As hospital rooms become more high tech, an important concern for today's medical device designer is electromagnetic interference (EMI). Much attention has already been given to the infamous cell phone case which highlights interference from radio frequency signals. This article will focus on shielding devices from lower frequency electromagnetic interference.
Nanocrystalline ferromagnetic coating (Nanovate EM) on (from top to bottom) a polymer part, an aluminum housing, and selectively deposited.
Andrew Wang is the VP of product development at Integran Technologies Inc. He is responsible for helping customers solve engineering challenges with the use of Integran's nanocrystalline materials. Wang can be reached at 416-675-6266, x263 or firstname.lastname@example.org
Unlike cell phones and walkie-talkies that operate at high frequencies, sources of EMI are typically low frequency magnetic signals. Common sources in a hospital would include electrical lines (60 Hz), MRI machines, and electrical motors. Electrical devices that may be affected by EMI can include pacemakers, ECG equipment, defibrillators, imaging devices, analog sensors, and hearing devices.
The EMI problem has typically been dealt with by shielding either the source or the device, or by keeping certain equipment away from others. Materials used to shield EMI signals are ferromagnetic metals with high permeability that can range from low cost steels to highly tailored mumetal (a special alloy with extremely high permeability). For example, MRI rooms are typically shielded using silicon steel sheets that are welded together in the floors, walls, and ceiling. In devices, mumetal foils are often used, which must be cut and applied manually or formed into shape. After deformation, the foils must be annealed to recover their magnetic properties, requiring an additional process step.
Direct Coating of Parts
A new way to shield medical device components is to apply a high permeability coating directly onto the device housing. Ferromagnetic coatings with nanocrystalline grain size offer permeability close to that of mumetals but do not need to be annealed. Due to their fine grain size, these coatings also have very high hardness and strength and can even add structural rigidity to polymer components. Other substrates such as aluminum castings, machined parts, or
| Typical method for shielding a component with mumetal foils. |
composite parts, can also be coated to add magnetic shielding. Coating thickness is usually on the order of 0.001? to 0.008? for most applications, but can even be varied in different locations. Selective coating is also possible if a particular area needs to be uncoated. Several companies have tested parts with nanocrystalline ferromagnetic coatings, showing adequate attenuation of magnetic signals, and are eager to move away from foil lay-up solutions on complex shaped parts. Although this solution is not practical for shielding entire MRI rooms, it can be very valuable to the medical device designer. Small to medium sized components can be easily coated, and the process is scalable to medium and high volumes, resulting in cost effective solutions.
For additional information on the technologies and products discussed in this article, see MDT
online at www.mdtmag.com
or Integran Technologies Inc. at www.integran.com