Staying Connected - April 2013

Cable Design for Increased Flexibility



Two individual four-conductor cables, both with
the same diameter exhibit a large difference in
flexibility due to materials and construction

One of the first impressions of the quality of a medical cable comes from how it looks and feels to the user.  Regardless of components within, it is the outside of the cable that is the most observable indication of the quality of the product.  The tactile feel of a cable and is how it is perceived by the user is largely influenced by the jacket material and by cable flexibility.

Cable Flexibility vs. Flex Life

The flexibility of cable material is a different characteristic than the flex life of cable material.  When designing a cable assembly, it is important to understand the roll of each characteristic for the intended use.

A long flex life, that is a high number of flex cycles before failure, is commonly an important characteristic for cables that will be used for an extended period of time.  High flexibility is a characteristic that allows a cable to be bent to a small radius without a reduction in performance or damage to the cable.  For some applications, both characteristics may be necessary.

The Cable Specification

Establishing detailed specifications for cable material is typically done in at an early stage of a project.  This is necessary because other aspects of the product design cannot be completed without knowing the physical properties of the cable material.  Initial specifications for cable material typically include requirements such as:

  • Number of conductors and conductor material
  • Conductor gauge and stranding
  • Conductor insulation type, thickness and durometer
  • Type of shielding if any
  • Cable jacket material, thickness and durometer
  • Outer diameter of cable


Typical cable specification calling
out materials and construction


One additional requirement that is not as easily specified is cable flexibility.  When discussing cable requirements with device manufacturers, requirements may indicate that the cable be “soft and flexible,” a characteristic that is more difficult to quantify than electrical requirements or other physical properties.  And, for some applications, cable flexibility, including torsional resistance, can be critically important.



The wall thickness of the cable jacket
can play a large role in determining the
flexibility of cable material

Diameter Affects Flexibility

Given the same construction, cable or wire flexibility is inversely proportional to the fourth power of the radius of the cable. For example, a 50 percent smaller cable will be about 90 percent more flexible.  With this in mind, cable flexibility is typically improved by using the fewest and lightest gauge conductors suitable for the application, thereby reducing the diameter.  Shielding, regardless of the type, typically decreases flexibility because it increases the diameter of the cable.

Cable Components

  • Outer jacket material – The type of material, thickness and durometer all effect cable flexibility.  Materials used in medical cables that may come in contact with the body need to meet FDA and ISO biocompatibility and cytotoxicity requirements.
    The durometer or hardness of jacket material also affects its flexibility.  Most materials used for wire and cable jackets are available in different degrees of hardness.  A softer grade of a specific material will be more flexible than a harder grade.  However, there is a trade-off in that softer grades are typically less durable than harder grades. One of the most flexible materials used to jacket medical cable is silicone.  However silicone is one of the least durable jacket materials, being easily cut or torn.  PVC is commonly used as a jacket material and can offer good flexibility but with the tradeoff of limited flex-life performance.  A good balance between flexibility and durability is often achieved by using a medical-grade thermoplastic elastomer (TPE) such as Santoprene®.

  • Insulation Material – The type and amount of insulation material can have a large impact on cable flexibility.  The most common conductor insulation is rubberized PVC, but other insulating materials are also used.  In a multi-conductor cable, a small increase in the thickness of the insulation of individual conductors can result in a large increase in the overall diameter, reducing flexibility. 

  • Conductor Material and Size – The size of the conductor plays a significant role in cable flexibility.  To a lesser extent, the conductor material can affect cable flexibility.  For most cables, tinned copper is the material of choice.  Copper alloys are available with much higher flex-life characteristics; however these materials are typically less flexible than pure copper.

  • Solid vs. Stranded Conductors – Stranded conductors are considerably more flexible than solid conductors.  Virtually all medical cables use stranded conductors to increase flexibility.  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

26

42

36

44

40

44

65

46

The higher the number of strands is, the greater the flexibility of the conductor.  However, a greater number of strands increases the cost of the conductor material.

