Cables are highly engineered products that must per-form consistently and reliably in a wide range of envi-ronmental extremes. Cables for medical applicationsare complex, heavily regulated, and demanding to de-sign and manufacture. They must work after repeatedexposures to one or more sterilization processes, in-cluding autoclave, ETO, gamma, e-beam, and high-level disinfectants and sterilants. A little backgroundon components and considerations can help select abetter medical cable.

Engineering a medical cableCable design includes choosing the right material for

insulation, tape-shield laminate, and jacket. Each of

Specifying a

medical cable

means

considering

flexibility,

chemical

resistance, and

more.

Sandy FultonAccount ManagerKevin DePratterEngineering ManagerThomas CollenDirector of MarketingNorthwire Inc.Osceola, Wisc.

Medical cables have tomeet requirements thatare more demandingthan the nonmedicalvariety. MedFlex is afamily of cables fromNorthwire Inc. that offersa combination offlexibility, toughmechanical properties,and non toxic features.The design is suited forapplications such assurgical devices andpatient monitors, andcan be customized foreither permanent orsemi-disposable use.

MedicalCables

What Designers Should Know AboutENGINEER’S SOURCE FOR CONTRACT MANUFACTURING

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these can be a different plastic, so the de-signer should understand plastics and theirvalue in product design.

It is important to select plastics formu-lated for functional attributes as well as thespecific sterilization method they will un-dergo. Dramatic changes in physical proper-ties can occur immediately after steriliza-tion while some can show up months later.Gamma and e-beam sterilization, for in-stance, produce high-energy ionization inpolymers at the molecular level wherebycross-linking can actually occur. Cross-link-ing occasionally enhances a few properties,but the opposite occurs more often. Excesscross-linking could likely cause materials tobecome brittle and stiff, discolor, and smell.In some materials the molecular-chain con-tinuity is destroyed and the plastic ulti-mately breaks down. Although differencesmay be slight, it has been reported that poly-meric materials become less brittle from

electron-beam sterilization than in thegamma-ray process. In addition to steriliz-ing finished parts, cables may also be testedfor bacteria and germs before assembly.Plastics are melted during extrusion andformed at high temperatures. Water at amuch lower temperature, however, is usedfor quenching the molten plastic. This wa-ter requires treatment to prevent bacteriumpropagation. Microbial levels are con-trolled by regularly treating the water withchemicals and maintaining a clean work en-vironment. Isolating the extrusion areafrom workers’ food and beverages and dis-infecting reels and packaging supplies,keep microbial counts low and let assem-blers work on the cable in a clean room.

Flexibility versus flex lifeOf course, medical cables have to be flex-

ible, but they must also have sufficient flexlife. Flexibility refers to a cable’s ability toremain soft and pliable. Flex life is a cable’sability to maintain consistent performanceafter repeated flexing cycles, often at highspeeds and for millions of repetitions. In dis-posable or limited-use applications, flex lifeis not as critical.

In a flexing environment it’s important toknow how cable materials interact witheach other. A material’s coefficient of fric-tion, flex modulus, and elasticity are key forflexural endurance. Special tapes and lami-

How to read the chartATC: Autoclave, steam sterilization at 121 to134°C and 15 to 19 psi. Exposure time: 5 to 45min.

ETO: Ethylene oxide gas at 25 to 75°C and atsubatmospheric, 25 psi. Gas concentration: 450to 1,000 mg/liter

GAM: Gamma radiation from Cobalt 60 or anelectron beam (Ebeam). Radiation dosage: 2.5Mrads minimum

STS: Steris Paracetic acid (0.2%)

VHP: Hydrogen peroxide (gas plasma/vaporization)

CID: Cidex, activated dialdehyde solution (Glu-daraldehyde up to 3.4%)

O: Outstanding, E: Excellent, VG: Very good,

G: Good, F: Fair, P: Poor

Hold your judgment on PVCsPVC is probably one of the most misunderstood of all thethermoplastics. Despite the minor controversy regardingthe material’s use and the use of a few plasticizers inmedical-grade cables, PVCs are high on the list of candi-date materials. This is largely because these compoundscan be effectively formulated to provide many attributesimportant in medical-grade cables.

