Medical Cables Choosing the Right Materials for Medical Cables

Medical cables are manufactured in a variety of materials. Knowing which type of medical grade insulation or jacket material to choose from medical cable manufacturers is important. Before designing a cable, you need to answer these basic questions:

• Is the cable disposable or reusable?
• Will there be patient contact? Bio-compatibility requirements?
• Will the cable be sterilized? If so how?

§ Flexibility requirements?

§ How will the cable be used?

Medical cables fall into two categories, reusable and disposable. Disposable one patient use cables are normally made of less expensive material such as PVC and are not subject to repetitive sterilization due to short term use. Reusable cables often times have to withstand many cycles of sterilization such as, autoclave, gamma, E-Beam, and chemical sterilization. Knowing how the cable will be sterilized is important in order to narrow down your jacket options as some materials will break down or can’t withstand certain types of sterilization.

Medical grade insulation and jacket materials differ depending on the type of contact it will have with the patient. Implantable grade insulation materials are usually Teflon or silicone rubber. Common jackets are silicone rubber or implant grade Polyurethanes. For other uses medical grade materials include PVC, TPE, polyethylene, FEP, PFA, and polyurethane.

Medical cables application and use vary from sensors to complex surgical devices. Relaying this information to your cable manufacturer is important. Cables can be subjected to repetitive flexing, abrasion, getting stepped on or rolled over by carts. Choosing a material that can withstand the day to day use will prolong the life of the cable.

The aerospace industry has applications which can be very critical of weight

The aerospace industry has applications which can be very critical of weight; Such as, missiles, rockets and satellites. When looking for ways to cut down equipment, payload, or satellite weight, the cable can be the last thing considered. One way to accomplish weight reduction in aerospace cables is to use metal clad aramid fibers for the shield and braid material instead of the standard copper. Aracon® is one such material which provides exceptional EMI shielding, strength and flexibility. Typically Aracon® fibers can reduce weight 60% in the braid, an overall reduction in cable weight of 26% on the average. Aracon® is combined with DuPont Kevlar® to provide high tensile strength and excellent conductivity. The large number of fine threads and the tendency of yarn to flatten and spread give this material more shielding coverage than copper. In the first photo you can see the amount of fibers on the end of the braid

Benefits for switching from copper braid to Aracon include:

• Overall cable weight reduction
• Added flexibility
• Increased strength
• Withstands repetitive flexing
• Improved EMI shielding coverage

Aracon® fibers are manufactured by only one company Micro-coax Inc. The Aracon® braid can be purchased either with the braid only (sock) or having the braid manufactured onto cables by cable manufacturers such as Calmont Wire & Cable. Calmont has had great success manufacturing cables using Aracon® for the space and military related applications.

The cost of using this type of material is considerably more expensive than the standard copper braid. The cost savings for applications such as satellites where weight savings equate to rocket fuel cost, definitely makes it a value. Depending on your shielding requirements, Aracon® is available with several metallic coatings, thickness and fiber sizes to meet your resistance needs. Consult your cable manufacturer or Micro-Coax for options available to you.

If you would like to learn more about EMI shielding, Aracon® or similar materials contact Calmont today. Your comments are welcome and if you find this article useful, please share it.

ARACON® is a registered trademark of Micro-Coax, Inc. KEVLAR® is a registered trademark of E.I. du Pont de Nemours and Company

Conductor Stranding: How important is it?

Conductor stranding can be a bit confusing. Wire and cable is manufactured in a variety of configurations, not all wire is the same. The conductors for wire (aside from solid core) are made of multiple strands of fine wire bunched or twisted together. Conductor stranding plays an important part of the flexibility and the performance life of the wire. The more individual wire strands in a wire, the more flexible, break-resistant and stronger the wire is. Hi flex applications where repetitive flexing is required use different stranding than wire used to wire a building or home. "Off the shelf wire normally is available in solid, 7 strand and 19 strand constructions which is fine for applications not requiring flexibility or repetitive movement.

Repetitive flexing can cause conductor strands to break over time which lowers the conductivity of the wire. Using higher strand wire helps eliminate this problem. High strand wire is used in medical handheld devices, robotics and even headphones.

Many custom wire and cable manufacturers offer high strand wire which you cannot obtain easily from distribution. For example, a 20AWG wire can be manufactured with a solid conductor or solid core all the way to 168 strands. Knowing which conductor stranding option will best suit your needs is important.

Identifying the conductor call out: A typical call out is XX (YY / ZZ). The "X" is the gauge of the wire, "Y" is the number of strands and "Z" is the gauge size of those strands. Once you understand what these numbers mean, identifying the conductor construction is simple. A wire gauge chart is a handy tool to view wire gauge stranding options

The more common conductor stranding configurations are concentric (true concentric, equilay concentric, unidirectional concentric, and unilay concentric), bunched and rope.

Rope stranding is conductor construction consisting of single strands assembled together into concentric or bunched configurations. Rope constructions consist of concentric or bunched members stranded together into the final concentric or bunched configuration. Rope stranding has the advantage of increasing flexibility by using a larger number of finer strands while maintaining a tighter diameter tolerance than a simple bunched construction. Ropes are more evident in the larger AWG sizes, such as 8 AWG and larger, but there are also many applications that require the flexibility of rope constructions in the smaller gauges. Constructions vary and can contain hundreds or thousands of strands.

