Designing UV light curing adhesives into medical devices

Light curing adhesives are extensively used in medical device manufacturing due to their process advantages, finding applications in products like catheters, syringes and needles, anaesthesia masks, reservoirs, tube sets, and medical electronics assembly.

Adhesives have advantages over alternative fastening methods like solvent bonding, welding or mechanical interlocks that include even distribution of load or stress, elimination of joint fatigue, improved impact resistance, and good aesthetics.

Taking a holistic view of designing in light curing adhesives during the early stages of a medical device project involves considering the entire bonding process including everything from substrate materials to joint design, surface preparation, quality control, adhesive application, and cure

Bond design considerations

When designing a device which needs bonding, a recommended approach is to consider both the functional nature of the bond strength and robustness, and its processability.

The highest possible strength adhesive bond will be useless if the bonding process turns out to be too expensive or takes too long. So the adhesive application and cure must be achievable instead within the constraints of the product geometry and production time.

One key benefit of using UV light curing materials is that they cure in seconds and are single part which means no mixing and reduced waste. This reduces work in progress, reduces the need for jigs and tooling, and frees up factory space.

Bondline design - adhesive needs space of its own

An important consideration in bondline design is that the adhesive needs some space of its own, so friction fit joints aren't usually suitable - the adhesive will not work if it is squeezed out of the joint.

Consider an optimal bondline gap of around 0.125 mm. UV curing adhesives are available in a wide viscosity range, from wicking and self-levelling grades to thixotropic pastes, and have the ability to cope with bondline variations and the need to gap fill if necessary.

Choose the right materials

To design in a UV light curing material, start with selecting bondable substrates. This typically means avoiding very low surface energy materials like PTFE or polyolefins, although if these are desirable for functionality you can consider surface treatment to improve wettability. Most UV curing materials have poor adhesion to rubbers and silicones, although the latest hybrid cyanoacrylate/UV adhesives can be considered for these substrates.

For the adhesive to cure, light of the correct wavelength must reach the entire bondline. This means, in most cases, one of the substrates will need to be clear so that the bondline is visible. If this is not possible, there are products on the market that are dual cure offering a cyanoacrylate surface cure as well as UV light curing, meaning the material will cure interfacially in opaque areas.

Many polymers contain UV blocking agents to prevent yellowing and embrittlement from ambient UV light. Adhesives that cure using a combination of UV and visible light can be specified to ensure cure speed and depth for bonding these plastics.

UV curing adhesives are resilient and tough

Generally, UV curing adhesives are resilient and tough with high tensile, shear, and peel strength, and a cured hardness range of Shore A80 to D80. Medical device manufacturers can consider them as a structural bonding agent for multiple substrates, from glass, to metal, to many types of plastic, including those used in certain medical device assemblies like PEEBAX or PEEK.

Adding UV curing adhesives into production is easy

UV curing adhesives are easy to implement due to their fast cure, solvent-free single component formulation, and choice of viscosity. They can be applied using a wide variety of dispensing techniques with repeatable precision, and are readily automatable.

Yields are typically high because the process is very controllable and repeatable. Because cure happens rapidly, you can implement quality assurance procedures immediately after assembly, or even in-line. Because there are few variables, locking down and validating the process is readily achieved.

UV curing lamps based on LED technology have become the default choice due to low energy consumption and stable output intensity. Modern UV curing adhesives have been formulated to cure with the narrow spectrum of LED UV curing lamps, and many are wavelength specific.

Dymax MD UV adhesives for medical device assembly

Dymax MD Medical Device Adhesives are light curing adhesives formulated with medical device assembly in mind. These adhesives meet ISO 10993 standards and biocompatibility and are resistant to common sterilisation methods including EtO and gamma. They have excellent adhesion to substrates common in medical device manufacturing such as PC, ABS, PVC, stainless steel, polycarbonate, glass and more.

Dymax MD light curing adhesives have several advantages that can provide assembly cost savings compared to, for example, two-part epoxies. As single part materials, they are straightforward to dispense directly from their syringe packaging and easily automated. Once dispensed, the adhesive placement can be checked in-line as the adhesive changes colour as it cures. Dymax MD adhesives cure nearly instantaneously on demand with UV light and can be moved on to the next stage of assembly immediately. Their in-line quality control features combined with precise, repeatable automated dispensing result in reduced material waste and fewer scrapped parts.

Intertronics is the leading supplier of Dymax UV curing adhesives and UV Curing equipment in the UK. You can talk to the company at any time about UV adhesives, encapsulants and other UV curable materials. Intertronics' experienced team of specialists will help you plan how to add UV curing into your research labs and production environments. You can also visit the company's Technology Centre in Kidlington, Oxfordshire to test adhesives and automation processes with your own substrates.