Cable Material Design Criteria for the Medical Industry by Floyd Henry Director of Manufacturing and Engineering Bioconnect, Division of RF Industries

Transcription

1 Cable Material Design Criteria for the Medical Industry by Floyd Henry Director of Manufacturing and Engineering Bioconnect, Division of RF Industries Abstract In the medical device industry, there is a much smaller margin of error that can be tolerated than in most other industries. What typically may work well and be economical in non-medical applications can create a myriad of situations that can become problematic in the medical industry. Medical devices are highly precise and sensitive instruments. As such, medical bio-connect cable, and more significantly, the material used in such apparatuses must be carefully chosen to perform under a unique set of considerations that are often significantly more constricting than standard, off-the-shelf cable products. Therefore, the design of medical interconnect cables is a rather specialized science that requires specific engineering expertise. This expertise insures that developing and fabricating such interconnect devices means that they meet exacting standards, under often less than ideal conditions. This paper will address the particular design and fabrication challenges of medical cables. It will examine the design parameters, material choices and manufacturing requirements that medical device manufacturers need to know to ensure that medical device interconnect is precise, reliable and durable and protect them from liability. Introduction Today, the pervasiveness of interconnect cables runs though virtually every facet of technology. There are countless types of cables for countless applications. Even though the wireless revolution explodes with its mantra of tetherless 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 specialized components, such as drain wires and strength members, which enhance medical cables. However, these enablement components make the manufacture of medical device cables 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 utilize cables of various types and having a myriad of design requirements.

2 Figure 1. Examples of cable with specialized components. 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 specialized 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 miniaturization 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 subplatforms 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 are a multitude of permutations of options within each parameter such as conductor thickness, wire configuration (stranded vs. solid, oxygen-free, size and coating), shield material, even color and terminations. This means that cable design involves a complex collection of factors that must be toleranced so that sensed or transmitted signals retain their original or conditioned state. 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 necessitate 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 2

3 determines the cable s intrinsic ohmic value. This is the parameter that must align with the device it will be connected to. 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 count. 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 multiconductor high-tech cable. 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 Figure 2. Multi-strand cable cross-section. 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 than this number of conductor strands. The overall gauge of the wire will be determined by the number and gauge of the conductors. For the highly stranded conductors, often the issue will be 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 3 lays of 35 strands, with all of the individual strands made of 44awg wire 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 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 Considerations Insulation has two main properties that need to be well understood dielectric 1 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 3

4 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 be taken into consideration including the potential 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 Figure 3. Cable jacket examples. 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 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. 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. 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. Overall, 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 hospital/medical facilities. Following is a brief discussion of the more common types. A more detailed list of jacket and insulators are listed in Appendix A. TPE/TPR Thermoplastic elastomer or thermoplastic rubber. This is a very common material with excellent chemical resistance and is easy to sterilize. Sterilization can be accomplished via 4

5 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 have rough handling. 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 site, air ambulances, etc.). PVC Polyvinyl Chloride PVC is a mature, inexpensive compound that is easy to work with, 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 very chemically resistant, autoclaveable, lightweight and flexible. While it may be the most preferred type, it does have one limiting factor. Silicon is delicate and tears/rips easily and is more expensive than many of the other compounds. As is the case with conductors and insulators, the physical properties of jacket materials affect its functionality. Flexibility of the cable 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 and can change jacket dynamics in the application. Cable Dynamics There are also other design options available for the engineer. For example, intrinsic cable electrical and physical characteristics can be modified by adding certain materials in the form of tape between layers. Teflon tape can be used as a binder between the cable conductors and shield to reduce the friction between them as they move against each other during use, for example. Friction is actually a significant issue in microwatt signal conditions and dealing with static electricity within the cable. When friction occurs within the cable, it generates what is called triboelectric effect, or noise. It is a result of the friction between the conductors or insulators against the shield and creates contact electrification of the materials. It is essentially a voltage potential that develops within the cable due to the differences between the elements along the cables (think static electricity). However, this charge can transfer into the devices attached to the cables and produce undesired phantom readings in the equipment. In medical equipment that often appears as mv, µa and mw, 50 mv of static noise. This is significant and potentially life-threatening if it skews an EEG signal, for example. Another potential interference variable that needs to be addressed in medical cable design is radio frequency interference (RFI). This type of noise can come from a number of potential sources such as antennas, electronic communications equipment, radars, terrestrial navigation aids, wireless networking devices even consumer electronics such as computers and entertainment equipment. Without proper construction, cables can act as capacitive or inductive elements when exposed to stray RF. In the clinical setting, transmitting devices, X-ray, computed tomography (CT) scans and magnetic resonance imaging (MRI) can create these signals, as well as surgical devices like electrocautery or even a drill used by an orthopedic surgeon. 5

