Medical electronic devices can run the gamut from imposing MRI and x-ray imaging machines to miniscule implantable circuits designed to detect physical conditions and deliver programmed therapy. As is the case with most electronic gadgets, there is a growing trend in medical electronics toward miniaturization. Advances in circuit miniaturization, low-power architectures, and lightweight components are facilitating improvements to hearing aids, pacemakers, and other devices, while giving rise to promising new applications.
As medical electronic systems become more sophisticated, their design and assembly become more challenging. Adhesives can offer big cost savings and performance advantages over traditional mechanical fasteners, such as screws and snap fits, as well as other bonding methods, including welding, brazing, and soldering. The increase of microelectronics in medical devices calls for greater use of adhesive bonding of assemblies that are smaller and lighter than previous designs. However, selecting the right adhesive is almost always a delicate balancing act of proper performance and manufacturability requirements. Medical device engineers also have to contend with strict industry regulations and different types of sterilants, including cycles of sterilization. (See Figure 1)
Industrial and Environmental Challenges for Adhesives
Since a malfunctioning medical device could potentially harm a patient, reliability is paramount, and medical electronic devices are highly regulated. A common standard for adhesives in medical devices is biocompatibility. In North America, the most widely accepted test for biocompatibility is United States Pharmacopeia (USP) Class VI. Intended to evaluate the suitability of polymeric materials for direct and indirect patient contact, the USP Class VI regimen consists of a series of in vivo reactivity and toxicity tests. These tests involve injecting and implanting samples of the material under consideration into mice and rabbits, which are then evaluated at fixed time intervals for any ill effects. ISO 10993 is another standard that tests cytotoxicity and is used to determine an adhesive’s compatibility with blood and body fluids according to the requirements of the Elution Test, ISO 10993-5 guidelines.
It is important to note that passing USP Class VI testing doesn’t guarantee that a device will gain FDA approval. All it tells an engineer is that a material has low levels of in vivo toxicity under the test conditions. Yet, given that a large number of standard adhesives cannot be considered biocompatible, USP Class VI testing represents a valuable tool for identifying products suitable for medical applications.
Medical devices that may come into contact with patients obviously need to be sterilized, either once for disposable devices like syringes or even dozens of times for reusable devices, such as surgical instruments. The healthcare industry has an arsenal of methods to make devices sterile—including autoclaving and exposing devices to ethylene oxide (EtO), radiation, electron beams, or chemical treatments.
While these sterilization methods do kill microbes, they can also be nearly as tough on polymeric materials. Consider autoclaving, for instance. By far the most popular method for sterilizing reusable devices, autoclaving can subject devices to temperatures up to 135°C and steam pressures up to 20 lb. Many polymeric materials, including a variety of standard adhesives, will start to degrade—primarily through hydrolysis—under repeated exposure to this kind of damp heat. Other sterilization methods tend to be less aggressive than autoclaving, but they each have at least some potential to break down polymeric materials as the number of sterilization cycles increases.
Adhesive Properties Protect Electronics and Patients
High performance adhesive systems offer design engineers dozens of options to choose from for the assembly of electronic medical devices of all shapes, sizes, and substrates. One- and two-part epoxies, silicones, and light-curable compounds are widely used for potting, encapsulating, sealing, coating, and bonding electronic components in medical devices. Through advances in adhesive technology, epoxies and other compounds can be formulated to provide a variety of mechanical, thermal, optical, electrical, and physical properties, enabling them to offer much more than their primary function.
By adding inorganic fillers to epoxies and other compounds, manufacturers can give them more utility, thereby improving properties like strength, viscosity, thermal expansion, heat and chemical resistance, electrical and thermal conductivity, and shrinkage, to name a few. For example, adhesives are formulated to be electrically conductive by adding certain amounts of silver, nickel, silver-coated nickel, or graphite. (See Figure 2) Incorporating aluminum oxide or aluminum nitride fillers enhances thermal conductivity properties. Adding silicas and other quartz particles will increase the viscosity of the system.
Selecting an Adhesive for a Medical Electronic Application
Medical design engineers have many options to choose from when selecting one or more compounds for bonding, coating, potting, and encapsulating components. While manufacturers can control many of the properties of these compounds, the underlying polymer chemistry is a critical factor to consider. Each family of compounds—epoxies, silicones, and light-curables— offers a different set of performance parameters and processing requirements.
Epoxies are among the most versatile polymer compounds used in medical electronics. They offer excellent cohesion and resistance to chemicals, adhere well to a large variety of materials, and operate over a wide range of temperatures from cryogenic (4K) to greater than 550°F. Because they are 100 percent reactive, epoxies produce no volatiles during cure and exhibit little or no shrinkage during polymerization.
Epoxies can be formulated to be electrically or thermally conductive or insulative, to withstand thermal cycling, resist vibration and shock, absorb thermally induced stress, or to exhibit a combination of features. Manufacturers can also control the color, hardness, and viscosity of epoxies. Epoxies offer superb protection from chemicals, abrasion, moisture, and mechanical shock and vibration, and are used for a multitude of functions in medical electronics.
One-component epoxies are often desired because they are easy to use and do not need to be mixed. These systems also offer “unlimited” working lives at room temperature, cure at moderately elevated temperatures, and are available in a range of viscosities. However, one-part epoxies generally have a more limited application range than two-component epoxies. Most two-part epoxies cure at ambient temperatures once mixed and typically offer higher physical strengths. Also, many two-component epoxies are available as premixed frozen compounds, eliminating the need for mixing while offering the convenience of room temperature curing.
Silicones are known for combining flexibility with high temperature resistance, but have limited strength and are less resistant to chemicals and steam than epoxies, especially at elevated temperatures. Because of their outstanding flexibility, silicone compounds are often used as stress absorbers in applications that involve large temperature excursions. (See Figure 3) One-part silicones cure at room temperature when exposed to moisture in the air, while two-part formulations cure at ambient or elevated temperatures after the addition of a curing agent. Like epoxies, silicones can be formulated to be optically clear and electrically or thermally conductive or insulative, and many two-part silicones exhibit low outgassing.
Light-curing compounds require no mixing and offer extremely rapid processing, but are limited to applications in which light can contact the compound to cure it. Specific formulations of UV/visible light curable compounds vary in hardness, viscosity, and thermal and electrical properties, and are used for bonding, sealing, and coating. LEDcurable compounds provide tack-free, moisture-resistant surfaces after exposure to visible light emitted at a 405 nm wavelength and are ideal for bonding thermally sensitive electronic components.
In selecting a compound, designers should be aware that there are always tradeoffs, both in performance and manufacturability. Adding silver particles to an epoxy, for example, increases its electrical conductivity, but it may diminish its adhesive strength, temperature resistance, and chemical resistance, since the filler particles effectively displace some of the polymer material. Rapidly curing compounds help reduce production time, but set quickly, leaving little room for error. Designers should be aware of such tradeoffs and prioritize performance requirements prior to selecting a compound for a specific application.
Versatile Compounds and Future Medical Electronic Innovations
High-tech adhesives give medical design engineers a new range of tools to use in the manufacturing of medical electronic devices. They play a central role in aiding manufacturing processes, including lowering costs, improving overall performance, and reducing the size and weight of electronic devices while meeting the specialized, demanding needs specific to the medical industry. Along with circuit miniaturization, low power architectures, and other ongoing research, advanced adhesive systems can help facilitate further development of innovative medical electronics.
This article was written by Robert Michaels, Vice President of Technical Services, Master Bond Inc., Hackensack, NJ. For more information, please visit http://info.hotims.com/49744-161 .