Originally Published MDDI May 2003

ADHESIVES

A review of the use of adhesives for device assembly, focusing on optical radiation curing using UV light.


by B. G. Yacobi, Chris Le Conte, Kevin Davis, and Manfred Hubert

Compared with other bonding methods, the use of UV-cured adhesives offers several advantages: lower energy requirements, savings on both curing time and floor space, improved production rates, and ease of automation. The technique is also compatible with the stringent requirements involved in medical device assembly, which incorporates various plastics, glass, and metals—all of which must be biologically inert, temperature and corrosion resistant, and highly durable.

In the past, UV-cured adhesives were believed to have limitations, when compared with heat-cured adhesives, such as shadowing effects, CTE mismatch, and shrinkage on cure. With the development of new UV curing systems and improved adhesive formulations (e.g., delay-cure adhesives), many of these issues have been resolved. The current generation of light curing sources can typically prevent shadowing effects by distributing focused UV radiation from different angles to the joined areas. 

Adhesive Use in Medical Devices

Device Biocompatibility. In the medical device industry, the main objectives for an assembly process are control and repeatability for both the adhesive and the curing system. Manufacturers must do rigorous testing on the adhesive to qualify it for United States Pharmacopeia (USP) Class VI status (signifying that the material or device may come into contact with bodily fluids), and FDA must validate the entire process. According to the USP guidelines, the systemic effects of adhesives on cells (i.e., cytotoxicity), blood constituents (i.e., hemolysis), and adjacent tissues are tested following implantation. ISO 10993, a widely accepted standard for biocompatibility testing, is also applicable. This standard includes additional tests and test durations, which include acute systemic toxicity, intracutaneous toxicity, and implantation testing.

Assembly Methods

Devices are typically assembled from components joined together through various techniques. The general categories of joining include chemical bonding (adhesive bonding), solvent welding, thermal bonding, and various mechanical methods.
Solvent welding, which dissolves the substrate, employs solvents to fuse thermoplastic parts. The method is fast and inexpensive; however, many materials (glass, metal, thermoset plastic) cannot be bonded using this method. In addition, some solvents are considered possible carcinogens, or are often harmful to the environment, or are highly flammable. In addition, solvent welding does not offer sufficient ability for filling rough surfaces.

Thermal bonding includes ultrasonic welding, which is based on generating heat at the polymer surfaces. This method can only apply to thermoplastic and miscible materials, and it is hard to form a hermetic seal. The method is also not sufficient for filling rough surfaces. 

Another thermal technique, laser-based microwelding, can also be employed in the joining of microdevices, such as stainless-steel catheters and stents. Laser joining, which is basically a relatively low-heat and well-controlled thermal bonding process, is especially suitable for joining small-diameter tubes. 

The mechanical methods typically include mechanical fastening—which has some applications in medical device manufacturing, but is relatively costly and is typically limited by the joint design—and insert molding, in which the critical issue is low yield related to the necessity for maintaining strict tolerances.

Adhesive bonding offers a versatile alternative to the above techniques: it provides an efficient means for joining different materials, with the added ability to fill rough surfaces and form a hermetic seal between two substrates. The main advantages of adhesive bonding are threefold: adhesives provide a continuous bond, with uniform distribution of load over wide areas; adhesives can bond irregularly shaped surfaces; and adhesive bonds are more resilient to stress and mechanical vibrations. In addition, adhesive bonding is especially suitable for the assembly of miniature components. 

Some limitations of adhesive bonding include a limited service temperature range. This is primarily related to the glass-transition temperature or chemical degradation of adhesives, and the fact that adhesive strength may depend significantly on the condition of the surface being bonded.

One effective bonding method is to use UV light for adhesive curing. In such cases, the bond design of the assembly must permit the UV light to reach the joint. The main advantages of using UV adhesive bonding in various applications, including medical devices, are:

• Instant cure.
• Cure on demand.
• Economical (saves time and cost).
• Solventless.
• Low energy requirements.
• High production speed.
• Ease of automation.
• Accurate alignment of components.
• One-component adhesive (no mixing).