6 When design calls for cables that are in close proximity to one another, the specter of crosstalk becomes a problematic element. Crosstalk occurs when signals leak from one closed signal path to another and induce stray electric fields. Good design practices include well-designed outer shielding and interweaving conductors (twisted) to nullify or reduce the effects of inter-cable signal coupling. Addressing the issue of crosstalk in insulating materials can become complex, and requires the consideration of various types of insulators, based on their dielectric strength, in order to maintain an acceptable level of crosstalk but keep a cable small. This is one of the permutations of selecting the proper insulating material, thickness versus insulation value. Finally, electromagnetic interference (EMI) can have a negative effect upon such low-power cables as those used in medical equipment. Large inductive pulse sources, such as elevator motors, industrial relays, solenoids, generators, etc., all have the potential to induce anomalies into sensitive complex cable assemblies. Jackets and shielding are the major components that insulate cables from these types of induced noise. Cable Manufacturing As has been discussed in this paper, the design criteria that applies to medical cables means that, in most cases, common cable manufacturing methods cannot be used, or need to be modified. The packaging and medical manufacturing industries are heavily regulated and must maintain a high level of hygiene to prevent end-users from being exposed to harmful microorganisms. In that vein, a standard, ANSI/ AAMI EC53, was developed to address the specific segment of ECG cables and leads. While it only applies to ECG cables, it is often used as a base upon which to build specifications for other medical cable applications since there is no global medical cable standard. The designer should be aware of this standard so a brief discussion is warranted. What this standard does is try to assure a level of reliability and safety as a baseline for cable specifications. Originally, it was developed to prevent the inadvertent cross-connect of patient leads to eliminate the potential of such leads coming in contact with a power source. A corollary was to facilitate patient movement in situations where time is of the essence so cable management was not an issue in transport or transfer. And while the standard is voluntary, most reputable cable manufactures will use it as a formal standard, with custom manufacturers going to a much higher level of quality and precision. Although there are a seemingly limitless number of choices for cable interconnect solutions, the ANSI/ AAMI specifications, as well as those from IEC, UL, CE and other such societies, can and do create specifications for the development of safe cable and wire systems. There are four basic sections to the standard: electrical, mechanical, testing and environmental. Following is a brief overview of the more significant elements of the standard. The designer is encouraged to investigate this standard in depth, if more than a cursory interest in cable design is the goal. Electrical In general, accepted practices of electrical source isolation are required. This is a bit of a moving target since defibrillation equipment has high levels of voltage that is sent to the patient through its cables, while heart rate, pulse and other monitors have none. So, for example, depending upon the customer s application, isolation design can have varying degrees of criticality to protect both the patient, and the monitoring equipment. 6

