MANUFACTURING
The challenge
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Adhesives, in particular, cyanoacrylates and ultraviolet (UV) curing acrylics are widely used for bonding a range of medical devices. Selecting the most appropriate adhesive grade for a production application can be a challenging design task. This is especially true when joining dissimilar materials or when bonding certain types of plastics. In many cases, adhesives provide the best, indeed, often the only, suitable assembly method.
When bonding catheter balloons, the adhesive can undergo severe peel loadings and, as a result, it is important that the right choice of product and an appropriate dispensing method are employed. Before that, however, it is important to understand how an adhesive works, especially with regard to the plastics used in catheter balloons.
For an adhesive to bond satisfactorily it must “wet” the surface of the components to be joined. To achieve this “wetting,” the adhesive requires a lower surface tension than the surface energy of the part. It follows, therefore, that because the surface tension for most adhesives is in the range of 33–35 mN/m, the surface energy of the parts must be greater than 33 mN/m.
Most industrial adhesives such as cyanoacrylate, epoxy, polyurethane, room temperature vulcanising silicone and many acrylics will not adhere to polypropylene (PP) and polyethylene (PE). It is for that reason that adhesives are frequently packaged in bottles made of these materials to prevent curing before dispensing. Although processes such as corona discharge, flame and plasma treatment can be used to increase the wettability and reactivity of component surfaces, these techniques can be expensive. However, by utilising adhesives that have been especially formulated for materials such as PP or PE, costs can be greatly reduced.
A quick look through an adhesives catalogue will reveal a bewildering number of products. Naturally, all will have their own detailed benefits for the manufacture of catheters and other medical devices. Ultimately, designers and engineers must balance a variety of adhesive properties to obtain the required bond strength and ease of use in production. The adhesive manufacturer’s technical helpdesk can be of considerable help for specific situations. Nevertheless, some general principles concerning the two most popular types of adhesives for medical devices: cyanoacrylates and UV acrylics will be helpful.
Cyanoacrylates
When presented in liquid form, cyanoacrylate instant adhesives are stored in high-density PE bottles to minimise the ingress of moisture into the liquid. These adhesives are generally available as colourless liquids, with grades varying in viscosity from 3 mPa.s to a thixotropic gel for application on vertical surfaces or highly porous materials.
However, whatever the viscosity, when the instant adhesive is placed between two close fitting surfaces, the moisture on both surfaces will neutralise the acidic stabiliser and cause the cyanoacrylate to cure rapidly. In these instances, the gap between parts should ideally be less than 0.1 mm.
The optimum cure conditions for these adhesives is when the relative humidity (RH) is between 40% and 60% RH. A lower RH will result in a slower cure, and a higher RH produces a faster cure. However, a high RH can be detrimental because the cyanoacrylate may cure so quickly that the adhesive has not had time to properly adhere to the surface and the result will be a poor bond. It should also be noted that a cyanoacrylate may never achieve full strength if it is disturbed during the critical time when it is changing from a liquid to a solid.
In most circumstances, cyanoacrylates will achieve handling strength within approximately the first minute, but will continue to develop strength over the next few hours. When the parts are subjected to a high load, for example, during the inflation of a balloon, the adhesive should be left for as long as possible prior to testing.
Bonding polyolefins
As mentioned above, some materials such as polyolefin plastics are always a challenge to bond because of their low surface energy. Silicone rubber, polytetrafluoroethylene and acetal could also be added to that list of challenging materials. In the late 1980s primers were introduced that significantly enhanced the adhesion of cyanoacrylates to polyolefins. These primers change the surface condition of the plastic and thereby create suitable bond areas for the cyanoacrylate. The effect of a polyolefin primer when used with a cyanoacrylate on PP should not be underestimated. Bond strengths are frequently 25 to 40 times higher than those achieved when using the same adhesive without primer.
UV curing cyanoacrylates
Another recent introduction to cyanoacrylate technology are single part, UV light curable grades. Although an investment in UV curing equipment is necessary, these light curing cyanoacrylate products combine the ability to cure rapidly with the reassurance of “shadow cure.” A shadow cure is required when an area of adhesive is not reached by the UV light source. In other words, it is in a “shadow” caused by some other part of the component being joined. When this area of adhesive is not cured by the light, the standard cyanoacrylate technology allows the adhesive to cure by means of the moisture present on the surface of the component. UV rays must of course penetrate through to the bond line for the adhesive to cure, therefore, at least one substrate must be clear or translucent to allow sufficient light to be transmitted. In some applications this is not always possible and the UV curable cyanoacrylates will “shadow cure” in dark areas as a result of the presence of surface moisture.
The “cure-on-demand” properties of UV adhesives, together with rapid polymerisation, have led to their use in high-volume assembly processes. In these situations, the fast throughput and reduced work-in-progress levels offer genuine cost savings.
