Reusable medical devices pose some unique challenges for design engineers, particularly when it comes to the proper selection of materials, adhesives, sealants, coatings and encapsulants.
Surgical instruments, implantable devices, catheters, endoscopes and ultrasound probes are just some of the medical devices that require adhesives, each requiring their own specific material, adhesive, sealant, coating and encapsulant properties for their particular applications.
Medical devices are often made from engineering resins such as ULTEM, PEEK, RADEL, PPS and Polyolefins. For lower temperature applications, polymers are also used, like polycarbonates, ABS or polyamides (Nylon), for example. Metals such as titanium, nickel, aluminum and stainless steel are also common.
Adhesives ranging from epoxies to silicones, polyurethanes, polysulfides and other adhesive systems offer many assembly advantages and each works best in specific applications, with specific materials. Advanced adhesives can also replace mechanical fasteners for improved performance, reduced costs and greater resistance to sterilization processes.
However, today’s reusable medical devices have stringent performance and biocompatibility requirements that they must meet. Typically, medical device adhesives must be biocompatible, according to standards like the USP class VI rating or ISO 10993-5 rating.
In addition to meeting these standards, modern sterilization techniques employ steam, ethylene oxide (EtO), hydrogen peroxide gas plasma, glutaraldehyde, peracetic acid and irradiation. The application conditions dictate not only the product selection but also underline the importance of processing techniques.
“You have to pay close attention to the processes, from measuring, mixing and curing the epoxy, because everything has to be done in a certain way,” explained Rohit Ramnath, senior product engineer at Master Bond Inc.
“Conditions can vary from application to application. As an example, if we were looking at surgical equipment, you might need autoclaving resistance, so you’d be looking at temperatures of around 260° F (~ 130° C). The key is for the epoxy or adhesive to be able to handle those environments. Selecting material that has a high glass transition temperature (Tg) in that environment is important, but on the other end of the spectrum, you might have applications with exposure to different PH levels. In cases like these, typically adhesives must be cured at elevated temperatures to optimize the cross linking and achieve the highest possible bond strength.”
Designing and Pre-treating for Adhesives
Using advanced adhesives to meet structural and biocompatibility requirements can change how engineers approach the design of a medical device, depending on the requirements of the application.
“The more the surface area, the better,” said Venkat Nandivada, manager of technical support at Master Bond. “The aim is to keep the [bondable] surface area at a maximum.”
These changes in design also extend to the production process of the medical device, from pre-treatment to assembly. Low surface energy plastics such as polyethylene, polyolefin or even PTFE (Teflon) require surface pre-treatment, like a primer. For example, X21MED can be used for polyolefins to make the surface bondable.
But even plastics and metals that are friendlier to bonding still need the right surface preparation, as Nandivada explained:
“For those, we typically recommend roughing and cleaning, usually with at least a 300µin finish. In many cases where that’s not possible, you can consider chemical etching and plasma treatment. Ultimately, substrates have to be clean and dry before you apply the adhesive and there shouldn’t be any contamination on the surface.”
Many adhesive chemistries, which offer good toughness characteristics, are often formulated to achieve a strong bond between dissimilar substrates.
Mixing and Dispensing Adhesives
From a broad perspective, adhesive chemistries come in either one-part or two-part mixes. Biocompatible solutions are available in both formats.
A key difference between the chemistries comes down to curing.
“One-component systems usually have the advantage of an unlimited working life at room temperature and can cure much faster with heat, at temperatures like 250° F (120° C),” said Nandivada. “Two-part chemistries can cure at room temperature, or you can accelerate the cure by adding heat to temperatures such as 150 F (65° C). However, there are certain two-part chemistries that only cure with heat, at around 250° F (122° C).”
Two-component systems usually are packaged in static mixers compatible with a gun applicator, pre-mixed and frozen systems, FlexiPaks and cans; unlike one-component systems, which are usually packaged in syringes or cans. However, both systems can be dispensed through automated systems.
Syringes are typically used for one-component systems and are designed to be applied with either manual or air-actuated automatic dispensers. Two-component systems which require static mix heads, are ideal for semi-automated assembly lines, but this doesn’t have to present challenges for automated dispensation systems.