Adhesive Types

Synthetic organic adhesives are composed of polymers, which are typically flexible and able to spread and interact on the surface of the substrate material. Some of the important physical properties of polymers (relevant to adhesive bonding) include melting point, viscosity, solubility, and tensile strength. Typically, adhesives are liquids; curing is the conversion of an adhesive from a liquid to a solid state. Such a state can be realized, for example, by optical radiation curing (i.e., ultraviolet, visible, and infrared photons).

In general, adhesives can be classified in several different ways. Relevant here are the classifications by chemical composition of UV-cured adhesives. The former classifies adhesives into three types: thermoplastic adhesives, which usually have low heat and creep resistance and are typically used for low-load conditions; thermosetting adhesives, which typically exhibit good creep resistance and are used for high-load conditions; and elastomeric adhesives, which are typically polymeric resins with good toughness and elongation. These last are especially suitable for bonding materials with dissimilar expansion coefficients. 

Some important properties of an adhesive that play a crucial role in various applications are adhesive strength, cure shrinkage, outgassing, moisture resistance, glass transition temperature, and degree of cure. The nature of spot curing ensures that bonding procedures will be repeatable and provide a consistent cure quality.

UV-cured adhesives are characterized as having relatively low process temperatures and relatively high polymerization rates (up to about 60 seconds). They are mainly based on acrylic and urethane chemistries, as well as some epoxies and silicones. These adhesives consist of monomers, various agents and modifiers (e.g., wetting agents, stabilizers, and fillers), and photoinitiators, which are only activated with the exposure to light of the specific wavelength and intensity. The energetic free radicals generated by the photoinitiators start the formation of monomer chains, and after several steps, the cross-linked polymer chains are completely reacted—i.e., cured. Combining the UV light with a visible light is advantageous, since UV curing of thick materials can lead to a cure gradient. By using a UV/visible adhesive, it is possible to attain a more consistent cure profile. Note that for optomedical assembly, optically clear adhesives are employed.

Adhesives that are employed in various medical device applications include light-curable acrylics (e.g., needle assembly, anesthesia masks, oxygenators), cyanoacrylates and light-curable cyanoacrylates (e.g., catheter components, tube-set bonding), polyurethanes (e.g., bonding of tips onto various components and assembling components requiring considerable flexibility), UV/moisture-curable silicones (e.g., bonding and sealing of silicone-based assemblies and coating of components and assemblies), and epoxies (e.g., needle assembly). Typically, a variety of formulations of adhesives with varying viscosities, cure times, temperature resistance, and strength properties are available for specific applications. 

Light-Curable Acrylics. Some of the advantageous characteristics of light-curable acrylic adhesives, which cure as thermoset plastics, include a capability to bond a wide variety of substrates; enhanced thermal, chemical, and environmental resistance; and rapid cure (down to several seconds) following exposure. The maximum depth of cure for light-curable acrylic adhesives is typically about 10 mm. Also, light-curable acrylic adhesives require a precise level of radiant power for the poly- merization reaction to take place, and thus it is important to select an appropriate light source for a specific adhesive. 

Cyanoacrylate Adhesives. Cyanoacrylates adhesives (also referred to as superglues), which form high shear and tensile strength, can be used to join nearly all materials. Light-curable cyanoacrylates combine the advantages of cyanoacrylate and light-curable acrylic adhesives. In these adhesives, photoinitiators are incorporated into the formulation.
Polyurethanes. Polyurethanes, which cure rapidly and have good impact resistance, are versatile for bonding to various substrates, and are typically used for joining plastics. 

Silicones. Silicones can be cured with UV, moisture, or their combination. These adhesives, which cure to rubber elastomers, adhere to a variety of different surfaces and offer a reliable seal for leak-proofing applications. One of the major applications of silicones is in joining and sealing components that undergo movement. Their tensile and surface peel strengths are low, however, indicating that these adhesives are most suitable as a structural sealant. 

