Whether a million-dollar diagnostic machine or a humble syringe, almost every medical device relies on adhesives for some aspect of assembly. In an industry where quality control and hygienics top design priorities, many medical products share two characteristics: They use plastic, and they're disposable.
Adhesives appeal to engineers for bonding plastic disposables because of their relatively low capital-investment costs. In fact, engineers often choose adhesives over mechanical clamping, heat sealing, or solvent bonding for their ability to provide uniform, space-efficient bonds. And they're compatible with automated dispensing and curing equipment. This gives manufacturers control over joint quality and can speed production.
Selecting the right adhesive is seldom a simple prospect for the medical design engineer, however. Bond performance, substrate compatibility, process parameters, and FDA requirements complicate the picture.
"Efforts to develop better adhesives are under way, and adhesives manufacturers would do well to accelerate those efforts," advises Jeffrey Ellis, a consultant specializing in plastics for medical devices. "In our highly regulated and competitive economy, the medical device industry needs these improvements "yesterday.'" Many adhesives makers are moving in that direction. Here are a few examples:
Disposable diagnostics. Recent adhesive advances allow engineers to match a variety of demands and bolster the design of disposable medical devices. For instance, engineers at ActiMed Laboratories, Burlington, NJ, use a UV-cure adhesive to assemble their ENA.C.T(R) cholesterol home-test product.
The disposable system consists of a thermoformed PETG base with compartments for pads and chemicals for diagnostics. Engineers bond the product's 10-mil-thick PETG cover to the base with an acrylate adhesive from Grace Specialty Polymers, Woburn, MA.
The first challenge in bonding the cover and base arose when engineers chose a white silkscreen for the cover. The silkscreen provides space to print the company logo and a brief patient-data form. Although the silkscreen meets FDA label-legibility requirements, there's a catch: It filters UV light.
Seeking a cure. "The white ink has a UV filter so that it doesn't yellow, and so that UV light doesn't damage our product," explains Senior Mechanical Engineer Hal Solberg. "The majority of UV adhesives wouldn't cure between the two plastic sheets because of that filter."
The silkscreen allows only about 0.02% transmission of light in the 360 nm spectrum, where adhesives typically initiate curing. So engineers needed an adhesive sensitive to small amounts of UV light. "Grace gave us one that had a visible component to the photo-initiator. So even though the white ink filters the UV, frequencies of around 400 nm-visible light-get through," explains Solberg. Grace's Eccobond(R) UV 9110 adhesive, based on the formula ActiMed uses, will be commercially available this month.
Before the adhesive was available, ActiMed engineers masked areas of the cover, leaving them clear of silkscreen for UV-light penetration. The attempt proved unsuccessful. "We're basically laminating two flat pieces of plastic together," says Solberg. "The problem was that the glue, being fairly thin liquid, would spread between the plates and go under the silkscreen and not get cured." Uncured adhesive gives off vapors. These vapors would be trapped in the product's hermetically sealed package and damage the diagnostic enzymes.
Inspired flashes. Typically, engineers use mercury-vapor lamps to generate UV light. But for Solberg and his colleagues, the heat generated by such lamps posed an additional problem. In early tests, the lamps were melting the substrate before the adhesive cured; prolonged UV exposure also caused the plastics to turn brittle.
Fans and ducts to remove heat and ozone generated by the lamps created a fairly complex problem, Solberg explains. "And there's no way to turn the lamps on and off quickly. If the line stopped, it would be like old film projectors when the film burns up. That's exactly what would've happened to our production line."
Instead, engineers turned to a pulsed UV-curing system from Xenon Corp., Woburn, MA. Controlled by a host PC, the system lends itself well to ActiMed's PC-controlled automation, says Solberg. Users can control the pulsed light via an RS232 communications port and select the pulse rate to suit the application. This allows engineers to alter process parameters and maintain the same amount of light energy striking each unit independent of line speed-a plus during manufacturing ramp-up. "It eliminates a production variable," says Solberg.
