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. In addition to evaluating the different types of adhesive formulations, device manufacturers also need to critically assess several other factors in the adhesion process, including the substrates to be bonded, their surface energy, and even the overall device manufacturing process. Without addressing these factors early in the design process, medical device manufacturers risk costly delays and potential device manufacturing issues.

Considering the Substrate

A two-part thixotropic, or non-slump, adhesive that holds its shape over time.

Medical devices may consist of multiple parts that must be permanently joined and sealed. In most cases where silicone adhesives are used, the adhesive is the sole element that adheres or bonds two surfaces together. But in some cases, adhesives may be used in conjunction with a physical or mechanical element to join two device surfaces — a slot and groove or a pin or other physical joining component.

In general, most medical devices, either implanted, inserted, or used externally, are made of one or more of the following types of materials:

  • Silicone.

  • Metals, including aluminum, stainless steel, and titanium.

  • A wide range of plastics, including polytetrafluorethylene (PTFE), polycarbonate, polyurethane, and polyimide, among others.

An increasing number of medical devices, such as oxygenators and blood reservoirs, are made with polycarbonate due to its ease of molding and light weight. Another plastic, polyamide (often known as Nylon®), can usually be found in common products such as medical tubing and catheters. Titanium and medical-grade silicone rubber are often used for many implanted medical devices, such as pacemakers, cochlear implants, hydrocephalus shunts, and implantable infusion pumps. 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. Device manufacturers might also discover that the substrate materials they selected, such as PTFE, may be very difficult to bond using silicone adhesives without first doing some type of surface treatment.

Fig. 1 - In general, a contact angle below 90° has high surface energy and provides good surface wettability, while a contact angle greater than 90° has low surface energy.

Adhesion must absolutely be secure for long-term use, preserving the medical device's functionality, especially those that are implanted in the body. Many medical devices are inherently small, and their joints are also quite small, requiring careful manual work. If it becomes difficult and time-consuming to achieve a proper and complete seal, medical device manufacturing throughout and productivity can suffer; additional inspection steps or rework of seals may need to be done if substrates fail to fully accept the adhesive application.

With the numerous substrates available and multiple adhesion tests to potentially choose from, there may be challenges with expecting a supplier to have data specific to an application. A custom formulation may also be required, so it's important to work with a capable silicone adhesives manufacturer early in the process to discuss critical factors. To achieve proper and reliable adhesion, three main factors should be considered and effectively addressed, preferably during the device design stage:

  • The wettability or surface energy of the different substrate materials being adhered — silicones, plastics, metals, or other elastomers — used to construct medical devices.

  • Surface cleaning to avoid inhibiting effective adhesion.

  • The use of appropriate primers to better prepare surfaces to fully accept silicone adhesives.

Fig. 2 - The effect of surface cleaning on surface wettability: How multiple cleaning passes impact the contact angle of the polyurethane substrates.

Wettability and Understanding Surface Energy

The receptiveness of a substrate to an adhesive is generally related to its surface energy, or wettability, which refers broadly to substrate acceptance of a coating. Low-surface energy materials, like polyethylene, do not allow a liquid adhesive to easily “wet out” or spread outward across its surface.

The contact angle of the substrate surface helps quantify the wettability of the surface. In general, a contact angle below 90° has high surface energy and provides good surface wettability, while a contact angle greater than 90° has low surface energy, with a greater likelihood of adhesion failure due to blank spots on the substrate surface where no adhesive is present (see Figure 1).

Plastics, such as polyethylene, have very low surface energies, which can make it challenging to use silicone adhesives without proper surface preparation. Better wetting also enables greater interaction with the substrate, filling in the microscopic valleys found on the surface to provide better adhesion.

Ways to Improve Surface Energy

Cleaning, activating, and/or priming a substrate's surface can maximize the available bond sites and wettability to improve adhesion. When developing a surface preparation procedure, there are several variables to consider:

Substrate Cleaning. 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 (e.g., isopropyl alcohol or heptanes) using swabs or lint-free cloths.

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 a gas, such as argon. Corona discharge technique is another treatment method, which 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 to ensure proper silicone adhesive joining and sealing. Primers act as coupling agents, increasing and strengthening the covalent bonds between the adhesive and substrate. It has also been shown to greatly increase the silicone adhesive's ability to wet out the substrates. Note that primers do not work as one-type-fits-all solutions. The substrate material, the adhesive being used, the device manufacturing process, and even certain regulatory requirements all play a role in determining the best primer for a given application.

