Selecting and Using the Right Adhesive

The three main technologies
Type the words “adhesives for medical devices” into an Internet search engine and the outcome can be almost one million offerings. Of course, not every result represents a specific product, however, the task of selecting the right adhesive for use on medical devices can be daunting. There follows some general guidelines that will help end users understand what is involved in selecting the right adhesive and, just as important, in making that choice work satisfactorily.

Although many different types of adhesives are used in the medical industry, three main types of adhesive are discussed here: light curing, epoxy and cyanoacrylates (CAs). Each provides unique benefits, and only by using the most appropriate product will the best results be achieved.

Light curing
These adhesives offer the unique benefit of curing on demand (on exposure to a suitable light source) and also provide the high degree of flexibility that is often required for medical applications. However, developments during the past few years have led to the introduction of adhesives that react to the visible light spectrum.

UV adhesives. These require the proper wavelength and the correct intensity of lamp for a complete cure. That means, of course, that an appropriate UV cure system is a vital part of the process. Although these adhesives will start to cure as soon as they are exposed to a UV source, it is important that the manufacturer’s datasheet is consulted to ensure that the product “receives” the light for the required duration. For example, a 20 s cure is not necessarily the same as two 10 s exposures, because the products work best when exposed to the light in one operation. UV adhesives withstand ethylene oxide (EtO) and gamma sterilisation processes and some grades will also survive the more aggressive autoclave cycles, but care must be exercised.

Visible light. As mentioned, a relatively new development is the introduction of adhesives that cure when exposed to visible light. These have been formulated for situations when the substrates to be bonded contain a colour that restricts light in the UV part of the spectrum. Some of the additives that are used in plastic manufacture to prevent discolouration actually restrict light in the required wavelength. Visible light curing products overcome these problems by incorporating photoinitiators that react only with light that is available in visible wavelengths, that is, those in excess of 425 nm. These products not only act faster than UV oriented adhesives, indeed, less than 10 s for cure times is not unusual, but they also have a cure depth of more than 12 mm in many instances. The latest visible light curing products provide good adhesion for materials such as polycarbonate and polyvinylchloride. Beyond that, visible light adhesives are available in low and high viscosity formulations to provide flexibility, for example, their elongation at break can be approximately 250%.

Furthermore, they generally include fluorescence properties that enable quality control inspection to be easily performed.
Light curing adhesives are used for a wide range of medical applications, including bonding tracheotomy cuffs and balloon catheters (Figure I). Bonding blood collection units (Figure II) is also an ideal application for this adhesive technology, because it results in a clear bond line that provides a good cosmetic appearance. In addition, light curing adhesives are widely used for joining stainless steel cannulae to polycarbonate or polypropylene syringe hubs. For this application, the adhesive is wicked down into the joint and cured in a few seconds.

For most adhesive applications, it is preferable to keep the adhesive bond line thin (ideally about 0.05 mm). Low viscosity adhesives should be chosen for wicking applications; higher viscosity products are more suited for large gaps and where the adhesive is required to be carefully positioned prior to assembly.

Epoxies
These adhesives provide shear strength on a wide variety of plastics and metals, for example, tensile strength on steel is
21.4 N/mm² and on glass is 24.4 N/mm². On steel parts, cured epoxies maintain bond strengths even after 1000 hours at 95%
relative humidity (RH) at 38°C, and also show similar results after immersion in
solvents such as acetone and isopropyl alcohol. Most epoxy grades are packaged side-by-side in cartridges, which allows them to be dispensed as easily as a one part system.

Epoxies are commonly used on endoscopes, catheters, artherectomy devices and blood heat exchangers, as well as on dental, surgical and orthopaedic instruments. They are also used for potting applications where a larger volume of material is required. These adhesives are often specified for applications where parts may be subject to repeated autoclaving.

Cyanoacrylates
CAs, also known as instant adhesives, have been used in the medical industry for many years for devices and for surgical use. CAs are frequently the first choice for bonding elastomers, especially when the application is essentially nonstructural, because they have superior adhesion to elastomers. The benefits of CAs include a fast cure (2–5 s), ease of use in manual processes and excellent adhesion to a wide range of plastics. Instant adhesives are generally clear in appearance and available in viscosities ranging from 3 mPas to a gel consistency.