7 Noise was previously discussed in some detail. The primary goal of this section of the standard is to insure that an adequate level of shielding is designed in, to minimize or eliminate the various noise sources. The standard establishes the maximum peak-to-peak noise of 50 µv and a standard test method to confirm the noise levels. Other electrical requirements such as hi-pot, sink current, and contact resistance are called out as well. Mechanical Mechanical call outs include elements that relate to stress, forces, flexibility tensile strength, etc. Typical parameters include numbers for mate/un-mate cycles, pull/retention forces and flex cycles, generally for contacts and connectors that are cable inter-mates. In each case, the designer should know the specifics for the application and use the standards accordingly. Testing The testing of cable involves a wide variety of parameters, from electrical to physical to functional. To cover them all in this section would be unrealistic, therefore, the focus for this section will be on the electrical. Because medical cables currents can span the gamut from a few micro amps to several amps, if the standard were to attempt to address each scenario, it would be unwieldy at best and likely unrealistically complex at worst. It would have to contain a large number of specific testing methodologies for each scenario. To follow this logic would be a daunting task. Therefore, rather than having the standard address each situation specifically, it attempts to establish procedures, rather than methodology that can be applied to most cables with respect to their applications. Such a test standard gives the engineer a level playing field when designing to the customer s requirement. Test equipment, circuits, and specific procedures are detailed and allow uniform and consistent testing, while the test standard provides test methods and procedures by which compliance to the standard can be verified. The value of the test segment of the standard is that the test methods defined in the standard can be applied to most medical cable assemblies so the playing field is fairly level. For instance one electrical test covers the critical aspect of current flow between conductors. Generally it is at a given voltage value, e.g., 1 KV for one second duration for all combinations of conductors within a given cable assembly (generally referred to as hi-pot testing, referencing the high potential of 1000 volts). This is one aspect of the testing phase that assures that exposure to the patient will be non-fatal, as a very small current can induce an arrhythmia. Also in the medical setting the issue of the generation of heat must always be considered. In an oxygen rich environment, the slow build-up of heat can end up in a disastrous conclusion. Again the consideration of wire size and the factors that combine to create electrical resistance must be attended. Also grounding issues are of critical importance. You need a device (ECG, ultrasound or whatever device is in use) to be well grounded, but you have to isolate the patient from ground to keep current from flowing to and/or through the patient. In general, the final goal, as the creators of the standards and the FDA define it, is the safety of the patient, as well as signal integrity. Environmental For the bio-environment wherein most medical cables exist, cleaning and sterilization are paramount. Its importance was touched upon earlier and perhaps no other cable environment has such critical considerations. Inside this segment of the standard is a set of procedures that allow the engineer to design cable components to withstand the harsh chemicals, extreme environments, as well as procedures that will ensure patient safety. For example, the standard stipulates that cables must withstand being cleaned fifteen times using green soap, glutaraldehyde, or bleach. Depending on the environment and the use of the device, eventual sterilization methods may include extreme applications 7

8 such as steam autoclave, EtO, or E-beam sterilization. However, in other environments a simple wipe may be the sterilization method of choice such as a wipe down with high-concentration chlorine (generally in a pre- packaged wipe form, such as Clorox Dispatch wipes) or Isopropyl alcohol 70% concentration. These types of products may need to be considered in the design of the cable as well. The choice of sterilization method however must be identified in the early design stages of the device when materials are being selected and life cycles are being calculated. The final application determines actual environmental design requirements. However the adherence to a specification ensures that the cable will be capable of withstanding the sterilization method of choice of the end user. For further information, the reader is directed to the ANSI/AAMI EC53 standard for review (2008 being the latest revision). The standard is an initial guide which provides a wealth of fundamental information for the medical cable designer. Conclusion This paper has attempted to provide a high-level overview of some of the issues that face the designer in the medical cable industry and provide some insight. Pertinent topics such as material and components used in cable fabrication have had some of the layers peeled back and the design issues exposed. As well, manufacturing topics have been discussed and solutions presented, which should help the medical cable and device designer sort through the myriad of considerations that are part of medical cable engineering. A brief discussion of medical standards was presented and how they benefit the designer as a reference baseline. A detailed discussion of the electrical considerations and parameters of cable design was also presented. Finally, this paper attempted to present some concepts to the design challenges of medical cables. It is hoped the reader will come away with a reasonable overview of the design issues that face the high-tech medical cable and cable device designer. 1 The dielectric constant is a mathematical representation of the insulating ability of a material at a given thickness when subjected to certain potentials. Appendix A Description of Common Cable Insulation Materials Cellular Fluorinated Propylene, expanded (CFEP) or foam FEP, consists of individual closed cells of inert gas suspended in a Fluorinated Ethylene/Propylene medium, resulting in a desirable reduction of the dielectric constant. Cellular Polyethylene, expanded (CPE) or foam Polyethylene, consists of individual closed cells of inert gas suspended in a Polyethylene medium, resulting in a desirable reduction of the dielectric constant. Ethylene-Propylene-Diene (EPDM) is a chemically cross-linked elastomer with excellent flexibility at high and low temperatures (150 to -55 C). It has good insulation resistance and dielectric strength, as well as excellent abrasion resistance and mechanical properties. EPDM also has better cut-through resistance than Silicone rubber, which it replaces in some applications. 8

