Christine Marotta, Henkel
Intricate medical devices are often constructed of thermoset and other engineered plastics, which require advanced adhesive technology. In addition to considerations of bonding, sealing, gap filling, and manufacturability, developers need to consider the sterilization plan that all materials, including the adhesives, will need to withstand.
The challenge is high particularly for reusables and non-disposables. These devices are often sterilized by steam autoclave, hydrogen peroxide, and chemical immersion because these methods are conducive to quick turn around and are considered low toxicity.
Autoclaving, the sterilization method for a high percentage of these categories of medical devices, presents the greatest challenge to device manufacturers due to the combination of temperature, pressure, and moisture. Manufacturers must seek substrates and joining methods that are versatile and easy-to-use, yet still hold up in the most rigorous environments.
Cyanoacrylate, light curing cyanoacrylate, light curing acrylic, dual UV/moisture curable silicone, epoxy and urethane adhesives are commonly used for the assembly of medical devices.
Cyanoacrylate adhesives are polar, linear molecules which undergo an anionic polymerization reaction. A weak base, such as moisture present on essentially all surfaces, triggers the reaction causing the linear chains to form. The products are maintained in their liquid form via the addition of weak acids which act as stabilizers. A variety of cyanoacrylate formulations are available with varying viscosities, cure times, strength properties and temperature resistance. Cyanoacrylates form thermoplastic resins when cured.
Testing of cyanoacrylate adhesives with a number of sterilization methods has yielded varying results, depending on formulations. In general, however, Cyanoacrylate adhesives have been shown to withstand up to fifty cycles of liquid sterilization immersion as well as hydrogen peroxide. In addition, select cyanoacrylate adhesives have exhibited moderate resistance to autoclave exposure – with some specialty ethyl grades maintaining nearly 100% of their initial strengths following exposure to fifty autoclave cycles. A critical factor in maintaining bond strengths with cyanoacrylate adhesives following autoclave exposure is the selection of substrates that offer moderate to high initial strengths as well as substrates capable of withstanding the rigorous temperature, pressure and steam environment of the system.
Light curing acrylics cure via a free radical reaction to form thermoset resins when exposed to light of the appropriate wavelength and intensity. Like cyanoacrylates, light curing acrylic adhesives are available in a range of viscosities. In addition, light curing adhesives vary in final cured form from hard, glasslike resins to soft flexible resins.
As with cyanoacrylate adhesives, light curing acrylics vary in bond strength retention following exposure based on formulation, substrates selected and initial strengths achieved. Testing has indicated that, in general, light curing acrylic adhesives maintain from 50% to 100% of initial strengths following fifty autoclave cycles.
Light-cured cyanoacrylates are ethyl based products which have photoinitiators added to the formulation. The end result is fast fixturing (like that of a traditional light curing acrylic) and cure in shadowed areas. Light curing cyanoacrylates would be expected to perform similarly to standard ethyl cyanoacrylates following sterilization exposure including autoclave.
Silicone adhesives are similar to polyurethane adhesives in that they form flexible polymers when cured. Silicones, however, possess no rigid segment and therefore exhibit lower cohesive strengths – the strength of the polymer itself. The sterilization resistance of silicone adhesives is typically measured on the bulk polymer rather than on assembled specimens due to the low cohesive strength of the polymers. Testing of dual light cure/moisture silicone adhesives following exposure to fifty autoclave cycles indicated a slight effect on the percent elongation of the adhesives, but an approximate 60% drop in tensile strength.
Epoxy adhesives, like the previously mentioned light curing acrylic adhesives, cure to form thermoset plastics. Like several previously mentioned chemistries, polyurethane adhesives form thermoset resins when cured, thus exhibiting good chemical and environmental resistance. It is important to note, however, that the overall thermal resistance of cured polyurethanes is less than that of cured epoxies.
Polyurethane adhesives are substrate versatile but do, on occasion, require the use of a surface primer to increase the reactivity of the surface to be bonded. Epoxy and urethane adhesives are often selected for applications due to their enhanced chemical and thermal properties. Such resistance makes the adhesives suitable candidates for sterile reusables and non-disposables. With the potential of repeated autoclaving exposure, it is critical that reusable/non-disposable device manufacturers select adhesives which have the ability to withstand high temperatures and high steam pressure conditions.
Christine Salerni Marotta is the North American medical business and market manager for Henkel.