Selecting an adhesive when designing a wearable medical device is not a trivial exercise. Too often the adhesive selection process is executed with the mindset “tape is tape” — a major oversimplification; or, it's assumed that the strongest adhesive for every application is the best choice.
From its own characteristics in different situations to the way it interacts with other device materials — including plastics, adhesive impacts device functionality and accuracy, so it should be thought about early in the design process. Once device housing material or finishing processes are decided, choosing the wrong adhesive could mean product launch delays or unforeseen costs. Though a seemingly small component, adhesive plays a role in the overall success of a device.
So, what can happen when adhesives aren't given careful consideration? It can result in manufacturing issues, device malfunctions, and even harm to the user. This article explores these three levels, progressing from bad to worse.
The Bad: Manufacturing Issues
You've made it through the design process, and it's now time to bring your product to life.
Adhesives can cause manufacturing issues for a variety of reasons. Two of the main causes are that the adhesive is too soft, or the other materials are incompatible with the speed of manufacturing.
When the adhesive is too soft, it has the potential to gum up equipment during production and converting processes, causing a fast-paced assembly line to stop for unscheduled cleaning. This is where unforeseen costs and project delays can become a reality.
When materials are incompatible with the speed of manufacturing, it can become painfully obvious that not all materials are capable of withstanding high-friction processes. An inadequate liner or support film, for example, that is removed from its adhesive before the adhesive is applied to a substrate can break or fail.
Adhesive may seem like it pales in significance when compared to other components, but it has an impact and can greatly affect a manufacturing schedule and overall project cost.
From the beginning of the design process, work with a materials supplier to determine which adhesive and backing combinations are best for a device's manufacturing process.
Getting Worse: Device and Adhesive Fails
Even once a device is manufactured, adhesive can cause adverse effects. One of the biggest concerns involves device accuracy. Below are three applications in which this plays out.
For stick-to-skin applications — Accuracy depends on the adhesive adhering to skin for a specific duration. If an insufficient adhesive is used, a device could prematurely detach from the skin.
For adhesives sticking components together — How well the components stick together impacts accuracy. When parts move, the device will no longer be as precise. An extended border, or skirt, around the device can help keep the device in its intended location, as well as improve stability and increase wear time.
For lab-on-a-chip devices — The wrong adhesive can negatively impact the bioassay being tested. Remnants from manufacturing, finishing, or other processes can jeopardize the assay's integrity, affecting the reading's accuracy. Including a cleaning step in the manufacturing process can help keep this from happening.
Beyond accuracy, device reliability is an area of concern because it can also have a detrimental effect on the device's success. For instance, when an insufficient bonding method is used, the flex circuit could debond from a printed circuit board. Using a conductive pressure sensitive adhesive (PSA) can, in some situations, replace solder connections, which can also make for a quicker, more efficient conductive connection. Be mindful of which PSA is used because some can create an inconsistent or incomplete connection.
Incompatible adhesive and other material combinations can also cause adhesives to combine with other device materials. One category to be aware of is plasticizers. As chemical compounds that, when added to other plastic or rubber device base materials, create softer, more flexible features, they can cause bonding issues when the adhesive is stuck to one of these altered materials. The initial bond is typically good, but as time goes on, the plasticizer can migrate onto the adhesive and, just as it does with plastic or rubber, soften the adhesive. It takes time for the issue to become apparent — meaning it isn't often seen early in the device's evaluation — making it a particularly frustrating issue. Engage a knowledgeable adhesives supplier to identify more tolerant solutions if you need to use a plasticizer.
The Worst: Risk to Users and Their Skin
Unlike other substrates, skin is a living, breathing organ. It helps keep us healthy and at just the right temperature, while also protecting us from infection and environmental harm. In order to do that, skin must be able to move, flex, and expel moisture as it normally would, meaning that when it comes to device design, skin must be prioritized.
In the world of adhesives, not all are skin-friendly. On one end of the spectrum, some will leave residue on the user's skin or attract dirt and lint. No real damage is done, but these annoyances are preventable with the right adhesive. On the other end of the spectrum, there are more serious and potentially harmful outcomes. Two of the most common issues to be aware of are allergic reactions and medical adhesive-related skin injuries (MARSIs). Allergic reactions can happen when an adhesive is incompletely cured or finished, causing materials to migrate into or onto the skin. MARSIs are mostly caused by choosing an inappropriate adhesive for a specific application. While a seemingly mild consequence, MARSIs range in severity from skin irritations, which can result from, for example, trapped moisture (maceration), or irritated skin follicles (folliculitis), to more serious outcomes, like skin tears or stripping, tension injuries, and blisters. Because skin is a person's first line of defense against infection, and adhesives can compromise that, it's important to know which adhesives are appropriate for stick-to-skin applications.
Once a user experiences pain, they may not want to use that device again or recommend it to others. Fortunately, these issues can be prevented when an optimal adhesive is chosen. Before starting the design process, consider these three questions to get your device design started down the right path:
What kind of substrate will my device adhere to? There are major differences between skin as a substrate and metal or plastic as a substrate. Keep that in mind when choosing an adhesive, and select one that is compatible to the intended substrate.
In what type of environment will my device live? Feeling the differences between a humid environment and a dry environment isn't difficult. Consider humidity levels when selecting an adhesive because materials that hold up in one climate may not function as intended in another. If moisture needs to move through the device and adhesive, the adhesive should be breathable to avoid swelling.
What's the device's intended wear time? A key factor that comes into play here is adhesive strength. Not all adhesives adhere in the same manner over the same length of time. Therefore, if an adhesive only needs to adhere for a maximum of three days, don't pick one that's meant to stick for 10–14 days. If an adhesive is in its prime strength when it needs to be removed, a skin injury could occur. Be careful to not overdesign a device.
While not a comprehensive list, these considerations can get a project started down the right path. Harder-to-answer, project-specific questions will also need to be addressed. For more guidance in selecting the right adhesive for your next medical device project, visit FindMyAdhesive.com and answer a series of questions specific to your project to identify a list of the most appropriate medical adhesive suggestions.
This article was written by Tony Kaufman, New Business Ventures in 3M's Medical Materials & Technologies, and Del R. Lawson, PhD, R&D manager in 3M's Medical Solutions Division, Maplewood, MN. For more information, click here.