  • Fillers – Fillers are often added to “round-out” cable.  Commonly used filler materials are: cotton, vinyl, jute, polyethylene.  Fillers used only to round-out a cable typically have little effect on cable flexibility.



  • A spiral shield covered by PTFE tape
    can enhance flexibility of a cable

    Serves and Tapes – These are materials wound spirally around cable components to hold them in position for subsequent processing.  The tightness or looseness of the wind can affect flexibility.  A tight wind tends to restrict movement of components and reduce flexibility while a loose wind allows components to move and increases flexibility.  A slippery serve material such as PTFE can enhance cable flexibility.

  • Shielding - Shielding can have a significant effect on cable flexibility.  Given the same percentage of coverage, a spiral shield is typically more flexible than a braided shield.  However a spiral shield may separate with continued flexing reducing the effectiveness of the shield. Braided shields typically offer the greatest amount of shielding, but when coverage is very high, flexibility may be compromised.  In high-flex applications, the braid can wear from friction and break down.  When this happens, severed strands can become abrasive and can reduce the service-life of the cable assembly.  A foil shield, which is typically aluminum laminated to a film, is typically not as flexible as a spiral or braided shield and also will withstand fewer flex cycles before failure.



  • Aramid fiber acts as both strength
    member and filler for cable with
    braided shield

    Strength member – When additional tensile strength is needed in a cable, it is common to add a strength member within the cable assembly.  One method is to add a core of synthetic fiber such as Kevlar which has very high tensile strength for its size and weight.  The materials used for a strength member are typically very flexible, however if they increase the overall diameter of the cable, flexibility may be decreased.

Tinsel Wire

For low voltage applications, tinsel wire offers the greatest degree of flexibility.

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.


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


Diagram of tinsel wire made up of
seven strands of tinsel conductor

Due to the nature of the construction, tinsel wire is more expensive than common stranded copper wire.  However in applications where both high flex life and tensile strength is required, tinsel wire, or cable made up of tinsel conductors, may be the best design choice.


Testing the torsional flexibility
of cable material


Cable Torque

While not directly related to cable flexibility, cable torque, also referred to as torsional flexing, is an important consideration for some medical applications.  If either the patient or instrument needs to rotate or twist during use, a cable with low torsional resistance may be desirable.  Low cable torque is a characteristic that can compliment high cable flexibility.

Similar to cable flexibility, the resistance to torsional flexing can be reduced by cable design and material selection.  In addition, the way the conductors and other components are twisted together – the lay of the materials – can have a large influence on the torsional characteristics.  Comparing identical materials, the looser the lay, the less the torsional resistance will be.

Summary

The desired flexibility of a cable assembly should be considered early in the design process.  When a high degree of cable flexibility or low torsional resistance is required, the specification of materials, components and cable construction becomes significant.

The Affinity engineering team has experience and expertise in designing high flexibility and low torque cables.  For more information or to discuss your requirements contact Affinity at +1 949 477-9495 or email to customercare2@affinitymedical.com.

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Meet Marilu Calderon – Affinity Production Associate



Marilu Calderon,
Affinity Production Associate

Marilu joined the Affinity production team in August 2011.  Her uncle, Abel Calderon, also works at Affinity and told her of the job opening.  Before joining Affinity, Marilu had worked in restaurants for 13 years.  She said “I was ready for a change – ready for a new challenge.  I had worked in restaurants for years and wanted to learn something different.”

Marilu started on first shift in the work cell that produces lead wires.  First shift starts work at 5:00 A.M.  Asked about if it was difficult to start work that early, Marilu said “No, I am a morning person.  I like the early shift because I can be home for my children in the afternoon.  Even when I am not working, I get up early.”  She now is assigned to work cell 33 which builds a variety of different cable assemblies.  When her Cell Champion, Nayeli Rodriguez is out, others look to Marilu for leadership.


Marilu is part of the Work Cell #33 Team


Asked about her first impressions of Affinity, Marilu replied, “I was really impressed with what Affinity did.  Everyone takes their job seriously and we always strive to do our best because we build cables that help save people’s lives.”

