Understanding Low-Friction Coatings for
`Medical Devices
`
`An expert offers a primer on popular medical device coatings and their
`
`applications.
`
`January 24, 2017in Surface
`
`
`keywords/surface-treatment-O)
`
`Treatment(/primary- £ (A#facebook)=(/#twitter) in (/#linkedin) (/#email)
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`
`
`An expertoffers a primer on popular medical device coatings and their applications.
`
`Kevin Guenther
`
`Acatheterwith a lubricious coating.
`
`In the world oflowfriction, lubricious coatings for medical devices are diverse and complex. There
`are manydifferent typesof coatings, each with their own advantages, disadvantages, and
`applications. This is a basic overview of the most popular medical device coatings and whatcan be
`expected from eachtype.
`
`Lubricious coatings are most commonlydistinguished by their degree of lubricity, or the amountof
`reduction in friction they provide. To understand what these measurements mean, we mustfirst
`learn how these measurementsare obtained. In the medical device industry, friction is measured by
`whatis known as a pinchtest.In a pinchtest, a coated device is secured on an instrument that pulls
`the device between the jaws of a clamp which produces a load on the part. While clamped, the coated
`
`‘
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`alwayssignificantly higher than dynamic friction.
`|
`
`
`Now that we have an understanding of how the CoF is obtained and the
`significance ofthe different amountsof gramsofpull force required, we
`
`cantakealookatsomecommonlyusedcoatingsandcomparethem.
`
`|
`
`part is pulled at a fixed speedfora fixed distance, and the resistanceto the pull is measuredin grams
`of pull force. The gramsofpull force needed to overcome the friction produced by the clamps divided
`by the clampforceyields the coefficientof friction (CoF). CoF = Forcepyii/ Forcectamp
`Visit Harland Medical Systems at Booth #1615 during MD&M Westin Anaheim, CA
`
`
`February 7-9, 2017. (http://mdmwest.mddionline.com/?
`
`
`_mc=arti_x_mddir_le_tsnr_thibault_mddir_med_44_x-harlandmedicalsystems)
`A500-gram clamp force (Force clamp) is acommonly usedsetting. The higherthe pull force needed,
`the higher the CoF and the lower thelubricity of the surface. The lowerthepull force, the lower the
`CoFand the higher the lubricity.
`
`When measuringfriction between two surfaces, anothercharacteristic
`is that of static friction pull force vs. dynamicfriction pull force. Static
`friction is the force needed to breaktheinitialfriction force so that the
`two surfaces can moverelative to each other. Dynamic friction pull
`force is that force needed to maintain the motion onceit has been
`initiated. Static friction (sometimes referred to as "stiction") is almost
`
`Before we look at thedifferent typesof coatings,let's begin by looking
`at the properties of an uncoated sample to provide a referencepoint. A
`very commonmaterial used in medical devicesis PEBAX®(a registered
`trademark of Arkema,Inc.), the brand nameof polyether block amide.
`Whenuncoated, PEBAX® (hardnessof 55 ShoreD)generally produces
`pinchtest results of about 500 to 600 gramsofpull force. Assuming a
`500-gram clampforce was used, the CoF would be (1.0-1.2), a very low
`level of lubricity. Other uncoated catheter materials of construction
`such as nylon 12, polyethylene, and others exhibit similar CoF. Such a high level of friction would
`make manyof today's catheter-delivered minimally invasive proceduresnearly infeasible.
`
`A friction test system to
`measurelubricity.
`
`Device manufacturers have turned to a variety of coatings to provide the surface lubricity not
`offered by the bare catheter material. Thefirst class of coatingis siliconeoil (siloxane). This type of
`coating has long been usedto reduce surfacefriction on medical catheters, introducers,etc.It is
`generally applied by the manufacturer, andis relatively inexpensive.It offers a much reduced CoF,at
`about 0.13, and approximately 125-175 gramsof pull force. However, devices coated with silicone oil
`canbedifficult to apply as they are very mobile and can spread to other manufacturing areas where
`they are undesirable. For example,the presenceofsilicone oil on a surface can prevent other
`surface-enhancing coatings from adheringto the device surface. Oncesilicone oil is present, it can be
`verydifficult to clean from a device or work surface. As a result, these coatings can be a major
`contaminantrisk to the manufacturer.
`
`The second class of coating is PTFE (Polytetrafluoroethylene), known best by the brand name
`Teflon®, another popularly used material to reducefriction. It has been employed as a material of
`construction for catheter-type products or as an addedcoating for guidewires and other devices
`made fror metal. PTFEtypically exhibits between 150 and 200 gramsofpull force, or a CoF of about
`0.3 - 0.4. On bare metals such as stainless steel and nitinol, PTFE offers performance similar to that
`of silicone oil. The main advantage of PTFE coatingsis that they provide someleveloflubricity while
`dry and do not require any sourceof moisture to perform.
`
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`Hydrophilic coatings are the third class of coatings employed to reduce
`surface friction and enhancelubricity. The term "hydrophilic" means
`thatit is water loving. In other words,it readily wets out when exposed |
`to moisture. Most hydrophilic coatings consistof a loosely cross-linked
`
`polymer coatingthatwill readily uptake liquid when exposed toa
`
`source.This coating that surroundsthe device acts like a microscopic
`sponge and the water(or otherliquid) provides excellent low friction
`process.
`characteristics as long as the coating remains wet. Compared to PTFE
`coatings, which are hydrophobic and repel water, hydrophilic coatings provide superiorlubricity
`(lower CoF) when wet. Devices coated with these coatings demonstrate pull forces as low as 3 grams
`and CoFvalues as low as 0.01.In clinical use, devices coated with hydrophilic material demonstrate
`
`The dip coating and curing
`
`superior control, exhibit lowerstiction(static friction) behavior, and require far less force to perform
`any given movement. Ranging from 3-25 gramsofpull force, hydrophilic coatings offer the best CoF,
`averaging between .01 and .05.This high level of lubricity enables devices to navigate tortuous
`anatomical pathways while reducing tissueirritation and adding to patient comfort.
`
`Sometypes of hydrophilic coatings may generate particulates from their surface in use. For these
`applications where particle generation from a coatingis critical, such as cardiovascular and
`neurovascular, the selected formulation must be one designedspecifically for low particle
`generation. The only reallimitation of this category of coatingsis that they must be hydrated to
`perform.
`
`Commercially available hydrophilic coatings average between 8 and 12 gramsofpull force that
`reducesfriction over an uncoated surface by as much as 98%.This type of low-friction performance
`increases the device's ability to navigate through tortuous anatomical pathways,improves device
`control, reduces tissue damage, and addsto patient comfort.
`
`Advancesin surface enhancements,especially low-friction hydrophilic coatings, have accelerated
`minimally invasive surgery's growth. These coatings now impacta variety of surgical specialties,
`including cardiovascular, orthopedic,urological, peripheral, and neurological procedures.
`
`Kevin Guentheris the methods director at Harland Medical Systems(http://directory.qmed.com/harland-
`medical-systems-inc-comp237833.html), a medical device coating technologies companybased in Eden
`Prairie, MN. He can be reached at info@harlandmedical.com (mailto:info@harlandmedical.com).
`
`[Images courtesy of HARLAND MEDICALSYSTEMS(http://www.harlandmedical.com/)]
`
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