Testing the Effectiveness of Surface Preparation

Each substrate being joined using silicone adhesives has a unique surface energy. In order to ensure that complete and effective adhesion is achieved in production processes, there are significant advantages to accurately characterizing and quantifying the surface energy of the substrates involved. With this data, medical device manufacturers can make informed decisions about surface cleaning, surface preparation, and the right surface primer to use — decisions that can be made before device designs are completed and production and assembly processes are finalized.

To evaluate how adhesion can be improved through these surface preparation steps, NuSil silicone adhesive experts conducted tests to demonstrate the effectiveness of substrate cleaning and use of adhesive primers.1 The following materials were used:

  • Two polyurethane substrates:

    • Pellethane® 2363 80A, an 80 shore A durometer thermoplastic polyurethane elastomer.

    • Pellethane® 2363 55D, a 55 shore D durometer thermoplastic polyurethane elastomer.

  • Two NuSil silicone adhesives:

    • MED-1137, a one-part tin-catalyzed room temperature vulcanizing (RTV) silicone adhesive.

    • MED3-4213, a two-part platinum-catalyzed fast-curing silicone adhesive.

  • Two NuSil primers:

    • MED-160, a general all-purpose primer used with moisture- and platinum-cure silicones.

    • MED-162, designed for use with polyurethane, polycarbonate, and polyetheretherketone (PEEK) substrates and platinum-cure adhesive systems.

The first surface preparation factor studied was the effect of surface cleaning on surface wettability, specifically whether and to what extent multiple cleaning passes impact the contact angle on the two polyurethane substrates.

Table 1. Contact angle (average ± std dev). Note: Ink rub tester weight wrapped with 12 ply of Kimwipes and substrates wetted with 70 percent IPA.

The surfaces of both substrates were wiped using isopropyl alcohol with a mechanical wiping device to eliminate inconsistencies in manual wiping, such as pressure against the substrate and area to be cleaned. The contact angle was measured after 20, 40, and 60 wipes and compared with the contact angle without cleaning. Results showed that cleaning the surface significantly improves the contact angle (see Figure 2 and Table 1).

Once cleaning was shown to improve surface energy, the silicone primers were applied to the same two precleaned polyurethane substrates. For each substrate, the peel force required to separate the polyurethane panel from the silicone was tested, assessing the effect of surface cleaning and the use of adhesive primers.

Fig. 3 - Peel force required to separate silicone sheeting adhered to an 80A durometer polyurethane with MED3-4213 and MED-1137.

The first test assessed adhesion and the effect of surface treatments on the Pellethane® 2363 80A polyurethane (see Figure 3). The polyurethane panel and a sheet of silicone were adhered using NuSil MED3-4213 and MED-1137 silicone adhesives with and without the use of the two primers.1 Peel force tests were conducted using an Instron 3345 tensile tester set at a 20 in./min pull rate, and the peak force was reported. Significant improvements were shown with surface cleaning alone, and even more so when primers were used, with the NuSil MED-162 primer providing the most effective improvement in adhesion for both silicone adhesives tested.

Fig. 4 - Assessing surface treatment effects on the Pellethane® 2363 55D polyurethane.

The tests were repeated on the Pellethane® 2363 55D polyurethane, using the same surface cleaning, silicone adhesives, and surface primers (see Figure 4). For this substrate, although only slight improvements in peel force were shown with surface cleaning alone, significant improvements were shown through the use of both NuSil primers, with the NuSil MED-160 primer on MED-1137 adhesive outperforming the rest.

Final Factors for Best Results

Silicone adhesives are a trusted and widely used material for many types of medical devices. These results demonstrate that for the specific polyurethanes tested, both surface cleaning and the use of primers significantly improves adhesion. In general, it shows that medical device manufacturers using silicone adhesives can benefit from assessing the surface energy or wettability of substrates they are joining with silicone adhesives.

Along with choosing the best silicone adhesive for a given device, manufacturers should take steps to ensure that they understand the surface energy of the substrate so that it can be effectively prepared and suitably wetted to ensure maximum adhesive dispersal and reliable adhesive performance. Because of the variety of adhesives, substrates, and manufacturing processes, there are advantages to working with an adhesives supplier that can characterize and perform different tests to qualify adhesion requirements specific to the application; they can also supply insight on the appropriate silicone adhesive and primer to use for the specific application. In some cases, due to unique requirements, a special formulation may be required. It is beneficial for the supplier to be equipped to develop and manufacture custom adhesives.

Finally, with silicone adhesives and primers used for medical devices, it's especially important not to overlook a supplier's manufacturing and purity qualifications as well as support during the regulatory process. Besides achieving the optimal adhesion requirements, an adhesives supplier that uses quality manufacturing processes and can provide Master Files to FDA and international authorities that document product details about ingredients, manufacturing, processing, packaging, and storage may save a device maker valuable time when taking the final product to market.

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