Selecting an adhesive is often a difficult task because it involves give and take between the end-use and manufacturing requirements. When it comes to bonding applications in the medical industry, the challenge is even greater because of strict specifications and safety constraints. 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. In tended 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.
There are a variety of adhesive formulations that meet these medical specifications and are designed to withstand rigorous requirements. They offer the highest level of reliability, such as epoxies, ultra-violet (UV) curable adhesives, and light emitting diode (LED) curable adhesives. Epoxies are the most common medical adhesives and are available in one-part or two-part versions. These systems offer exceptional physical strength properties, high temperature resistance, and a remarkable ability to withstand sterilization exposure. The main difference between one- and two-component systems is the way they are prepared and cured. (See Figure 1)
A two-component epoxy is capable of being cured at room temperature but it requires precise measuring and mixing of the resin and the hardener. A one-component epoxy eliminates the need for meticulous mixing and also offers the added bonus of an unlimited working life, but it can only be cured with the addition of heat (typically 250 to 300°F), which may not be suitable for certain applications.
UV adhesive systems also meet many medical industry requirements and offer an array of advantages including rapid, room temperature cure schedules, high bond strength, and easy application. They offer excellent adhesion to glass, metals, ceramics, rubber, and most plastics. These one-part products require no mixing, and eliminate waste. However, like the other previously mentioned formulations, UV curable adhesives are not without their trade-offs, such as the need for UV light exposure to cure.
Advantages of LED Curing
Recently, adhesive experts have developed a formulation that uses cutting edge, LED curable technology to provide a safer alternative to traditional UV light curable adhesives and one- and two-component epoxies. There are many advantages of LED curing over more conventional heat or UV methods including faster processing speeds, improved user safety, and environmental responsibility. (See Figure 2)
LED curing represents a new frontier in adhesive technology. For years, adhesive manufacturers have been researching ways to cure adhesives in the visible region of the UV spectrum. The engineers at Master Bond, Inc., have developed a system that is able to cure under a visible light wavelength of 405 nm. This latest technological breakthrough enables safer bonding, encapsulation, and sealing applications that were not suitable for UV curable adhesives that cure under light sources with wavelengths between 250 and 365 nm.
While the wavelength difference doesn’t seem very significant, shifting to visible light eliminates the need for expensive UV lighting equipment in favor of more affordable LED lamps. Because commercial UV lamps have the potential to cause serious eye injuries and skin burns, LED light improves worker safety.
Like UV curable systems, LED curable adhesives feature excellent physical properties, including superior dimensional stability, electrical insulation, chemical resistance, and fast cures — typically in 30 seconds or less. They also withstand various sterilization methods such as gamma radiation, ethylene oxide, and many chemical sterilants. (See Figure 3)
Unlike some one-part epoxies and UV curable formulations, LED curable systems have a lower exotherm and can cure in sections up to and beyond 1/8 inch thick. They also cure at room temperature, which allows them to cure rubber and plastic components that are very sensitive to heat exposure.
Two-component epoxies are the mainstays of bonding, but they may have trade-offs when compared to newer technology systems. Similar to LED curable products, two-part epoxy adhesives offer room temperature cures, but most epoxies take several hours or even days to cure, whereas an LED system takes just seconds. A two-component epoxy requires complicated mixing and has a limited working life once both parts are mixed. However, an LED adhesive has a single component with an unlimited working life, giving users more flexibility when processing these devices.
LED curable adhesives mark a new direction for bonding technology that feature many benefits over traditional adhesives in the medical and other industries. These easy-to-use systems do not require mixing, cure quickly upon exposure to an appropriate LED light source, and can be cured in thicker sections. In addition to passing USP Class VI and ISO 10993-52 cytotoxicity tests, LED systems meet RoHS specifications for environmental friendliness. They offer increased safety, reducing the risk of operator eye injuries and burns, while offering the bonding and physical properties manufacturers have come to expect from traditional epoxies and UV curable adhesives.
This article was written by Jenifer Hooker, Technical Editor for Master Bond, Inc., Hackensack, NJ. For more information, Click Here