The pulsed-curing technique's brief exposures and the adhesive solved Actimed's assembly problems, Solberg adds. "It's a nice marriage of the two technologies."
Achieving a full cure wasn't the only hurdle for ActiMed engineers. "We looked at some competitive adhesives that would cure in the same lighting conditions, but the Grace product was made specifically for the medical industry and several engineering plastics," Solberg notes. Tests showed that the adhesive bond is typically stronger than the plastic substrate. "Some of the other adhesives cured similarly, but they just would not adhere to our plastic."
Solberg emphasizes the benefit of teamwork among engineers from Grace, Xenon, and ActiMed. "It's a pretty tight triangle of technological people."
Lasting bonds. Like Solberg, engineers at Becton Dickinson (BD) teamed with an adhesive manufacturer for a disposable test-kit design. To develop kits such as home pregnancy tests, engineers at BD's Microbiology Systems group, Sparks, MD, worked closely with Adhesives Research, Inc., Glen Rock, PA.
Here, the challenge was to develop a way to bond delicate substrates to be slit into test strips for the kit. "The home-test market is moving to a different format," explains Carrie Arndt, senior project manager at BD. "Instead of letting materials flow down through a test, they now go in a lateral migration, on a flat surface."
To accommodate such fluid migration, BD engineers needed to stack several materials without hampering fluid flow. They required both a solid support and an adhesive surface to bond the test strips.
BD engineers had two main hurdles to overcome: They needed an adhesive that would not migrate, and it had to withstand heat, cold, and very dry or humid environments, since the kits are shipped world-wide. Says Arndt: "It wasn't good enough that it work sitting on your desk in the U.S."
Finding a bonding solution wasn't easy. Several large tape manufacturers Arndt approached were unwilling to create custom configurations, or they did not have formulas compatible with the substrates and slitting process.
"Adhesives Research helped identify an adhesive that wouldn't migrate and gunk up the pores of our material," says Arndt. "Their adhesive is not only compatible, it bonds nicely to our material without causing it to distort, shred, or crack.
Vendor's value. Equally important was the vendor's participation during prototyping and scale-up, says Arndt. "Adhesives Research understood what was going on from a development basis." In the end, engineers chose a solvent-based acrylic adhesive on mylar plastic film. BD engineers bond a glass-fiber material and a layer of delicate nitrocellulose. In some applications, they also use porous polyethylene. Finally, the adhesive is cured through forced-hot-air ovens. "These are pretty dissimilar materials, and the adhesive holds while we chop the composite into 0.25-inch-wide test strips," adds Arndt.
No matter what the substrate, cure method, or performance demands, when it comes to bonding medical disposables, chances are there's an adhesive for the job. Often, however, engineers must balance priorities in order to achieve a successful bond. Says ActiMed's Solberg: "At first glance, it seems that these projects should be easy to do. But it's not Elmer's glue."
Adhesives encapsulate protective mask
Adhesives must sometimes do more than bond medical products. For instance, engineers at Acutek, Inglewood, CA, recently encapsulated a protective lead mask using pressure-sensitive adhesives.
Dentists use the shield to block x-rays from patients' eyes. And while the lead protects them from radiation, a clear copolyester bonded with adhesives protects them from the lead. Small holes in the shield allow light to come through so that patients aren't disoriented when they remove the mask.
The mask consists of conformable lead foil with a protective polyester cover. Acutek applies acrylic-based adhesives from 3M and Avery with a silicone-coated paper liner to the lead roll, then die-cuts the mask's outline. A rotary die cuts the light holes. Next, an operator removes the adhesive liner, excess adhesive, and lead from the holes, and bonds the mask to a clear polyester film.
On the other side of the mask, operators apply a clear polyester/copolyester substrate, supplied from the manufacturer with adhesive already on it. Finally, they die-cut the outer shape. The shield, which can be used as many as 50 times, is worn in a single-use polyester sleeve.