Epoxies. Epoxies, which bond to many materials, offer high strength in various loading modes. They also offer outstanding temperature, chemical, and electrical resistance. These typically cure slowly, however, and are susceptible to long-term moisture pickup in humid environments. 

It is important to emphasize that medical adhesives for in vitro devices must undergo the sterilization procedures that are usually employed in device manufacturing. These may include exposure to ethylene oxide, gamma radiation, and autoclaving. (Note that sterilization resistance depends on a specific design and substrate.) As mentioned above, sterile reusable devices must undergo multiple sterilization cycles. In this context, the most common methods include steam autoclaving and ethylene oxide, as well as treatment with chemical gases (e.g., hydrogen peroxide plasma). These sterilization cycles require careful choice of the appropriate adhesive.

Adhesive-Dispensing Systems

Dispensing units for medical device assembly can be distinguished between manual, semiautomatic and automatic, and stand-alone systems, or those integrated into automated assembly systems. The current generation of programmable adhesive-dispensing systems for UV-cured adhesives are designed to dispense adhesive onto cylindrical components by inserting the part into the applicator. In such cases, requiring no dispensing needles or syringes, the reservoir filled with UV adhesive is incorporated into a production line (including a fully automated line). Such a method provides the capability to dispense adhesive evenly onto the whole bond surface of the component, resulting in highly consistent and repeatable adhesive bonds. 

Adhesive Choice and Design Considerations

The typical considerations for employing adhesive bonding relate to joining materials that cannot be joined by welding, such as thermosets and certain thermoplastics. The procedure is generally used in the following applications for bonding dissimilar materials (including those with inadequate mechanical property compatibility), for joining components that are too thin to be welded, for joining when optimal stress distribution, and for joining various types of subassemblies.

In general, adhesive selection criteria for medical device applications are similar to those for other applications. The selection of an  adhesive for a specific application depends on such properties (cured and uncured) as viscosity, pot life, cure time, postcure strength and hardness, chemistry (which determines those materials that can be adhered), shrinkage, glass-transition temperature, moisture resistance, and density. 

The general adhesive selection criteria can be related to the following characteristics: the type of surface to be bonded, the bond area and geometry, the required bond strength and line thickness, the requirement for the bond to be electrically or thermally conductive, the requirement for the bond to be transparent, and the possible environmental exposures during assembly and during the final end use (e.g., temperature limits and cycling, mechanical vibrations, water or chemical exposure), and the stress experienced by the assembly during its use. In addition to these requirements, there are also those related to the dispensing and curing considerations, such as, for example, the viscosity of the adhesive, its cure speed, and the permissible handling time.

It should be emphasized that a carefully planned joint design is as crucial as an appropriate choice of adhesive. One of the principal objectives in joint design is ensuring the distribution of the load throughout the bonded area and, thus, minimizing stress concentrations. In general, joints bonded with adhesives may experience various types of stresses, such as tensile, compressive, shear, cleavage, or peel, which may also be present in various combinations. Thus, the design of joint geometry is of great importance, considering that adhesively bonded joints are relatively strong under tension, compression, and shear loading, and that such joints are not as strong in cleavage and peel. The details of the joint design, related to its geometric characteristics and the manner of the transmission of the applied load, have a major effect on the mechanical properties of the adhesively bonded joint. In general, as a primary characteristic, better-quality adhesive joint designs are those that have larger contact areas between the components to be joined.

Surface treatment of the substrate is one of the most important issues related to adhesive bonding, with insufficient or inappropriate treatment being one of the most probable causes for adhesive bond failure. Indeed, given that adhesive bonding is fundamentally related to surface attachment, the properties and condition of the substrate surface (such as the presence of any contamination) are of vital importance. 

