Staying Connected - January 2014

Effective Shielding of Medical Cables



Medical Cables, including those used for
EKGs, often benefit from effective shielding

Because medical cables and lead wires can act as unintended antennas to both receive and to radiate electromagnetic interference (EMI) signal shielding is often necessary and is an important consideration of medical cable design.

Shields are additional conductors added to wire or cable to help isolate the electromagnetic fields of conductors within the shield from those outside of the shield.  Shields may be placed over individual conductors, over groups of conductors, or over the entire bundle of conductors within the cable.  It is often necessary to incorporate multiple shields, in which case, they are typically electrically isolated from each other.

The effectiveness of shielding is due to the shield reflecting a portion of the electro-magnetic interference but also absorbing a portion and directing it to ground.  In both cases, some energy may still reach the signal conductors, but it is typically highly attenuated and does not cause problematic interference.

Physiological signals are analog by nature and are generally very low in signal strength.  This makes any interference undesirable and therefore effective shielding of the signal may be an important consideration.


Drawing of a cable with
braided, spiral and foil shields


The most common location for a shield is immediately below the cable or wire outer jacket.  However, more complex cables may be made up of several components (sub-cables) with one or more shields.

Shielding helps prevent unwanted external interference but is often used to prevent interference within the cable.  This may be a significant factor when a cable has both power and signal lines, which is common for many medical applications.

Various shielding methods have advantages and disadvantages to be considered when selecting the most effective option for a particular medical cable application.

Spiral or Serve Shields

A spiral shield, also known as a serve shield, consists of wire (usually tinned or silver plated copper) wrapped in one or more layers spirally around insulated conductors.  Excellent flexibility and flex life as well as 85 to 95 percent coverage are characteristics of spiral shields.  Spiral shields are also easy to terminate and are most effective at providing low frequency protection.


Spiral shield with 95% coverage under Teflon
wrap offers excellent flexibility and flex life

Spiral shield showing strands opening
after high number of flex cycles

Spiral shields can lose effectiveness if the individual strands of the shield separate, something that can occur after a high number of flex cycles.

Braided Shields

Braided shields provide excellent protection from both high and low frequency interference while maintaining good flexibility and flex life.

A braided shield usually consists of two groups of copper strands woven in opposite directions; one group is applied in a left-hand lay and other in a right-hand lay.  Because of this design, a braided shield offers good flexibility.



Example of a braided shield with 95%
coverage. 100% coverage is not possible
because of spaces between braid

Most medical cables incorporating a braided shield specify 80 to 95% shield coverage.  The higher the percentage of braid coverage, the more effective the EMI shielding is.  Coverage of 100% is not possible with a braided shield because leakage will always occur at points where the strands cross each other.

In addition to the percentage of coverage, the tightness of the braid affects performance.  A tight braid with a high percentage of coverage does a better job of shielding but makes the cable or wire less flexible.  A loose braid offers greater flexibility but with reduced effectiveness of the shield.


A drain wire running the length of the
braided shield facilitates terminatio
n




Braided shields can be difficult to terminate unless the braid is “combed out and pigtailed – that is combining conductors from both the left and right lay into a single twisted wire.”  The extra labor to “comb-out” and terminate a braided shield can add cost to the cable assembly.  In some configurations a drain wire running the length of the shield is added to make termination of the shield easier.

When subjected to a high number of flex cycles, a braided shield may breakdown due to the cross lapping strands abrading each other.  When this happens the broken strands can damage insulation leading to cable failure.  For applications requiring a very high number of flex cycles, a braided shield may not be the best choice. 



A foil shield requires a drain wire
to facilitate terminating the shield

Foil Shields

A foil shield consists of metallic foil, typically aluminum, laminated to a polyester or polypropylene film.  Foil shields provide 100% coverage with the foil typically wrapped in overlapping layers around the cable core.  The 100% coverage is a physical property and does not mean that a foil shield provides 100% EMI shielding.  Foil shields are effective at shielding both high and low frequency interference. Foil shields are lighter weight, less bulky, and typically add less cost to a cable or wire assembly than braided or spiral shields.

Foil shields may be more flexible than a braided shield but typically have a shorter flex life. A drain wire, which runs the length of the cable in contact with the foil shield, makes a reliable electrical termination of a foil shield possible.



Example of a complex multi-core
cable with a foil shield on inner
conductors and an outer braided shield


Combination Shields

Combination shields consist of more than one layer or type of shielding and provide maximum efficiency across a wider frequency spectrum. The combination braid-over-foil combines the strength and flexibility of a braided shield with the advantages of 100% coverage of a foil shield.

