In working with various medical equipment such as needles, syringes, trocars, cannulas, guide-wires, catheters, and valves, medical device designers must account for friction in the form of insertion, drag, and break-loose forces. A biocompatible silicone lubricant can significantly reduce friction at interfaces between components and between components and human tissue.
Silicone has a long and proven history of use with medical devices. When choosing a lubricious silicone for an application or a specific device, it’s important to consider several key factors to ensure that the lubricant properties deliver the expected result for both the device manufacturing process and the end use.
Different Lubricants for Different Substrates
A main consideration is to understand the nature of the various substrates that need to be lubricated and identify why the materials and surfaces require different types of silicone lubricants. Medical devices can incorporate a variety of substrates, including silicone, metal, glass, and plastics. Each material has different characteristics that can pose unique lubrication requirements.
Silicone Substrate. The surfaces of cured silicone elastomers often exhibit a high coefficient of friction (COF). These surfaces can be tacky, causing problems when molded, or extruded parts must move or slide. Silicone elastomers also tend to block, meaning they stick to each other due to chemical affinity. Blocking is particularly evident in slit valves, where the two sides of the silicone part touch each other and “heal” or close the slit. Considerations for silicone parts include the following:
Surface interaction factors: Consider a lubricant with a low chemical affinity to the elastomer. For molded silicone parts, it’s important to account for the difference in chemistries between the part and the lubricant itself. Otherwise, the lubricant may diffuse into a chemically similar material, and the molded component will swell. If this occurs, the fluid is depleted from the surface, which will reduce or eliminate the lubricating effect. Most silicone components are produced using a dimethyl silicone elastomer. Choosing a fluorosilicone lubricant, which has minimal chemical affinity to the dimethyl silicone, will result in minimal diffusion into the substrate.
Viscosity factors: Consider a higher-viscosity fluid for longer lubrication periods. Since diffusion or the chance of migration decreases as the silicone lubricant’s viscosity increases, higher-viscosity fluids may lubricate a silicone elastomeric surface for a longer period of time than lower-viscosity types.
Curable coatings: Consider alternative technologies to eliminate the need for a traditional lubricant. Technological advances have resulted in some alternative options. One specific example is a curable, nonmigrating coating that when applied to a substrate’s surface, reduces the COF. Once cured, these coatings chemically bond to the underlying substrate and mimic its mechanical properties. The result is a durable, flexible coating on moving, sliding, and rubbing parts that substantially reduces the COF. Specific formulations are available for platinum-cured or tin-cured silicone substrates.
Reducing processing time: Self-lubricating silicone elastomers may be chosen to reduce the number of processing steps. They do not require the additional processing step of adding a lubricant, coating, or grease to the surface of a component or device. Instead, the lubricity is built into the silicone elastomer, which yields a lubricious surface on the final molded component, eluting over time. The elastomer can be chosen with the physical properties and level of lubrication needed for the application.
Moisture sensitivity factors: Consider ambient humidity. When working with one-part dispersed silicone fluids that readily de-volatize, it’s important to remember that they are moisture-sensitive. Consequently, if adjustments are performed to optimize viscosity or solids content, they should take place in a moisture-free environment.
Other general factors: When planning the device manufacturing process, consider either applying the lubricant directly as an oil or dispersed in solvent to provide the coverage needed for the required properties. To reduce COF and enhance abrasion resistance, consider thin, wettable coatings. To minimize break-loose forces, consider thicker greases.
The metal surfaces and edges of hypodermic and suture needles, scalpels, or other cutting edges have an inherently high surface friction. During incision or penetration in human tissue, friction damages the substrate surface and, of course, makes the patient uncomfortable as the metal penetrates tissue. To counteract penetration and drag forces, the design of a component can play a role. For example, hypodermic needles are tri-beveled with an elliptical opening, followed by an elongated tube. This shape makes penetration easier and prevents coring effects, but the metal substrate still exhibits surface friction that prevents a smooth, more comfortable, puncture. Considerations for metal include the following:
Surface interaction factors: Consider penetration frequency and lubricant longevity. To minimize the effects of surface friction, silicone lubricants can be applied to lower the COF of the metal surface without compromising penetration or cutting efficacy. For applications involving repeated use, the lubricant must be robust. Taken together, factors that reduce friction include lowering puncture force, lowering drag force, reducing rub-off, and providing consistency throughout multiple uses.