James Sisneroz, the Production Supervisor that Marilu reports to commented, “Marilu understands what she is doing.  She understands the drawings and documents and makes sure the Work Instructions are followed.  She pays attention to detail which is important to quality and takes charge when needed.  Marilu is a very good worker!”

Marilu grew up in nearby Garden Grove, California.  She and her husband Victor have been married 14 years and have two daughters, seven and thirteen year’s old.  Describing her family Marilu said, “We have a lovely and caring family.  Most importantly, we like to help others in need.”

For hobbies, Marilu likes to bake, cook, read and use the computer.  “I really like to bake.  I bake for our family and often bring things that I bake into Affinity.  My husband Victor is a cook, but at home, I do all the cooking.”

“I’m really happy to be a part of Affinity,” said Marilu.  “Affinity is a place where I can learn and continue to grow as an employee and as a person.”

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Markings and Keying to Make Mating Intuitive



Keyway molded into plug and receptacle
facilitates proper connection

In a well-designed system, the user should instinctively know how to properly plug the cable into the device it is intended to be connected to.  Too often, the technical requirements, including electrical and mechanical properties are focused on and the user interface is given inadequate attention.

Keying Facilitates Connection

Keyways are commonly used to assist in proper mating.  Keyways can also be used to prevent a connector from being plugged into the wrong receptacle.  Keyways should be large and strong enough to prevent either misalignment or damage that would make the keyway ineffective


Large raised arrow facilitates proper
mating of the plug to receptacle


Markings

Often markings on the connector are employed to help users understand how the connector is mated to the device.  When markings are employed, care should be taken to ensure that they are sufficiently prominent to be effective.  If the design calls for a custom connector, raised markings can be molded into the connector which can provide both a visual and tactile reference.

Another option for marking is printing or applying a permanent label directly onto the connector.  The materials and shape of most connectors limit printing to simple graphics in a single color.  If multiple colors or complex graphics are desired, a permanent and durable label can be affixed to the connector.  In either case, printing or graphics should be easily read or recognized to be effective.



Color-coded connector plugs
into color-coded receptacle

Color-Coding

One way to achieve a cable design that is intuitive to connect is to use color-coding on both the cable and device.  This is especially helpful when the device has more than one receptacle.  To achieve color designations on the connector, one common method is to overmold the strain relief in a color that will match the color indicators on the device or receptacle.  Off-the-shelf connectors can often be ordered with colored strain reliefs.

Summary

When designing a medical interconnect device (a cable assembly or a connector), consideration should be given to making the intended connection as intuitive to use as possible.  If the design of a medical connector or cable is intuitive, the intended user should be able to connect or disconnect with little or no thought and without the opportunity to damage the device or connector.

The Affinity engineering team can help you design an interface that is intuitive, robust and cost effective.   Take advantage of our experience and expertise and help complete your project on-time by making us part of your team.

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Announcements, Information and Trivia



Semi-domesticated
bees produce excess
honey for human
consumption

 


Canned herring is
commonly known as
Sardines
!


Food Trivia

Honey – Honey is produced by bees as a food source for themselves.  By semi-domesticating bees and providing artificial hives, excess honey is produced and harvested for human consumption.

Tomatoes – each American consumes about 22 pounds of tomatoes a year.  Over half this amount is in the form of tomato sauce and catsup!

Celery - Celery has “negative calories.”  The calorie content of celery is so low that it takes more calories to eat celery than the celery has in it to begin with.

Sardines - The canning process for herring was developed in Sardinia, which is why canned herrings are commonly known as sardines!

Rice – rice is the most important food in terms of nutrition and caloric intake making up more than 20% of calories consumed worldwide by humans.

Mai Tai – The Mai Tai is a rum-based cocktail made popular at two California restaurants: Trader Vic’s and rival Don the Beachcomber.  The name of the drink comes from the Tahitian phrase “maita’i roa ae” typically translated as “very good.”