Comparison of Common Shielding Methods

The following chart offers a comparison of the three types of shielding discussed and a summary of electrical and mechanical performance for each:

Cable Shielding Characteristics
Type of Shield Percent Coverage Low Frequency Effectiveness High  Frequency Effectiveness RFI/EMI Effectiveness Flexibility Flex Life
Braided 80-95% Excellent Excellent Good Good Good
Spiral 80-95% Good Fair Poor Excellent Excellent
Foil 100% Excellent Excellent Poor Good Fair

Ferrite Beads


Ferrite bead fixed over cable


While ferrite beads are not a shield, they can have the effect of shielding.  Where high-frequency noise may be a problem, ferrite attenuators are often used.  These are placed around a cable to absorb extraneous and unwanted energy traveling on the cable.  Low frequency and DC signals see only the conductor and are unimpeded, but higher frequency signals are suppressed and dissipated, reducing EMI interference.

Summary

EMI and RF interference can degrade signals making diagnosis difficult or even impossible. Effective shielding can reduce unwanted interference and reduce the amount of active filtering needed within the device.

The Affinity engineering team has experience and expertise to assist you in designing medical cable assemblies with appropriate and adequate shielding for your specific application.

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Meet Laura Ramirez – Affinity Engineering Assistant



Derik Larson,
Affinity Quality Engineer


Meet Derik Larson, Quality Engineer

Derik joined Affinity in early of 2013 when the company he had worked at for 6 ½ years downsized.  “When I realized that my job would be eliminated, I started looking for a new company to continue my career,” said Derik.  “I was fortunate that I found Affinity, went through three interviews and was offered a position within less than a month.”

Derik is from Minnesota and came to Southern California to attend college.  He applied to and was accepted at both Cal Tech and Harvey Mudd College.  Derik chose Harvey Mudd, a small private college and one of the top schools in the country, for math, science and engineering.  He earned a Bachelor of Science degree in chemistry.

As a Quality Engineer, Derik reports to Affinity Quality and Regulatory Affairs Manager, Jack Sowen.  Asked about Derik, Jack commented, “Derik is very knowledgeable in many areas: technical issues, design controls, quality systems, regulatory requirements, process improvement and lean manufacturing methods.  He has a high level of enthusiasm, enjoys interfacing with people in all departments and he focuses on getting results.  He is a valuable asset to both Affinity and Molex.”

“Derik is a great addition to the Affinity team,” said Hank Mancini, Marketing Manager.  “He has a lot of knowledge and experience and also a lot of energy.  It is a pleasure working with him.”

Asked what he likes about working for Affinity, Derik replied: “There is always so much going on here.  It is never boring.  Everyone seems to like being here and so do I.”


Derik occasionally finds himself in
Receiving Inspection when there is an
issue with an incoming part





“Every day has its challenges,” said Derik, “but that is one of the things I like about my job.  I find myself always having to balance unexpected issues with the things I had planned to do.  Or course, the critical issues get taken care of whether they are planned or unplanned.”  When asked about his aspirations, Derik said “With additional experience, I hope to move into the role of Quality Manager.”

Derik lives in Lakewood, California, about twenty miles from the Affinity plant.  When not working he enjoys playing softball and bowling.

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Multi-channel Cable Systems



Eliminating even a few of the connections
in an ICU may be beneficial


Patients in hospitals are typically monitored simultaneously for multiple physiological parameters. This generally means that there are three or more cables connected between the patient and monitors.  Add to these one or more IV lines and an oxygen line there can be a “tangle of wires” as nurses have described it.

One solution to help reduce the number of lines is to employ multi-channel cables.  Multi-channel cable systems are most commonly used when multiple invasive blood pressure transducers are used.  One end of the cable features multiple monitor plugs and the opposite, patient end, multiple transducer receptacles.


Example of multi-channel trifurcated
invasive blood pressure cable


Affinity has designed and manufactures multi-channel cables with two to four channels combined into a single trunk cable.  If you would like more information or samples contact your local Molex Sales Engineer or contact Affinity at CustomerCare2@affinitymedical.com.

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Announcements, Information and Trivia



The mountains near
Sochi Russia are
known for ample snow


Ice hockey is one of
the most popular
events of the
winter games

 

Winter Olympics Trivia

Krasnaya Polyana – is Russian meaning “Red or Beautiful Glade.”  Located in the Western Caucasus is home to Rosa Khutor Alpine Ski Resort which will host the alpine and Nordic events at the 2014 Winter Olympics.

Opening Ceremony – While the 2014 games will start on Thursday February 6th with Figure Skating and Freestyle Skiing the Opening Ceremony will be held a day later on February 7th.

Winter Olympic Events - This year’s winter games will feature 98 different competitions for the 15 different events

First Winter Olympic Games - The first Winter Olympics were held in Chamonix France in 1924 and featured alpine and cross country skiing, figure skating, ice hockey, Nordic Combined, ski jumping and speed skating.

Northern Preference - The Winter Olympic Games have never been held in the southern hemisphere