Too often the adhesive selection for a medical device is left to the end of a project. It doesn’t help that there are so many adhesives as to make the choice overwhelming. With so many potential negative outcomes, here are tips on making the right choice.
Tony Kaufman, New Business Ventures in 3M’s Critical and Chronic Care Solutions Division
Del R. Lawson, Ph.D., R&D manager in 3M’s Critical and Chronic Care Solutions Division
If adhesive is an integral component in your medical device design, it needs to be thought about sooner rather than later. It’s often a factor left to consider at a point too late in the product development process – where changes in, say, device housing material or plans for final finishing processes would be too expensive or cumbersome to execute.
Too often adhesive selection is approached with the mindset “tape is tape.” Perhaps different kinds might be interchangeable in everyday life, but that’s a dangerous oversimplification when designing a product someone will eventually be wearing and perhaps depending on to make critical health decisions. An adhesive’s characteristics and how it interacts with other device materials directly impacts the integrity and success of a device, regardless of whether the adhesive is adhering device components together or sticking the device directly to skin.
So, what can happen when adhesives aren’t given careful consideration? Below are three levels of potential outcomes, progressing from bad to worse.
Level one: Manufacturing issues
You’ve designed a device and the prototype looks, feels and even functions how it’s intended. You and your team are rightfully proud of what you’ve created and are excited to keep it moving forward. Next stop, the production floor. Adhesives can cause manufacturing complications for a variety of reasons. For example, an adhesive that is too soft can gum up the equipment during production or converting (cutting to shape). Now a fast-paced assembly line is halted for cleaning, causing production delays that add unforeseen costs to the project. The choice of liner (the support film that is stripped away from the adhesive before application to the target substrate) is another potentially complicating factor, if the liner properties are not tuned to or consistent with the speed of production. If the liner release is too high, the materials can break or fail during production. By working closely with your supplier early in the process, the adhesive and backing combinations can be formulated to work seamlessly for the performance of the device and optimized to match the speed and friction of manufacturing.
Adhesive issues can also show up after the final product has been made. For example, imagine you are making a sterile bandage or wound care dressing. The adhesive cut well and is sealed and packaged all in-line. However, the adhesive chosen may be soft and would therefore have a tendency to slightly ooze on the outer edges of the bandage or dressing. Upon opening the package, the bandage or dressing sticks to the packaging, which compromises the sterile delivery of the product. Not only is this an annoyance for the customer, it could also potentially compromise the safety of the product.
Adhesives, though seemingly a small component, can have an immense impact on keeping manufacturing on schedule and its associated costs in check.
Level two: Device and adhesive fails
Even when the device is successfully manufactured, an incompatible adhesive can rear its ugly head. Let’s explore a few of the larger concerns.
Adhesives impact the device’s accuracy. In stick-to-skin applications, accuracy largely has to do with keeping the device adhered to the user’s skin for a specific amount of time. If an insufficient adhesive is used, the device is more likely to prematurely fall off. In applications in which the adhesive is adhering components together, those individual components could move, or sheer in some cases, altering the device’s precision. The addition of an extended border, or “skirt,” around the perimeter of a device can help avoid movement, improving wear duration and stability. Devices, like cardiac monitors, rely on sticking to specific locations for accuracy and proper function of the device.
Another issue that can affect the integrity of a device is called de-bonding. As an example, de-bonding can have a particularly detrimental effect in the case of flex circuits de-bonding from the printed circuit board. If the two are unable to stay connected, the device’s reliability comes into question. In some situations, conductive pressure sensitive adhesives (PSAs) can replace a solder connection, allowing for a quicker, more efficient conductive connection. However, if the wrong PSA is used, the connection can start to come apart, creating inconsistent or incomplete connections, which results in device reliability issues.
