Adhesive advancements are allowing simple and consistent joining of materials, used in the production of catheters and disposable medical devices, and once thought impossible to glue.

Which adhesive systems are applicable to medical devices, and how do device manufacturers get the best results from them? Read on.

Medical adhesives 101

The basic adhesive systems for disposable medical devices are epoxy, cyanoacrylate, acrylic-based light (UV) cure, and solvent bonding. The most widely used for disposable plastic medical device assembly are cyanoacrylate (CA) and UV cure (UVC). Epoxy is often used for metal-metal bonding and potting, and solvent bonding (SB) with solvents such as cyclohexanone are used in the assembly of low-cost tubing sets. SB can be inexpensive and efficient; however the flammability and toxicity of these solvents and the venting of VOCs can be a significant problem. CA and UVC are one-part, easy–to-use, highly versatile, work at room temperature, and do not require solvents or catalysts, making them the most popular adhesives in medical device assembly. Each system has its own set of capabilities, so it is important to become familiar with the range of adhesive systems available to the medical device designer, and have a “toolkit” of capabilities for choosing the best adhesive system for your application.

Medical grade epoxy is available from several suppliers, such as Epotek, Master-Bond, and Loctite. Cyanoacrylate (CA) is popularly known by the consumer trade name, Krazy Glue. CA was originally developed by the Eastman Chemical, then a division of Kodak, and is now made and marketed in a very wide range of commercial and medical versions by Loctite (division of Henkel). Medical grade UVC adhesives are manufactured by Dymax and Loctite.

CA and epoxy are readily available for R&D (non-clinical) use in “hardware store” grades. UVCs are typically only available from commercial suppliers, and can require a relatively expensive light source to cure. For R&D, an inexpensive dental UV light source or LED “light pens” from your vendor can be used for prototyping. It is important to remember when using any UV source to use UV blocking eye protection. These safety glasses are available from your vendor. If a UVC system is used in a production environment, it must be calibrated regularly to ensure the light is completely curing the adhesive. These calibration devices are available from the light source manufacturer (e.g. Efos, Dymax, and Loctite)

There are special formulations in a wide range of viscosities for joining nearly every plastic and surface treatments for difficult-to-bond substrates. Loctite offers a comprehensive course in the use of their adhesives.

To understand how best to bond parts together, it helps to know the basic potential failure modes of a bond. These are cohesive, adhesive, and substrate failure. Cohesive failure is when the adhesive fails to bond to one surface or the other. Adhesive failure is when there is a bond to both surfaces, but the adhesive takes the load and fails. Substrate failure is when the adhesive bond is stronger than what is being bonded, and it fails. Success is when the adhesive forms a molecular bond to the substrates, and the two parts joined act as one material. Adhesives join parts in one of two basic ways, either as a molecular bond, where the adhesive joins chemically to the substrates, or mechanically, where the adhesive grabs onto a substrate’s surface without bonding chemically. Sometimes, secondary operations such as grit blasting metal or etching non-reactive substrate materials such as PTFE (Teflon) or silicone are required to achieve a successful mechanical bond.

Some tips for a happy union are fresh adhesive, clean parts, and compatible materials. One way to avoid adhesive bonding problems is to choose materials that bond more easily when possible, such as styrenes, ABS, polycarbonates, acrylics, and urethanes. “Blocked” polymers such as Pebax and high surface energy plastics such as polypropylene may be bonded with proper surface preparation. These surface preparation “primers” are available from the adhesive manufacturer. Lowdurometer materials with high levels of plasticizer can be problematic. Obvious difficult-to-bond materials are the fluorinated polymers—

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