“There are cases where you might have a large volume application, wherein static mixers in combination with gun applicators can be used for mixing two-component systems, and that could be attached to a pneumatic system for large volume production,” said Ramnath. “Alternatively, you can use a semi-automated system where you manually pull the trigger from the gun and dispense the adhesive for smaller production runs.”
For particularly small production runs, manufacturers can use FlexiPaks , where you have two components separated by a rod, which can be removed. The flexipack can then be manually mixed by hand and dispensed on the device.
“Packaging plays a crucial roll to ease some manufacturing concerns,” Ramnath added.
Potting and Curing Adhesives
Encapsulation is another process that requires some research for best results. The potting process presents a significant issue when it’s time to cure your adhesive: the co-efficient of thermal expansion (CTE).
To eliminate CTE complications, Ramnath recommends using products filled with elements like aluminum oxide, which is a ceramic filler.
“It’s not going to match something like a metal housing, but it might come close and that is definitely useful if you were to look for a low CTE system,” he explained. “On the other hand, if you’re looking to handle thermal cycling stresses, there are flexible compounds and epoxy urethane-hybrid systems or even silicones, which are widely used as potting compounds.”
Potting can also be a tricky business: achieving high tack to keep the product in place, but getting a long enough pot life that small adjustments can be made before a hard set can be difficult.
Ramnath’s team at Master Bond also uses novel systems combining UV light and heat as a dual curing mechanism for sealing and encapsulating small electronic components. “You tack it with a UV lamp and you can then cure it or post-cure it in an oven, depending on the geometry of the device,” he said. “If you can get enough access to UV light, you can get enough tack where you don’t need external support for the heat curing portion. It’s quite popular because it is fast. However, the trade off is that the shrinkage would be higher than typical two component potting compounds. Master Bond is trying to develop new nano-filled systems, which might help reduce some of the shrinkage in such systems.”
Adhesive Bond Strength, Testing and Off-Gassing
The bond strength of adhesives used in medical devices is largely dependant on the substrate.
Most metals can achieve up to 4000psi in lap shear tests with the strongest adhesives. However, plastics usually achieve up to 1200-1500psi. “At that point, the substrate itself might fail,” Nandivada said.
“It all depends on the substrates, but we have experimented with different epoxy chemistries that are stronger than silicone or urethane types, and they are able to give really high bond strengths for different plastics and metals or ceramic-based systems.”
The Master Bond team tests the quality of their bonds with lap shear tests, but Nandivada also attests to the effectiveness of hardness tests.
“The most basic and fastest test is the hardness test, wherein you’re just curing a small amount of the adhesive by itself and checking the hardness after it cures, which can be used as a retain,” he explained. “If you’re talking about a rigid curing epoxy with a hardness greater than 80 on Shore D, if it’s curing like a silicone or if its very soft or tacky than something went wrong in the way it was measured, mixed or cured, so that is a red flag you can catch before it goes into assembly.”
Like all products made with plastics and industrial grade adhesives, medical devices will have a period of off-gassing, in which harmful chemicals will be released from the product as gases – think of that “new car” smell, for example.
Off-gassing can be an important issue when working with epoxies and silicones, the latter of which off-gasses similarly to polyurethanes, Ramnath explained. “In these kinds of scenarios, leaning towards epoxies are the best bet, because they will probably give you the least amount of off-gassing levels versus the other chemistries available.”
Adhesives for Medical Devices
Medical device manufacturers need to plan ahead and work with their suppliers to achieve the right chemical compositions for their adhesives, both for general use and intense sterilization. It’s best to approach industrial adhesive companies like Master Bond as early as possible – even before the design phase – to know what can or can’t be done and to guarantee best outcomes.
“We’re trying to guide manufacturers as to what adhesives can be used and with what substrates and so on,” Nandivada said.
“A good example is, an engineer might call with a design that involves the use of plastics like polyolefins and Teflon. If it’s that early, they may have the option to change the plastic and so we tell them that if they go with something like a fluoropolymer, there might be multiple steps involved in the process. They can go back and see which plastics they can use and come back to us with different options, so we can recommend the friendliest ones. That’s the most important thing, because choosing the right substrates is important and, of course, depending on what their handling and processing requirements are, we can guide them accordingly. It depends on the sterilization procedure that they’re planning on using as well.”
For more information on Master Bond and adhesives for medical devices, visit the Master Bond website.
Master Bond has sponsored this post. All opinions are mine. -- Kagan Pittman