In medical devices, the physical property most commonly used to define lubricity of a device is Coefficient of Friction (CoF). Unlike true material properties, CoF is more accurately described as a system property since it involves the relationship with another mating surface and can be affected by environmental conditions such as temperature and moisture. CoF is the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used. Static friction refers to objects starting at rest relative to each other. Dynamic, or kinetic, refers to objects that are moving relative to each other. Static friction is usually larger than dynamic for given objects. Low CoF implies better lubricity in medical devices.
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. Two common options for reducing dry frictional properties of plastics in medical devices include compounding fluorinated oil or PTFE powder into the base polymer.
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%. 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 to improve frictional properties. PTFE by itself is one of the most lubricious materials available; however, it is not melt processible and therefore difficult to form in complex shapes. Small amounts up to 15% added to other melt processible plastics can improve dynamic frictional properties in dry applications. Because the PTFE is dispersed in the base polymer, which often has a ‘resin rich’ at the surface of a molded or extruded part, the effect of the PTFE additive is most noticeable after a ‘break in’ period in which the mating surfaces have rubbed against each other and work off the resin rich surface, thus exposing the PTFE particles. Given the initial resin rich surface, plastics modified with PTFE additive can achieve good bondability with adhesives.