Staying Connected - April 2011

Designing Cables for Increased Flexibility



Two four-conductor cables, both with the
same diameter that exhibit a large difference
in 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 the jacket, it is the outside of the cable that is the most observable indication of the quality of the product.  The way a cable feels and is how it is perceived by the user is largely influenced by the jacket material and by its flexibility.

Initial specifications for cable material typically include requirements such as:

  • Number and type of conductors
  • Gauge of conductors
  • Type of shielding if any
  • Type of 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 quantify than electrical requirements or other physical properties.  And, for some applications, cable flexibility and torque can be critically important.

Smaller Diameter Equals Greater Flexibility


Cable flexibility is influenced by the
choice of materials and components

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.  Regardless of the type of shielding, because it increases the diameter of the cable it will decrease flexibility.

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.

The durometer or 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.  But, the trade-off is that softer grades are typically less durable than harder grades.

One of the most flexible materials used to jacket 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 trade off 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 multi-conductor cable, a small increase in the thickness of insulation of conductors can equal 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
1 28
7 36
16 40
19 40
40 44
65 46

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


28 gauge conductor with 16 strands
of 40 gauge tinned copper

28 gauge conductor with 40 strands
of 44 gauge bare copper

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.



Teflon tape and a spiral shield can
improve 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 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.

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.


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

Example of an aramid synthetic
fiber strength member as the core
of a shielded cable

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 will 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

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.

Cable Torque


Affinity Lab Manager, Bob Evans,
testing 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 torque 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 important.

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 Karena Karena Bejarano – Buyer



Karena Bejarano, Affinity Buyer

Karena started her career at Affinity in January as a temporary employee brought in to help prepare for moving from Irvine to Costa Mesa.  As a temp, Karena helped organize and pack thousands of files.  Immediately after the move Karena continued, organizing the same files in Affinity’s new building.  Once all of the files were in order, Karena was asked to assist the Quality Team

Karena reports to Affinity Materials Manager, Sue Alessi.  When asked why she chose Karena to fill the open Buyer’s position, Sue said, “We were fortunate to have Karena work at Affinity as a temp for about three months.  We saw that she worked hard regardless of what she was asked to do.  We had a good idea of how she would fit in before we hired her.”

(Insert photo “KnS.jpg) Caption: “Karena and her supervisor Sue Alessi reviewing materials reports”)


Karena and her supervisor Sue Alessi
reviewing materials reports

Continuing, Sue said “Karena brings a wealth of experience to Affinity including purchasing and project management. Karena is a hard worker and is very organized.  She is also a quick learner.  If you show her something once, she understands and then needs little or no supervision.  She really understands the flow of materials”

Asked what her biggest challenge has been so far, Karena said “The most difficult thing so far has been learning the thousands of different components that are used to manufacture our products. 



Karena checking material inventory
in Affinity warehouse

Karena was asked of her impressions of Affinity so far and answered, “I work with a group of people who truly care about the products and services we provide to our customers.  I am very excited to part of the Affinity team.”

Karena grew up on Oahu in Hawaii.  She and her husband live nearby in Tustin.  They have three children and will be celebrating their 10th wedding anniversary this year.

Outside of work, most of Karena’s time is spent on family-oriented activities.  She is also Vice President of the Tustin National Junior Basketball League and enjoys watching and even playing basketball.  Vacations are usually spent traveling back to Hawaii to visit family.

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Announcements, Information and Trivia


Food Trivia

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

Refried Beans – Refried beans are only fried once.  An error in the translation of “frijoles refritos, which actually means “well fried beans,” yielded the incorrect name “refried beans.”



For many, purple carrots
may seem to be an oddity

Carrots – Before the 17th century, carrots were purple.  The orange carrot that we know today was first cultivated by Dutch farmers in the late 16th century.

Orange and Oranges – The name for the color we now call orange came from the name of the fruit.  The Spanish name for oranges is “naranja” which was modified in English to orange.


Lobster – Until the 20th century, lobster was considered a “poor man’s food” and was so plentiful that in some areas it was used as fertilizer.

Worcestershire Sauce – This pungent sauce produced by Lea & Perrins is made by soaking anchovies in vinegar until they have completely dissolved.

 

Affinity Customer Care and Hours of Operation



Candy, Suzann and Cesar – the Affinity Customer
Care team - the Affinity Customer Care Team

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 Coordinators is to take care of our OEM customer partners.

The Affinity Customer Care team includes Candy Golding, Suzann Sitka and Cesar Jara.  Candy is the team supervisor and has over twenty years experience working with medical cables.  Suzann joined Affinity over five years ago.  She has extensive experience having worked for several medical device manufacturers before joining Affinity.  Cesar is the newest member of the team, joining Affinity in mid 2009.

Affinity Medical Technologies Customer Care specialists are available to assist you from 7:00 A.M. until 4:00 P.M., Monday through Friday, U.S. Pacific Time, except holidays.

 

Affinity Medical Technologies

3545 Harbor Boulevard, Suite 150
Costa Mesa, CA 92626 USA
Phone: +1 949 477 9495
Fax: +1 949 477 9499

Email: Customercare2@affinitymedical.com
Website: www.affinitymedical.com
Blog: http://blog.affinitymed.com