The reusable medical device market is an area in which adhesive technologies are developing at a rapid rate. These devices are frequently made from engineering resins such as Polyolefins, PPS, ULTEM, PEEK, and RADEL as well as metals, particularly titanium, nickel, aluminum, and stainless steel.
This wide range of materials means that medical adhesives can include silicones, epoxies, polysulfides, polyurethanes, and more, and the choice of adhesives will depend on the medical device’s uses as well as the materials from which it is made. Apart from offering greater efficiency in the assembly of medical devices, adhesives often outperform traditional fasteners in their resistance to sterilization processes.
Extra Care with Adhesives
Choosing the right adhesives for medical devices does, however, pose a few challenges. First and foremost, biocompatibility must be in accordance with standards. Secondly, they must maintain bond integrity despite harsh sterilization processes. These demands not only affect the adhesives chosen but also the way in which they are used.
The way in which adhesives are measured, mixed, applied and cured must be precise. The type of sterilization process that the device will be subjected to will determine the conditions the adhesive must withstand. For example, autoclaving implies that an adhesive must withstand temperatures of up to 260° F (130° C). Typically, this implies the choice of adhesives that are cured at high temperatures.
Adhesives Affect Device Design
It’s not only a matter of choosing the right adhesive for a specific medical device design – the decision to use adhesives for better performance can affect the design of the device itself. For example, the greater the surface area to be bonded, the stronger the bonds will be.Also, some materials used in medical devices will require pre-treatment before the surfaces can be bonded. This may involve roughening surfaces, chemical etching, or plasma treatment.
Adhesive Mixing and Dispensing
Medical adhesives can be one or two-part adhesives, and the way in which they are cured can be broadly divided between these two types of formulation. One-part adhesives will typically remain workable at room temperature and will cure much faster when exposed to heat. Two-part adhesives will typically cure at room temperature, but curing can be hastened with exposure to heat. Having said this, some will only cure at very high temperatures.
Mixing two-part adhesives precisely can be achieved by packaging them as gun applicators or by using pre-mixed formulations that are preserved through freezing. Either one or two-part adhesives will work in automated production systems. Whether working in large or small-scale manufacture, dispensing systems that suit the volume requirements can be implemented.
Potting and Curing
At times, entire components must be encapsulated with adhesive, and here the coefficient of thermal expansion becomes particularly important. Currently, products using a ceramic filler like aluminum oxide are used in this context.
Getting high tack but sufficient pot life to allow for small readjustments presents another challenge. One company, Masterbond, overcomes this by using systems that combine UV light and high temperatures in dual curing systems. Typically, tack will be achieved under UV light, with final post-curing taking place in a high-temperature environment. Shrinkage can present a problem, but Masterbond says it is working to overcome this problem.
Bond Strength, Testing, and Off-Gassing of Adhesives
Lap shear testing and hardness tests help adhesive manufacturers to test bond strength, but the substrate on which adhesives are used will also affect the results. For example, metal components will perform better in lap sheer tests than plastics, with plastic substrates often giving way before the adhesive does.
Medical adhesives manufacturers also have to limit off-gassing of harmful agents. Experts agree that epoxies are currently the solution with the lowest off-gassing.
Medical Device Manufacturers and Adhesives Industry Cooperation
When the bonding of materials requires adhesives with specific properties, adhesives industry chemists are better able to recommend products or formulate new ones when they know precisely what will be required. Industry insiders recommend cooperation between medical device designers and adhesive industry partners from the early design concept stage. Should the available adhesives not yet be able to fulfill requirements for specific substrates and the demands of sterilization procedures, medical device designers can then explore alternative materials.