In medical devices, the physical property most commonly used to define lubricity is Coefficient of Friction (CoF). CoF is the ratio of force of friction between two bodies and the force pressing them together. The coefficient of friction varies based on the materials used and can be affected by environmental conditions, such as temperature and moisture. For medical devices, low CoF implies better part lubricity and less overall friction between components and contact surfaces.
There are two kinds of measurable friction: static and dynamic. Static friction refers to objects starting at rest relative to each other, while dynamic (or kinetic), refers to objects that are moving relative to each other. Static friction is usually larger than dynamic for given objects.
Applications Requiring Low Friction
Both molded and extruded medical devices and components often require low coefficient of friction. Medical device applications that require low CoF in dry conditions (i.e. not commonly exposed or immersed in liquids while in use) include, handle triggers, buttons and gears. Applications that often require lubricity in wet conditions (with blood or fluid contact) include medical catheters.
Foster ProPell™ Low Friction Polymer Technology
Foster ProPell™ Low Friction Compounds is a technology platform for modifying the surface energy and frictional properties of medical polymers. This surface modification substantially reduces the inherently high friction and tackiness of low durometer polymers commonly used in catheters for improved manufacturing and handling. Depending on formulation and end application, Foster has categorized formulations as ProPell S™, ProPell C™ or ProPell™“T”.
ProPell S™ – ProPell S™ is considered to be the standard and original ProPell™ formulations. These compounds are comprised of either PEBA or TPU resins, compounded with a select blend of proprietary additives for surface enhancement and ease of processing. ProPell S™ formulations have passed USP Class VI testing for biocompatibility and are available with radiopaque fillers and custom colors. These formulations are not available for applications requiring transparency.
ProPell C™ – ProPell C™ formulations are customer specific and additives are selected based on individual requirements. Resins are not limited to Pebax and TPU blends and formulations are available for applications requiring transparency, bondability and printing, among others. Since these blends are custom formulated, USP Class VI documentation is not available for these compounds.
ProPell™ “T” – ProPell™ “T” provides the same low friction properties as ProPell S™ and ProPell C™ , while also providing part translucency. These compounds are formulated without the prescence of a radiopaque filler for visual monitoring of medical devices and catheters. If a filler is desired, the compound will remain opaque.
Perfluoropolyether synthetic oil is colorless, odorless, stable and very inert. It meets USP class VI biocompatibility and is usually added to plastics in very low loadings (below 5% by weight). Fluorinated oil works well in low durometer thermoplastic elastomer materials for dry applications. Because of the low loading, it allows for bonding to the substrate, which can be a problem for many low friction materials.
PTFE powder is perhaps the most common additive for plastics in low friction applications. PTFE by itself is one of the most lubricious materials available; however, it is not melt processible and therefore difficult to form into complex shapes. Small amounts (up to 15%) of PTFE powder can be added to other melt processible plastics to improve dynamic frictional properties in dry medical device applications. Plastics modified with PTFE additives often achieve good bondability with medical adhesives.