Thus, the careful treatment of surfaces before adhesive application is essential for realizing a strong adhesive bond. The adhesive strength is typically reduced by the presence of contaminants on the substrate surface, including weakly adsorbed organic molecules or condensed moisture. In general, the surface can be treated chemically, using solvents, oxidants, strong acids, or bases. Alternative methods of treatment for metallic and ceramic surfaces involve excimer or CO2 lasers, or bombardment with high-energy ions (plasma), resulting in ablation of surface contaminants. In addition, this can lead to increased surface roughness arising from the removal of the outer layers of the substrate. Typical contaminants present may include oils and weak oxide layers on metals, or fluorocarbons and silicones on polymers. 

Optical Radiation Curing

The curing of adhesive using optical radiation, in the UV, visible, and infrared ranges, has become one of the most effective methods for the accelerated curing of various adhesives in a wide range of applications. The combination of UV and visible light provides improved cure speeds and depths and permits a wider range of applications. In addition, curing of miniaturized components or devices with focused radiation minimizes the effect of irradiation on the surrounding areas.

A cure gradient might be obtained with inadequate penetration of light into the bulk of the material. In this case, the cure depth depends on the wavelengths of light used to treat the adhesive, the absorption properties of the adhesive material, and the thickness of the adhesive bond. 

The initiators in light-cured adhesives can possess an absorption profile with an intense maximum either in the visible, UV-A, UV-B, or UV-C regions. (The ranges for the UV are as follows: 315–380 nm for UV-A, 280–315 nm for UV-B, and 100–280 nm for UV-C.) Typically, however, the breadth of the profile incorporates more than one of these regions. As a result, the majority of light-cured adhesives can be treated with a wide range of wavelengths; therefore, an initiator with absorption maximum centered in the UV-C region can still be treated with longer wavelengths appropriate for achieving a greater depth of cure. It is merely necessary to modify the intensity of the light source according to the absorption cross section for the wavelengths. 

Light-based systems are able to deliver faster cures for adhesives than do traditional assembly methods. Efficient delivery of energy from a lamp source to the material is achieved using either a light guide with a narrow diameter or a focusing lens. The technique is referred to as spot curing, and in this case, the light energy is deposited in a localized area where the adhesive has been dispensed. The lamp source is selected to provide the appropriate wavelength range of light to cure the material. Control of the exact dose of light delivered to the material through the measurement and setting of the intensity and duration of the exposure, in addition to the wavelength of light used, allows for customization of the curing profile for a particular application. Repeatability of the curing procedure can then be achieved using spot-curing methods, giving rise to consistent properties of the cured material. 

In typical applications, optical radiation curing offers great advantages and improved manufacturing efficiencies, since such curing methods readily render an in-line and in situ processing step that can directly follow or precede other processing steps. Cure depth is a critical characteristic of the curing process. Light absorption by any material depends on wavelength. Higher-energy (i.e., shorter-wavelength) UV is typically absorbed very near the surface region of the material, and thus it is limited to applications for very thin layers, whereas lower energy (i.e., longer-wavelength) UV penetrates further. Some materials do not transmit UV light well, and some have UV-blocking species that are added to avoid UV light degradation. Thus, UV curing of thick materials can lead to a cure gradient with the material on top being cured better than the material at the bottom. 

UV light curing, which is the most widespread type of light-induced curing of adhesives, can be employed with either a continuous wave or pulsed wave irradiation. Pulsed UV light is a rapid curing method that allows control of cure at low average power and low temperatures. It is important that the spectral output (i.e., the intensity of light at each wavelength over the whole wavelength range emitted by the lamp) is matched with the absorption characteristics of the photoinitiator. 

Some important wavelength regions include 250 nm, which is associated with surface cure; those greater than 350 nm, which improve depth cure; and the range between 400 and 435, which employs visible photoinitiators to absorb light and to provide even greater depth of cure. In general, in order to match a light source to the UV-curable adhesive, one must determine both the photoinitiator system that is incorporated in the adhesive and the transmission characteristics of the substrate. 

