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.