9 Fluorinated Ethylene/Propylene Copolymer (FEP) has excellent electrical properties, temperature range and chemical resistance. It is not suitable where subjected to nuclear radiation and does not have good high voltage characteristics. FEP is extrudable in a manner similar to PVC and polyethylene. This means that long wire and cable lengths are available. TFE is extrudable in a hydraulic ram type process. Lengths are limited due to amount of material in the ram, thickness of the insulation, and preform size. TFE must be extruded over a silver or nickel-coated wire. The nickel and silver-coated designs are rated 260 C and 200 C maximum, respectively. Halar is a thermoplastic fluoropolymer material with excellent chemical resistance, electrical properties, thermal characteristics, and impact resistance. The temperature rating is -70 C to 150 C. Neoprene is a very good material for outdoor cables because it is both oil- and sunlight resistant. Its temperature range varies from between -55 C and +90 C. The actual range would depend on the formulation used. The most stable colors are Black, Dark Brown, and Gray. The electrical properties are not as good as other insulation materials. Because of this, thicker insulation should be used. Typical designs where this material is used are lead wire insulation and cable jackets. Polyethylene (PE) is a very good insulation in terms of electrical properties: low dielectric constant, a stable dielectric constant over all frequencies and has a very high insulation resistance. In terms of flexibility, polyethylene can be rated stiff to very hard, depending on molecular weight and density low density being the most flexible, and high-density, high-molecular weight formulation being very hard. Polyethylene also has an excellent moisture resistance rating. Correct Brown and Black formulations have excellent weather resistance. The dielectric constant is 2.3 for solid insulation and 1.64 for foam designs. Flame retardant formulations are available with dielectric constants ranging from about 1.7 for foam flame retardant to 2.58 solid flame retardant polyethylene. Polypropylene (PP) is similar in electrical properties to polyethylene. This material is primarily used as an insulation material. Typically, it is harder than polyethylene. This makes it suitable for thin wall insulations. UL maximum temperature rating may be 60 C maximum. The dielectric constant is 2.25 for solid and 1.55 for foam designs. Polyurethane (PU) is primarily used as a cable jacket material. It has excellent oxidation, oil, and ozone resistance. Some formations also have good flame resistance. It is a sturdy compound with excellent properties, making it an ideal jacket material for retractile cords. Polyvinyl Chloride (PVC) has the ability to be formulated for a wide variety of temperature ranges. Extremely high or low temperature properties cannot be found in one formulation. Certain formulations may have -55 C to 105 C rating. Other common vinyls may have -20 C to 60 C. Typical dielectric constant values can vary from 3.5 to 6.5. The many varieties of PVC also differ in flexibility and electrical properties with price ranges varying accordingly. Polyvinylidene Fluordide (PVDF) is a high-molecular-weight polymer with the predominant repeating unit CH2 CF2. It is a crystalline material with an extremely high melting point. While generally not used in typical medical device interconnect, its high UL fire rating makes it suitable in some instances where extreme heat may be an issue. 9

10 Santoprene is a very commonly used standard rubber material used in the medical device market. Rubber is an excellent insulator and generally includes both natural rubber and SBR compounds. Both of these materials can be used for insulations and jackets. There are many formulations of these basic materials. Each formulation is for a specific application. Some formulations are suitable for -55 C minimum, while others are suitable for 100 C maximum. Santoprene is a trademark of Exxon Mobil Corporation Silicon is a very soft insulation that has a temperature range from -80 C to 200 C. It has excellent electrical properties plus ozone resistance, low moisture absorption, weather resistance, and radiation resistance. It typically has low mechanical strength and poor scuff resistance. Appendix B Applications for Medical-Grade Cables Catheter Defibrillator Diagnostic Imaging Dialysis EKG and EEG Electrodes Endoscopy Handheld Instruments Implant Devices Laparoscopy Lasers LVAD Implant Medical Implants Medical Robotics Other Electrodes Pulse Oximetry Patient Monitoring Systems Surgical Instruments Tattoo Hand Pieces Ultrasonic Scanners Ultrasound X-ray Devices 10