PVC is readily available in a broad range of durometersthat provide elasticity and flexibility. When formulated withspecialized plasticizers, antioxidants, stabilizers, lubri-cants, and other additives, PVCs become more durable.New PVCs are being introduced that do not use plasticiz-ers at all, further opening new applications for PVCs inmedical-grade cables.

For example, the material can be engineered to meet awide range of temperature requirements from –50 to105°C, resist chemicals, and be produced in translucentor opaque variations. The material is also available in a va-riety of molecular weights, including an ultra-high molecu-lar weight where it becomes a thermoplastic elastomer(TPE). These formulations exhibit excellent gamma stabil-ity, ETO compatibility, and resistance to low tempera-tures, high-pressure disinfectants, and sterilants.

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Copyright © 2004 by Penton Media, Inc.

nates help protectsurfaces from abra-sion during flexingand extend a cable’sflex life. The helicallay of primary com-ponents, sub-assem-blies, tapes, andfillers all play a role.The extrusion tech-nique used in jacket-ing the complete ca-ble or an extrudedsub-assembly mem-brane can also con-tribute significantlyto a cable’s flexibilityand flex life. Pres-sure, semi-pressure,and tube techniquescan have a positive ornegative effect onflexural properties of extruded materials.The tooling used and specific extrusionmanufacturing techniques are all criticalvariables in extrusion. Consequently, two

cables made by different suppliers with ex-actly the same raw materials and built to thesame bill of materials can have differentflexibilities. ■

A discussion of cables andtheir construction is likely toencounter these terms:Fillers: Materials that helpmaintain the shape of an in-ner cable bundle. Flex life: A number of cy-cles of twisting, or bending,or both that a cable mustwithstand during its workinglife.Flexibility: The limpnessof a cable. Jacket: A cable’s outerprotective covering. Onsome constructions, the ca-

ble’s electrical insulation isalso the jacket. Pressure extrusion: Aproduction method that in-volves extruding plastic orrubber over conductors soall internal voids (interstices)are occupied. This is a filler-less construction.Semi-pressure extru-sion: A reduced-pressureextrusion process that mini-mizes deformation of the in-ner cable core. Tape-shield laminate:An electrically conductive

foil or Mylar tape that actsas an electrical shield.Tape: Any of several tapeswrapped over a bundle tohold it together. Tapes canmake a jacket more easilystripped, and they act as aninternal lubricant especiallyin applications that call for aflexible cable or high flex-lifecable. Tube techniques: Any ofseveral methods that ex-trude a tube that fits overthe fillers and conductors.

Plastic ATC ETO GAM STS VHP ..CID

How plastics stand up to sterilization

Polypropylene (copolymer) T/E: 3,500/>600Dielectric constant: 2.25

GVGVGGFF

Polyethylene (LLDPE)T/E: 3,000/>300 Dielectric constant: 2.27

GVGVGEEP

PVC (polymeric)T/E: 2,500/300Dielectric constant: 4.20 @ 1 MHz

EEEGEP-F

PVC (non-toxic, gamma stabilized)T/E: 2,300/300Dielectric constant: 4.50 @ 1 MHz

VGVGVGEEP

Polyurethane (ether base)T/E: 4,000/500 Dielectric constant: 7.0

GVGVGEEP

Northwire MedFlex (UHMW)T/E: 2,200/300 Dielectric constant: 4.25 @ 1 MHz

VGVGVGVGEF

Northwire EnduroFlex (TPR/TPO)T/E: 1,500/400Dielectric constant: 3.35

EEEVGEE

SiliconeT/E: 1,100/500 Dielectric constant: 3.1

VGVGVGGEO

FEP TeflonT/E: 3,000/275 Dielectric constant: 2.1

OOOP-FOO

0.240 in.

Anatomy of a cable22 gauge tinnedcopper stranding

PVC insulation60°C, 300 V

Paperseparator

Pressure extruded MedFlex jacket 60°C300 V Gamma stable jacket material

0.150in.

A glossary for cable terms

The matrix provides a quick reference for selecting various plastics for specific types ofsterilization. Property values are representative of the actual products and not of a plasticgrade in general.

The cross section of a MedFlex cableshows its components.

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