Our 17th Year of Publication
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Cover Story

Lubricating Medical Devices with Silicones

Brian Reilly,
Product Director – Healthcare Materials and Nathan Wolfe, Technical Sales,
NuSil Technology

For over six decades silicones have been used extensively in the design and manufacture of medical devices intended for short and long-term human implantation. Silicone’s success in the healthcare industry is due to its proven bio-inertness and chemically dynamic nature, which yields a diversity of raw materials including fluids, gels, adhesives, coatings, and moldable materials. Typically, silicones are used in the cured form to produce a device’s part, such as a tube, protective coating or balloon. Because of hydrogen bonding, cured silicones tend to exhibit an affinity for themselves and other surfaces such that they want to stick to each other rather than slide against each other. 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. The success of many medical devices depends upon minimizing friction at the interface between cured silicone rubber and other various components, and between those components and human tissue. A silicone coating or fluid is a very viable solution to reduce this friction. This article details these silicone solutions and provides the relative benefits of each.

Silicone Fluids

Silicone fluids are non-curing silicone polymers comprising repeating Si-O units (see Figure 1). 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.

Figure 1. Schematic of a silicone polymer

During the polymerization process when producing the fluids, there are two main factors that are controlled: the pendant groups (a.k.a. R-groups), and the degree of polymerization. These pendant groups (R) are typically methyl, phenyl or methyltrifluoropropyl groups. Phenyl fluids are rarely used as lubricants due to their large bulky nature. Methyl and fluoro groups are most common in silicone lubricants. 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 almost a solid (in the millions of cP). For lubrication purposes common viscosities are 350, 1,000 and 12,500cP.

Methods of applying the fluid 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 fluoro polymers may be dispersed in chlorinated hydrocarbons and ketones, and to a lesser extent, aromatic hydrocarbons, mineral spirits, and isopropyl alcohol. For ease of use and minimal processing, some medical device manufacturers purchase fluids predispersed down to a specified percent solids content.

When lubricating a molded silicone part, if more than a few hours of lubrication is needed from the time of application, it is important to select a fluoro fluid. If a dimethyl fluid is used, it will diffuse into the silicone 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, higher viscosity fluids lubricate a silicone elastomeric surface for a slightly longer period than lower viscosity fluids.

Silicone Coating-Ambient Cure

Beyond fluids (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. Unlike fluids, dispersed silicone formulations minimally bond to the substrate they coat. This feature makes these products ideal for lubricating needles, such as suture, cannula, and syringe. 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.

Self Lube Molding Materials

Technological advances have resulted in some unique molding materials. Specifically, there are now some liquids silicone rubbers (LSRs), silicone elastomers designed for liquid injection molding (LIM), that self-lubricate. The silicone is molded as normal, released from the mold, and then after an hour or so a fluid weeps from the material onto its surface to lubricate it. With this formulation, as with all of the above, there is some potential for uncontrolled migration of the lubricious fluid.

Silicone Coating-Heat Cure

The problem of all the previous detailed lubrication methods having potential migration has led to the most recent breakthrough in the evolving technology of silicone coatings: a heat-curable silicone dispersion that covalently bonds to a cured silicone substrate and results in a dry, yet slippery finish.

The development of this material 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. By spraying this friction-diminishing silicone coating onto a silicone substrate, a dramatic reduction in the coefficient of friction of that part or device, is observed, see Figure 2.

Figure 2. Cured 50 durometer HCR coated with 10 microns of a low coefficient of friction coating.

This material can achieve this critical dry lubrication goal with negligible impact on the mechanical properties of the silicone substrate it coats. 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 medical devices requiring safe and effective lubrication between various materials. The above-described silicone fluids, low-crosslink density ambient coatings, and self lube molding materials are well-established performers that routinely deliver on these critical criteria in a host of applications. With the addition of the new “dry lubricant” cured coating, it is clear that the body of silicone solutions will only continue to grow.

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