Staying Connected - November 2011

Flex Life of Cables and Lead Wires

While flex life and flexibility are often discussed as similar attributes, the flex life of a cable assembly is not directly related to its flexibility.  Flexibility is the property of a cable which allows it to be deformed by bending without, breaking or sustaining other damage.  Flex life is a characteristic describing how many times a cable can be bent or flexed before failing.

The flex life of cable or wire is typically much greater than the flex life of a cable assembly.  Because of the way they are used, a medical cable assembly will virtually always fail first at one of the termination points, where the conductors are soldered or crimped, rather than within the cable or wire material.



ANSI/AAMI EC53 establishes
minimum flex life requirements
for ECG cables and lead wires

Flex Life Requirements

ANIS/AAMI EC53 establishes minimum flex life requirements for both reusable and disposable ECG cables and patient lead wires as well as a method to perform testing.  “The flex life at the instrument connector/trunk cable, trunk cable/cable yoke…shall all withstand flexes of +/- 90 degrees as specified in the following table.”

Test Location Reusable Disposable
Trunk cable – both ends 1,000 n/a
Leadwire – both ends 500 30

EC53 flex life requirements have proven to be reasonable for ECG cable assemblies.  However other medical applications may require higher or, in some instances, lower flex life.  As an example, a medical cable that is designed to connect two stationary devices may only be disconnected and reconnected infrequently and therefore may not be subject to any flexing.  On the other hand, a reusable device that is worn by an ambulatory patient for an extended period of time may require a flex life of tens or even hundreds of thousands cycles.

Determining how a cable or lead assembly will likely be used and how long it is expected to last will help establish how many flex cycles it should be designed to achieve before failure.

Flex Testing


Affinity’s custom flex tester
can test six cables
simultaneously & monitor
each one independently


Flex testing is a means to quantify and confirm the intended flex life of a cable assembly in a controlled environment.  In section 5.5.5 of ANSI/AAMI EC53, a standardized method to test the flex life of a cable or lead assembly is established.  It includes the degree of flexing (plus and minus 90 degrees), the weight to be used, a description of failure modes and also a diagram of the suggested test apparatus.

At Affinity, we test most samples at a rate of 20 cycles per minute to avoid “work hardening” of copper conductors.  The resistance and stiffness of copper conductors increases as the material is flexed.  This phenomenon, known as “work hardening” is not normally noticeable when the rate of flexing is slow.  But, when flexed repeatedly at higher rates, copper becomes stiffer and harder and may skew the results of flex testing.

Role of a Strain Relief

Because the most likely failure of a cable or lead assembly is at the point where the conductors are soldered or crimped, a good design will attempt to prevent stress on these connections.


A well designed strain relief can help
prevent the stress from cable movement
transferring to terminations which can
lead to failure


Flat surfaces show cable clamp that has
been crimped onto the cable jacket
increasing tensile strength and flex life
when captured in overmold

A strain relief typically provides a transition from a flexible cable to a rigid connector or connection point.  A well designed strain relief will prevent mechanical force applied to the exterior of a cable from being transferred to the electrical terminations within the connector which could lead to failure.  A strain relief should prevent an axial load or any flexing being applied at the wire termination point within the connector or device.

Additional tensile strength can typically be achieved by incorporating a clamp which is crimped onto the cable jacket prior to molding.  When the strain relief is overmolded, the clamp is embedded into the mold material which increases the tensile strength and flex life of the cable or lead assembly.

Material Selection

The selection of materials plays a very large role in determining the flex life of a cable assembly.  Common stranded copper conductors are suitable for many medical cable applications; but where a higher flex life is required, copper alloy or tinsel conductors are often specified.

Common stranded copper is used for the majority of medical cables because its performance matches the requirements of most applications and it is a low cost material that is widely available.  Where either increased flex life or tensile strength is required, a high strength copper alloy may be specified.  A variety of copper alloys are available that offer many times the flex life of common copper conductors.  However, some of these materials are not RoHS compliant because they contain restricted metals and all are significantly more expensive than common tin or silver-plated copper.



Micro photo of a single strand of tinsel
wire – flattened conductors wrapped
spirally around a strong fabric core

For low voltage applications, tinsel conductors wrapped around a strength member can offer excellent flex life, flexibility and high tensile strength.

Tinsel wire is made by flattening the conductor material into a ribbon and then spirally wrapping one or more conductors around a strong fabric core.  Tinsel conductors are typically made of copper and are often plated with tin or silver.  Because the fabric core is what gives tinsel wire its strength, the conductors can be made very thin and flexible.

Conductor Stranding and Cable Diameter

Stranded conductors are considerably more flexible than solid conductors while offering almost identical tensile strength.  Smaller diameter conductors offer greater flex life due to the outer surface of the conductor being closer to the central axis.  Reducing the diameter of the cable results in an exponential increase in flex life if the bend radius remains constant.

