Understanding Low-Friction Coatings for
`Medical Devices
`
`An expert offers a primer on popular medical device coatings and their
`
`applications.
`
`January 24,2017 in Surface
`Treatment (/primary-
`keywords/surface—treatmentvo)
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`An expert offers a primer on popular medical device coatings and their applications.
`
`Kevin Guen ther
`
`A catheter with a Iubricious coating.
`
`In the world of low friction, Iubricious coatings for medical devices are diverse and complex. There
`are many different types of coatings. each with their own advantages, disadvantages, and
`applications. This is a basic overview of the most popular medical device coatings and what can be
`expected from each type.
`
`Lubricious coatings are most commonly distinguished by their degree of lubricity, or the amount of
`reduction in friction they provide. To understand what these measurements mean, we mustfirst
`learn how these measurements are obtained. In the medical device industry, friction is measured by
`what is known as a pinch test. In a pinch test, a coated device is secured on an instrument that pulls
`the device between thejaws of a clamp which produces a load on the part. While clamped, the coated
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`:
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`part is pulled at a fixed speed for a fixed distance, and the resistance to the pull is measured in grams
`of pull force, The grams of pull force needed to overcome the friction produced by the clamps divided
`by the clamp force yields the coefficient of friction (CoF). CoF = Forcepun/ Forcedamp
`Visit Harland Medical Systems at Booth #1615 during MD&M West in AnaheimI CA
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`Februag 7-9 2017. (http://mdmwestmddionlinecoml?
`_mc=arti_x_mddir_le_tsnr_tl1ibaull_mddlr_med_44_x-harlandmedicalsystems)
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`
`A SOO-gram clamp force (Force Clamp) is a commonly used setting. The higher the pull force needed,
`the higher the CoF and the lower the lubricity of the surface, The lower the pull force, the lower the
`COP and the higher the lubricity.
`
`When measuring friction between two surfaces, another characteristic
`is that of static friction pull force vs. dynamic friction pull force. Static
`friction is the force needed to break the initial friction force so that the
`two surfaces can move relative to each other. Dynamic friction pull
`force is that force needed to maintain the motion once it has been
`initiated. Static friction (sometimes referred to as "stiction") is almost
`
`always significantly higher than dynamic friction.
`
`Now thatwe have an understanding of how the COP is obtained and the
`significance ofthe different amounts of grams of pull force required, we
`
`i
`
`cantakealookatsomecommonlyused coatingsandcomparethem.
`ll
`very common material used in medical devices is PEBAX® (a registered
`
`Before we look at the different types of coatings, let's begin by looking
`at the properties of an uncoated sample to provide a reference point. A
`
`trademark of Arkema, Inc.), the brand name of polyether block amide.
`When uncoated, PEBAX® (hardness of 55 ShoreD) generally produces
`pinch test results of about 500 to 600 grams of pull force. Assuming a
`SOO-gram clamp force 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 many of today's catheterdelivered minimally invasive procedures nearly infeasible.
`
`A friction test system to
`measure lubricity.
`
`Device manufacturers have turned to a variety of coatings to provide the surface lubricity not
`offered by the bare catheter material, The first class of coating is silicone oil (siloxane). This type of
`coating has long been used to reduce surface friction on medical catheters, introducers, etc. It is
`generally applied by the manufacturer, and is relatively inexpensive. It offers a much reduced CoF, at
`about 0.13, and approximately 125-175 grams of pull force. However, devices coated with silicone oil
`can be difficult to apply as they are very mobile and can spread to other manufacturing areas where
`they are undesirable. For example, the presence of silicone oil on a surface can prevent other
`surface-enhancing coatings from adhering to the device surface. Once silicone oil is present, it can be
`very difficult to clean from a device or work surface. As a result, these coatings can be a major
`contaminant risk to the manufacturer.
`
`The second class of coating is PTFE (Polytetrafluoroethylene), known best by the brand name
`Teflon®, another popularly used material to reduce friction. It has been employed as a material of
`construction for catheter-type products or as an added coating for guidewires and other devices
`made from metal. PTFE typically exhibits between 150 and 200 grams of pull 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 coatings is that they provide some level of lubricity while
`dry and do not require any source of moisture to perform.
`
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`Hydrophilic coatings are the third class of coatings employed to reduce
`
`
`
`surface friction and enhance lubricity. The term "hydrophilic" means
`that it is water loving. In other words, it readily wets out when exposed '
`to moisture. Most hydrophilic coatings consist of a loosely cross-linked
`polymer coating that will readily uptake liquid when exposed to a
`source. This coating that surrounds the device acts like a microscopic
`sponge and the water (or other liquid) 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 superior lubricity
`(lower CoF) when wet. Devices coated with these coatings demonstrate pull forces as low as 3 grams
`and CoF values as low as 0.01. In clinical use, devices coated with hydrophilic material demonstrate
`
`The dip coating and curing
`
`superior control, exhibit lower stiction (static friction) behavior, and require far less force to perform
`any given movement. Ranging from 3-25 grams of pull 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 tissue irritation and adding to patient comfort.
`
`Some types of hyd rophilic coatings may generate particulates from their surface in use. For these
`applications where particle generation from a coating is critical, such as cardiovascular and
`neurovascular, the selected formulation must be one designed specifically for low particle
`generation. The only real limitation of this category of coatings is that they must be hydrated to
`perform.
`
`Commercially available hydrophilic coatings average between 8 and 12 grams of pull force that
`reduces friction 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 adds to patient comfort.
`
`Advances in surface enhancements, especially low-friction hydrophilic coatings, have accelerated
`minimally invasive surgery's growth. These coatings now impact a variety of surgical specialties,
`including cardiovascular, orthopedic, urological, peripheral, and neurological procedures.
`
`Kevin Guenther is the methods director at Harland Medical Systems (http://directory.qmed.com/harland-
`medical-systems-inc-comp23 7833.html), a medical device coating technologies company based in Eden
`Prairie, MN. He can be reached at inftharlandmedicaIcom (mailto:info@harlandmedicgl,coml.
`
`[Images courtesy of HARLAND MEDICAL SYSTEMS lhttpglmmm,harlandmedicalcomll]
`
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