Adhesives also have the potential to combine with other device materials, such as plasticizers, if they’re incompatible. Plasticizers are chemical compounds added to other plastic materials to create softer, more flexible features for the base material of the device. PVC and many rubber materials use plasticizers to change their properties. Although the initial bond to these materials is typically good, it is the adhesion over time that poses a concern. The plasticizer can migrate from the device’s base material into the adhesive. Just like it does for the plastic or rubber material, the plasticizer softens the adhesive. This can create a very “gooey” situation, changing the properties of the adhesive and base material. This is a particularly frustrating defect, as it is not often seen early in the evaluation of the device – it shows up over time. If you are using plasticizers in your device design, a knowledgeable adhesives supplier can help you identify adhesives that are more tolerant of these materials.
Another important note, specifically for lab-on-a-chip devices, is that the wrong adhesive choice can compromise the integrity of the bioassay that is being tested, rendering it useless. Adhesives can also have remnants in them from manufacturing, finishing or other processes that can interact with the assay, affecting the accuracy of its read.
Level three: Risk to user and their skin
Skin cannot be treated like just any other substrate– it is a living organ. It’s one of our body’s best sensors – detecting subtle temperature shifts and letting us know when we are hot and cold – and we have to be careful with it. Skin also knows and can be impacted when an inappropriate adhesive is applied to it.
Some outcomes in this area are more annoying than catastrophic, like the adhesive leaving residue on skin or picking up lint. These are more unsightly than the cause of real damage. But still – these annoyances don’t have to happen, and they shouldn’t.
On the other hand, some outcomes are more serious and can potentially result in harm to the user. Allergic reactions and medical adhesive-related skin injuries (MARSIs) are common adverse reactions to applying the wrong adhesive to skin. Incomplete curing or finishing of an adhesive can result in residual contaminants migrating into or onto the skin; these are common causes of allergic reactions. It’s also important to know which adhesives are meant for stick-to-skin applications. MARSIs are most commonly caused by choosing an adhesive that’s inappropriate for the application. They can compromise the integrity of the wearer’s skin. Some common types of MARSI are skin irritations like folliculitis (small rash-like bumps on the skin’s surface that are a result of irritation of hair follicles) and maceration (whitened layer of skin caused by trapped moisture, essentially drowning the skin), which increase the vulnerability of the skin; tension injuries and blisters (typically caused by adhesives that are stretched when applied and create excess tension on the skin as the tape relaxes); skin tears or skin stripping (when the stratum corneum and the epidermis, the first two layers of the skin, pull apart from another during adhesive removal).
The biggest takeaway here, and why skin is so important to the success of a device, is that once the user experiences pain, they won’t want to use that device again or recommend it to others. And in a world where reviews can be proliferated quickly by word of click, an entire company can experience and be affected by the backlash.
A lot can go wrong with an inadequate adhesive selection. The good news is that they are largely preventable. In order to avoid these potentially costly and time consuming reworks, there are three key points to consider before starting the design process:
–The substrate it’ll stick to: There are distinct differences in property characteristics between steel and skin. Choose an adhesive that’s congruent with and friendly to the substrate to which it will stick.
–The environment in which it’ll live: Let’s say you design a device that functions and stores perfectly in the cold Minnesotan climate, but the device will be shipped to and used in a humid South American country. Materials that hold up in one climate may not function as intended in another. For example, consider humidity levels when selecting an adhesive. If moisture needs to move through the device and adhesive, the adhesive needs to be breathable to avoid swelling.
–The length of its lifetime: A device’s intended lifetime impacts the level of durability the device will need to withstand. If a stick-to-skin device needs to adhere for three days, pick an adhesive that’s capable of adhering for that duration, not longer. Over-designing by choosing the strongest adhesive possible is just as, or more so, dangerous than one meant to adhere for a matter of hours (doing this may cause the device to prematurely fall off).
While not a comprehensive list, these considerations will get a project started down the right path. Additional project-specific questions, which are tougher to answer and often neglected until problems arise, will also need to be addressed.
Even with so many potential negative outcomes, adhesives aren’t as fickle as they may seem. That being said, the plethora of adhesive options can make the choice seem overwhelming. For more help selecting the right adhesive for your next project, check out FindMyAdhesive.com, and answer a series of project-specific questions to identify a list of the most appropriate medical adhesive suggestions.
Filed Under: Adhesives • epoxies, Design World articles, Medical