Formulation factors: Consider dispersion and bonding behavior. Dispersed silicone formulations minimally bond to the metal substrates they coat, making them ideal to lubricate needles. Polydimethlysiloxane (PDMS) fluid is typically considered for this substrate. Inert PDMS fluid dispersions function as generic lubricants for various penetration and cutting surfaces. They improve lubricity but are more suitable for one-time use. For multiple usage, a dispersed high-viscosity fluid is more advantageous.
Other general factors: Consider either applying the lubricant directly or dispersed in solvent. To reduce migration compared to fluids, also consider using a silicone grease to help mitigate potential migration issues.
Silicone fluids have a silicon-oxygen chemical structure similar to glass, quartz, and sand. Consequently, they tend to bond very well with glass. Cross-linking to enhance bonding over the glass substrate may be achieved by heating the silicone beyond its operating temperature. Considerations for glass include the following:
Formulation factors: Consider a hydrophobic lubricant. To reduce drag forces in glass prefilled syringes, for example, the insides can be coated with a PDMS silicone oil. Hydrophobic coatings are available for syringe barrels to promote container drainability.
Curing factors: Consider high-temperature heating to activate cross-linking. Keep in mind that PDMS fluid by itself is nonfunctional and does not cure. However, this may be compensated by exposing the syringe to extremely high temperatures to activate polymer cross-linking, as previously described. The result is a functional interaction between the siliconized lubrication of the glass barrel and plunger stopper to make the system operate efficiently.
Other general factors: Consider either applying the lubricant directly or dispersed in solvent. To reduce migration compared to fluids, consider using a silicone grease. To enhance durability, consider heat treatment.
A wide variety of plastics are used in medical products such as valves and stopcocks. Friction points in these applications may benefit from silicone lubricants. Considerations for plastics include the following:
Formulation factors: Consider a very-high-viscosity grease. To enhance gliding with plastic and plunger stoppers, for example, consider using a silicone grease to lubricate the device. In these applications, the grease provides a lubricant that is less likely to migrate when applied to a plastic surface.
Other general factors: Consider either applying the lubricant directly or dispersed in solvent. Consider a combination with a PDMS fluid for enhanced/customized lubricant properties.
Other Key Considerations
Other factors to consider include the biocompatibility and manufacturability of the device.
Biocompatibility: Lubricious silicones used in medical applications should be biocompatible and in conformance with ISO 10993. As an inorganic material, silicone lubricants are chemically inert and stable over extended periods of time. The molecular backbone of silicone fluids is much stronger than the carbon-to-carbon chain in hydrocarbon polymers. Consequently, silicone lubricants are more resistant to chemical attack, oxidation, shear stresses, and extreme temperatures. Silicone can be readily sterilized by ethylene oxide, dry-heat or autoclaves, or other standard techniques without degradation. Device designers should be sure to consider high-purity, medical-grade silicone lubricant products supported by Master Files with U.S. Federal Drug Administration (FDA) and international authorities. These Master Files include biological tests conducted on each product.
Manufacturability: Application methods include dipping, spraying, or wiping. If a very thin film is desired, silicone fluids may be further diluted down as far as 1–5 percent silicone solids in a compatible solvent. Methyl polymers may be dispersed in nonpolar organic solvents, whereas fluoropolymers (and copolymers) may be dispersed in chlorinated hydrocarbons and ketones. Dispersion to a lesser extent can also be accomplished in aromatic hydrocarbons, mineral spirits, and isopropyl alcohol. For convenience, some medical device manufacturers select polymers predispersed down to a specified percent solids content. Be sure the silicone material selected can work with these options.
As medical device designers evaluate lubricants, it’s important to note there isn’t a one-size-fits-all silicone solution for each application. With the many factors involved in lubricant selection, device designers and manufacturers may wish to collaborate with suppliers of medical-grade lubricious silicone to meet the unique force reduction and material requirements of their particular medical device.
This article was written by Brian Reilly, Business Development Director – Biomaterials, for NuSil™ – part of Avantor®, Carpinteria, CA. For more information, visit here .