Figure 1: A UV curing adhesive is employed to bond these balloon catheters.

It is worth emphasising that the viscosity of the adhesive is a significant factor in adhesive choice. Low viscosity adhesives (< 50 mPas) should be utilised where gaps are small and the adhesive is required to flow or wick into parts that have been pre-assembled. A medium viscosity adhesive (up to 1000 mPas) may be more appropriate for bonding cylindrical parts where the diametral gap is in the order of 0.1 mm. High viscosity adhesives (>10000 mPas) are specified for larger gaps or where the adhesive must not flow away from the joint prior to assembly and cure.

CAs contain an acidic stabiliser, which is neutralised when it comes into contact with surface moisture. When a CA is placed on a substrate, the layer of adhesive in contact with that surface will cure almost immediately. However, the remainder will stay in liquid form until a full cure is achieved, typically between 12 and 24 h.This means CAs should be applied sparingly, because overuse can slow the cure. The actual cure speed also depends on the ambient RH; the best results are achieved with 40–60% RH. The other factor affecting cure speed is the size of the gap. Large gaps will result in poor curing and low strengths.
It is important to remember that CAs become thermoplastics when cured and, as a result, will melt at high temperatures. With that in mind, these adhesives are not recommended for repeated autoclave applications, but they will withstand EtO and gamma sterilisation processes.

Some substrates are slightly acidic and a standard instant adhesive will not cure on those surfaces. Special grades of CAs contain an additive (ester) that absorbs the surface acidity and thereby allows the adhesive to continue with the normal cure process.

CAs are widely used to bond components in the assembly of blood pressure transducers, infusion pumps, endoscopes, intravenous sets, catheters, orthopaedic devices, cast boots, hearing aids and
diagnostic imaging equipment.

Polyolefin primer
Some plastics, in particular, polypropylene, polyethylene and acetal grades, have a low surface energy and although a CA will cure, it will not wet the surface properly and the resulting adhesion will be poor. To remedy those situations, primers are available that modify the surface condition of the plastic and allow the adhesive to bond to the surface. These primers are only compatible with instant adhesives and will not work with UV adhesives, epoxies or other adhesive technologies.

Dispensing equipment
Once the adhesive has been selected, consideration must be given to how it is applied. The easiest way, of course, is straight from the bottle or container, and for some applications that is sufficient. Yet, the use of application equipment can save money because there is little or no wastage, and save time because dispensing can be performed quicker than a manual operation and produce more consistent results.

The most common principle of dispensing is the use of a pressure–time system. Here, the adhesive is placed under pressure and forced out through a nozzle via a dispense valve. The quantity dispensed is controlled by the air pressure and the time the valve is open.

Figure 2: This blood collection unit is securely bonded using an adhesive dispensed from an automated system.

When dispensing in fully automatic manufacturing lines is required, special valves are available to suit individual production requirements. In those circumstances, it is recommended that advice is sought from the valve supplier or the line manufacturer. For smaller applications, a bench top curing system is ideal for batch assembly or the production of specialised parts.
For light curing systems, a variety of equipment is available ranging from light emitting diode lamp units to light chambers. A check should be made to ensure that light cure equipment is engineered to match the photoiniators in the adhesives. For this, it helps to call on the assistance of a supplier that manufactures both the adhesive and the curing equipment. Only by matching the adhesive to the correct curing system will the assembly process achieve the fastest, most consistent cures.
 

Call the specialists
The selection of the adhesive is all too often the last item on the production agenda and that can frequently result in serious and costly delays. Just as there is no one plastic that is suitable for every application, there is no universal adhesive and the bonding process should be regarded as an integral part of the entire design and assembly operation.

This article has provided an overview. More detailed assistance is available from the helpdesk of reputable adhesives manufacturers. By providing those specialists with details of the application, the substrates involved, the production process and other relevant factors, they will be in a position to provide the expert advice needed to make the right selection of adhesives and equipment.

Bob Goss is a Senior Technology Specialist at Henkel Ltd, maker of the Loctite brand, Technologies House, Wood Lane End, Hemel Hempstead HP2 4RQ, UK
tel. +44 1442 278100
e-mail: technicalservice.loctite@uk.henkel.com [6]
www.henkel.com