Sticking to Light-Cure Adhesives

COVER STORY >> ADHESIVES

Over the past four decades, the medical device market has experienced many changes in the types of devices produced, substrates chosen, and sterilization requirements. In the early 1970s, devices such as syringes and surgical instruments were made of glass, rubber, and metal, and they were typically fastened, machined, or molded to the appropriate configuration.

The intricate and high-performance medical device designs that evolved in the 1980s required different substrates and assembly methods. Due to growing concerns about contagious disease, single-use medical devices became the norm—a trend that forced engineers to evaluate engineering plastics such as acrylic, polycarbonate, and polyvinyl chloride (PVC) for their designs. Assembly was completed predominantly with room temperature–curing cyanoacrylate, epoxy, polyurethane, and silicone adhesives that were ideally suited for these early single-use devices.

In the late 1980s, the adhesive industry introduced acrylic-based adhesives that cured or solidified on exposure to ultraviolet (UV) light, and UV light-cure equipment became commercially available. By curing much faster, adhering to a wider variety of substrates, and being easier to automate, this early UV technology offered distinct advantages over traditional room temperature–curing adhesives.

This article explores the additional light-cure technologies that have been introduced to assist medical device manufacturers with their assembly processes over the last decade. Such technologies include biocompatible light-curing epoxies, cyanoacrylates, and silicones, as well as acrylics that cure with pure visible light.

How They Cure

Figure 1 depicts a typical light-curing reaction. All liquid light-cure adhesives contain chemical species called photoinitiators, which are indicated in step 1 as double red spheres. When light of the appropriate wavelength and intensity is introduced, as illustrated in step 2, the photoinitiators absorb the light energy and divide or fragment into reactive species. These reactive species form the linkages that are created to generate the polymer or cured adhesive (steps 3 and 4).

Depending on the adhesive chemistry, these cured polymers are either thermoplastic or thermoset resins. Light-curing cyanoacrylates are thermoplastics with limited temperature and chemical resistance. The remaining chemistries form thermoset resins that offer superior temperature and chemical resistance. To facilitate and complete the curing reaction, the type and intensity of light exposure is critical.

The electromagnetic spectrum (see Figure 2) organizes radiant energy by wavelength. Most adhesive curing is accomplished using light ranging from approximately 200 to 450 nm. The wavelengths below 400 nm are considered UV, while the output from 400 to 500 nm is visible. Also included in the spectrum are other forms of radiant energy including gamma, infrared, and microwave.

For successful adhesive cure, the absorbance of the adhesive photoinitiators must match the output of the selected light source. For example, if a UV-curing acrylic adhesive contains photoinitiators that absorb and break down with 365-nm light, then the light source selected must emit sufficient intensity at this same wavelength. A mismatch may result in only a partially cured or a fully uncured adhesive.

First-generation light-curing adhesives responded only to UV light in the 254–365-nm range. As the second generation of the technology added photoinitiators that reacted at 405 nm, they were deemed UV/visible (UV/V) curing. The addition of the visible initiator allowed for slightly faster cure times and the ability to cure through UV-blocked substrates.

Key Benefits

Light-curing technology offers the significant benefit of rapid fixture and cure following exposure to as little as 5 seconds of light for select joints. The rapid cure minimizes work in process and allows for nearly immediate quality testing to ensure that assembled devices fall within defined specification ranges.

Light-curing adhesives bond a wide variety of substrates and yield a clear bond line when used in thin sections. Many light-cure formulations offer fluorescent tracers that enable detection of the applied adhesive in the uncured or cured state to monitor placement and coverage. Unlike fasteners or other mechanical means of assembly, light-cure technology is easily automated on a production line. Because the technology cures on demand, adhesive remains liquid during the application process and will not cure until exposed to the curing source. Because the majority of light-cure adhesives require specific light wavelengths and intensities, ambient light is typically not sufficient to induce polymerization. However, typical handling procedures of light-cure adhesives include black or opaque product packages, feed lines, and dispense tips.

