Staying Connected - October 2009

Cable Flexibility Design Issues


Two four-conductor cables both
with the same .187” diameter that
exhibit different degrees of flexibility
due to materials and design

One of the first impressions of the quality of a medical cable comes from how it looks and feels to the user.  Regardless of what’s inside, it is the outside of the cable that is the most obvious indication of the quality of the product.  The look and feel of a cable is largely influenced by the jacket material and by the flexibility of the cable.

Initial specifications for cable material often include only minimum requirements such as:

  • Number of conductors
  • Conductor size
  • Type of shielding
  • Cable jacket material

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

Smaller Diameter Equals Greater 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.  Because shielding increases the diameter of cable, shielding will decrease the flexibility.


Cable flexibility is influenced by the choice of components


Cable Components

  • 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 biocompatibility and cytotoxicity requirements.  A medical-grade thermoplastic elastomer (TPE) such as Santoprene® is one of the most common materials specified for jacket material.

    The hardness of jacket material also affects its flexibility.  Most materials used for wire and cable jackets are available with different degrees of hardness.  A softer grade of a specific material will be more flexible than a harder grade.  Softer grades are typically less durable than harder grades.

    For increased durability, polyurethane is often specified.  Silicone offers excellent flexibility, and flex life and optimal autoclave performance.  PVC is most commonly used as a jacket material for lower-cost cables and can offer good flexibility but with limited flex life performance.

  • 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 multi-conductor cable, a small increase in the jacket thickness of any conductor can equal a significant increase in diameter. 

  • 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 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 built up with 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
1 28
7 36
16 40
19 40
40 44
65 46

The higher the number of strands, the greater the flexibility of the electrical conductor.  The trade off is that a higher number of strands increases the cost of the conductor.


28 gauge conductor with 16 strands
of 40 gauge tinned copper

28 gauge conductor with 40 strands
of 44 gauge bare copper

  • 11-conductor cable showing filler
    in center to both increase
    diameter and round-out the cable
    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.
  • Serves and Tapes – These are materials spirally wound around cable components to hold them in position for subsequent processing.  The tightness or looseness of the wind can affect flexibility with a tight wind tending to restrict movement of components and reduce flexibility.  A slippery serve material such as Teflon 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.

Teflon tape wrapped around a
spiral shield before jacketing

A spiral shield typically offers
greater flexibility than either a
braided or foil-wrapped shield

Braided shields typically offer the greatest amount of shielding, but when coverage is high, flexibility is compromised.  In high-flex applications, the braid can break down becoming abrasive and actually reduce cable life.  A foil shield, which is typically aluminum laminated to a film offers reduced flexibility and a shorter flex life.


  • Example of a strength member
    as the core of a shielded cable
    Strength member – When additional tensile strength is needed in a cable, one method is to add a strength member within the cable assembly.  One common method is to add a core of synthetic fiber such as Kevlar which has very high tensile strength for its size and weight.


Tinsel Wire

Another way to increase flexibility is to use flattened tinsel conductors instead of round copper or copper alloy conductors.  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 copper and are often plated with tin or silver.  This type of wire construction offers excellent flexibility due to the ribbon conductors and excellent tensile strength due to the strong fabric core.


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

Tinsel wire made up of seven
strands of tinsel conductor

Due to the thinness of the conductors, tinsel wire is not suitable for high voltage applications and 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.

Cable Torque

While not directly related to cable flexibility, cable torque 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 torque may be desirable.  Low cable torque is a characteristic that can compliment high cable flexibility.

Summary

The desired flexibility of a cable assembly should be considered early in the design process.  When a high degree of cable flexibility is desired or required, the specification of materials, components and cable construction becomes important. Involving the Affinity engineering team early in the design stage can help achieve your design goals, including cable flexibility.

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Affinity Customer Care

At Affinity Medical, we don’t have a customer service department.  In place of customer service, we have Customer Care Coordinators.  While our Customer Care Coordinators perform many of the same functions that customer service representative would, we strive to offer our OEM partners more than that.  The job of our Customer Care Coordinator is to care for and take care of our OEM customer partners.
Affinity’s Customer Care team consists of Candy Golding, Suzann Sitka and Cesar Jara.  Candy is the team supervisor and has over twenty years experience working with medical cables.  Suzann Sitka joined Affinity over four years ago, and has extensive experience having worked for several medical device manufactures before joining Affinity.  Cesar is the newest member of the team, joining Affinity last summer.


Candy, Suzann and Cesar – the Affinity Customer Care team

The Affinity Customer Care team is always busy!  Some of the day-to-day duties of our Customer Care team include:

  • Being the primary contact within Affinity for our OEM customer partners via phone, email, and fax
  • Accept and process customer orders and customer change orders, including contract review and confirming pricing and delivery dates
  • Track orders through production to help assure on-time delivery
  • Prepare and send quotations to customers on behalf of  engineering or sales
  • Coordinates deliveries of samples and customer-supplied parts to and from customers
  • Coordinate project meetings between customers, engineering and sales
  • Attend project meetings, takes meeting minutes and distributes minutes and action items to all team members

Besides the above duties, one of the main responsibilities of the team is to be available when customers call.  “When a customer calls, we need stop whatever it is we are doing and take their call,” said Candy.  “If for any reason we can’t take their call, we need to get back to them right away.”

“Our OEM partners love Candy, Suzann and Cesar,” said Business Development Manager Hank Mancini.  “When I visit with our customers I always receive unsolicited compliments and positive comments about our Customer Care team.  They take good care of our customers and the rest of the Affinity team really appreciates that.”

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the Jack-O-Lantern is one
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Halloween

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