Depending on the use, the type of conductor is important. Hi flex and load bearing applications would require conductors with higher tensile strength such as alloys. Copper conductors are normally for less rigorous flexing

Flexible wire is a term which can be interpreted in many ways from hi flex to the actual flexibility of the wire. Letting your cable manufacturer know what your expectation is for the flexibility of your wire or cable will help in choosing the right conductor stranding. Calmont has manufactured extremely flexible large gauge wire and cable as big as 4/0 which is limp and flops over the end of a table. Of course insulation and jacket material also plays an important role in making a flexible cable, but the conductor stranding is where you start

Medical Cables: Often Overlooked

Medical cables are as important to a medical device as the device itself. Medical device designers many times focus on the device and overlook the wire and cables that is needed for the device to perform its function. Medical cables tend to be more specialized due to the many types of uses and environments it is subjected to. The wire and cable is many times not thought of until the end of the design process, which may be too late. Knowing your medical cable limitations and the options available to you in the beginning is important especially when designing housing or specifying connectors. Some things to keep in mind when specifying wire and cable for a medical device:

• Size- With the size of devices becoming smaller many times smaller gauge wire and cable is needed. Off the shelf wire and cable usually go as small as 28AWG. Fine wire and custom cable designs are usually needed.
• Flexibility- Handheld devices and medical robots require flexible cables which can withstand repetitive flexing. Higher stranded conductors and sometimes alloys are needed to achieve this.
• Sterilization- Sterilization can be done with alcohol, steam or gamma to name a few. Cable jackets need to withstand the sterilization process.
• Bio-compatibility- Medical grade cables are needed depending on the level of contact with the patient. Bio-compatible and implantable grade compounds are available for medical devices.
• Construction- Cables sending signals, power and even gas/fluid can be constructed in on jacket. Constructions can vary from the simple wire to complex hybrid cable.
• Unique- Medical devices are all unique in regards to application, requirements and environment. You should not use an off the shelf mass produced cable designed for to meet the generic needs of a variety of industries for a specialized medical device.
  

Considering your wire and cable needs from the early stages of your product design will prevent problems down the line in regards to failures in the field, design issues and time constraints. Designing custom medical cables for your application ensures you have a medical device that will exceed performance and longevity you require.

Depending on the type of use a variety of medical grade insulation materials are available. Reusable medical cables are built to endure constant flexing and extended cycles of sterilization. Insulation may be Teflon®, TPE or silicone rubber with TPR or silicone jackets. For disposable medical cables PVC and PE are the insulation most commonly used. Discuss your application with your custom cable manufacture in order to choose the right material for your medical device.

Working with a custom wire and cable manufacture is easier than you think. Custom wire and cable engineers work with you every step of the way to determine and suggest the best materials and construction for your medical application. It's like having a wire and cable expert on your team. Cable designs are usually modified throughout the design process to accommodate changes to the device design. Many custom wire and cable manufacturers are able to produce small quantities for prototypes and clinical trials.

Silicone Ribbon and Flat Cables: More Flexibility

When you think of extremely flexible wire, silicone insulated wire comes to mind because of its flexible properties. But what about using silicone in ribbon cable? Manufactures turn to silicone ribbon cables when applications require flat cables with specialized requirements or extreme flexibility. Flat cables and ribbon cables are often used when space is a factor ribbon cables can also be easily mass terminated using

an insulation displacement connector (IDC). Ribbon cables are manufactured in a variety of materials and configurations including silicone; the most common and least expensive insulation is PVC.

Space is a luxury as devices become smaller. Manufacturers are using flat and ribbon cables for more applications.

Industries include:

Aerospace
Automation/Robotics
Medical
Industrial
Automotive
Telecommunication

These types of applications often need insulation materials other than PVC due to special requirements such as extreme temperature, low smoke, flammability, flexibility, sterilization and chemical contact. Silicone is an excellent choice for these types of requirements. Not only is silicone wire extremely flexible, it can withstand millions of flex cycles, and has good steam and chemical resistance. Silicone ribbon cables can be extruded or bonded depending on the type of termination that is used. Extruded ribbon cables have a tighter center to center tolerance than the bonded ribbon cable. When using a bonded ribbon cable, terminating with IDC is recommended. Extruded ribbon cables normally require tooling to control shape and pitch which may add to initial cost. Bonded ribbon cables normally do not have special tooling used.

Flat cables can be manufactured in a variety of constructions which may include zip cords, ribbon cables and flat cables with an outer jacket. Custom cables can also be constructed with shielded twisted pairs, overall shield, and with tubing or lumens. Custom flat cables are also available in smaller gauges which you cannot find "off the shelf".

Silicone insulated wire and cables do have their drawback. Silicone has poor abrasion resistance, which can be a critical problem for applications where the cable comes in contact with sharp objects. Higher abrasion resistant formulas are available for silicone. Consult your wire and cable manufacturer regarding your application to find out the options available to you.

To find out more about flat cable options or if silicone cables are right for your application contact Calmont for assistance with your design or request a quote. Feel free to leave a comment.