Magnetics Business & Technology - September/October 2019 - 11

FEATURE ARTICLE

manent magnets, it tends to be an insignificant effect in medical
devices-except where magnetics are used as sensors.

Designing for Electromagnetic Interference (EMI)

Many medical devices utilize sensors which are susceptible to
having their readings affected by EMI. That means that it's a core
functional need for all medical devices to control their emissions
(which can affect nearby medical devices), and if they have sensitive electronics they must also control their susceptibility (their
ability to be affected by other devices). To accomplish this, they
are typically shielded with metal panels, spray metal coating products, or faraday cage-type structures. For sensitive equipment,
the largest hole defines the EMI shielding level (where a hole is
measured linearly), this means an extremely narrow seam that's
not completely closed may be a very significant EMI hole, depending on the target wavelength.
Shielding materials are chosen based on their requirements, as
each material has different properties - certain metals provide
better shielding per unit thickness than others. The best electricfield shields are Nickle-silver, BeCu, and other specialty metals.
Mu-metal or permalloy are common choices where magnetic field
shielding is a priority, which can be brought on by nearby permanent magnets or AC currents. Mild steel, stainless steel and zinc
plated steel are also fairly effective and much more common. Aluminum isn't an effective shield for DC currents or magnetic fields
and requires more thickness to achieve the same AC shielding as
steel but may be more appropriate for corrosive medical environments.

These shields can be single purpose, but in a complex medical
device typically also become a part of thermal management - a
sealed metal box right near PCBs tends to contain heat even if
you don't want them to. Multi-use parts (for example, metal panels
that seal against EMI but also prevent ingress and protect the
internal components) will reduce part count, but you may find
the competing requirements lead to a very complex design with
numerous vested parties and an overall increase in manufacturing
cost despite the lower part count. That being said, taking advantage of the large metal sheets for structural elements may be a
golden opportunity too.

All About Metals

(Note that only 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.)
As noted, iron, cobalt, and nickel alloys are generally ferromagnetic. 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 accompanying chart shows very common medical device metals and their relative magnetism, though
it's highly dependent on the manufacturing, treatment, and handling processes used to produce the part.

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September/October 2019 * Magnetics Business & Technology

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Magnetics Business & Technology - September/October 2019

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

Magnetics Business & Technology - September/October 2019
Contents
Editor’s Choice/ Features
Advancements in Transcranial Magnetic Stimulation Open Up Promising New Therapies for Array of Neural Conditions
Barcoded Magnetic Beads and New Diagnostic Systems Revolutionize Battle Against Infectious Disease
European Space Agency Swarm Mission Pinpoints New Magnetic North and Geomagnetic Jerks
Research & Development
Product News
Industry News
Marketplace/ Advertising Index
Magnet Summer Camp Spontaneous Thoughts by Dr. Stan Trout
Spontaneous Thoughts
Magnetics Business & Technology - September/October 2019 - Magnetics Business & Technology - September/October 2019
Magnetics Business & Technology - September/October 2019 - Cover2
Magnetics Business & Technology - September/October 2019 - Contents
Magnetics Business & Technology - September/October 2019 - Editor’s Choice/ Features
Magnetics Business & Technology - September/October 2019 - 5
Magnetics Business & Technology - September/October 2019 - Advancements in Transcranial Magnetic Stimulation Open Up Promising New Therapies for Array of Neural Conditions
Magnetics Business & Technology - September/October 2019 - 7
Magnetics Business & Technology - September/October 2019 - 8
Magnetics Business & Technology - September/October 2019 - 9
Magnetics Business & Technology - September/October 2019 - 10
Magnetics Business & Technology - September/October 2019 - 11
Magnetics Business & Technology - September/October 2019 - 12
Magnetics Business & Technology - September/October 2019 - 13
Magnetics Business & Technology - September/October 2019 - Barcoded Magnetic Beads and New Diagnostic Systems Revolutionize Battle Against Infectious Disease
Magnetics Business & Technology - September/October 2019 - 15
Magnetics Business & Technology - September/October 2019 - 16
Magnetics Business & Technology - September/October 2019 - 17
Magnetics Business & Technology - September/October 2019 - Research & Development
Magnetics Business & Technology - September/October 2019 - 19
Magnetics Business & Technology - September/October 2019 - Product News
Magnetics Business & Technology - September/October 2019 - 21
Magnetics Business & Technology - September/October 2019 - 22
Magnetics Business & Technology - September/October 2019 - 23
Magnetics Business & Technology - September/October 2019 - Industry News
Magnetics Business & Technology - September/October 2019 - 25
Magnetics Business & Technology - September/October 2019 - 26
Magnetics Business & Technology - September/October 2019 - 27
Magnetics Business & Technology - September/October 2019 - Marketplace/ Advertising Index
Magnetics Business & Technology - September/October 2019 - 29
Magnetics Business & Technology - September/October 2019 - Spontaneous Thoughts
Magnetics Business & Technology - September/October 2019 - Cover3
Magnetics Business & Technology - September/October 2019 - Cover4
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