For over 50 years, silicones have been used extensively in the design and manufacture of medical devices intended for short- and long-term human implantation. Silicone’s overwhelming success in the healthcare arena is not only due to its proven bio-inertness, but also to silicone chemistry’s dynamic nature, which yields a diversity of raw materials including fluids, gels, adhesives, dispersions, and elastomers. These two assets explain why many therapies rely heavily on silicones and why some medical devices are fabricated entirely of several types of silicone.
Even so, the success of many medical devices depends upon minimizing friction at the interfaces between various components and between those components and human tissue. Whether due to a critical need for bio-inertness, wetability, lubricity, or all three, in many cases a silicone coating or lubricant is the only viable solution to reduce this friction. This article details these silicone solutions and delineates the relative benefits of each.
First, and most simply, there are nonfunctional silicone polymers. Silicone polymers are chains comprised of repeating Si-O units. While there are no carbon-carbon double bonds on the polymer backbone, the pendant groups coming off the backbone do contain carbon, making it appropriate to describe silicone polymers as organo-polysiloxanes.
During the polymerization process, there are two main factors that are controlled: the pendant groups and the degree of polymerization. The non-functional radical/pendant groups (R) coming off the polymer backbone may be methyl, phenyl, or methyltrifluoropropyl. Phenyl fluids are rarely used as lubricants due to their large, bulky nature. Generally, a material is referred to as a “polymer” if a molecule contains only one type of pendant group, and a “copolymer” if a molecule contains a combination of pendant groups. Furthermore, all silicone polymers can be synthesized to a very specific degree of polymerization. The degree of polymerization dictates the average molecular weight, which in turn governs the viscosity. A silicone polymer may possess a viscosity close to that of water (20 cP) or so high that it’s a solid (in the millions of cP).
Regardless of chemistry, all polymers will have varying levels of success as a lubricant or a hydrophobic coating on a variety of substrates including molded silicone parts, metal, glass, and many plastics. Methods of applying the polymer include dipping, spraying, or wiping. If a very thin film is desired, these silicone fluids may be further diluted down as far as 1-5% silicone solids in a compatible solvent. Methyl polymers may be dispersed in non-polar organic solvents, whereas fluoropolymers (and copolymers) may be dispersed in chlorinated hydrocarbons and keytones, and to a lesser extent, aromatic hydrocarbons, mineral spirits, and isopropyl alcohol. For ease of use and minimal processing, some medical device manufacturers select polymers predispersed down to a specified percent solids content.
When attempting to coat and lubricate a molded silicone part, it is important to be mindful of the chemistry of the molded part versus that of the lubricant. At the time of application, if more than a few hours of lubrication is needed, it is important to select a fluoropolymer or a copolymer. Otherwise, the silicone fluid will diffuse into the elastomer, both swelling the molded component and depleting the fluid’s surface, thereby reducing or eliminating all lubricating characteristics. Since a fluid’s rate of diffusion into a silicone elastomer decreases as the fluid’s molecular weight increases, the higher-viscosity fluids lubricate a silicone elastomeric surface for a slightly longer period than the lower-viscosity fluids.
It’s also worth noting that on temperature-resistant materials such as glass, ceramic, and metal, silicone polymers and copolymers can be converted to highly durable hydrophobic films by heating the treated surfaces.
Beyond polymers (predispersed and otherwise), some dispersed silicone formulations are designed to cure at ambient conditions. These formulations yield a very minimal crosslink density such that they result in a sticky yet slippery finish, not unlike the finish of the above described polymers. Unlike polymers, dispersed silicone formulations minimally bond to the substrate they coat. This feature makes these products ideal for lubricating cutting edges, needle cannula, etc. These formulations are one-part dispersions that devolatilize, cure at ambient conditions, and may be applied by dipping or wiping. When working with these formulations, it is important to remember that they tend to be moisture-sensitive. Consequently, if adjustment to the percent solids or viscosity is needed, it is important to use a moisture-free organic solvent.
Technological advances have resulted in some newer formulations. Specifically there are now some silicone elastomers designed for molding that self-lubricate. With this formulation, as with all of the above, there is some potential for the uncontrolled migration of the lubricious component. This introduces the most recent breakthrough in the evolving technology of silicone coatings: heat-curable silicone dispersions that covalently bond to their silicone substrate and result in a dry, yet slippery finish.
The development of formulations such as these came as the result of two industry goals: design a coating for molded or extruded silicone parts that overcomes their inherent blocking characteristic, and achieve this without the potential for the migration of any formulary component. Because of hydrogen bonding, cured silicones tend to exhibit an affinity for themselves and other surfaces such that they want to stick rather than slide. This presents obvious problems in a host of applications where a molded or extruded silicone part must move or slide with a modicum of friction. By spraying or dipping these friction-diminishing silicone coatings on a substrate, one can expect to see a dramatic reduction in the coefficient of friction of that part or device.
These formulations have enjoyed an enthusiastic reception in the healthcare industry, not simply for their unprecedented performance relative to friction reduction and regulatory concerns, but also because they achieve these critical performance goals with negligible impact on the mechanical properties of the silicone substrates they coat. It may be said that the coatings mimic the mechanical properties of the elastomers they coat. Therefore, a silicone device that must bend, twist, elongate, etc. may be relied upon to do this coated the same as if it were uncoated, and without worrying about cracking, flaking, or peeling.
Clearly, there are many silicone solutions for devices requiring safe and effective lubrication between various materials. The above-described silicone polymers and low-crosslink density coatings are well-established performers that routinely deliver on these critical criteria in a host of applications. Additionally, in light of the just-described state-of-the-art “dry finish” cured coatings, it is clear that the body of silicone solutions will only continue to grow.
This article was written by Nathan Wolfe, Technical Sales, at NuSil Technology in Carpenteria, CA. For more information, Click Here