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James Darlucio, applications engineer, NuSil, provides key tips on selecting the right silicone adhesives for medical device manufacturing. 

Medical device manufacturers often use silicone adhesives to bond parts together when assembling products such as catheters, pacemakers, cochlear implants, aesthetic implants, and gastric balloons, due to their biocompatibility and versatility.

Selecting the appropriate adhesive can ensure a proper, long-lasting bond, protecting the integrity and performance of the medical device. Manufacturing considerations such as cure rates, adhesive application factors, and surface preparation requirements also affect adhesive selection. 

By understanding these factors, selecting the optimum silicone adhesive solution can help medical device manufacturers improve their products, increase manufacturing productivity, and gain a competitive edge.

Silicone adhesive options

The main types of silicone adhesives available for use with medical devices include condensation cure systems and addition cure systems.

Condensation cure silicone adhesives

Traditional silicone adhesives cure at room temperature upon exposure to atmospheric moisture and are available in one- or two-part formulations.

One-part adhesives, which are the most widely used medical device silicone adhesives, do not require heat to cure, making them ideal for temperature-sensitive components, such as electronics and thermoplastics. They are also well-suited for bonding silicone to other silicones, metals, some plastics, and glass.

• Cure characteristics: One-part adhesive cure rates are influenced by several factors, including humidity levels (20–60% relative humidity recommended), cure temperature (25–50 °C recommended), cross-sectional thickness and the amount of surface area exposed to air. 
• Application considerations: One-part adhesives tend to have slower cure rates compared with two-part adhesives. In addition, liberation of byproducts of the curing process, such as mild acids, alcohols, or other volatile substances, can cause these adhesives to shrink during cure, typically between 3–6% of its original volume. 
• Specialty applications: In certain applications, adhesives can be supplied in a compatible solvent to help facilitate a specific process or application of the adhesive. This method is especially useful in conformal coating applications, which require thin sections or small intricate areas that might otherwise be hard to access. 

Addition cure silicone adhesives

Two-part silicone adhesives offer a high degree of versatility and device assembly efficiency because they do not require ambient moisture to cure. These silicones can meet unique assembly requirements, such as forming bonds at interfaces that have little or no access to air and thick cross sections.

Two-part adhesives are also ideal for products where a relatively low temperature must be maintained. They adhere well to a variety of substrates and can be used for sealing, potting, or encapsulating.

• Cure characteristics: With two-part adhesives, curing starts by mixing two components together in a 1∶1 mix ratio. The cure rate is influenced by temperature, and unlike one-part adhesives, practically no byproducts or leaving groups are released.
• Application considerations: The typical work times range from 15 minutes to 2 hours, depending on temperature, and the cure rate can be accelerated by increasing temperature. Some leading silicone adhesive suppliers can provide the product in ready-to-use cartridges, where the material is dispensed through a static mix tip where air is not introduced.
• Specialty applications: High temperature vulcanizing silicone adhesives are formulated to cure with the application of heat; if a manufacturing or device assembly process requires very fast curing after the adhesive is applied, the cure rate can be accelerated by increasing temperatures.  There are assembly operations where manufacturers may need a prolonged period from when the adhesive is applied until it is cured, yet they also need very rapid curing once the assembly process calls for it, so they apply heat for rapid curing. For example, high-temperature addition cured adhesives can be fully cured at temperatures and times ranging from 10 minutes at 116 °C to 2 minutes at 150 °C.

Medical device substrates and surface preparation 

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

• Silicone 
• Metals such as aluminum, stainless steel, and titanium
• A wide range of plastics, including polytetrafluorethylene (PTFE), polycarbonate, polyurethane, and polyimide.

Increasingly, device manufacturers are discovering that plastic substrate materials such as PTFE can be particularly difficult, if not impossible, to bond using silicone adhesives without first applying some type of surface treatment.

Materials such as polyethylene do not allow a liquid adhesive to easily ‘wet out’ or spread outward across its surface. Proper preparation of the substrate surface is essential to forming a long-lasting bond. Cleaning, activating and/or priming the surface can maximise the surface area’s available bond sites and wettability to improve long-term adhesion.

When developing a surface preparation procedure, there are several variables to consider:

• Substrate cleaning: Certain chemical elements and compounds can reduce or inhibit the adhesive’s curing process. Proper cleaning methods remove contaminants such as finger oils, dust particles, mold release agents, and machine oils on metal parts. Surface cleaning can be done by mechanically or manually wiping the surface with an appropriate solvent (for example, isopropyl alcohol or heptanes).
• Surface treatments: To help increase the substrate’s surface energy for better adhesion, several methods can be considered. Plasma treatment bombards the substrate surface with ions of gas such as oxygen or argon. Another treatment method is the corona discharge technique, which uses increasing voltage cyclically to generate a plasma known as ‘corona discharge’. The type of surface treatment used is typically dependent on the type of substrate and to what degree that substrate needs modification of its surface energy.
• Adhesive primers: Primers can be applied to substrate surfaces that have particularly low surface energy to ensure proper silicone adhesive joining and sealing. Primers act as coupling agents, increasing and strengthening the covalent bonds between the adhesive and substrate. They have also been shown to greatly increase the silicone adhesive’s ability to wet out the substrates.

Biocompatibility and purity

Medical-grade silicone adhesive applications are biocompatible and conform with applicable ISO and USP testing protocols. Consequently, silicone adhesives are resistant to chemical attack, oxidation, and shear stresses. They can be readily sterilised by ethylene oxide, dry heat, or other standard techniques without degradation. 

Device designers should be sure to consider high-purity, medical-grade silicone adhesives with extensive regulatory support and Master Files submitted to the FDA and international authorities. It is also important to select medical-grade silicone adhesives manufactured to strict purity standards with products that use carefully selected raw materials and utilise advanced purification technologies to protect the safety and integrity of the medical devices using those adhesives.