Key Considerations

UV-curing adhesives—those that react at light frequencies less than 400 nm—are limited by several adhesive and equipment-related factors. Transmission through substrates is critical because UV light must reach the adhesive bond line to achieve full cure and its associated performance properties. But most colored substrates do not transmit UV light. In addition, many grades of clear plastics include additives that act as UV inhibitors that can prevent curing. Similarly, when curing through large volumes of adhesive in a potting or filling application, the adhesive can limit light transmission and result in a low depth of the cure.

Also, traditional UV curing sources output high-intensity light over a broad spectrum of wavelengths. In addition to light, these systems typically emit infrared energy (heat)and ozone. High-intensity light, high levels of ozone, and significant heat can be dangerous to operators. Therefore, manufacturers must provide shields and vents to protect operators.

Recent Advances in Light-Cure Technology

Three new types of light-cure adhesives are targeted for use in medical device manufacturing: high-wavelength visible light–curing adhesives, light-and-moisture-curing silicone adhesives, and flexible light-curing acrylic adhesives.

High-Wavelength Visible Light–Curing Adhesives. The latest generation of light-cure adhesives offers photoinitiators that react solely with light in the visible wavelengths that exceed 425 nm. These adhesives cure in less than 10 seconds and are compatible with metals, glass, and a wide array of plastics. They can be used on UV-blocking substrates and select colored materials, particularly translucent grades of purple, blue, gray, and white.

The adhesion of visible light–curing products is comparable to most commercially available UV/V acrylic adhesives. These new adhesives offer particularly high adhesion on polycarbonate and PVC. Current grades of visible light adhesives meet strict ISO 10993 biocompatibility requirements and can cure to depths in excess of 0.5 in., making them suitable for potting applications.

Much of the benefit of visible light–cure technology is directly tied to the efficiency of the cure equipment. An ever-growing range of focused visible light sources provides considerable processing advantages for manufacturers.

Visible light–cure systems are available in both point and flood configurations that can be lamp or bulb based, similar to some early UV systems. These light sources produce a considerably narrower band of output than current commercially viable UV light systems. As typical bulb-based visible light sources provide output ranging from approximately 400 to 600 nm, they minimize excess unusable light and infrared (heat) energy output. Due to substantial heat reduction, visible light–cure technology is ideal for use on medical devices made of temperature-sensitive materials. For example, thermoplastic substrates that can be negatively affected by elevated temperatures include select grades of styrenic thermoplastic elastomers, acrylic, ionomers, and polystyrenes.

The initial cost and ongoing maintenance expenses for visible bulb systems are considerably less than those of traditional UV and UV/V systems. For example, the initial cost for most commercially available visible systems is well under $2000. With bulb lives twice those of standard UV and UV/V bulbs, medical device manufacturers can realize a nearly immediate cost savings in maintenance alone.

A second type of visible light–cure equipment, light-emitting diode (LED) technology, emits focused visible-light wavelengths in a significantly tighter output range than visible lamp technology. In most cases, LED-curing systems emit at one primary wavelength such as 420 nm, and offer slight amounts of residual light in nearby wavelengths (±15 nm).

LED systems are efficient because they do not emit excess and unnecessary broadband light and heat/infrared energy. LEDs produce higher outputs that more effectively cure the adhesives. A traditional UV light source may offer output irradiance of 150 mW/cm2, but a visible LED system offers more than 2 W/cm2.

Currently available as point or spot sources, LED-curing systems are predicted to have light output lives in excess of 10,000 hours and are typically built into solid-state housings that make them extremely durable and portable. Such long life and durability translate to immediate and ongoing cost savings. These systems take up less space than UV cure equipment and are easy to automate.

LED curing systems are available in both UV and visible wavelengths. Although most commerically available LED systems are point sources, broader cure area floods with increased output are on the horizon. One challenge has been the fact that some materials are extremely tacky when cured with visible light. Adhesives have been specifically formulated to cure tack-free with such systems. However, not all light-curing formations will afford a tack-free surface, so device designers should consult their adhesive suppliers to determine which products are best suited to their requirements.

