Silicone has a long and proven history of use with medical devices and can provide many benefits, from flexibility to cushioning. When working with some medical devices, however, silicones can present challenges due to an inherent surface tack. The surface of cured silicone elastomers is sometimes characterized by a high coefficient of friction (CoF), some degree of tackiness, and a tendency for blocking (sticking to itself by virtue of chemical affinity). These characteristics may need to be addressed in applications that require a molded or extruded silicone component to move or slide with minimal friction. Blocking is particularly evident in slit valves, for example, where the two sides of the silicone part touch each other and “heal,” or close the slit.
As medical device designers account for friction, one solution is to use self-lubricating liquid silicone rubber (LSR), which eliminates an extra step of applying lubrication. Another solution is the use of a low CoF silicone coating applied to the surface and then cured to chemically bond the coating to the substrate for a non-tacky surface.
The traditional solution to these friction challenges requires medical device manufacturers to apply a silicone lubricant onto the molded part as a separate manufacturing step. However, self-lubricating LSRs are now available that do not require the additional processing step of adding a lubricant, lubricious coating, or grease. Instead, the lubricity is built into the silicone elastomer system, which yields a lubricious surface on the molded component. The elastomer system is formulated with a proprietary additive that elutes out over time after vulcanization. The performance of many medical devices depends upon minimizing friction at the interfaces between various components, and self-lubricating LSRs provide the basis on which to produce effective results. Potential uses for self-lubricating silicone elastomers include:
Silicone devices with moving or sliding parts.
Parts that require assembly.
Low CoF Silicone Coatings
Another method to overcome the high degree of surface tack and blocking with silicone elastomers includes using cured silicone coatings. When applied to the surface of the part and cured to chemically bond to the substrate, the coating mimics the mechanical properties of the underlying substrate. The outcome is a “dry” lubricity that reduces the coefficient of friction on the surface of silicone parts and eliminates concerns about migration, leaking, or rubbing off that are commonly associated with traditional lubricants such as fluids or oils and greases.
Coating options differ in how they are designed to cure — for example, heat cure (HTV) or room temperature vulcanization (RTV) are available — and can accommodate a multitude of applications.
With approximately the consistency of water, the coatings can be applied by dipping, but spraying is recommended. It achieves a smooth finish that results in more than a 50 percent decrease in the coefficient of friction when compared to a non-coated silicone.
NuSil™ technology experts recently conducted a study to evaluate the static and kinetic coefficient of friction of coated versus non-coated silicone surfaces. The force it takes to initiate movement between the silicone rubber and a steel panel (static CoF) was measured, as well as the force needed to keep the silicone rubber moving against a steel panel (kinetic CoF). Both a 70 shore A durometer liquid silicone rubber and 50 shore A durometer high-consistency rubber were tested (the coated samples were tested according to ASTM D 1894 at ambient conditions. During the test, samples were placed on a stainless-steel panel with a mirror finish). On average, the coated samples demonstrated up to 74 percent reduction in static CoF, and up to 59 percent reduction in kinetic CoF depending on the silicone rubber substrate.
Both lubrication options provide advantages for the healthcare industry. Low CoF coatings are ideal, not only for their performance relative to friction reduction and regulatory concerns, but also because they achieve critical performance goals with negligible impact on the mechanical properties of the silicone substrates they coat. Therefore, a silicone device that must bend, twist, elongate, etc., can handle this movement, coated or uncoated, without cracking, flaking, or peeling. Self-lubricating silicone elastomers may be chosen to reduce the number of processing steps required; the elastomer can also be specified with the physical properties and level of lubrication needed for the application.
This article was written by John Freedman, Business Unit Director, Biomaterials for NuSil™, part of Avantor®. For more information, click here .