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Cable material design for medical applications

17 Dec 2014  | Floyd Henry

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Today, the pervasiveness of interconnect cables runs though virtually every nook and cranny of technology. There are countless types of cables for countless applications. Even though the wireless revolution explodes with its mantra of tether-less interconnect, it is far from universal in many industries.

In some segments, such as medical device/patient interconnects, cables are the only solution and the tether is still king. As well, medical cables are held to a higher standard when it comes to electrical specifications and patient safety. While cables aren't necessarily the most glamorous element of today's medical technology, without a doubt they are still critical and very significant components of medical devices.

Modern cables do not vary from their earlier counterparts in one basic aspect. In the end, they still have only two fundamental components: conductors and insulators. However, modern, high-tech designs of cables include multiple elements of these components as well as special constructs such as air, water, vacuum, suction reinforcing elements and drain wires, as well as others, within the "cable" assembly. These are used to enhance medical cables and insure that patient safety and cable reliability are maintained as well as enable a diverse applications base. Figure 1 presents some examples of specialised components, such as drain wires and strength members, which enhance medical cables.

 Cables with specialised components

Figure 1: Examples of cable with specialised components.

However, these enablement components makes the manufacture of medical device components much more complicated than simple cables with just a few current circuit pairs. It is this complex structure that needs to be thoroughly understood, and the topic of this white paper. Appendix B gives a list of devices within the medical device field that utilise cables of various types and having a myriad of design requirements.
The challenge
Cutting-edge cables for aerospace and medical applications are often required to possess the "best of both worlds" design attributes. That is, they must be as flexible as "spaghetti" yet strong and durable and withstand a wide variety of environmental conditions. As well, they can be subject to specialised standards, such as bio-compatibility, that are unique. This is the challenge that medical device cable designers face today.

Discussion: One example, of many in today's medical equipment science, is the uptick in the use of robotics. The extensive integration of robotics has a number of benefits, such as decreasing device foot print and power efficiency, but there is a downside, it increases design complexity, sometimes by orders of magnitude.

In many cases miniaturisation is a primary driver in device footprint and it, in turn, demands cables and wires to be smaller and more flexible yet containing an ever increasing number of conductors. To the inexperienced designer, meeting these conditions while maintaining the vision of the value proposition for the customer sometimes seems like an insurmountable task.

Even when the global issues of this particular industry are well understood, the designer still has to address the multitude of choices that are available for cable construction across a myriad of sub-platforms within the industry. Among them are types of conductors, insulators shields, jackets, chemical compositions and more. And there are the construction parameters of the cable itself.

As well, there is a veritable cornucopia of options within each parameter such as conductor thickness, wire configuration (stranded vs. solid, oxygen-free, size and coating), shield material, even colour and terminations. This means that cable design involves a complex collection of factors that must be exact so that sensed or transmitted signals retain their original or conditioned state.

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