Improved reliability and cool LED light-curing top the trends in adhesives
The adhesives area is expanding more than trending, as more and smaller electronics require bonding into wearable devices. The trend for wearables in turn is fuelling demand for increased reliability and the ability to withstand harsh sterilization environments. And new methods for curing adhesives are on the forefront.
More companies are dealing with electronic applications that call for bonding tiny printed circuit boards to different surfaces. That makes reliability a challenge for medical devices, which require the bonding of dissimilar surfaces including plastics, metal, ceramic, and glass.
“So much of the adhesive selection is device dependent,” notes MasterBond application engineer James Brenner. “It’s difficult to define generalities.”
One concern for designers is thermal stability and sterilization. When bonding two dissimilar surfaces, be aware that they involve two different coefficients of thermal expansion and contraction. The good news, says Brenner, is that there are adhesives that withstand such thermal cycles. The earlier designers seek the advice of an adhesive company, he suggests, the better off they’ll be.
The FDA is concerned that reusable devices be able to withstand multiple cycles of sterilization, including autoclaving, radiation, ethylene oxide, and cold chemical sterilization. For higher-strength and multiple sterilization cycles, one or two-component epoxies work best while silicones are good for flexible bonds, tolerate high temperatures and have a low shrinkage on cure. The standard to shoot for is USP class 6 certification, whether a device must withstand one cycle or multiple cycles, to show a strong indication the adhesive will not cause problems when submitted to the FDA for approval.
Biocompatibility is a key trend for designers of implantable devices, which must be compatible with blood and body fluids. There are two widely used – and stringent – tests for biocompatibity: ISO N 993-5, detailing a cytotoxicity requirement, and USP Class 6.
Perhaps the latest trend in adhesives is LED-cured material, as explained by Brenner.
“The advantages of it are that it is a one-part material – it requires no mixing, unlike expoxies – and the LED is a cool light so adds no heat (as would a UV lamp) that would distort the substrates being bonded,” he says. “Just as with UV light curing, the parts must be transparent.”