Key Factors for Choosing Silicone Solutions in Medical ...

Key Factors for Choosing Silicone Solutions in Medical ...

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When working with various medical equipment such as needles, syringes, trocars, cannulas, guide-wires, catheters, and valves, medical device designers must consider friction at interfaces, including insertion, drag, and break-loose forces. A biocompatible silicone lubricant can significantly reduce friction at these touchpoints between components and between components and human tissue.

Dispersed silicone formulations minimally bond to metal substrates, making them ideal for needle lubrication, while hydrophobic coatings are available for syringe barrels to enhance container drainability. (Credit: iStock.com/Liuhsihsiang)

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Silicone has a long and proven history of use with medical devices. When choosing a lubricious silicone for an application or specific device, it’s critical to consider several key factors to ensure that the lubricant properties deliver the expected results for both the device manufacturing process and the end use.

Different Lubricants for Different Substrates

A primary consideration is understanding the nature of various substrates that require lubrication and identifying why different materials and surfaces need specific types of silicone lubricants. Medical devices can incorporate substrates such as silicone, metal, glass, and plastics, each presenting 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 need to move or slide. Silicone elastomers also tend to block, sticking to each other due to chemical affinity. Blocking is particularly evident in slit valves, where the two sides of the silicone part touch and "heal" or close the slit. Keep in mind the following considerations for silicone parts:

• Surface interaction factors: Choose a lubricant with low chemical affinity for the elastomer. For molded silicone parts, it’s crucial to account for the difference in chemistries between the part and the lubricant to prevent the lubricant from diffusing into the material, which can cause swelling and reduce the lubricating effect.

• Viscosity factors: Use higher-viscosity fluids for longer lubrication periods, as diffusion or the chance of migration decreases with higher viscosity, ensuring prolonged surface lubrication.

• Curable coatings: Technological advances have introduced curable, non-migrating coatings that reduce the COF. These coatings bond chemically to the substrate and mimic its mechanical properties, providing a durable, flexible coating on moving parts.

• Reducing processing time: Self-lubricating silicone elastomers can decrease the number of processing steps by incorporating lubricity into the elastomer itself, providing a lubricious surface on the final molded component.

• Moisture sensitivity factors: One-part dispersed silicone fluids that de-volatize easily are moisture-sensitive. Any adjustments to optimize viscosity or solids content should occur in a moisture-free environment.

• Other general factors: Apply the lubricant directly as an oil or dispersed in solvent to achieve desired properties. Consider thin, wettable coatings for reduced COF and thicker greases to minimize break-loose forces.

Metal

The metal surfaces of hypodermic and suture needles, scalpels, and cutting edges have high surface friction, which can cause discomfort during incision or penetration in human tissue. To mitigate this, silicone lubricants can be applied to lower the COF without compromising the metal's cutting efficacy. Consider the following for metal surfaces:

• Surface interaction factors: For repeated use applications, a robust lubricant is necessary to minimize surface friction, lowering puncture and drag forces, reducing rub-off, and ensuring consistent performance.

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• Formulation factors: Consider dispersion and bonding behavior. Dispersed silicone formulations minimally bond to metal substrates, making them ideal for needle lubrication. PDMS fluids are suitable for single-use applications, while high-viscosity dispersed fluids are better for multiple uses.

• Other general factors: Use a silicone grease to minimize migration compared to fluids, enhancing durability and performance.

Glass

Silicone fluids’ silicon-oxygen chemical structure bonds well with glass. Bonding can be enhanced by heating the silicone beyond its operating temperature. Consider the following for glass:

• Formulation factors: A hydrophobic lubricant like PDMS silicone oil can reduce drag forces in glass prefilled syringes and promote container drainability.

• Curing factors: High-temperature heating activates cross-linking, creating a functional interaction between the lubrication of the glass barrel and plunger stopper for efficient operation.

• Other general factors: Consider using a silicone grease to minimize migration and enhance durability through heat treatment.

Plastics

Various plastics are used in medical products, with friction points that can benefit from silicone lubricants. Consider the following for plastics:

• Formulation factors: Use a very-high-viscosity grease to enhance gliding with plastic and plunger stoppers, as grease is less likely to migrate when applied to plastic surfaces.

• Other general factors: Apply the lubricant directly or dispersed in solvent, potentially combining it with PDMS fluid for customized lubricant properties.

Other Key Considerations

Other factors include the biocompatibility and manufacturability of the device.

Biocompatibility: Silicone lubricants used in medical applications should be biocompatible and meet ISO 10993 standards. Silicone’s chemically inert nature makes it stable over long periods, resistant to chemical attack, and suitable for sterilization methods like ethylene oxide and autoclaves. Medical-grade silicone lubricant products supported by Master Files with the FDA and international authorities should be considered.

Manufacturability: Silicone fluids can be applied by dipping, spraying, or wiping and may be diluted to 1–5 percent silicone solids in a compatible solvent for very thin films. Dispersion can be achieved in various organic solvents or predispersed down to a specified percent solids content. Ensure the silicone material selected can be applied using these methods.

Since there isn’t a one-size-fits-all silicone solution for each application, medical device designers should collaborate with suppliers of medical-grade lubricious silicone to meet the specific force reduction and material requirements of their devices.

This article was written by Brian Reilly, Business Development Director – Biomaterials, for NuSil™ – part of Avantor®, Carpinteria, CA. For more information, visit here .

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