Delivering the optimum cable for the medical industry is much more challenging than it was just a few short years ago: It is no longer just pieces of copper within a non-conductive sleeve adapted to the application. Floyd Henry of Bioconnect examines cable material design criteria for the medical industry.
Cutting-edge cables for aerospace and medical applications often need to possess best-of-both-worlds design attributes: They must be as flexible as spaghetti yet strong and durable to withstand a wide variety of environmental conditions. They may also be subject to unique specialized standards, such as bio-compatibility. This is the challenge that medical device cable designers face today.
Every cable designer must address the multitude of choices available for cable construction across a myriad of sub-platforms within the industry. Among them are types of conductors, insulator shields, jackets, chemical composition, and more. There are also the construction parameters of the cable itself, which include a multitude of options within each parameter, such as conductor thickness, wire configuration (stranded versus solid, oxygen-free, size, and coating), and shield material, even color and terminations.
Delivering the optimum cable 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 step in cable design involves the conductor, which necessitate the assessment of parameters such as average wire gauge (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 to which it will be connected.
Today, as many medical devices become smaller and more portable, so must the cable. This is especially true for multifunctional cables with high conductor strand counts. Since the conductor’s size is determined by the circuit resistance, 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.
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 its fatigue life cycle. This is true regardless of the strand-diameter ratio. For example, a 22AWG 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.
The overall gauge of the wire is determined by the number and gauge of the conductors. For highly stranded conductors, the issue is often addressed by having the individual conductors put into individual bundles of strands, or lays. Then individual lays will be intertwined to create a symmetrical conductor of high stranding. For instance, a 24AWG, 105-strand wire may consist of 3X35/44 or three lays of 35 strands, with all of the individual strands made of 44AWG wire.
The thing to keep in mind 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.
Of late, carbon conductors have been introduced for medical applications. The reason carbon conductors are rapidly being adopted is because they are radiolucent, which means they are transparent to x-rays and imaging equipment, so they can be left in place while a patient is x-rayed. These conductors are defined a bit differently than other 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 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 or electroencephalogram 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 is crucial, especially if there is potential exposure to a number of environmental elements such as water, alcohol, beverages, and 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 in contact with the environment. The design geometry and construction of the connector/cable interface must address the ability to facilitate ease of cleaning and sterilization and minimize or eliminate areas that can harbor biohazards. The initial design consideration here is to choose insulator materials that minimize the bio-activity intrinsically.
Once the insulators and conductors have been determined, the jacket is the next consideration in 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. Jackets and insulators share many of the same design considerations. While insulators may see some contact with environmental compounds or elements, it is usually at the connector sites and not along the cable’s length. The major difference is that the jacket is the component that comes into direct contact with the environment, so its design needs to be able to withstand all of the possible conditions to which it may be subjected.
In general, for medical cables, the jacket material of choice would be one that is non-reactive to the patient and resistant to chemicals and temperature extremes typically found in hospitals and other medical facilities. The most common jacket types are:
- TPE/TPR (thermoplastic elastomer or thermoplastic rubber) – This is a very common material with excellent chemical resistance that is easy to sterilize. Sterilization can be accomplished via chemical or heat (autoclaving, ETO, Gamma, etc.). This compound is an excellent choice for surgery rooms, isolation conditions, and intensive care instruments.
- Polyurethane – This compound offers excellent wear characteristics and is used for cables that will be roughly handled. The caveat with this compound is that it displays poor resistance to high heat and certain cleaning agents and techniques. It is a good compromise where cables are likely to be handled regularly in a non-sterile environment (ambulances, portable/temporary medical sites, air ambulances, etc.).
- PVC (polyvinyl chloride) – This is a mature, inexpensive compound that is easy to work with, is easily moldable, and offers good resistance to many commonly used sterilizing chemicals. It is not suitable for autoclaving or high-temperature sterilization methods.
- Silicon – The gold standard for medical jacket and insulator material. In many ways, silicon is the compound of choice in a broad range of applications. It is highly chemical-resistant, able to be autoclaved, lightweight, and flexible. While it may be the most preferred type, it is delicate and tears/rips easily, and it is more expensive than many of the other compounds.
As is the case with conductors and insulators, the physical properties of jacket materials affect the functionality. The cable’s flexibility is directly related to the jacket thickness and durometer (hardness). The thicker and harder the jacket material, the more rigid it is. Also, the molecular characteristics of jacket materials are affected by temperature, cleaning agents, and sterilization techniques, which can change jacket dynamics in the application.
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