6 tips for choosing silicone adhesives in medical devices

Medical-grade silicone adhesives are one of the most commonly used materials to seal and join medical devices, with many assembled from multiple components and materials that must also be permanently joined and sealed. They are widely used in these applications because of biocompatibility, purity, and ease of customization for adherence to critical requirements and industry specifications.

In most cases, when medical devices are sealed, the silicone adhesive is the sole element bonding the two surfaces together. Sometimes adhesives may be used with a physical or mechanical element to join two surfaces – slot and groove, a pin, or another physical joining component. In addition, functional, technical, and surface preparation influences adhesive choice. Considering these factors early in the process can help manufacturers run assembly operations with greater efficiency and assurance that their medical devices are fully and reliably joined.

#1: Understand different silicone adhesive types

Medical device manufacturers can choose from many silicone adhesives, each with unique characteristics. Consider the substrates, their surface energy, and compatibility with the preferred adhesive.

Room temperature vulcanizing (RTV): Traditional RTV silicone adhesives cure at room temperature upon exposure to atmospheric moisture. Newer, more versatile RTV formulations do not require moisture, and the process can be accelerated if minimal heat is applied. RTV adhesives are available in one-part or two-part formulations.

One-part moisture-cure RTV adhesives are the most common silicone adhesives for medical devices, can be applied directly, don’t require premixing, and are typically self-leveling formulations that flatten or spread out throughout time. Due to the chemical properties, adhesive can shrink between 3% to 6% of original volume.

Two-part RTV adhesives are versatile, have much shorter cure times, do not require moisture to cure, and can meet assembly requirements such as forming bonds at interfaces that have little or no access to air. Two-part RTV adhesives are also ideal for temperature-sensitive components and assembly processes where a relatively low temperature must be maintained.

High temperature vulcanizing (HTV): HTV silicone adhesives cure quickly with heat and are only available in two-part formulations. HTV adhesives are ideal for bonding parts that are not heat-sensitive or for a quick cure to save device assembly time.

#2: Consider the substrate

Most medical devices – implanted, inserted, used externally – are made of one or more of the following types of materials:

• Silicone
• Metals (stainless steel, titanium)
• Plastics (polytetrafluorethylene, polycarbonate, PEEK, polyimide)
• Polyurethane

In some cases, two pieces of the same substrate – plastic, silicone, or metal – are sealed with silicone adhesive. In other cases, two different types of material – such as silicone-to-metal or silicone-to-plastic – need to be joined and sealed. Depending on the substrates being adhered, simply applying the silicone adhesive may not provide the desired level of bonding.

It’s crucial to understand the wettability or surface energy of the different substrate materials being adhered, especially if two different substrates are being joined. Wettability defines how a substrate will accept a coating. Low surface energy materials, such as polyethylene, do not allow a liquid adhesive to easily spread outward across its surface. If an adhesive doesn’t spread correctly, blank spots can form with a greater likelihood for adhesion failure.

Understanding surface energy of the substrates being joined is a guide on how to prepare the substrate surface for proper silicone adhesive application.

#3: Prepare substrate surfaces before adhesion

If you have determined that one or more of the substrates used in your medical device have low surface energy, several options can improve it to assure proper adhesive application.

Substrate cleaning: Surface cleaning can be accomplished mechanically or by manually wiping the surface with an appropriate solvent (e.g., isopropyl alcohol or heptanes) using swabs or lint-free wipes to remove contaminants such as finger oils, dust particles, mold release agents, and machine oils on metal parts.

Surface treatments: Several methods are available to sufficiently modify a substrate’s surface energy to improve wettability. Plasma treatment bombards the substrate surface with ions of a gas, such as argon. The corona discharge technique uses increasing voltage cyclically to generate a plasma known as corona discharge.

Adhesive primers: Primers can be applied to substrate surfaces that have particularly low surface energy, acting as coupling agents and increasing the covalent bonds between the adhesive and substrate. They also have been shown to increase the silicone adhesive’s ability to wet-out the substrates.

Note: Primers do not work as one-type-fits-all solutions. The substrate material, adhesive, device manufacturing process, and certain regulatory requirements all play a role in determining the best primer for a given application.

#4: Consider adhesion requirements

Physical, mechanical, and chemical forces act against a bond, so determining the acceptable failure point differs from device to device and manufacturer to manufacturer. Conduct tests in different conditions to simulate real-world use and confirm adhesion is suitable for the given application.

It’s important to work with an adhesive manufacturer equipped to perform different tests to qualify adhesion for the application. Several factors can be tested and measured, such as lap shear and peel strength of cured bilayers from silicone elastomers.

#5: Avoid cure poisons, contamination risks

When working with silicone adhesives, it’s important to consider solvents, chemicals, or substrates they may contact in their uncured state. Certain chemical elements and compounds can retard or inhibit the adhesive’s curing process during device handling, storing, or assembly.

These cure poisons can lead to unacceptable variations in the manufacturing process and finished product, and the presence of inhibitors may cause the silicone to appear wet or tacky at the substrate interface.

For platinum-catalyzed silicone systems, most poisons typically have a sulfur-containing material (e.g., natural rubber, latex, neoprene), a nitrogen-containing material (e.g., amines), or an organotin-containing material (e.g., condensation-cured silicones). For RTV or tin-catalyzed silicone systems, the most common inhibitor is the presence of an alcohol, such as isopropyl.

Proper surface preparation prevents inhibition for a contaminate-free surface. To determine whether substrate surfaces have been properly prepared, test a small amount of silicone on the surface as a trial to evaluate potential inhibition effects.

#6: Work with silicone technology experts

Every medical device has unique materials and configurations – and that includes the design of the joints that need sealed. In addition, each manufacturer has unique production and assembly processes. Because of these variables there are advantages to working with an adhesives supplier that can perform different tests to qualify adhesion requirements specific to your production processes or device. These experts can also provide insight on the silicone adhesive and primer for the application.

Sometimes it may require a special formulation, so it’s best if the supplier is equipped to develop and manufacture custom adhesives.

When using silicone adhesives and primers for medical devices, it’s especially important to look at a supplier’s manufacturing and purity qualifications as well as support during the regulatory process. Besides achieving optimal adhesion requirements, adhesive suppliers that use quality manufacturing processes and can provide Master Files to the U.S. Food and Drug Administration (FDA) – documenting product details about ingredients, manufacturing, processing, packaging, and storage – may save a device maker valuable time when taking the final product to market.

NuSil
https://www.nusil.com

About the author: Brian Reilly is business development director, biomaterials, for NuSil – part of Avantor. He can be reached at 805.684.8780 or silicone@nusil.com.