Sticking to the basics

Advanced manufacturing technology has had multiple effects on the fabrication and assembly of medical devices. The level of complexity that can now be achieved is unprecedented, with the capability to integrate electronics, drug delivery systems, and even smart technology into internal and external devices. The ability to customize medical devices to individual patients’ needs results in better efficacy and comfort.

This technology isn’t magic, however – you can’t just press a button and have a finished product pop out. Many elements go into creating a product such as a medical device, and some of the most important pieces of the puzzle are the least glamorous, especially when compared to flashy 3D printers or other modern fabrication machines. What’s an example of those humble yet critical elements? Adhesive.

Adhesives are much more complex than they may seem to the layperson. There’s a wealth of options when it comes to choosing an adhesive, and they must be selected carefully and used properly. Medical device performance depends on the type of adhesive along with application technique, surface preparation, and consideration of the surface type and manufacturing method.

Without careful selection and preparation, weak adhesion and delamination can occur, causing medical devices to fail in testing. Failure in testing is obviously better than failure when in use by a patient, but it’s still a significant problem in terms of lost time, money, and confidence. To avoid this, manufacturers should learn as much as possible about their adhesive options, ensuring they’ll choose the right one and apply it correctly. Silicone has, for many years, been commonly used in medical devices, as a substrate and an adhesive, but it still presents plenty of complexity.

The best adhesive for the job

“There’s never a one size fits all,” says James Darlucio, senior technologist for biomaterials at NuSil Technology LLC, the silicone manufacturing division of product development company Avantor. “Depending on the substrate, silicone adhesive may work by itself or sometimes, if that surface has low surface energy, meaning it’s hard to adhere to, for example polytetrafluoroethylene (PTFE) or polyethylene (PE) or polypropylene (PP), then it may require other surface preparation such as plasma treatment and silicone primer to improve its adhesion to the silicone substrate.”

Silicone adhesives have numerous advantages in medical device manufacturing. They’re biocompatible, chemically inert, and stable in a wide range of temperatures. They’ve been thoroughly tested and used in medical devices for a long time, so they’re trusted by the medical industry. They’re tough, flexible, and gas permeable, and they’re well-suited to applications ranging from catheters to cochlear implants. Even with such a reliable material, however, it’s important to make sure you’re using the appropriate adhesive.

NuSil silicone adhesives fall into two main categories – one-part and two-part adhesives. One-part adhesives are applied as-is, without the need for mixing, and are moisture cure systems that don’t require heat to cure. These are well-suited for temperature-sensitive components. Two-part adhesives are addition cure systems and have, as the name suggests, two parts mixed together prior to use which can cure at room temperature or can cure more rapidly with the use of heat. They’re better for applications requiring rapid throughput. NuSil has several different adhesive products within these classifications and offers custom formulations.

The first thing to consider when deciding on a type of adhesive is the device end application. Will it be implanted inside the body and for how long, or used externally? What’s it made of – plastic, metal, more silicone? What special features does it have? Electronics are an example of a newer type of feature requiring particular attention, especially where temperature is concerned. Often, electronics will have limited heat tolerance, which influences the selection of adhesive – lack of heat tolerance rules out two-part adhesive, for example, which uses heat for curing. Choosing the correct adhesive is often something of a process of elimination – a checklist of needs to be met that narrows down the options until the optimal solution presents itself.

“There are a lot of variables,” Darlucio says. “During the selection process, it’s almost like an interview where we’re trying to figure out the potential variables that the end user’s trying to do with the silicone adhesive, so we can provide them the best possible silicone adhesive for a positive outcome.”

Energetically aligned

Deciding on the best adhesive for a particular application is only step one in the device assembly process. Once the user has settled on the adhesive that best meets their needs, they need to ensure it performs at its best – and that involves some work, especially if the surfaces to be bonded are less than ideally compatible with the adhesive.

When it comes to substrates, they can be low surface energy or high surface energy. Substrates with high surface energy have more chemically reactive components on their surfaces, allowing adhesives to chemically attach to them. Low surface energy substrates, on the other hand, have very minimal reactive components on their surfaces, making it more difficult for adhesives to stick. With these types of materials, additional preparation is often needed before the adhesive can be applied. This can be as simple as starting with a silicone primer, which NuSil also offers. Other times, surface activation may be needed, such as corona discharge or plasma treatment, to allow the substrate to be more receptive to the adhesive. This involves the application of high voltage to an electrode, creating a plasma that actually alters the chemical composition of the substrate, increasing its surface energy.

Keeping it clean; looking toward the future

So the substrate is now primed – is that it? Not yet. Have you ever tried to use tape or sticky tack to attach a poster or flier to a dusty or dirty wall? It doesn’t work very well. Even if the surface isn’t visibly dirty, invisible particles may be lurking to sabotage the stickiness. It’s the same with medical device components – they may look clean, but appearances can be deceiving.

“It’s easy to overlook, because you have parts either coming directly from assembly or from another supplier thinking that they’re already clean, but typically it’s not so,” Darlucio says. “We’re trying to remove any residual oils, dirt, or dust that may have come into contact with a surface. Any type of contaminant between the adhesive and the substrate is usually detrimental to the durability and strength of that bond, so cleanliness is the key step.”

Medical devices will continue to evolve and grow in function and complexity, and the adhesives used to manufacture them will need to evolve as well. According to Darlucio, the most important factor to consider when developing new types of adhesives will be the substrates – especially as more devices incorporate electronics. Adhesives must be formulated to prevent corrosion of delicate components, as well as accommodate certain temperature requirements. One thing Avantor and NuSil have made clear is there’s a lot you can do with a silicone adhesive. The material has been a reliable go-to for medical device manufacturing for years, and with a few tweaks to adhesive formulas here and there, it’s likely to continue to maintain its reputation well into the future.

NUSIL
https://nusil.avantorsciences.com/nusil

About the author: Clare Scott is associate editor for Today’s Medical Developments magazine. She can be reached at CScott@gie.net or 216.393.0314.