Safety is perhaps the most significant benefit afforded by higher-wavelength visible light–cure systems. Visible light output minimizes or eliminates the need for shielding and operator protective equipment associated with UV systems. Although safety glasses are often still recommended to protect operators from the brightness of the visible light sources, visible light systems do not require heat-protective equipment or costly ventilation systems to protect workers from infrared and ozone.

Light-and-Moisture-Curing Silicone Adhesives. Room temperature vulcanizing (RTV) silicone adhesives and sealants have long been the choice for medical device manufacturers using silicone substrates or dealing with extremely flexible bond lines. RTV silicones are available in a variety of formulas that offer various viscosities, cure times, durometers, and appearances (clear and colorless to opaque and colored).

However, the primary limitation of RTV silicones is their cure time. Most of these adhesives require a minimum of 24 hours of exposure to humidity at room temperature to a maximum of 72 hours to ensure full cure and evolution of corrosive by-products, such as acetic acid.

In an effort to reduce the cure time of traditional RTV silicone adhesives, light-curing silicones and light-and-moisture-curing silicones have been developed that offer significant benefits to device manufacturers. Both of these categories of silicone adhesives maintain high adhesion to silicone substrates and offer significant flexibility while delivering cure times of approximately 60 seconds. Neither technology contains corrosive by-products, and they do not require ventilation to dissipate any residues or strong odors. The silicones are also tested to meet the biocompatibility requirements set forth in ISO 10993.

Much like traditional light-curing acrylics, light-cure silicone technology requires that all of the adhesive be exposed to light. These adhesives react with moderate- to high-intensity UV or visible light (minimum 70 mW/cm2). The cured polymers are transparent and colorless. This technology offers high adhesion to thermoplastics such as polycarbonate, acrylic, and PVC. In addition, the cured adhesives deliver high tear strengths to ensure tough and strong bond lines.

Light-and-moisture-curing silicone technology cures on exposure to moderate- to high-intensity light and includes a secondary moisture cure similar to traditional RTV silicones. The secondary cure allows adhesive located in shadowed areas to cure if light cannot reach it (see Figure 3). Light-and-moisture-curing silicone adhesives are translucent in appearance and offer high elongation and tear properties.

Flexible Light-Curing Acrylic Adhesives. Traditional UV/V light-cure acrylic adhesives are available with a wide range of physical properties—from rigid, high-modulus polymers to materials offering moderate flexibility. These traditional light-cure adhesives are often selected due to their high-strength bond to a wide range of plastics, metals, and elastomers. However, assembly applications requiring high adhesion and high levels of flexibility presented problems for UV/V technology.

Recent advances in formulating UV/V acrylic adhesives have resulted in extremely flexible acrylic-based polymers. With hardness values of Shore A 70 and elongations greater than 100%, these flexible light-cure acrylics are suited for medical device applications that undergo extreme flexing and bending. They are also appropriate when substrates with varying coefficients of thermal expansion are being joined and must undergo thermal cycling.

Flexible light-curing acrylics cure on exposure to low to moderate UV/V light sources and will fluoresce under black light for inspection purposes. Because they are acrylic-based, their high adhesion to a wide variety of materials is a key advantage over silicone adhesives.

Conclusion

Light-cure technology continues to be adopted by medical device manufacturers worldwide. With its rapid cure and wide product offering, there is a category and product to suit a variety of medical device applications. Recent advances, including high-wavelength visible acrylics, light- and light-and-moisture-curing silicones, and flexible acrylics, have further broadened the applicability of light-cure adhesive technology for medical device assembly challenges.

New product development of both adhesives and their associated cure systems is ongoing. New versions of LED-based spot-curing units are on the horizon, offering high intensity output in 365-nm, 405-nm, and >450-nm wavelengths. Wide-area LED-curing systems are expected to hit the marketplace in mid- to late 2009 that offer cure areas of approximately four square inches.

Adhesive development continues as well with alternative fluorescent agents for pre- and postcure adhesive detection, additional high-wavelength visible-curing adhesives, and light-curing epoxies that meet ISO 10993 biocompatibility requirements.

Christine Salerni Marotta is medical market development manager for Henkel Corp. (Rocky Hill, CT).

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