or electromagnets in a simplified way. They act as a conductor of magnetic fields, allowing the magnetic fields to take the path of least resistance, much like electricity prefers metallic conductors to air. In magnetics, this not only causes their magnetic domains to align but also affects the magnetic field map. If the magnetic field lines are small streams, ferromagnetic materials are the river that absorbs all streams that try to cross it. Similarly, the “streams” affect the “river” itself—if the streams are small (the nearby magnet is weak), then the river will be largely unchanged (little magnetic alignment). If the streams are large (the nearby magnet is strong), then the river will swell (become a magnet itself). Again, this is also true for electricity, which will travel along conductors to conserve energy by following the path of least resistance. The practical application of all this is that magnetic fields can be altered and shaped in the desired way by understanding the path of least resistance and using ferromagnetic materials in the vicinity of the magnetic field source. Modeling software is the simplest way to visualize this effect and show the resultant fields.