The curing process
Some UV products cure at a wavelength of 365 nm (UVA), although a number of plastics, including polycarbonate (PC), can act as a UV filter and therefore inhibit the cure. As a result of the wide use of PC and flexible poly(vinyl chloride) (PVC) in the medical industry, a range of adhesives has been developed. These are not only considerably more flexible, but also cure at 400–420 nm. Although these products are described as “visible light curing” adhesives, an investment in UV equipment is still necessary for rapid curing.
The UV lamp is an essential part of the process and many different light sources are available to suit a variety of applications and budgets. These sources can range from a simple bench-top open unit through to a fully automated conveyor system with several lamps that incorporate special fixturing to hold the components in place during the cure cycle. UV light guides or “wand” systems are often specified for small components because these units produce high intensity light over a diameter of approximately 10 mm.
It is essential that the output spectrum of the selected UV source matches the photoinitiator system used in the adhesive. The adhesive requires the correct wavelength and the correct intensity of UV light at the bond line to achieve optimum cure. A low intensity lamp offers 5–10 mW/cm2, and a high intensity lamp will provide 100–1000 W/cm2. Beyond that, some light guide units will produce even higher intensities (>5000 mW/cm2). In general, the higher the intensity, the faster the cure and for most production applications the UV intensity should not be less than 50 mW/cm2.
UV curing acrylics bond exceptionally well to PVC and PC, but not so successfully to PP. Therefore, for PP or PE parts, the UV cyanoacrylate should be used in conjunction with a primer.
Bonding balloon catheters
The materials used to manufacture balloon catheters vary widely according to the type of catheter and the properties required for the specific catheter application. Furthermore, the joint design can also differ considerably between different manufacturers.
As already observed, adhesives used in balloon catheters must sustain considerable peel loadings during use. Some factors that need to be considered at the design stage include the following.
- Viscosity: It is important to select an adhesive of the correct viscosity to prevent it from wicking into the working length of the balloon.
- Fluorescence: It is often difficult to detect the presence of the adhesive in the joint. Some adhesives have a fluorescent additive to aid inspection.
- Flexibility and peel strength: As a general rule cyanoacrylates are relatively rigid adhesives, but can offer high adhesion to many different polymers.
- Curing of the adhesive: One significant benefit of the UV acrylics is that they will “cure on demand“ and this can significantly help to ensure that the balloon is correctly positioned at the proximal and the distal ends prior to curing. To maintain a uniform bond around the perimeter of the catheter, the adhesive must be able to cure through 360°. It is also critical that the dose of irradiance delivered to the part is consistent and can be repeated with accuracy. Optical accessories such as cure rings provide this 360° facility by means of fibres that surround the part. Indeed, in some cases just two light guides located opposite each other are sufficient to expose the complete circumference of a balloon.
The components used in catheters are frequently close fitting and, consequently, the bond line thickness is often 0.02 mm or less. When cyanoacrylates and UV acrylics are employed, it is generally the case that the thinner the bond line, the stronger the joint. However, when the balloon is inflated, this bond line can be subjected to high peel loads and under those conditions it may be beneficial to accommodate a slightly thicker bond line with a fillet of adhesive. Figure 1 [6] shows a drawing of a catheter joint.
Dispensing adhesives
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Application of the adhesive over the complete bond area is desirable. For the manufacture of balloon catheters in a production environment, it is important to devise a technique to ensure adequate joint coverage and minimise the possibility of the adhesive being scraped off during assembly. The type of application and workplace conditions such as production rate, parts handling and cure speed will determine whether manual, semi-automatic or fully automatic dispensers will be required.
The most common method of dispensing is to use a pressure/time system. Here, the adhesive is placed under pressure in a container and forced out through a nozzle via a dispense valve. The quantity of adhesive dispensed is controlled by the air pressure and the time the valve is open. When the application requires dispensing in fully automatic manufacturing lines, special dispense valves are available to suit the particular production requirements. In these cases, it is recommended that advice be sought from the valve supplier or the line manufacturer.
Use expert advice
When selecting an adhesive it is important to look at the entire assembly process, including the preparation of substrates and the dispensing and curing of the product. In addition to expertise on adhesive technology, the supplier can offer specialised knowledge in the areas of materials selection, specially adapted dispensing equipment and full support in production processes. Furthermore, ongoing development work that involves the adhesive supplier will lead to a good working relationship and the improvement of a reliable and reproducible process.
Bob Goss is Senior Technology Specialist at Henkel Loctite Adhesives Ltd, Technologies House, Wood Lane End, Hemel Hempstead HP2 4RQ, UK, tel. +44 1442 278 000 email: technicalservice.loctite@uk.henkel.com [7], www.loctite.co.uk [8].