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Cable material design for medical applications

17 Dec 2014  | Floyd Henry

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Therefore, delivering the optimum cable of today to a broader market segment with higher expectations is much more challenging than even a few short years ago. It is no longer just pieces of copper within a non-conductive sleeve adapted to the application.

General conductor design
The first element of cable design involves the conductor. The conductor requirements require the assessment of parameters such as average wire gage (AWG), material, coatings or platings, composition (stranded or solid), and coefficient of flexibility. The type and material composition of the conductor determines the cable's intrinsic ohmic value. This is the parameter that must align with the device it will be connect to.

Today, as many medical devices become smaller and more portable, so must the cable. This is especially true for multi-functional cables with high-conductor count wires. Since the conductor's size is determined by the circuit resistance it will see, the conductor's composition becomes a significant factor in cable size. The general rule is to design the conductor's size to the maximum resistance of the circuit, then engineer the conductor's composition to the cable size requirement. Figure 2 presents a cross-sectional schematic of a typical multi-conductor high-tech cable.

 Multi-stand cable cross-section

Figure 2: Multi-stand cable cross-section.

Most flexible cable will have at least 19 strands of conductor wire. The higher the stranding, the more flexible it will be. As well, the higher the strand count, the longer will be its fatigue life cycle. This is true regardless of the strand/diameter ratio. For example, a 22 AWG wire may contain 19 conductor strands of 34 gauge wire. Or it may consist of 68 strands of 48-gauge conductor wire. The upper limit of stands is about 120 and it is uncommon to find cables with more than this number of conductor strands. The overall gauge of the wire will be determined by the number and gauge of the conductors.

The thing to keep in mind here is that the conductor flexibility is proportional to the number of strands, and that the smaller the conductor gauge, the higher the resistance. Other factors that contribute to conductor performance are its material composition and construction. Hence, the desired result of the final product is related to a number of different variables.

Of late, there has been the introduction of carbon conductors for medical applications. The reason carbon conductors are rapidly being adopted is because they are "radio-lucent" which means they are transparent to x-rays and imaging equipment so they can be left in place and imaged through. These conductors are defined a bit differently than wire types and offer a unique characteristic that is ideally suited for medical cable applications. Carbon conductors are rated in "k" strands because they are so small – i.e., 2k or 3k within a given conductor.

Insulation considerations
Insulation has two main properties that need to be well understood: dielectric and composition. Insulating materials can vary widely in their dielectric constant, performance and specifications based upon the composition. Insulation composition affects cable flexibility, life, bio-compatibility and resistance to environmental factors. It also acts as the primary safeguard to prevent current leakage to the patient where the devices are attached or connected. For smaller cables that are to be used in bundles or in close proximity to one another (electrocardiogram/graph – EKG; or electroencephalogram – EEG cables) the issues of conductor/conductor and conductor/insulator contact and the consequential friction must be considered in the design of the insulating jacket.

In medical applications, the possibility of electrical leakage to a patient must be addressed as one of the more critical design parameters, because medical cables are often in contact with the patient. Therefore, the design of the insulator has to take into consideration the exposure to a number of environmental elements such as water, alcohol, beverages, even medical preparation.

While most insulators do not come into direct contact with the elements, there are points of ingress, such as connectors, that can be exposed to the environment. Occasionally, jackets may be cut or torn, which compromises the cable integrity and can allow the insulator to come into contact with the environment.

The design geometry and construction of the connector/cable interface must address the ability to facilitate ease of cleaning and sterilisation and minimise or eliminate areas that can harbor biohazards. The initial design consideration here is to choose insulator materials that minimise the bio-activity, intrinsically.

Cable jackets
Once the insulators and conductors have been addressed, the jacket comes into the design cycle. Since cables can have a number of conductor and insulating layers, the primary purpose of cable jackets is to encompass the cable components while protecting and supporting them. Figure 3 is an example of some cable jacket types. Jackets and insulators share many of the same design considerations.

 Cable jacket examples

Figure 3: Cable jacket examples.

While, as was noted above, insulators may see some contact with environmental compounds or elements it us usually at the connectors and not generally along its length. The major difference between them is that the jacket is the component that comes into direct contact with the environment so their design needs to be fully cognizant of all of the possible conditions to which they may be subject.

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