A UV cure can be achieved in as little as 5 seconds or less. There are two types of UV curing systems: flood curing and spot curing. Spot-curing systems deliver precise doses of UV energy to a specific cure site. This energy may be delivered in a “spot” of light, or a customized line or shape delivered though customized optics. Some commercially available UV spot-curing systems have internal software that allows the user a level of control and repeatability that is not achievable with flood-cure UV or heat-cure systems.

Optimizing the Curing Process

There are several control variables in the curing process. These include the time-intensity profile of the applied illumination, the spectral range of light used, the number and profile of curing steps, the positioning of the piece being cured relative to the light, and the method of distribution of light onto the piece. In addition, all the above should also be related to the characteristics of both the piece being cured and the adhesive. 

Such interdependence between the variables of the light-based curing process necessitates its optimization providing for more control over the process and improving its yield and throughput. Such an optimization for some adhesives may entail an initial slow curing at low powers—a pause for a given period of time followed by curing at a higher power level, which can contribute to a greater mechanical stability of the cured joint. In particular, employing such multistep curing profiles for controlling the curing process can help minimize shrinkage and thus help to facilitate alignment between components throughout the curing process. 

The optimization of the curing process through multistep curing profiles can be accomplished by using a computer program related to a number of separate curing steps, each with its specific cure time, irradiance, and rest interval. Such programmable cure profiles provide a valuable means for optimizing a curing process. The results indicate that employing such programmed cure profiles provides reliable cures in the shortest possible time and with improved adhesive bond properties that meet stringent standards. 

Process optimization methods developed for curing various materials in different applications are based on four things: information monitoring during curing and repetitive temperature analysis to maintain a predetermined cure temperature; generating cure constants for different compounds and storing the data in the computer database, so that these can be used later for optimizing the cure of a specific compound; the comparison of actual characteristics to predicted values derived from computer simulations of the cure cycles; and using the program code for deriving the gradients related to process parameters. This method allows the monitoring and control of processes by determining the rate and direction of change of variables in relation to other process variables.

One of the key characteristics of cured adhesives is the precise measurement of the degree of cure, which is essential in order to ensure component reproducibility. A possible criterion for complete cure could be some form of stability of a given process or property, or their observation. In this context, other relevant factors could be related to shrinkage versus creep, or environmental factors (e.g., temperature variations) leading to certain curing-related effects.

Future Developments

Ongoing developments in polymer science are expected to provide new and improved (and tailor-made) adhesives, based on the assumption that, in principle, a wide variety of polymers with designed properties can be synthesized for joining different combinations of materials. There is a continuing effort directed at developing new adhesive formulations. These will include adhesives with dual photoinitiators—i.e., for surface cure and bulk cure—and delay-cure adhesives, which are activated by UV light and will cure within a specified time window that allows time for alignment.

The trend toward full automation of device assembly will also continue. The total integration of the adhesive bonding process (including automatic alignment, adhesive dispensing, curing, and cure monitoring) is highly desirable, since it could provide a reliable and reproducible means of adhesive bonding in a mass production environment.

Light-emitting diode arrays having various adaptable configurations with sufficient radiation intensity may also become the candidates for sources of radiation having tailored spectral content. 

Conclusion

UV-cured adhesive bonding facilitates the economical automated assembly of medical devices. Current generations of UV curing systems, solvent-free UV adhesives that cure in seconds, and advanced adhesive-dispensing systems provide effective means for the formation of consistent and repeatable bonds for assembling medical devices. Future developments related to light-curable solvent-free systems (100% solid) with improved adhesion characteristics and low shrinkage are expected to make this joining method superior to other joining techniques. Adhesive bonding already offers several advantages over traditional mechanical and welding methods. The recent advances in both adhesive formulations and UV curing methods make certain that the UV-adhesive bonding will continue to perform a crucial role in medical device assembly applications.

Bibliography

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2.Ritter, G.W., “Using Adhesives Effectively in Medical Devices,” Medical Device & Diagnostic Industry 22, no. 11 (2000): 52.
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5.Yacobi, B. G., et al., Journal of Applied Physics, Vol. 91, May 15, (2002): 6227. 

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