Staying Connected - August 2012

Shielding Medical Cables and Wire



ICU bed with multiple monitors shows
why shielding is an important
consideration for most medical cables

Shielding medical cables is beneficial and often necessary to protect from external electrical interference.  Shielding can also help confine the electrical field within the wire or cable.  Because medical cables and lead wires can act as unintended antennas to both receive and 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.

Physiological signals are analog by nature and are generally very low in signal strength.  This makes any interference problematic and therefore effective shielding of the signal is 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 internal signals from interfering with each other within the cable.  This becomes a more significant issue when a cable carries 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 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 some amount of effectiveness with repeated flexing if the individual strands of the shield separate.

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.



Macro photo of braided shield with 95%
overage. 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 termination

Braided shields can be difficult to terminate unless the braid is “combed out and pigtailed.”  The extra labor to terminate a braided shield can add cost to the cable assembly.  In some configurations a drain wire running the length of the shield is used to make termination of the shield easier.

For applications requiring a very high number of flex cycles, a braided shield may not be the best choice.  With repeated flexing the braid may breakdown due to the cross lapping strands abrading each other.  When this happens the broken strands can damage insulation with continued flexing.



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 the 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 shield 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 Types of Shielding

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 bead fixed over cable

Ferrite Beads

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

Cables designed for medical electronic applications present some unique problems, both because of the need for patient safety and also because of the nature of physiological signals.

EMI and RF interference can degrade signals carried by medical cables 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 the 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 John DePillo – Affinity Staff Engineer



John DePillo

John DePillo is one of the newest members of the Affinity team, joining the company in early August as a Staff Engineer.

And, while John is new at Affinity, he is not new to Affinity.  While attending college John worked for one of Affinity’s tooling partners, Engineering Today, as a tool and die maker for the past seven years and has worked on many of the production tools and fixtures used at Affinity.

“I really wanted to stay in the medical industry when I made the move from tool maker to engineer and the position at Affinity was just what I was looking for,” said John.  “I saw how Affinity much had grown in the last few years and wanted to help the company continue to grow.”


John working on a tool design
in SolidWorks


Asked what his impressions of Affinity have been so far, John answered, “Everyone is very helpful and accommodating in helping me learn all the things that I need to get started in my new job.  I also like the way that all of the departments exchange information to keep things flowing smoothly.”
 
“John’s background in tool design and tool making will be valuable to us as we continue to grow,” said Bob Frank, Affinity’s Director of Engineering.  “In his previous position John worked on many of the production tools used at Affinity.  He already knows how we tool products to match our manufacturing processes.”

John grew up in Hawthorne, California.  He graduated in May from California State University Long Beach with a Bachelors Degree in Manufacturing Engineering Technology.

John and his wife Silvia live in Lake Elsinore with their dog Sunny.  John enjoys staying physically fit and working on old cars and boats.  He also enjoys camping, fishing and hiking and is trying to learn how to wake board.

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Modular Medical Connector System



Custom modular connector system with
panel mount receptacles for four plugs

While Affinity incorporates connectors from virtually all connector companies into the cable assemblies that we manufacture, the Affinity engineering team loves the challenge of designing new connectors.  This example of a proprietary, modular connector is one that Affinity designed for a customer-partner.

Customer requirements included a panel mount for up to four connectors, mechanical keying so that the plug would only fit the appropriate receptacle, and color keying to the proper receptacle.  In addition, the connector needed to have a positive lock, but one which would un-mate if a cable was pulled, without damage to either the instrument or cable.


Connector features mechanical
keying and color coding


Positive latch locks connector in place
yet allows accidental disconnect
without damage

The customers design goals were a challenge, but one that the Affinity engineering team was able to meet.  The completed modular connector system was intuitive to use, robust, and met the design requirements.

Affinity has expertise and experience in designing and manufacturing custom connectors.  If you would like more information on custom connectors, custom cable assemblies or any of our other products, please contact Affinity Medical Customer Care at +1 949-477-9495 or email to CustomerCare2@affinitymed.com.

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Announcements, Information and Trivia



Tokyo, the world’s largest
metropolitan area


Beautifully marbled
Kobe beef


Shinjuku Station is the
world’s busiest train station

 

Tokyo Trivia

The Largest City in the World – Wow, according to National Geographic, Tokyo Japan is the largest metropolitan area in the world with 36 million inhabitants!  Tokyo is nearly twice as large as second place Mexico City with 19 million inhabitants.

Most Expensive Restaurant in the World – Tokyo boasts many firsts including Aragawa, a small, exclusive steakhouse specializing in Kobe beef.   Expect to spend at least $550 per person for a meal in this ten seat restaurant which is nearly hidden at the end of a dark hallway in the basement of an office building in Tokyo’s Shinbashi district.

Busiest Train Station in the World - Shinjuku Station is officially the busiest train station in the world, averaging over of 3.6 million passengers per day.  Eight subway and train lines converge at Shinjuku, but it is only one of 503 train and subway stations in Tokyo.  More people pass through Shinjuku station every day than live in the U.S. state of Connecticut!

東京 or Tokyo – The name means Eastern Capital.  The city now known as Tokyo was originally a small fishing village named Edo.  The name was changed when it became the capital of Imperial Japan in 1868.

One Million Trees – The “Ten Year Project for Green Tokyo” was enacted in 2006 with a goal of doubling the number of roadside trees in the city from just under 500,000 to one million by 2016.  There will be a lot of leaves to sweep up in the fall!