Staying Connected - September 2014

Understanding Cable Flexibility



Example of a 34-conductor cable
with excellent flexibility


The Perception of Quality

How a cable is perceived by the user is often influenced by the tactile feel of the cable and by how flexible it is.  Regardless of the quality of the internal components and construction, it is the outside of the cable that is the most observable indication of the quality of the product.

A cable that is pliable and flexible commonly is regarded as higher quality than one that is stiff and difficult manipulate.

Cable Flexibility vs. Flex Life

The flexibility of cable material is a different characteristic than the flex life of a cable assembly.  When designing a cable assembly, it is important to understand the roll of each of these characteristics and how they apply to 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 best 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
  • Wire gauge and wire stranding
  • Conductor insulation material, thickness and durometer
  • Type of shielding if any
  • Cable jacket material, wall thickness, durometer and color
  • Cable jacket application – extruded, extruded with air gap, tubed
  • Outer diameter of cable


Two four-conductor cables, both
with the same diameter exhibit a large
difference in flexibility due to
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.

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.



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


Because diameter affects flexibility, there is a trend to reduce the diameter of cable material by reducing conductor size.  Most medical cables designed ten or more years ago incorporated 22 to 24 gauge conductors.  Today, most cables are designed with 28 to 30 gauge conductors reducing the diameter and offering greater flexibility.

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:

The higher the number of strands, 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.

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.



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


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.  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.

Cable Construction

Cabling is the process of wrapping conductors and other components around each other to form cable material.  Cabling is required so that the cable can be flexed without placing excessive tension on the conductors on the outer radius of the bend when a cable is flexed.



Cable with black and white conductors
and a 2.5 inch (63.5mm) lay


One aspect of cabling is the lay.  Lay is the measurement along the axis of a cable component, such as a conductor, to make one complete turn about the axis.  Typically, the shorter the lay, the more flexibility the cable will exhibit.

The cable jacket covers the conductors and other components protecting them from damage and providing a user interface.  In addition to the previous discussion of cable jacket material, how the cable jacket is applied to the cabled components affects flexibility.

Extruding resin directly onto the cabled components is by far the most common method of jacketing cables.  During this process resin comes in direct contact with the cabled components filling the naturally occurring voids and forming a physical bond which makes the cable somewhat less flexible.



Examples of a cable jacket that has been
extruded with an air gap and one extruded
directly onto the cabled components


If greater flexibility is required a small gap between the inner wall of the cable jacket and the cabled components is desirable.  One method to achieve this is to extrude the jacket with an air gap between the jacket and components.  Not all cable manufacturers have the capability to extrude a cable jacket with an air gap.

Another method is to use pre-extruded cable jacket, referred to as “tubing.”  Tubing is typically done on shorter lengths of cable and involves pulling the cabled components into the pre-formed tube.

By leaving a gap between the cable jacket, the inner bundle can move independently from the cable jacket which increases flexibility.



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


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.

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 that the cable is connected to 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

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

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 your local Molex Sales Engineer, Account Manager or call Affinity at +1 (949) 477-9495 or email to custcare2@molex.com.

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Bonded Leadwire

For some medical interconnect applications, bonded wire is the most appropriate material.  Bonded wire can offer an easier user experience than individual leadwires for ambulatory monitoring applications.

Bonded leadwires help eliminate wire tangle that is common when individual leadwires are used on Holter, event, or telemetry monitors.  The longer the length and the greater the number of leads, the greater the opportunity is for leadwires to become tangled.  Tangled leadwires can lead to incomplete monitoring, patient discomfort and frustrated clinicians.


5-lead bonded wire exiting strain
relief of custom medical connector

Example of bonded wire splitting out to
individual leadwires for ambulatory monitoring

Bonded leadwires are offered in a variety of configurations:

  • Shielded or unshielded
  • Two to ten leads
  • Jacketed with TPU, TPE, PVC and Silicone
  • Tinned copper or high-performance copper alloys
  • Black, grey and custom colors
  • Wire diameter from .060” (1.5m) to .120” (3mm)


Molded rip stop prevents
bonded leadwires from splitting


Almost any length lead is possible and it is common that the bonded wires typically separate into individual conductors 8” to 10” from the patient connection.

For more information, or if you would like to see samples of cables incorporating bonded leadwire, contact your local Molex Sales Engineer or Customer Care at +1 (949)-477-9495 or email to CustCare2@molex.com

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Announcements, Information and Trivia



The NBA season typically
starts the end of October


Jamaican Blue
Mountain Coffee
– Image by Mario Roberto
Duran Ortiz, courtesy
of Wikimedia Commons

October Trivia

Sports – October is the only month of the year in which the NBA, Major League Baseball, NHL and NFL all schedule games.

Second Monday in October – in the United States, Columbus day is celebrated on the second Monday in October.  In Canada, the same day is celebrated as Thanksgiving Day.

Pizza, Popcorn and Pork – Culinary observances in the month of October include National Pizza Month, National Popcorn Poppin’ Month and National Pork Month.

Leif Erickson Day – or, in Icelandic Leifur Eiriksson is celebrated in America on October 9th to honor the Norse explorer who lead the first Europeans to North America.

Disney – The Walt Disney Company was founded on October 16th 1923 by Walt and his brother Roy Disney.

Beer Flood – the London Beer Flood occurred on October 17th 1814.  A vat at the Meux and Company Brewery burst sending over 1.4 million liters of beer into the streets killing eight.