Designing a medical device comes with many challenges throughout the process. When considering the complexities of the human body, headaches can be inevitable. Creating wearable medical devices that stick to skin can take these challenges to the next level.
For wearable devices to function as intended, they need to have a secure, reliable hold for the desired duration of use. Without a reliable hold, devices could prematurely fall apart or de-bond from skin, rendering it incapable of collecting the data users rely on to make critical health decisions. Adhesives can play a key role.
But selecting the appropriate adhesives is not as straightforward as it may seem. Not every adhesive is right for every application. It requires a thoughtful approach as well as many other considerations like how materials and device components affect whether an adhesive is right for the task.
Whether an obstacle has stumped your team, or you’re tired of the same approach, here are a few questions to ask yourself when selecting an adhesive and to keep in mind throughout the design process.
What Are the End-User Demographics at Play?
A major factor impacting adhesive selection is a user’s age. Not every adhesive is compatible with skin at every stage of life. Full-term newborn skin can reach structural and functional development equal to an adult within the first year of life.1 Adolescent skin presents other obstacles, like increased oil and sweat production. Healthy young adult skin can be the easiest stage of skin to stick to because it’s at its most durable and elastic. As skin continues to age, it loses hyaluronic acid, resulting in a stiffer, drier, less elastic and more fragile state.
Some adhesives could be too strong while others may not be breathable enough. Knowing your end-user’s age, as well as other demographic information, like where they live (more on this in the next section) and their culture, will help guide your adhesive selection process.
What Environmental Conditions Are at Play?
Climate can have significant behavior implications when it comes to adhesive performance. Adhesives that perform as intended in dry, mild climates may not exhibit the same properties in wet, humid climates. Climate also changes how skin acts. Humid climates may make skin dewier and produce more sweat than skin in arid climates. Considering these conditions ahead of time can enhance material choice.
Moisture-vapor transmission rates (MVTRs) can help predict adhesive behavior in high-moisture environments, and in turn, aid in the material selection process. However, devices often impede this function, preventing moisture from evaporating directly through the adhesive in the z-direction. This means that MVTRs typically reported may not be applicable for adhesives used underneath wearables. Selecting an adhesive with a nonwomen backing, which allows some x-y transmission of moisture beneath the device, can improve performance compared to film-backed alternatives.
What Will This Adhesive Stick To?
Device components and skin act differently. One of them has to breathe. It sounds like a no-brainer, but where that distinct difference can be forgotten is with adhesive selection. If the adhesive is sticking the device to skin, it needs to allow the skin to breathe, flex, move, and expel moisture as it normally would. Skin also grows hair and, as discussed earlier, changes with age, making conformability and gentleness upon removal particularly important.
Alternatively, if the adhesive is adhering device components together, breathability and gentleness aren’t as important. The intent is to keep parts from shifting or rubbing against each other, making strength and resistance to friction the important characteristics.
Where on the Body Will the End-User Wear the Device?
Thickness of the skin’s three layers — the epidermis, dermis, and subcutis — varies across the body. From the thickest areas of the epidermis and dermis, such as on the palms of hands and bottoms of feet, to the thinnest, such as the eyelid, the sensitivity changes and influences how skin may react to the device.
When it comes to wearable design, thinner and/or highly sensitive skin can be less tolerant of devices covering it or adhesives sticking to it. This consideration adds further criteria that adhesives and other materials must meet to help ensure that the device can fulfill its intended purpose.
What Wear Time Am I Aiming For?
Wear time can prompt difficult tradeoffs for designers who need to balance their desired duration with skin’s needs. The first step in deciding on an adhesive that is compatible with the intended wear time is knowing that adhesives adhere differently over specific amounts of time. Some stay stagnant in adhesion strength throughout the entire duration of wear, while adhesion strength increases over a few days.
It’s not uncommon to assume that the strongest adhesive is the ideal option because it will maximize wear time. When it comes to stick-to-skin applications, though, the strongest adhesive is not always the best. If a device only has a wear time of a few days, an adhesive that’s at peak strength during removal can strip away the top layers of the skin. Ideally, longer-term wear devices, potentially worn up to two weeks, not only require a strong adhesive, but also one that is gentle and breathable to keep skin healthy.
Will This Adhesive Be Compatible with My Other Materials?
As mentioned earlier, not every adhesive is right for every application. The other materials you plan to use can also influence your adhesive selection.
Silicone rubber, for example, is often used for its flexible nature, but it can be hard to stick to, limiting your adhesive choices. Other common material choices, such as polycarbonates, acrylics, and polyolefin, also aren’t compatible with every adhesive.
On top of this, plasticizers can challenge the design process, too. Plasticizers are usually used to make device materials softer and more flexible. Unfortunately, they can migrate into the device’s adhesive, creating an unintentionally softer adhesive and jeopardizing the device’s success.
Doing the leg work ahead of time to determine the best compatibility for each design can make or break the success of your product. The best way to ensure that materials are compatible is to discuss all of the options you plan to use with your materials supplier. An experienced supplier will be able to counsel and work with you to find the best-suited solution for your device.
What Sterilization Method Do I Plan to Use?
Sterilization can alter some materials’ properties. Considering which sterilization method you would like to use on the front end can help save designs from costly changes down the line. Take the time to understand the three most common sterilization methods — ethylene oxide, e-beam, and gamma radiation — and how each may affect the finished device so you can mitigate risks before the device is nearly finished.
How Will My Design Respond to the Manufacturing Process?
Not all adhesives are compatible with all manufacturing processes. Some adhesives are unable to withstand the speed and friction of certain processes, while some are too soft and will gum up equipment. Others are incapable of bonding at the needed performance level. Run experiments with your converting partner using a final device on a pilot or full-scale process equipment to learn how larger-scale production will go. It will help avoid redesigns, delays, or cost overruns.
While these questions and considerations will get you started, there are still more factors that could influence the medical device design process.
- Oranges, T., (2015). Skin physiology of the neonate and infant: clinical implications. Advances in Wound Care. Vol (4) 10: 595- 604.
This article was written by Del R. Lawson, PhD, R&D manager in 3M’s Medical Solutions Division, and Tony Kaufman, New Business Ventures in 3M’s Medical Materials & Technologies. For more information, visit here . For additional help in selecting the right adhesive, go here . To learn more about bringing a wearable medical device to market, download “The engineers guide to wearables: Lessons learned from design mishaps” here .