Medical cables and sterilization

Medical cables are manufactured using a variety of materials. Reusable medical cables often times are subjected to sterilization processes and are built to endure extended cycles of sterilization. Understanding how a wire or cable will be sterilized is crucial to determine what materials to specify when designing a medical device. Insulation and jacket materials vary when it comes to resistance to chemicals, heat and other sterilization techniques.

Common types of sterilization include:

• Heat/steam - Autoclave
• Chemical
• Irradiation - E-Beam & Gamma Ray

Autoclave (heat/steam) resistant materials for reusable medical cables include FEP®, TPE, silicone or with TPR or silicone rubber jackets. Radiation resistant medical cables can be made with silicone and Teflon® (if water is not present). TPE and PVC can be made for Gamma sterilization with limited sterilization cycles. PVC is not commonly used with chemical sterilization, alcohol based liquids can break down the plasticizers (an additive for PVC) and can make the PVC brittle and damage the insulation or jacket. PVC is more commonly used with disposable applications.

It is always best to discuss with your custom cable manufacturer what types of sterilization and how many sterilization cycles the cable will be subjected to. Cable manufacturers experienced with medical cables and their applications can help narrow down your choices based on sterilization techniques, bio-compatibility and other requirements.

Silicone Insulated Wire- Medical Grade Silicone Applications

Silicone insulated wire and cable is an excellent choice for medical cable applications. Besides being extremely flexible, silicone rubber can withstand autoclave and other forms of sterilization. Properties that make silicone suitable for medical applications include: Low thermal conductivity, low chemical reactivity, low toxicity and thermal stability (consistency of properties over a wide temperature range of 100°C to 250°C). Abrasion resistance is a concern when considering silicone insulation or jacket. Formulas for higher tear strength are available to increase resistance to abrasion.

Applications suitable for silicone cables include:

• Arthroscopic
• Surgical Robotics
• Electrosurgical
• Implantable
• Ophthalmic
• Foot Switch
• Patient Monitoring

What makes a material medical grade? Materials are tested for bio-compatibility and are categorized to be used for medical applications. Medical grade materials are generally grouped into three categories: non implantable, short term implantable and long-term implantable. Materials approved as USP Class V and VI can be considered medical grade, most medical grade silicone is at least Class VI certified.

Uses for medical grade silicone cables include:

• Tubing/Lumen
• Drains
• Feeding tubes
• Catheters
• Implants for long and short term use
• Sensors

Co-extruded and multi-layer silicone cables can be used for high voltage applications such as X-Ray and MRI equipment which require electrical noise suppression and limited power loss . Co-extrusion involves extruding a semi-conductive silicone layer around the conductor to remove air thus reducing corona. Extruding both semi-conductive layer and jacket simultaneously insures no air or particles are caught between the layers. Benefits for co-extruded silicone include corona, moisture & ozone resistance, lightweight and flexible cable design and radiation resistance to name a few.

When working with your custom cable manufacturer, it is crucial to provided them as much information as possible about the application, the environment, sterilization and bio-compatibility requirements. The more information you can provide to your silicone cable manufacturer will help in determining if silicone is right for your application and to offer you material choices that will enhance your product's performance.

Super Flexible Wire Designs: High strand count conductors

A super flexible wire design requires two things, a flexible insulation material and the right conductor stranding. Even with the most flexible wire insulation, the flexibility of the wire will depend on the conductor stranding. Conductor construction is key in achieving a noodle like flexible wire. There are two things to look at when choosing the right conductor:

1. Number of strands. Conductors are made of many fine strands which are cabled together to meet the required gauge size. Conductor stranding is available from one strand (solid) to hundreds depending on the gauge size. The higher the strand count the more flexible and resistant to flexing fatigue the wire will be. For example a 24AWG wire the stranding options are from 1 strand (solid) all the way to 105 strands. That’s quite a range.

2. Conductor construction. How these fine strands are put together is as important as the strand count. The most common configuration is concentric. Concentric conductor may be defined as: a central wire (strand) surrounded by one or more layers of helically laid wires in a geometric pattern. 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.

Flexing vs. Flexibility

High Strand Silicone Wire

To some flexibility means to withstand repetitive flexing, while to others it means how much the wire will bend and rigidity. For both types of applications a high stranded conductor is recommended. For repetitive flexing one should consider using an alloy material for higher strength and flex cycles. For noodle like flexibility a standard copper conductor is fine.

Flexible cables

Flexible cables also use high strand conductors, but there are more factors involved controlling the flexibility; Such as, Shielding, outer jacket material, cable lay construction and type of conductors just to name a few.

It's always best to discuss your flexibility needs and the conductors available with a wire and cable manufacturer, especially one that works with high strand count conductors. Off the shelf (commodity) wire manufacturers usually work with solid, 7 and 19 strand conductors which are less flexible than the high strand conductors.

Silicone Cable- 5 Things you didn't know about Silicone Wire

Silicone wire is extremely flexible and resistant to high temperatures, everyone knows that. There are some misconceptions regarding silicone wire and silicone cables. Listed below are five facts you probably didn't know about silicone insulated wire that I get asked frequently.

1. It doesn't have to be tacky. Recently at a trade show I handed a visitor a sample of our silicone wire and he said: "Wow, this isn't sticky, we stay away from silicone because it's always sticky." Silicone rubber is normally tacky, this can be a problem especially in medical applications where you don't want dirt sticking to the cable. Silicone cables can be made so they are not tacky or sticky. This is accomplished with manufacturing techniques and formulas can be used to make the jacket smooth and not tacky. Normally silicone wire and cable is coated with Mica to keep it from sticking to itself during the manufacturing process, this coating is not acceptable in some applications that requiring clean environments. The alternative is to manufacture the silicone so it is not sticky from the start.

2. Available in ribbon cables. Silicone cables can be manufactured as ribbon cable a few different ways. Silicone ribbon cables can be extruded, bonded and even molded into ribbon and flat cables. Silicone ribbon cables can be extruded as a ribbon cable which offers the tightest center to center tolerances. Bonding the ribbon cable involves extruding the individual silicone wires separately then they are bonded together, this method is less expensive than extruding the ribbon cable because there isn't any additional tooling. Bonded silicone ribbon cables work best with IDC connectors and soldering. Molded ribbon cables are usually silicone wires encapsulated with a silicone outer jacket, this method allows for unique flat shapes.

3. Low outgassing formulas are available. Aerospace, specifically space applications often have outgassing restrictions for the wire and cable. Recent advancements with silicone formulas and manufacturing techniques have created low outgassing silicone cable. For space craft and satellite applications Calmont manufactures silicone wire and cable products which meet NASA .01% TML requirement. Depending on your outgassing requirements, check with your silicone cable manufacturer to find out what is available. Click here to view Aerospace insulation properties chart.

4. Reinforced silicone is available. High strength silicone formulas are now being used to increase abrasion resistance of silicone wire and cable. Fluorosilicone has better abrasion resistance than the standard silicone. Additional measures can be taken to reduce abrasion such as strength members between layers and nylon outer braid to protect the silicone insulation.

5. Doesn't burn. Silicone insulation is self extinguishing, it will not burn. Flame retardant additives can be added to meet flame requirements such as VF1