The strict medical regulatory environment can give engineers a tough time finding an adhesive that meets conflicting end-use and manufacturability requirements. Materials must be biocompatible, able to withstand aggressive sterilization methods, and have high bond strength, thermal performance, and chemical resistance.
Materials and adhesives must work together as a system and minimize performance and manufacturability trade-offs. Finding the medical adhesive that can do it all may not be easy, but answering these four questions is a good start.
Is the adhesive biocompatible?
A common misconception about medical adhesives is that the United States Food and Drug Administration (FDA) regulations require that adhesives be tested for biocompatibility before they can be incorporated in a medical device. However, the FDA considers only the biocompatibility of the finished medical device, not the individual materials.
Using materials known to be biocompatible makes it more likely that the finished device will be biocompatible, so device engineers favor adhesives and materials that have been pre-screened by passing industry standard biocompatibility tests. In North America, the most widely accepted test standard is U.S. Pharmacopeia (USP) Class VI, which still does not guarantee that a device will gain FDA approval.
Master Bond has more than 30 USP Class VI-certified grades for bonding, potting, sealing, and coating applications, including epoxies, UV-curables, silicones, cyanoacrylates, polyurethanes, and solvent-based compounds.
How will it be used and sterilized?
Medical devices with any chance of patient contact must be sterilized, either once for disposable devices or dozens of times for reusable devices. Health care sterilization methods include autoclaving, exposing devices to ethylene oxide (ETO), radiation, electron beams, and chemical treatments. Microbe-killing sterilization treatments can be very tough on polymeric materials. Autoclaving can subject devices to up to 135°C and steam pressures up to 20 lb. Many polymeric materials, including a variety of standard adhesives, will degrade – primarily through hydrolysis – following repeated exposure to the damp heat. Other sterilization methods tend to be less aggressive than autoclaving but still have some potential to degrade polymeric materials as the number of sterilization cycles increases.
Various Class VI epoxies, UV-curables, cyanoacrylates, and silicon adhesives can be used with most bulk-sterilized disposable devices, so adhesive choice depends on non-sterilization factors.
Reusable devices have to endure multiple trips through an autoclave or are repeatedly treated with polymer-unfriendly chemical sterilizing agents. Epoxies make up the majority of autoclave-tolerant adhesives because they are the most chemical, moisture, and thermal resistant.
How important is manufacturability?
The relationship between designing for use and designing for assembly can be particularly troubled in medical applications because adhesives that offer the best resistance to sterilization are not always the fastest or easiest to use on the shop floor.
Disposable devices usually have the highest manufacturing volumes, so they need the most productive medical adhesives available. Many Class VI adhesives require little or no complex mixing, are easy to apply, cure quickly, bond without fixtures, and resist common bulk sterilization methods. Popular choices for these applications are cyanoacrylates, UV-curables, and some one-part epoxies.
The epoxies and other adhesives that withstand multiple autoclave cycles used on reusable devices are not easy to apply. They don’t cure as fast as cyanoacrylates or UV-curables, they may have mix ratios that are less straightforward than 1:1, and they may require fixturing and heat curing. A device’s sterilization method can have implications that reach all the way back to the productivity of assembly operations.
Is the adhesive an afterthought?
It is not uncommon for engineers to pick the right material to meet their device’s functional requirements, only to find out that they have severely constrained their bonding choices. Even though a solution for almost every substrate combination probably exists, the solutions require avoidable, expensive trade-offs. For example, olefinic materials can be adhesively bonded, but they need primers or surface treatments that add time and cost to assembly.
Many more bonding options are available when materials and adhesives are evaluated early in the design process.
Master Bond Inc.