Virtually all medical cables use stranded conductors to increase flexibility and flex life.  However, the same gauge conductor can be made up of different strand configurations which greatly affect flexibility.  Standard 28 gauge wire, commonly used in medical cable assemblies, may be made up of the following combinations of conductors:

Number of Strands

Gauge of each Strand

7

36

16

40

19

40

40

44

65

46

Using a greater number of strands improves flex life performance but there is an increased cost for cable material.


A braided shield typically has a shorter
flex life than a spiral shield due to the
metallic strands abrading against each
other when flexed


Shield and Flex Life

For shielded cable and wire, shielding adds another component that can eventually fail due to flexing.  If cable is shielded, the shield is often the first component of the cable material to fail.  This is because the shield is at a greater distance from the central axis of the cable than the conductors and therefore sees a greater amount of stress when the cable is bent.  For this reason, if materials and design are similar, a cable with a smaller diameter will generally not only be more flexible, but will also offer longer flex life.

While a spiral shield is more flexible than a braided shield, a spiral shield may lose effectiveness due to separation of shield conductors as a cable is repeatedly flexed.  A braided shield may fail due to friction of the intertwined strands during flexing.  If this happens, the metallic braid can break down and cut the surrounding insulation.

Jacket Material and Flex Life

The cable jacket protects the conductors and any shields.   For increased flex life, the most appropriate jacket materials are thin with high tensile strength and high resistance to tearing.  For these reasons, polyurethane is often a good choice for a medical cable jacket.  It is very durable, having excellent abrasion resistance, and resists most cleaners found in a clinical setting.  Polyurethane is available in medical grades suitable for skin contact but is not suitable for steam sterilization.

Another common jacket material is a thermoplastic elastomer (TPE) such as Santoprene®.  A TPE jacket can withstand sterilization by autoclave, is available in medical grade, but is not as durable as polyurethane.  Other characteristics of a TPE jacket are a good “feel” and very good flexibility.

Silicone is the most flexible material used for medical cable jackets but, due to poor durability, is typically used on instruments which are used in a controlled setting such as surgery.  Silicone does not resist cutting or tearing and therefore flex life would be compromised when used for many medical applications.

Summary

Flex life is typically an important characteristic of a cable assembly.  As such, flex life requirements should be established early in the development process so that appropriate materials and construction can be considered.

The Affinity engineering team can help establish what the appropriate flex life requirements are for your particular application.  Our team also understands the role that materials and cable design play in achieving good cable and lead wire flex life.

If you would like to discuss any aspect of cable flex life and how it relates to your project, contact the Affinity engineering team.

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MEDICA 2011 Recap


The Affinity team at Medica - from the left Bob, Didier, Hank, Mary, Jim,
and Roberto and Lorenz of MPS, our German partner

This was the largest Affinity team ever traveled to Medica in Dusseldorf, Germany mid November to meet with our OEM partners and visit with prospective OEM customers.

Affinity President and CEO Mary Phillipp, Director of Engineering, Bob Frank, OEM Sales Manager, Jim Itkin, Business Development Manager, Hank Mancini and European Business Development Manager Didier Chabault attended.  In addition, Lorenz Huber and Roberto Henker from Affinity’s new German partner, MPS, attended and helped staff Affinity’s stand.

“Medica is the largest medical show in the world, attracting virtually every medical device manufacturer,” said Affinity OEM Sales Manager, Jim Itkin.  “Besides our current OEM partners, we have the opportunity to meet and visit face to face with prospective customers from all over the world.”

During the four days of Medica, the Affinity team visited with 51 customers and prospective customers, with about half coming to the Affinity stand and the other half were visited at their stand.

“We are fortunate to be able to co-exhibit with Technomed, our Dutch partner in Hall 9,” said Affinity President and CEO Mary Phillipp.  “Hall 9 is the home to many of our OEM customers and it is one of the most heavily visited of all exhibit halls.  It was our best show yet and we are already planning our Medica strategy for 2012.”

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Announcements, Information and Trivia



Sinterklaas – celebrated on
December 5th in The Netherlands –
image courtesy of Wikimedia


Rizal day is celebrated in the
Philippines on December 30th and
honors Jose Rizal, one of the
Philippines’ national heroes –
image courtesy of Wikimedia

December Trivia

Around the world, many holidays are celebrated in December. Some of the more notable are:

Dec 1st - Romania – Union Day
Dec 2nd - United Arab Emirates – National Day
Dec 5th - Thailand – Father’s Day
Dec 5th - Netherlands – Sinterklaas
Dec 6th - Finland – Independence Day
Dec 12th - Kenya – Independence Day
Dec 16th - Bangladesh – Victory Day
Dec 16th - South Africa – Day of Reconciliation
Dec 23rd - Japan – The Emperor’s Birthday
Dec 25th - United States – Christmas
Dec 26th - United Kingdom – Boxing Day
Dec 29th - Spain – Constitution Day
Dec 30th - Philippines – Rizal Day

 

Affinity Holiday Schedule

Affinity will be closed to allow our team members to enjoy the holidays as follows:

Friday, December 23rd and Monday, December 26th for Christmas
Monday, January 2nd for New Years